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<div>:''This page is part of the topic [[Marine biology in the instrumental period]]''<br />
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The Antarctic spring is one of the planet&rsquo;s principal episodes of oceanic primary production (Hense et al., 2003<ref name="Hense et al, 2003">Hense, I., Timmermann, R., Beckmann, A.and Athmann, U.V. 2003. Regional and interannual variability of ecosystem dynamics in the Southern Ocean, ''Ocean Dynamics'', '''53''', 1-10.</ref>), reaching maximum values of 0.1 mg Chl/l in just a few weeks. As more than 10<sup>7</sup> km<sup>2</sup> of sea ice melts, it releases a huge trapped biomass (Thomas and Dieckmann, 2002<ref name="Thomas and Dieckmann, 2002">Thomas, D.N. and Dieckmann, G.S. 2002. Antarctic sea ice-a habitat for extremophiles, ''Science'', '''295''', 641-644.</ref>). Sunlight continues to increase from spring to summer, driving notable changes within an ecosystem just emerging from a long, dark winter. This explosion of life is immediately followed by a growth spurt in the life cycle of the krill, the organism standing at the base of the food chains for nearly all Antarctic marine vertebrates. As winter approaches, the continental shelf and large areas of the open ocean pass back towards a seasonal coverage of ice more than a metre thick, which is why most of the large predators abandon the high Antarctic at the start of the long austral winter.<br />
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This pattern led to the conception of a long-lasting paradox &ndash; that the ocean around Antarctica experienced pronounced marine seasonality (Clarke, 1988<ref name="Clarke, 1988">Clarke, A. 1988. Seasonality in the Antarctic marine environment, ''Comp. Biochem. Physiol.'', '''90'''(B), 461-473</ref>), with a period of low activity in winter as a consequence of reduced food availability, despite the fact that the sea water temperature remained practically constant all year round.<br />
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While the marked environmental seasonality naturally does influence and condition life in the water column, the first inklings that the Antarctic paradox might not be entirely accurate arose after the discovery of the rich marine fauna that dwells on the continental shelves in the high Antarctic (Gutt et al., 1992<ref name="Gutt et al, 1992">Gutt, J., Gerdes, D. and Klages, M. 1992. Seasonality and spatial variability in the reproduction of two Antarctic holothurians (Echinodermata), ''Polar Biol.'', '''11''', 533-544.</ref>). Over the past twenty years, the region has been shown to host one of the most diverse, high-biomass benthic communities in the ocean (Clarke and Johnston, 2003<ref name="Clarke and Johnston, 2003">Clarke, A. and Johnston, N.M. 2003. Antarctic marine benthic diversity, ''Oceanogr. Mar. Biol. Ann. Rev.'', '''41''', 47-114.</ref>). Suspension feeders constitute the bulk of these communities, which depend on the particles settling down from the upper layers of the water column or laterally advected to them by currents. Due to low temperatures, a large number of species have slow metabolic rates, associated with a low energy demand, yet they still attain considerable age and size (Peck et al., 2006<ref name="Peck et al, 2006">Peck, L.S., Convey, P. and Barnes, D.K.A. 2006. Environmental constraints on life histories in Antarctic ecosystems: tempos, timings and predictability, ''Biol. Rev.'', '''81''', 75-109.</ref>). This and other traits connected with reproduction patterns would at first glance appear to be in harmony with the tenets of the Antarctic paradox, rooted in the dormant state thought to prevail in winter. However, new features forcing the scientific community to reconsider the Antarctic Paradox have recently come to light. For instance, quite a few species exhibit reproduction rates similar to those in other regions, while others demonstrate higher growth rates than expected by quickly occupying areas scraped clean by icebergs (Teixid&oacute; et al., 2004<ref name="Teixid&oacute; et al, 2004">Teixid&oacute;, N., Garrabou, J., Gutt, J. and Arntz, W.E. 2004. Recovery in Antarctic benthos after iceberg disturbance: trends in benthic composition, abundance and growth forms, ''Mar. Ecol. Prog. Ser.'', '''278''', 1-16.</ref>). Experimental observations have furnished earlier selected results (Barnes and Clarke, 1994<ref name="Barnes and Clarke, 1994">Barnes, D.K.A. and Clarke, A. 1994. Seasonal variation in the feeding activity of four species of Antarctic bryozoa in relation to environmental factors, ''J. Exp. Mar. Biol. Ecol.'', '''181''', 117-133.</ref>, 1995<ref name="Barnes and Clarke, 1995">Barnes, D.K.A. and Clarke, A. 1995. Seasonality of feeding activity in Antarctic suspension feeders. Polar Biol., 15, 335-340.</ref>) supporting the assumption that the long Antarctic winter may not be as inactive as hitherto thought. These findings include:<br />
<ol start="1"><br />
<li>The existence of &ldquo;food banks&rdquo; extending over hundreds of kilometres, offering a potential food source for numerous bottom-dwelling organisms (Mincks et al., 2005<ref name="Mincks et al, 2005">Mincks, S.L., Smith, C.R. and Demaster, D.J. 2005. Persistence of labile organic matter and microbioal biomass in Antarctic shelf sediments: evidence of a sediment &lsquo;food bank&rsquo;, ''Mar. Ecol. Prog. Ser.'', '''300''', 3-19.</ref>). This phenomenon also known as &ldquo;green carpets&rdquo; tends to form at the beginning of the austral spring, when the high primary production generated by melting ice is not immediately exploited by planktonic grazers and settles on the shelf seabed in a time span of hours to days (Gutt et al., 1998<ref name="Gutt et al, 1998">Gutt, J., Starmans, A. and Dieckmann, G. 1998. Phytodetritus deposited on the Antarctic shelf and upper slope: its relevance for the benthic system, ''J. Mar. Syst.'', '''17''', 435-444.</ref>).</li><br />
<li>Widespread distribution of seabed sediment with high nutritive quality and grain sizes suitable for the anatomic structures of benthic suspension feeders. On average, measured concentrations of protein (3 mg/g) and lipids (2 mg/g) are higher than on other continental shelves and similar to the contents found in settling particles (Smith et al., 2006<ref name="Smith et al, 2006">Smith, C.R., Minks, S. and Demaster, D.J. 2006. A s&iacute;ntesis of bentho-pelagic coupling on the Antarctic shelf: Food banks, ecosystem inertia and global climate change, ''Deep-Sea Res. II'', '''53''', 875-894.</ref>).</li><br />
<li>Tides acting as an incessant mechanism to resuspend the &ldquo;food banks&rdquo; and supply particles to suspension feeders throughout the year (Smith et al., 2006<ref name="Smith et al, 2006">Smith, C.R., Minks, S. and Demaster, D.J. 2006. A s&iacute;ntesis of bentho-pelagic coupling on the Antarctic shelf: Food banks, ecosystem inertia and global climate change, ''Deep-Sea Res. II'', '''53''', 875-894.</ref>).</li><br />
<li>Benthic suspension feeders on Antarctic shelves feeding on small-sized particles in contrast to species from other latitudes that mainly ingest zooplankton (Orejas et al., 2003<ref name="Orejas et al, 2003">Orejas, C., Gili, J-M. and Arntz, W. 2003. Role of small-plankton communities in the diet of two Antarctic octocorals (Primnoisis antarctica and Primnoella sp.), ''Mar. Ecol. Prog. Ser.'', '''250''', 105-116.</ref>).</li><br />
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[[File:Figure 4.52 - Seasonal cycles of Antarctic shelf seabed organic matter transport processes.png|thumb|'''4.52''' Synoptic view of the processes described in the text showing the seasonal vertical flux of new organic matter originated mainly at the beginning of spring (green line), the seasonal variation of food banks and the lateral and resuspension transport just above the seabed (arrows close the bottom).]]<br />
The new evidence of the physical-chemical conditions on the shelf seabed at Antarctic latitudes makes it necessary to reconsider the paradox that had formerly served as a cornerstone for understanding polar ecosystems. After the summer, resuspension by tidal currents and the high nutritional quality of the seabed sediment allow benthic trophic conditions to remain almost constant throughout the year, which provides the basis for a new model of Antarctic seasonality. The new model helps to explain the high diversity and high biomass of benthic communities around Antarctica, even when the food input from the euphotic zone becomes scarce when ice covers the ocean surface during the winter months. These new findings must be taken into account when planning future research on the Antarctic bottom-dwelling fauna. Special emphasis should be placed on carrying out studies during the austral winter, when processes occurring near the seabed ([[:File:Figure 4.52 - Seasonal cycles of Antarctic shelf seabed organic matter transport processes.png|Figure 4.52]]) could be a key to understanding both the high productivity of the system in the early spring and the high biodiversity of the benthic ecosystem (Gili et al., 2006<ref name="Gili et al, 2006">Gili, J-M., Arntz, W.E., Palanques, A., Orejas, C., Clarke, A., Dayton, P., Isla, E., Teixid&oacute;, Rossi, S. and L&oacute;pez-Gonzales, P.J. 2006. A unique assemblage of epibenthic sessile suspension feeders with archaic features in the high-Antarctic, ''Deep-Sea Res. II'', '''53''', 1029-1052.</ref>).<br />
==References==<br />
<references /><br />
[[Category:The instrumental period]]<br />
[[Category:Antarctic biology]]<br />
[[Category:Marine biology]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_West_Antarctic_cryosphere_in_the_instrumental_periodThe West Antarctic cryosphere in the instrumental period2014-08-06T14:34:24Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[The ice sheet in the instrumental period]]''<br />
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West Antarctica has received particular attention because it remains as the last &ldquo;marine-based&rdquo; ice sheet, a configuration that was suggested to be inherently unstable, fated to oscillate between fully extended to the edge of the continental shelf or completely lost, having suffered accelerating retreat (Weertman, 1974<ref name="Weertman, 1974">Weertman, J. 1974. Stability of the junction of an ice sheet and an ice shelf, ''J. Glaciol.'', '''13''', 3-11.</ref>). Mercer (1968<ref name="Mercer, 1968">Mercer, J.H. 1968. Antarctic ice and Sangamon sea level, IASH publ. 79, 217-225.</ref>) suggested that full collapse of the West Antarctic ice sheet had occurred as recently as the last interglacial, 125,000 years ago and the concern driving much of the research of the West Antarctic ice sheet was whether such an eventuality was inevitable or even underway.<br />
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The West Antarctic ice sheet is conveniently divided into three sectors, each feeding ice into one of the major surrounding seas: the Ross Sea, the Amundsen Sea and the Weddell Sea. Being closest to the US research station at McMurdo, major US field research proceeded on the ice streams of the Ross Sea sector along the Siple and Gould Coasts. Meanwhile, UK field research was focused on the ice streams of the Weddell Sea sector that were similarly closer to their major station at Rothera. Ironically, the largest changes were observed by satellite to be occurring in the Amundsen Sea sector. Each area is discussed in the following sections, beginning with the area exhibiting the largest changes.<br />
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==Amundsen Sea Embayment==<br />
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The Amundsen Sea sector represents approximately one third of the entire WAIS. Recent observations have shown that this is currently the most rapidly changing region of the entire Antarctic ice sheet. Long before these observations were available the vulnerability and potential significance of retreat in this area was highlighted in a prescient paper by Hughes (1973<ref name="Hughes, 1973">Hughes, T.J. 1973. Is the West Antarctic ice sheet disintegrating?, ''J. Geophys. Res.'', '''78'''(33), 7884-7910.</ref>).<br />
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[[File:Figure 4.39 - Recent and longer term thinning of the Amundsen Sea Embayment sector ice sheet.png|thumb|'''4.39''' Recent and longer term thinning of the Amundsen Sea Embayment sector ice sheet. Regional colours represent rate of elevation change derived from two decades of satellite radar altimetry. Circled colours represent recent elevation changes derived from airborne altimetry flown in 2002. The difference in the two surveys indicates a recent increase in the rate of thinning (from Thomas et al., 2004b<ref name="Thomas et al, 2004b">Thomas, R.H., Rignot, E., Kanagaratnam, P., Krabill, W. and Casassa, G. 2004b, Force perturbation analysis of Pine Island Glacier suggests cause for recent acceleration, ''Ann. Glaciol.'', '''39''', 133-138.</ref>)]]<br />
Thinning of the ice in the Amundsen Sea sector occurred because of an increase in the discharge of several major outlet glaciers ([[:File:Figure 4.39 - Recent and longer term thinning of the Amundsen Sea Embayment sector ice sheet.png|Figure 4.39]]). Rignot (1998b<ref name="Rignot, 1998b">Rignot, E. 1998b. Fast recession of a West Antarctic Glacier, ''Science'', '''281''', 549-551, (doi:10.1126/science.281.5376.549).</ref>) first reported flow acceleration and subsequent grounding line retreat of Pine Island Glacier, one of the two largest West Antarctic outlet glaciers draining into the Amundsen Sea. This retreat has been accompanied by thinning of the ice sheet at a rate of 10 cm/year averaged over a drainage basin twice the area of Great Britain (Wingham et al., 1998<ref name="Wingham et al, 1998">Wingham, D.J., Ridout, A.L., Scharroo, R., Arthern, R.J. and Schum, C.K. 1998, Antarctic elevation change from 1990 to 1996, ''Science'', '''282''', 456-458, (doi:10.1126/science.282.5388.456).</ref>). Thinning rates reach well over 1 m/yr at the coast (Shepherd et al., 2001<ref name="Shepherd et al, 2001">Shepherd, A., Wingham, D.J., Mansley, J.A.D. and Corr, H.F.J. 2001. Inland thinning of Pine Island Glacier, West Antarctica, ''Science'', '''291''', 862-864, (doi:10.1126/science.291.5505.862).</ref>). This discovery of a 10% increase in flow speed in 4 years was anticipated based on oceanographic evidence of very high and increasing basal melt rates beneath the ice tongue fronting the glacier (Jacobs et al., 1996<ref name="Jacobs et al, 1996">Jacobs, S.S., Hellmer, H.H. and Jenkins, A. 1996. Antarctic ice sheet melting in the southeast Pacific, Geophysical Research Letters, 23(9), 23(9), 957-960.</ref>; Jenkins et al., 1997<ref name="Jenkins et al, 1997">Jenkins, A., Vaughan, D.G., Jacobs, S.S., Hellmer, H.H. and Keys, J.R. 1997. Glaciological and oceanographic evidence of high melt rates beneath Pine Island Glacier, West Antarctica, ''Journal of Glaciology'', '''43'''(143), 114-121.</ref>). Later direct measurement of elevation loss near the grounding line and an assumption of flotation at hydrostatic balance, revealed rates as high as 58 &plusmn; 8 m/yr with an ice shelf wide average of 24 &plusmn; 4 m/yr. (Rignot, 2006<ref name="Rignot, 2006">Rignot, E. 2006. Changes in ice dynamics and mass balance of the Antarctic ice sheet, Phil. Trans. R. Soc. A, 364, 1637-1655; doi:10.1098/rsta.2006.1793</ref>), exceeding the previously value of 15 m/yr (Shepherd et al., 2004<ref name="Shepherd et al, 2004">Shepherd, A., Wingham, D. and Rignot, E. 2004. Warm ocean is eroding West Antarctic ice sheet, ''Geophys. Res. Lett.'', '''31''', L23402, doi:10.1029/2004GL021106.</ref>). As basal melt increased, the grounding line retreated, possibly in two stages&mdash;during the 1980s and in 1994-96 - each leading to a separate increase in speed (Rignot, 1998b<ref name="Rignot, 1998b">Rignot, E. 1998b. Fast recession of a West Antarctic Glacier, ''Science'', '''281''', 549-551, (doi:10.1126/science.281.5376.549).</ref>; Joughin et al., 2003<ref name="Joughin et al, 2003">Joughin, I., Rignot, E., Rosanova, C., Lucchitta, B. and Bohlander, J. 2003. Timing of recent accelerations of Pine Island Glacier, Antarctica. Geophysical Research Letters, 30(13), doi:10.1029/2003GL017609.</ref>). Most recently Rignot (2008<ref name="Rignot, 2008">Rignot, E. 2008. Changes in West Antarctic ice stream dynamics observed with ALOS PALSAR data, ''Geophysical Research Letters'', '''35''', L12505, doi:10.1029/2008GL033365.</ref>) has shown that the grounding line at Pine Island has retreated still further, with a simultaneous increase in both speed and acceleration. Pine Island Glacier is now moving at speeds nearly double those in the 1970s. No data are available to determine if an earlier period of acceleration occurred, however, a study of past images of Pine Island Glacier&rsquo;s ice shelf indicate that thinning, possibly by as much as 134 metres, occurred in 28 years with significant shifts to the lateral margins, including a major flow shift, beginning perhaps as early as 1957 (Bindschadler, 2001<ref name="Bindschadler, 2001">Bindschadler, R., 2001. Flow History of Pine Island Glacier from Landsat Imagery, Eos Transactions (Fall 2001).</ref>).<br />
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Other glaciers in the Amundsen Sea sector have been similarly affected: Thwaites Glacier is widening on its eastern flank, and there is accelerated thinning of four other glaciers in this sector to accompany the thinning of Thwaites and Pine Island Glaciers (Thomas et al., 2004a<ref name="Thomas et al, 2004a">Thomas, R.H., Rignot, E.J., Casassa, G., Kanagaratnam, P., Acu&ntilde;a, C., Akins, T.L., Brecher, H., Frederick, E.B., Gogineni, S.P., Krabill, W.B., Manizade, S., Ramamoorthy, H., Rivera, A., Russell, R., Sonntag, J., Swift, R., Yungel, J., and Zwally, H.J. 2004a. Accelerated Sea-Level rise from West Antarctica. Science, 306, 255-258.</ref>). Where flow rates have been observed, they too show accelerations, e.g., Smith Glacier has increased flow speed 83% since 1992.<br />
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Calculations of the current rate of mass loss from the Amundsen Sea embayment range from 50 to 137 Gt/yr with the largest number accounting for the most recent faster glacier speeds (Lemke et al., 2007<ref name="Lemke et al, 2007">Lemke, P., Ren, J., Alley, R., Allison, I., Carrasco, J., Flato, G., Fujii, Y., Kaser, G., Mote, P., Thomas, R. and Zhang, T. 2007. Observations: change in snow, ice and frozen ground. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 337-384 (Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M, Miller HL, Eds), Cambridge University Press, Cambridge, UK.</ref>; Rignot et al., 2008<ref name="Rignot et al, 2008">Rignot, E., Bamber, J.L., Van Den Broeke, M.R., Davis, C., Yonghong, L., Van Deberg, W.J. and Van Meijgaard, E. 2008. Recent Antarctic ice mass loss from radar interferometry and regional climate modeling, Nature Geoscience, 13 January 2008; doi:10.1038/ngeo102.</ref>). Data sources and methodologies vary, but generally when uncertainties and the time intervals analyzed are considered, the estimates are consistent with accelerating rates of loss, in concert with the accelerations of the primary discharging glaciers. These rates are equivalent to the current rate of mass loss from the entire Greenland ice sheet. The Pine Island and adjacent glacier systems are currently more than 40% out of balance, discharging 280 &plusmn; 9 Gt/yr of ice, while they receive only 177 &plusmn; 25 Gt/yr of new snowfall (Rignot et al., 2008<ref name="Rignot et al, 2008">Rignot, E., Bamber, J.L., Van Den Broeke, M.R., Davis, C., Yonghong, L., Van Deberg, W.J. and Van Meijgaard, E. 2008. Recent Antarctic ice mass loss from radar interferometry and regional climate modeling, Nature Geoscience, 13 January 2008; doi:10.1038/ngeo102.</ref>; see also Thomas et al, 2004b<ref name="Thomas et al, 2004b">Thomas, R.H., Rignot, E., Kanagaratnam, P., Krabill, W. and Casassa, G. 2004b, Force perturbation analysis of Pine Island Glacier suggests cause for recent acceleration, ''Ann. Glaciol.'', '''39''', 133-138.</ref>). The increasingly negative mass balance is confirmed by several recent radar altimetry assessments of reduction in surface elevation of the Pine Island catchment (e.g., Zwally et al., 2005<ref name="Zwally et al, 2005">Zwally, H.J., Giovinetto, M., Li, J., Cornejo, H., Beckley, M., Brenner, A., Saba, J. and Yi, D. 2005. Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002, ''Journal of Glaciology'', '''51'''(175), 509-527.</ref>; Rignot et al., 2008<ref name="Rignot et al, 2008">Rignot, E., Bamber, J.L., Van Den Broeke, M.R., Davis, C., Yonghong, L., Van Deberg, W.J. and Van Meijgaard, E. 2008. Recent Antarctic ice mass loss from radar interferometry and regional climate modeling, Nature Geoscience, 13 January 2008; doi:10.1038/ngeo102.</ref>).<br />
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[[File:Figure 4.40 - Water masses in the Antarctic coastal zone.png|thumb|'''4.40''' Water masses in the Antarctic coastal zone. (Dieckmann, unpublished)]]<br />
Summer temperatures in the Amundsen Sea embayment rarely reach melting conditions, and there is little reason to assume that atmospheric temperatures have had any strong role to play in the changes that have occurred there. Similarly, the patterns of thinning, which are very clearly concentrated on the most dynamic parts of the glaciers, indicate that the changes are not the result of anomalous snowfall. The most favoured explanation for the changes (e.g. Payne et al., 2004<ref name="Payne et al, 2004">Payne, A.J., Vieli, A., Shepherd, A.P., Wingham, D.J. and Rignot, E. 2004. Recent dramatic thinning of largest West Antarctic ice stream triggered by oceans, ''Geophysical Research Letters'', '''31''', L23401, doi: 10.1029/2004GL02184.</ref>) is a change in the conditions in the sea into which this portion of West Antarctica flows ([[:File:Figure 4.40 - Water masses in the Antarctic coastal zone.png|Figure 4.40]]). ITASE ice core research indicates that marine air mass transport in the Amundsen Sea sector of the WAIS has increased in intensity as of recent decades (Dixon et al., 2005<ref name="Dixon et al, 2005">Dixon, D., Mayewski, P.A., Kaspari, S., Sneed, S. and Handley, M. 2005. Connections between West Antarctic ice core sulfate and climate over the last 200+ years, ''Annals of Glaciology'', '''41''', 155-166.</ref>). While there are no adjacent measurements of oceanographic change that can support this hypothesis, it appears to be the most likely option, and the recent observations of relatively warm Circumpolar Deep Water on the continental shelf and in contact with the ice sheet in this area suggest it is a reasonable one.<br />
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==Ross Sea Embayment==<br />
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Elsewhere within West Antarctica, the changes are not as extreme. Among the ice streams feeding the Ross Ice Shelf, there is a rich history of change on millennial and shorter time scales. A major event approximately 150 years ago was the stagnation of Kamb Ice Stream (formerly ice stream C) (Retzlaff and Bentley, 1993<ref name="Retzlaff and Bentley, 1993">Retzlaff, R. and Bentley, C.R. 1993. Timing of stagnation of Ice Stream C, West Antarctica, from short-pulse radar studies of buried surface crevasses, Journal of Glaciology, 39, No. 133, 553-561.</ref>). Since that time, ice upstream of the stagnated trunk has been thickening at a rate of nearly 50 cm/yr over an area tens of kilometres across. The next largest change is the gradual deceleration of the Whillans Ice Stream, immediately south of Kamb Ice Stream, at rates of between 1 and 2% annually (Joughin et al., 2002<ref name="Joughin et al, 2002">Joughin, I., Tulaczyk, S., Bindschadler, R.A. and Price, S.F. 2002. Changes in west Antarctic ice stream velocities, Journal of Geophysical Research, 107, No. B11, 2289, doi:10.1029/2001/JB001029.</ref>).<br />
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[[File:Figure 4.41 - Pictorial view of the ocean-ice system of the Ross Sea.png|thumb|'''4.41''' Pictorial view of the ocean-ice system of the Ross Sea. Colours on the ice sheet indicate flow speed with speed increasing from yellow to green to blue to magenta (see Figure 1.6). Colour in the ocean represents the major currents: light blue and yellow are the surface flows of the Antarctic Circumpolar Current and the opposing boundary current, respectively; red is CDW upwelling to reach the continental shelf and dark blue is the sinking Ice Shelf Water exiting from beneath the floating ice shelf. As CDW rises, some loses heat to the atmosphere (wavy vertical line) while the remainder circulated under the floating ice shelf causing basal melting. (illustration courtesy of National Geographic)]]<br />
Aside from these two phenomena, the remainder of the ice flow in the region appears to be near equilibrium. Overall, Whillans and Kamb ice streams skew the cumulative mass balance calculations in the region to a net positive, indicating slight growth. An earlier estimate of 26.8 &plusmn; 14.9 Gt/yr by Joughin and Tulaczyk (2002<ref name="Joughin and Tulaczyk, 2002">Joughin, I. and Tulaczyk, S. 2002. Positive mass balance of the Ross Ice Streams, West Antarctica, Science, 295 (Jan 18), No. 5554, 476-480.</ref>) has only been slightly modified to 34 &plusmn; 8 Gt/yr recently by Rignot et al. (2008<ref name="Rignot et al, 2008">Rignot, E., Bamber, J.L., Van Den Broeke, M.R., Davis, C., Yonghong, L., Van Deberg, W.J. and Van Meijgaard, E. 2008. Recent Antarctic ice mass loss from radar interferometry and regional climate modeling, Nature Geoscience, 13 January 2008; doi:10.1038/ngeo102.</ref>), but the errors overlap, indicating consistency. The ocean-ice system of the Ross Sea is shown in [[:File:Figure 4.41 - Pictorial view of the ocean-ice system of the Ross Sea.png|Figure 4.41]].<br />
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==The Weddell Sea Embayment==<br />
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This final third of the WAIS is about equal in size to the Amundsen and Ross Sea sectors, but appears to be more stable, at least for the past millennium. The ice streams are deeper than within the other sectors, but show few signs of flow rates or directions far out of the present equilibrium. The most recent calculation of its mass balance of -4 &plusmn; 14 Gt/yr (Rignot et al., 2008<ref name="Rignot et al, 2008">Rignot, E., Bamber, J.L., Van Den Broeke, M.R., Davis, C., Yonghong, L., Van Deberg, W.J. and Van Meijgaard, E. 2008. Recent Antarctic ice mass loss from radar interferometry and regional climate modeling, Nature Geoscience, 13 January 2008; doi:10.1038/ngeo102.</ref>) varies insignificantly from an earlier calculation of +9 &plusmn; 8 Gt/yr by Rignot and Thomas (2002<ref name="Rignot and Thomas, 2002">Rignot, E. and Thomas, R.H. 2002. Mass balance of polar ice sheets, Science, 297 (5586), 1502-1506 AUG 30 2002.</ref>).<br />
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Satellite altimeter records suggest, that there may be some areas within this sector (e.g. Rutford Ice Stream) where, in the last decade, there has been an excess of snow accumulation, although such records are too short to imply any likely ongoing change (Wingham et al, 2006a<ref name="Wingham et al, 2006a">Wingham, D.J., Shepherd, A., Muir, A. and Marshall, G.J. 2006a. Mass balance of the Antarctic ice sheet, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, DOI: 10.1098/rsta.2006.1792.</ref>).<br />
==References==<br />
<references /><br />
[[Category:The instrumental period]]<br />
[[Category:The Antarctic ice sheet]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_Weddell_Sea_sector_in_the_instrumental_periodThe Weddell Sea sector in the instrumental period2014-08-06T14:34:23Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[The Southern Ocean in the instrumental period]]''<br />
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The Weddell Sea hosts a subpolar gyre (e.g. Treshnikov, 1964<ref name="Treshnikov, 1964">Treshnikov, A.F. 1964. Surface water circulation in the Antarctic Ocean, Information Bulletin of the Soviet Antarctic Expedition, 5, 81-83 (English translation)</ref>), the Weddell Gyre (Figure 1.10), that brings relatively warm, salty circumpolar water (Warm Deep Water, WDW) south towards the Antarctic continent, and transports colder, fresher waters northward (Weddell Sea Deep and Bottom Waters, WSDW and WSBW, as well as surface waters). The transformation of this source water mass is one of the major climate-relevant processes in the Southern Hemisphere, affecting and involving ocean, atmosphere and cryosphere and a major contribution to the deep meridional overturning cell (Figure 1.9).<br />
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In respect to climate change, the most relevant property is the change in the intensity of the overturning circulation, i.e. the transport of shallow source water masses into the formation areas and the newly formed water masses leaving the Antarctic Systems into the world oceans. However, since these transport differences are relative small in comparison to the gyre scale circulation, the latter must be known to distinguish between recirculation and net transformation.<br />
<br />
The cyclonic Weddell Gyre is bounded to the west by the Antarctic Peninsula, to the south by the Antarctic continent and to the north by a chain of roughly zonal ridges at ~60&ordm;S. The eastern boundary is less well defined but is generally agreed to extend as far east as ~30&ordm; E (Gouretski and Danilov, 1993<ref name="Gouretski and Danilov, 1993">Gouretski, V.V. and Danilov, A.I. 1993. Weddell Gyre: Structure of the eastern boundary, Deep Sea Res., ''Part I'', '''40''', 561-582.</ref>). The flow associated with the Antarctic Slope Front, a boundary current tied to the steep topography of the Antarctic continental slope, contributes a major proportion of the gyre transport. The Weddell Gyre is primarily driven by the cyclonic wind field (Gordon et al., 1981<ref name="Gordon et al, 1981">Gordon, A.L., Martinson, D.G. and Taylor, H.W. 1981. The wind-driven circulation in the Weddell-Enderby Basin, ''Deep-Sea Res.'', '''28''', 151-163.</ref>), leading to a doming of isopycnals in the centre of the gyre. The volume transport is still an ongoing topic of research, partly because it is dominated by the barotropic component (Fahrbach et al., 1991<ref name="Fahrbach et al, 1991">Fahrbach, E., Knoche, M. and Rohardt, G. 1991. An estimate of water mass transformation in the southern Weddell Sea, ''Mar. Chem.'', '''35''', 25-44.</ref>) so it is difficult to measure. Integrating the wind field in a Sverdrup calculation revealed a volume transport of 76 Sv (Gordon et al., 1981<ref name="Gordon et al, 1981">Gordon, A.L., Martinson, D.G. and Taylor, H.W. 1981. The wind-driven circulation in the Weddell-Enderby Basin, ''Deep-Sea Res.'', '''28''', 151-163.</ref>) while the first attempts to reference the geostrophic shear to current meters yielded a transport of 97 Sv (Carmack and Foster, 1975<ref name="Carmack and Foster, 1975">Carmack, E.C. and T.D. Foster. 1975. On the flow of water out of the Weddell Sea, ''Deep-Sea Res.'', '''22''', 711-724.</ref>). For a section across the Weddell Gyre from the tip of the Antarctic Peninsula to Kapp Norvegia, transports have, uniquely, been referenced to a long time series current meter array, and this has produced lower transport estimates of 20-56 Sv (Fahrbach et al., 1991<ref name="Fahrbach et al, 1991">Fahrbach, E., Knoche, M. and Rohardt, G. 1991. An estimate of water mass transformation in the southern Weddell Sea, ''Mar. Chem.'', '''35''', 25-44.</ref>; Fahrbach et al. 1994<ref name="Fahrbach et al, 1994">Fahrbach, E., Rohardt, G., Schroder, M. and Strass, V. 1994. Transport and structure of the Weddell Gyre, ''Ann. Geophysicae'', '''12''', 840-855.</ref>). At the Greenwich meridian, larger transports of the order of 60 Sv were obtained from referencing shear to shipboard ADCP (Schr&ouml;der and Fahrbach, 1999<ref name="Schr&ouml;der and Fahrbach, 1999">Schr&ouml;der, M. and Fahrbach, E. 1999. On the structure and transport of the eastern Weddell Gyre, ''Deep Sea Research'', '''46''', 501-527.</ref>). Current meter arrays subsequently yielded 45&ndash;56 Sv (Klatt et al., 2005<ref name="Klatt et al, 2005">Klatt, O., Fahrbach, E., Hoppema, M. and Rohardt, G. 2005. The transport of the Weddell Gyre across the Prime Meridian, ''Deep-Sea Research II'', '''52''', 513-528.</ref>). The larger values at the Greenwich meridian may be due to recirculations within the central gyre not being captured by the Kapp Norgvegia section. However, a recent high resolution section at the Antarctic Peninsula also yielded ~46 Sv (Thompson and Heywood, 2008<ref name="Thompson and Heywood, 2008">Thompson, A.F. and Heywood, K.J. 2008. Frontal structure and transport in the northwestern Weddell Sea, ''Deep-Sea Research I'', '''55''', 1229-1251.</ref>), and it is suggested that referencing the geostrophic shear across the steep Antarctic continental slope to a relatively widely spaced current meter array may have underestimated the barotropic contribution to the total volume transport.<br />
<br />
WSBW is formed on the Antarctic continental shelves where they are wide. In the Weddell Sea, the Filchner-Ronne Ice shelf is one such source region (e.g. Foldvik et al., 2004<ref name="Foldvik et al, 2004">Foldvik, A., Gammelsrod, T., Osterhus, S., Fahrbach, E., Rohardt, G., Schroder, M., Nicholls, K.W., Padman, L. and Woodgate, R.A. 2004. Ice shelf water overflow and bottom water formation in the southern Weddell Sea, ''J. Geophys. Res.'', '''109''', C02015, doi:10.1029/2003JC002008.</ref>), and recent evidence suggests formation also on the eastern side of the Peninsula near the Larsen Ice Shelf. The water on the Antarctic continental shelves is typically fresher than its warmer, salty source to the north; it is believed to freshen by the addition of sea ice melt, glacial ice melt from the Antarctic ice sheet and floating ice shelves, and from precipitation. This freshening is a necessary precursor to the bottom water formation process, which involves salinification from brine rejection during sea ice formation, together with cooling. One contributing process to WSBW involves mixing with Ice Shelf Water, and the other process involves mixing with HSSW. The WSBW descends the continental slope and entrains ambient water as it goes (Baines and Condie, 1998<ref name="Baines and Condie, 1998">Baines, P.G. and Condie, S. 1998. Observations and modelling of Antarctic downslope flows: a review. In Ocean, Ice and Atmosphere: Interactions at the Antarctic Continental Margin, AGU Antarctic Research Series Vol. 75, S.S. Jacobs and R. Weiss editors, 29-49.</ref>). The descent of the WSBW affects the structure of a series of fronts along the rim of the Weddell Gyre, in particular the Antarctic Slope Front and the Weddell Front.<br />
<br />
The WSBW is too dense to be able to escape the Weddell Sea. It can only escape by mixing with the water above it, becoming warmer, saltier and less dense, and forming WSDW, which is sufficiently shallow to flow out through the passages in the topography surrounding the Weddell Sea. WSDW outside the Weddell Sea has the water mass properties of Antarctic Bottom Water, of which it is believed to be the major contributor. Estimates of the proportion of Antarctic Bottom Water originating in the Weddell Sea range from 50 to 90% (Orsi et al., 1999<ref name="Orsi et al, 1999">Orsi, A.H., Johnson, G.C. and Bullister, J.B. 1999. Circulation, mixing and production of Antarctic Bottom Water, ''Prog. Oceanog.'', '''43''', 55-109.</ref>). An inflow of water of WSDW properties from the east has been documented (Meredith et al., 2000<ref name="Meredith et al, 2000">Meredith, M.P., Locarnini, R.A., Van Scoy, K.A., Watson, A.J., Heywood, K.J. and King, B.A. 2000. On the sources of Weddell Gyre Antarctic Bottom Water, ''J. Geophys. Res.'', '''105''', 1093-1104.</ref>) that may form in the region of the Prydz Bay Gyre, or may even originate in the Australian Antarctic Basin and enter the Weddell Sea through the Princess Elizabeth Trough (Heywood et al., 1999<ref name="Heywood et al, 1999">Heywood, K.J., Sparrow, M.D., Brown, J. and Dickson, R.R. 1999. Frontal structure and Antarctic Bottom Water flow through the Princess Elizabeth Trough, Antarctica, ''Deep-Sea Research I'', '''46''', 1181-1200.</ref>).<br />
<br />
The export of WSDW to the world ocean is of the order of 10&plusmn;4 Sv (Naveira Garabato et al., 2002<ref name="Garabato et al, 2002">Naveira Garabato, A.C., McDonagh, E.L., Stevens, D.P., Heywood, K.J. and Sanders, R.J. 2002. On the export of Antarctic Bottom Water from the Weddell Sea, ''Deep-Sea Res. II'', '''49''', 4715-4742.</ref>). This can escape through gaps in the ridges to the north and east of the Weddell Sea, and subsequently invades all ocean basins.<br />
<br />
Measurements of water mass transports exist in relation to climate time scales only as snapshots and not over a long enough times to be able to detect changes. Therefore, one has to measure water mass properties and derive from their variations conclusions about changes of transport and formation rates. Carefully validated models play a significant role here. Even for water mass properties observations in sufficient spatial resolution and accuracy last only over one to two decades. Therefore it is still not possible to unambiguously distinguish between a trend and decadal to multidecadal variation. In spite of the fact that the variability in the deep water masses seems to be relatively small in comparison to those of the surface water masses, it is importance, because the large volume of the deep waters can store significant heat quantities or dissolved substances even if the changes in property is only minor.<br />
<br />
Considering its remote and inhospitable location, the Weddell Sea was well observed during WOCE and subsequently through CLIVAR, with (largely summer-time) hydrographic sections across the Weddell Gyre onto the Antarctic continental shelf, and with arrays of moorings. These sections indicate a number of decadal-scale changes in water mass properties. The WDW warmed by some 0.04&ordm; C during the 1990s (Robertson et al., 2002<ref name="Robertson et al, 2002">Robertson, R., Visbeck, M., Gordon, A.L. and Fahrbach, E. 2002. Long-term temperature trends in the Deep Waters of the Weddell Sea, ''Deep-Sea Research II'', '''49'''(21), 4791-4806.</ref>; Fahrbach et al., 2004<ref name="Fahrbach et al, 2004">Fahrbach, E., Hoppema, M., Rohardt, G., Schr&ouml;der, M. and Wisotzki, A. 2004. Decadal-scale variations of water mass properties in the deep Weddell Sea, ''Ocean Dynamics'', '''54''', 77-91.</ref>) and has subsequently cooled (Fahrbach et al., 2004<ref name="Fahrbach et al, 2004">Fahrbach, E., Hoppema, M., Rohardt, G., Schr&ouml;der, M. and Wisotzki, A. 2004. Decadal-scale variations of water mass properties in the deep Weddell Sea, ''Ocean Dynamics'', '''54''', 77-91.</ref>). This was accompanied by a salinification of about 0.004 (note salinity does not have any units), just detectable over the decade. A quasi-meridional section across the Weddell Gyre occupied in 1973 and 1995 revealed a warming of the WDW in the southern limb of the gyre by 0.2&ordm; C accompanied by a small increase in salinity, whereas there was no discernible change in the northern limb of the gyre (Heywood and King, 2002<ref name="Heywood and King, 2002">Heywood, K.J. and King, B.A. 2002. Water masses and baroclinic transports in the South Atlantic and Southern Oceans, ''J. Mar. Res.'', '''60'''(5), 639-676.</ref>). There has been debate as to whether these changes to the warm inflow to the gyre are caused by advection of warmer circumpolar waters, and/or by changes in the wind field (Fahrbach et al., 2004<ref name="Fahrbach et al, 2004">Fahrbach, E., Hoppema, M., Rohardt, G., Schr&ouml;der, M. and Wisotzki, A. 2004. Decadal-scale variations of water mass properties in the deep Weddell Sea, ''Ocean Dynamics'', '''54''', 77-91.</ref>, 2006<ref name="Fahrbach et al, 2006">Fahrbach, E., Hoppema, M., Rohardt, G., Schroder, M. and Wisotzki, A. 2006. Causes of deep-water variation: Comment in the paper by L.H. Smedsrud, 2005, &ldquo;Warming of the deep water in the Weddell Sea along the Greenwich Meridian: 1997-2001&rdquo;, ''Deep-Sea Research I'', '''53''', 574-577.</ref>; Smedsrud, 2005<ref name="Smedsrud, 2005">Smedsrud, L.H. 2005. Warming of the deep water in the Weddell Sea along the Greenwich Meridian: 1997-2001, ''Deep-Sea Research I'', '''52'''(2), 241-258.</ref>).<br />
<br />
During the 1970s a persistent gap in the sea ice, the Weddell Polynya, occurred for several winters. The ocean lost a great deal of heat to the atmosphere during these events. The polynya strongly affected the mode of overturning in the Weddell Sea, shifting it from shelf and slope processes to open ocean convection (Gordon, 1978<ref name="Gordon, 1978">Gordon, A.L. 1978. Weddell Polynya, Gyre and Deep-Water Convection, ''Transactions-American Geophysical Union'', '''59'''(4), 292-292.</ref>) with consequences for the overturning rates. After this one event the large polynya did not show up again, but weak ice cover was observed in the Maud Rise area several times and interpreted as a sign of possible emergence of a new polynya, though no such polynya emerged. In a recent study, Gordon et al. (2007<ref name="Gordon et al, 2007">Gordon, A.L., Visbeck, M. and Comiso, J.C. 2007. A possible link between the Weddell Polynya and the Southern Annular Mode, ''Journal of Climate'', '''20'''(11), 2558-2571.</ref>) attribute the occurrence of the Weddell Polynya to variations in the SAM.<br />
<br />
Weddell Sea Bottom Water is observed on the western continental slope and within the basin up to the Greenwich Meridian. Whereas the bottom water was warming and getting more saline in the basin (Fahrbach et al., 2004<ref name="Fahrbach et al, 2004">Fahrbach, E., Hoppema, M., Rohardt, G., Schr&ouml;der, M. and Wisotzki, A. 2004. Decadal-scale variations of water mass properties in the deep Weddell Sea, ''Ocean Dynamics'', '''54''', 77-91.</ref>) it became colder and fresher on the slope in the western Weddell Sea (Heywood et al, in preparation). It is a matter of debate if such regional changes in phase are caused by the time lag of 5-10 years involved when freshly formed bottom water spreads across the basin.<br />
<br />
Because much of the Weddell Sea is covered with sea ice for much of the year, there is a lack of observations on the continental shelf and slope, especially in winter. This was a priority area during IPY and moored arrays were deployed together with hydrographic sections to fill the observational gaps. Measurements beneath the sea ice in the Weddell Sea were obtained for the first time by acoustically tracked floats and by instruments carried by marine mammals such as elephant seals.<br />
==References==<br />
<references /><br />
[[Category:The instrumental period]]<br />
[[Category:The Southern Ocean]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_transition_to_Holocene_interglacial_conditionsThe transition to Holocene interglacial conditions2014-08-06T14:34:22Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[The last million years]]''<br />
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The transition from the LGM (beginning about 21 ka BP) to the present interglacial period (Termination I) was the last major global climate change event. The Northern Hemisphere experienced dramatic changes during the deglaciation starting with an abrupt warming (the B&oslash;lling-Aller&oslash;d) at 14.7 ka B2K (i.e. before AD 2000; an ice core dating convention introduced by Rasmussen et al. (2006<ref name="Rasmussen et al, 2006">Rasmussen, S.O., Andersen, K.K., Svensson, A.M., Steffensen, J.P., Vinther, B.M., Clausen, H.B., Siggaard-Andersen, M.-L., Johnsen, S.J., Larsen, L.B., Dahl-Jensen, D., Bigler, M., R&ouml;thlisberger, R., Fischer, H., Goto-Azuma, K., Hansson, M.E. and, Ruth, U. 2006. A new Greenland ice core chronology for the last glacial termination, Journal of Geophysical Research, 111 , D06102.</ref>) as an alternative to the conventional Before Present (BP) where &lsquo;Present&lsquo; is 1950), followed by a return to colder conditions (the Younger Dryas episode) and a final rapid warming leading to the Holocene at 11.7 ka B2K (Rasmussen et al., 2006<ref name="Rasmussen et al, 2006">Rasmussen, S.O., Andersen, K.K., Svensson, A.M., Steffensen, J.P., Vinther, B.M., Clausen, H.B., Siggaard-Andersen, M.-L., Johnsen, S.J., Larsen, L.B., Dahl-Jensen, D., Bigler, M., R&ouml;thlisberger, R., Fischer, H., Goto-Azuma, K., Hansson, M.E. and, Ruth, U. 2006. A new Greenland ice core chronology for the last glacial termination, Journal of Geophysical Research, 111 , D06102.</ref>). In contrast, long climate records extending back to 800 ka BP, recently obtained from deep Antarctic ice cores, have revealed the exceptional character of this last termination: neither the Antarctic temperature proxies nor the methane records capture such abrupt changes during the earlier terminations (Spahni et al., 2005<ref name="Spahni et al, 2005">Spahni, R., Chappellaz, J., Stocker, T.F., Loulergue, L., Hausammann, G., Kawamura, K., Fluckiger, J., Schwander, J., Raynaud, D., Masson-Delmotte, V. and Jouzel, J. 2005. Atmospheric Methane and Nitrous Oxide of the Late Pleistocene from Antarctic Ice Cores, ''Science'', '''310''', 1317-1321.</ref>; Jouzel et al., 2007<ref name="Jouzel et al, 2007">Jouzel, J., Masson-Delmotte, V., Cattani, O., Dreyfus, G., Falourd, S., Hoffmann, G., Minster, B., Nouet, J., Barnola, J.M., Chappellaz, J., Fischer, H., Gallet, J.C., Johnsen, S., Leuenberger, M., Loulergue, L., Luethi, D., Oerter, H., Parrenin, F., Raisbeck, G., Raynaud, D., Schilt, A., Schwander, J., Selmo, E., Souchez, R., Spahni, R., Stauffer, B., Steffensen, J.P., Stenni, B.S., Tison, J.L., Werner, M. and Wolff, E. 2007. Orbital and millenial Antarctic climate variability over the past 800,000 years, ''Science'', '''317''', 793-796.</ref>).<br />
<br />
Because it is the most recent transition between a glacial and an interglacial period, Termination I can be examined with high resolution data and a robust age control (EPICA, 2006<ref name="EPICA, 2006">EPICA community members. 2006. One-to-one coupling of glacial climate variability in Greenland and Antarctica, ''Nature'', '''444''', 195-198.</ref>; Rasmussen et al., 2006<ref name="Rasmussen et al, 2006">Rasmussen, S.O., Andersen, K.K., Svensson, A.M., Steffensen, J.P., Vinther, B.M., Clausen, H.B., Siggaard-Andersen, M.-L., Johnsen, S.J., Larsen, L.B., Dahl-Jensen, D., Bigler, M., R&ouml;thlisberger, R., Fischer, H., Goto-Azuma, K., Hansson, M.E. and, Ruth, U. 2006. A new Greenland ice core chronology for the last glacial termination, Journal of Geophysical Research, 111 , D06102.</ref>). Ice core records can be synchronised either regionally using aerosol tracers (e.g. dust particles including calcium from continental aerosols, or sulphate from volcanism) (Severi et al., 2007<ref name="Severi et al, 2007">Severi, M., Becagli, S., Castellano, E., Morganti, A., Traversi, R., Udisti, R., Ruth, U., Fischer, H., Huybrechts, P., Wolff, E., Parrenin, F., Kaufmann, P., Lambert, F. and Steffensen, J.P. 2007. Synchronisation of the EDML and EDC ice cores for the last 52 kyr by volcanic signature matching, ''Climate of the Past'', '''3''', 367-374.</ref>) or globally using well-mixed atmospheric gas records (such as CH<sub>4</sub> or o<sup>18</sup>O of O<sub>2</sub>) (Blunier et al., 1997<ref name="Blunier et al, 1997">Blunier, T., Schwander, J., Stauffer, B., Stocker, T., Dallebach, A., Inderm&uuml;hle, A., Tschumi, J., Chappellaz, J., Raynaud, D. and Barnola, J.M. 1997. Timing of the Antarctic Cold Revearsal and the atmospheric CO<sub>2</sub> increase with respect to the Younger Dryas event, Geophys. Res. Letters, 24, 2683 - 2686.</ref>; Morgan et al., 2002<ref name="Morgan et al, 2002">Morgan, V., Delmotte, M., Van Ommen, T., Jouzel, J., Chappellaz, J., Woon, S., Masson-Delmotte, V. and Raynaud, D. 2002. Relative Timing of Deglacial Climate Events in Antarctica and Greenland, ''Science'', '''297''', 1862-1864.</ref>; Blunier et al.. 2007). The accuracy of the Greenland ice core layer-counted age scale GICC05 is between 100 and 200 years over the last transition (Rasmussen et al., 2006<ref name="Rasmussen et al, 2006">Rasmussen, S.O., Andersen, K.K., Svensson, A.M., Steffensen, J.P., Vinther, B.M., Clausen, H.B., Siggaard-Andersen, M.-L., Johnsen, S.J., Larsen, L.B., Dahl-Jensen, D., Bigler, M., R&ouml;thlisberger, R., Fischer, H., Goto-Azuma, K., Hansson, M.E. and, Ruth, U. 2006. A new Greenland ice core chronology for the last glacial termination, Journal of Geophysical Research, 111 , D06102.</ref>). The error on the transfer of this age scale to Antarctic ice cores using CH<sub>4</sub> synchronisation is estimated to be at most 250 years for the Younger Dryas period (the abrupt cooling interrupting the Northern Hemisphere deglaciation). Since the pioneer works of the 1960s, Greenland and Antarctic ice core records have been used to determine the magnitude of climatic and environmental changes and the precise sequence of events during the last major climate transition.<br />
<br />
[[File:Figure 3.14 - Climate and environmental records of Termination I.png|thumb|'''3.14''' Climate and environmental records of Termination I. From top to bottom: yellow, 60&deg;N June insolation (W m<sup>-2</sup>) (Berger and Loutre, 1991<ref name="Berger and Loutre, 1991">Berger, A. and Loutre, M.F. 1991. Insolation values for the climate of the last 10 million years, ''Quaternary Sciences Reviews'', '''10''', 297-317.</ref>); black, global sea level change (m) (Lambeck and Chappell, 2001<ref name="Lambeck and Chappell, 2001">Lambeck, K. and Chappell, J. 2001. Sea level change through the last glacial cycle, ''Science'', '''292''', 679-686.</ref>; Waelbroeck et al., 2002<ref name="Waelbroeck et al, 2002">Waelbroeck, C., Labeyrie, L., Michel, E., Duplessy, J.C., McManus, J.F., Lambeck, K., Balbon, E. and Labracherie, M. 2002. Sea level and deep water temperature changes derived from bentic foraminifera isotopic records, ''Quat. Sci. Rev.'', '''21''', 295-305.</ref>); blue-grey, Greenland NorthGRIP ice core &delta;<sup>18</sup>O (&permil;), a proxy of Greenland temperature change (NorthGRIP-community-members, 2004); composite Greenland (light green) and EPICA Dome C (EDC) (dark green) atmospheric methane concentration records (ppbv) (EPICA, 2006<ref name="EPICA, 2006">EPICA community members. 2006. One-to-one coupling of glacial climate variability in Greenland and Antarctica, ''Nature'', '''444''', 195-198.</ref>) (Loulergue et al., submitted); red, Antarctic EDC and Vostok stacked atmospheric CO<sub>2</sub> concentration record (ppm) (Monnin et al., 2001<ref name="Monnin et al, 2001">Monnin, E., Indermuhle, A., Dallenbach, A., Fluckiger, J., Stauffer, B., Stocker, T.F., Raynaud, D. and Barnola, J-M. 2001. Atmospheric CO<sub>2</sub> Concentrations over the Last Glacial Termination, ''Science'', '''291''', 112-114.</ref>; Petit et al., 1997<ref name="Petit et al, 1997">Petit, J.R., Basile, I., Leruyuet, A., Raynaud, D., Lorius, C., Jouzel, J., Stievenard, M., Lipenkov, V.Y., Barkov, N.I., Kudryashov, B.B., Davis, M., Saltzman, E. and Kotlyakov, V. 1997. Four climatic cycles in Vostok ice core, ''Nature'', '''387''', 359.</ref>); non sea-salt calcium flux, a proxy for continental aerosols, from EPICA Dronning Maud Land (EDML) (brown) and EDC (orange) ice cores (&micro;g/m&sup2;yr) (Wolff et al., 2006<ref name="Wolff et al, 2006">Wolff, E.W., Fischer, H., Fundel, F., Ruth, U., Twarloh, B., Littot, G.C., Mulvaney, R., R&ouml;thlisberger, R., De Angelis, M., Boutron, C.F., Hansson, M., Jonsell, U., Hutterli, M.A,, Lambert, F., Kaufmann, P., Stauffer, B., Stocker, T.F., Steffensen, J.P., Bigler, M., Siggaard-Andersen, M.L., Udisti, R., Becagli, S., Castellano, E., Severi, M., Wagenbach, D., Barbante, C., Gabrielli, P. and Gaspari, V. 2006. Southern Ocean sea-ice extent, productivity and iron flux over the past eight glacial cycles, Nature, 440, 491-496 (doi:10.1038/nature04614).</ref>; Fischer et al., 2007a); sea-salt sodium flux, a proxy for marine aerosols, from EDML (light grey) and EDC (dark grey) (&micro;g/m&sup2;yr) (Wolff et al., 2006<ref name="Wolff et al, 2006">Wolff, E.W., Fischer, H., Fundel, F., Ruth, U., Twarloh, B., Littot, G.C., Mulvaney, R., R&ouml;thlisberger, R., De Angelis, M., Boutron, C.F., Hansson, M., Jonsell, U., Hutterli, M.A,, Lambert, F., Kaufmann, P., Stauffer, B., Stocker, T.F., Steffensen, J.P., Bigler, M., Siggaard-Andersen, M.L., Udisti, R., Becagli, S., Castellano, E., Severi, M., Wagenbach, D., Barbante, C., Gabrielli, P. and Gaspari, V. 2006. Southern Ocean sea-ice extent, productivity and iron flux over the past eight glacial cycles, Nature, 440, 491-496 (doi:10.1038/nature04614).</ref>; Fischer et al., 2007a); blue, EDML &delta;<sup>18</sup>O (&permil;), a proxy of EDML temperature (EPICA, 2006<ref name="EPICA, 2006">EPICA community members. 2006. One-to-one coupling of glacial climate variability in Greenland and Antarctica, ''Nature'', '''444''', 195-198.</ref>); and finally, in light blue, EDC &delta;D (&permil;), a proxy of EDC temperature (Jouzel et al., 2007<ref name="Jouzel et al, 2007">Jouzel, J., Masson-Delmotte, V., Cattani, O., Dreyfus, G., Falourd, S., Hoffmann, G., Minster, B., Nouet, J., Barnola, J.M., Chappellaz, J., Fischer, H., Gallet, J.C., Johnsen, S., Leuenberger, M., Loulergue, L., Luethi, D., Oerter, H., Parrenin, F., Raisbeck, G., Raynaud, D., Schilt, A., Schwander, J., Selmo, E., Souchez, R., Spahni, R., Stauffer, B., Steffensen, J.P., Stenni, B.S., Tison, J.L., Werner, M. and Wolff, E. 2007. Orbital and millenial Antarctic climate variability over the past 800,000 years, ''Science'', '''317''', 793-796.</ref>).]]<br />
Termination I is now documented at a high resolution in more than 10 Antarctic ice cores located in both East and West Antarctica ([[:File:Figure 3.14 - Climate and environmental records of Termination I.png|Figure 3.14]]). The beginning of the Holocene at ~11.7 ka BP (Rasmussen et al., 2006<ref name="Rasmussen et al, 2006">Rasmussen, S.O., Andersen, K.K., Svensson, A.M., Steffensen, J.P., Vinther, B.M., Clausen, H.B., Siggaard-Andersen, M.-L., Johnsen, S.J., Larsen, L.B., Dahl-Jensen, D., Bigler, M., R&ouml;thlisberger, R., Fischer, H., Goto-Azuma, K., Hansson, M.E. and, Ruth, U. 2006. A new Greenland ice core chronology for the last glacial termination, Journal of Geophysical Research, 111 , D06102.</ref>) can be found at various depths depending mainly on the accumulation rate and the ice flow at different locations: ranging from ~270 m at Vostok to ~1120 m at Law Dome; note that it is found even deeper in Greenland (~1490 m at NorthGRIP, ~1620 m at GRIP). The full length of the transition varies between ~20 m at Taylor Dome or Law Dome, 100 m at Siple Dome, ~135 m at Vostok, ~155 m at EPICA Dome C, ~290 m at Byrd and up to 300 m at EPICA Dronning Maud Land. Climate records of Termination I from different Antarctic ice cores can therefore offer varying temporal resolutions. The records are affected by a strong thinning at coastal locations and may be partly affected by changes in ice sheet elevation and ice origin from upstream areas at inland locations, especially those not located on domes. The quality of the ice core samples may be affected when the transition is located in the brittle zone &ndash; cores taken from the range approximately 400 to 1,000 m depth, often described as brittle, can be easily damaged by drilling and handling.<br />
<br />
[[File:Figure 3.15 - Map of Antarctica showing some deep ice core sites.png|thumb|'''3.15''' Map of Antarctica (created using http://www.aquarius.ifm-geomar.de/omc/) showing some deep ice core sites where climate records have been obtained back to the LGM and beyond: Byrd (Hammer et al., 1994<ref name="Hammer et al, 1994">Hammer, C.U., Clausen, H.B. and Langway, C.C.J. 1994. Electrical conductivity method (ECM) stratigraphic dating of the Byrd Station ice core, Antarctica, ''Annals of Glaciology'', '''20''', 115-120.</ref>; Blunier et al., 1997<ref name="Blunier et al, 1997">Blunier, T., Schwander, J., Stauffer, B., Stocker, T., Dallebach, A., Inderm&uuml;hle, A., Tschumi, J., Chappellaz, J., Raynaud, D. and Barnola, J.M. 1997. Timing of the Antarctic Cold Revearsal and the atmospheric CO<sub>2</sub> increase with respect to the Younger Dryas event, Geophys. Res. Letters, 24, 2683 - 2686.</ref>), Caroline (Yao et al., 1990<ref name="Yao et al, 1990">Yao, T., Petit, J.R., Jouzel, J., Lorius, C. and Duval, P. 1990. Climatic record from an ice margin area in East Antarctica, ''Annals of Glaciology'', '''14''', 323-327.</ref>), Vostok (Petit et al., 1999<ref name="Petit et al, 1999">Petit, J.R., Jouzel, J., Raynaud, D., Barkov, N.I., Barnola, J.M., Basile, I., Bender, M., Chappellaz, J., Davis, J., Delaygue, G., Delmotte, M., Kotyakov, V.M., Legrand, M., Lipenkov, V.Y., Lorius, C., P&eacute;pin, L., Ritz, C., Saltzman, E. and Stievenard, M. 1999. Climate and Atmospheric History of the Past 420000 years from the Vostok Ice Core, Antarctica, ''Nature'', '''399''', 429-436.</ref>), Komsolmolskaia (Nikolaiev et al., 1988<ref name="Nikolaiev et al, 1988">Nikolaiev, V.I., Kotlyakov, V.M. and Smirnov, K.E. 1988. Isotopic studies of the ice core from the Komsomolskaia station, Antarctica. Data of Glaciological Studies of the USSR Academy of Sciences, 63, 97-102.</ref>), Dome B (Jouzel et al., 1995<ref name="Jouzel et al, 1995">Jouzel, J., Vaikmae, R., Petit, J.R., Martin, M., Duclos, Y., Sti&eacute;venard, M., Lorius, C., Toots, M., Burckle, L.H., Barkov, N.I., Kotlyakov, V.M. 1995. The two-step shape and timing of the last deglaciation in Antarctica, ''Climate Dynamics'', '''11''', 151-161.</ref>), Law Dome (Morgan et al., 2002<ref name="Morgan et al, 2002">Morgan, V., Delmotte, M., Van Ommen, T., Jouzel, J., Chappellaz, J., Woon, S., Masson-Delmotte, V. and Raynaud, D. 2002. Relative Timing of Deglacial Climate Events in Antarctica and Greenland, ''Science'', '''297''', 1862-1864.</ref>), Taylor Dome (Steig et al., 1998<ref name="Steig et al, 1998">Steig, E.J., Brook, E.J., White, J.W.C., Sucher, C.M., Bender, M.L., Lehman, S.J., Morse, D.L., Waddington, E.D. and Clow, G.D. 1998. Synchronous climate changes in Antarctica and the North Atlantic, ''Science'', '''282''', 92-95.</ref>), Dome C with a first deep drilling (Lorius et al., 1979<ref name="Lorius et al, 1979">Lorius, C., Merlivat, L., Jouzel, J. and Pourchet, M. 1979. A 30,000-yr isotope climatic record from Antarctic ice, ''Nature'', '''280''', 644-648.</ref>) and the EPICA deep ice core (EPICA, 2004<ref name="EPICA, 2004">EPICA community members. 2004. Eight glacial cycles from an Antarctic ice core, ''Nature'', '''429''', 623-628.</ref>), Dome F (Watanabe et al., 2003<ref name="Watanabe et al, 2003">Watanabe, O., Jouzel, J., Johnsen, S., Parrenin, F., Shoji, H. and Yoshida, N. 2003. Homogeneous climate variability across East Antarctica over the past three glacial cycles, ''Nature'', '''422''', 509-512.</ref>), Siple Dome (Brook et al., 2005<ref name="Brook et al, 2005">Brook, E.J., White, J.W.C., Schilla, A.S.M., Bender, M.L., Barnett, B., Severinghaus, J.P., Taylor, K.C., Alley, R.B. and Steig, E.J. 2005. Timing of millenial-scale climate change at Siple Dome, West Antarctica, during the last glacial period, ''Quarternary Science Reviews'', '''24''', 1333-1343.</ref>) and EPICA Dronning Maud Land (EPICA, 2006<ref name="EPICA, 2006">EPICA community members. 2006. One-to-one coupling of glacial climate variability in Greenland and Antarctica, ''Nature'', '''444''', 195-198.</ref>) . The surface elevation is represented as grey contours (100, 200, 500, and each 1,000 m). Locations of existing deep ice cores going back to the LGM are displayed in white. Names in italics indicate recent ice cores spanning the last termination but not yet published. Further ice cores have recently been drilled at the Detroit Plateau on the northern Antarctic Peninsula and at Titan Dome near the South Pole. Future deep ice core drilling projects are displayed in black.]]<br />
[[:File:Figure 3.15 - Map of Antarctica showing some deep ice core sites.png|Figure 3.15]] shows selected examples of Antarctic ice core records of Termination I. These records offer the potential to compare local (Antarctic site temperature / accumulation), regional (sea salt and marine biogenic sulphur concentrations), hemispheric (concentration of different Antarctic dust fractions) and global (greenhouse gas concentrations in the atmosphere) climate and environmental parameters which are described in more detail below. The stable isotopic composition of ice (aD or <sup>18</sup>O) is classically used to quantify past Antarctic surface air temperature changes. The strong linear relationship observed between snowfall isotopic composition and temperature (Masson-Delmotte et al., 2008<ref name="Masson-Delmotte et al, 2008">Masson-Delmotte, V., Hou, S., Ekaykin, A., Jouzel, J., Aristarain, A., Bernardo, R.T., Bromwhich, D., Cattani, O., Delmotte, M., Falourd, S., Frezzotti, M., Gall&eacute;e, H., Genoni, L., Isaksson, E., Landais, A., Helsen, M., Hoffmann, G., Lopez, J., Morgan, V., Motoyama, H., Noone, D., Oerter, H., Petit, J.R., Royer, A., Uemura, R., Schmidt, G.A., Schlosser, E., Sim&otilde;es, J.C., Steig, E., Stenni, B., Stievenard, M., Broeke, M.V.D., Wal, R.V.D., Berg, W-J.V.D., Vimeux, F. and White, J.W.C. 2008. A review of Antarctic surface snow isotopic composition : observations, atmospheric circulation and isotopic modelling, ''J. Climate'', '''21''', 3359-3387.</ref>) is caused by the progressive cooling and distillation of air masses along their trajectories from oceanic moisture sources to inland Antarctica. General atmospheric circulation models have shown that this relationship remains stable in central Antarctica at the glacial-interglacial scale, leading to uncertainties of 20-30% on reconstructed temperatures (Jouzel et al., 2003<ref name="Jouzel et al, 2003">Jouzel, J., Vimeux, F., Caillon, N., Delaygue, G., Hoffmann, G., Masson-Delmotte, V. and Parrenin, F. 2003. Magnitude of the Isotope/Temperature scaling for interpretation of central Antarctic ice cores, ''J. Geophys. Res.'', '''108''',1029-1046.</ref>). Temperature estimates based on stable isotope records may be biased by deposition effects related to the seasonality of snowfall. Climate models suggest that this effect remains limited inland in Antarctica (Werner et al., 2001<ref name="Werner et al, 2001">Werner, M., Heimann, M. and Hoffmann, G. 2001. Isotopic composition and origin of polar precipitation in present and glacial climate simulations, ''Tellus'', '''53B''', 53-71.</ref>). However, past changes in evaporation conditions may affect Antarctic snowfall isotopic composition. The deuterium excess parameter (d=fD-8-<sup>18</sup>O), mainly affected by a kinetic fractionation effect, has been used to quantify past changes in moisture source and site temperatures more precisely (Stenni et al., 2001<ref name="Stenni et al, 2001">Stenni, B., Masson-Delmotte, V., Johnsen, S., Jouzel, J., Longinelli, A., Monnin, E., Rothlisberger, R. and Selmo, E. 2001. An Oceanic Cold Reversal During the Last Deglaciation, ''Science'', '''293''', 2074-2077.</ref>; Vimeux et al., 2002<ref name="Vimeux et al, 2002">Vimeux, F., Cuffey, K. and Jouzel, J. 2002. New insights into Southern Hemisphere temperature changes from Vostok ice cores using deuterium excess correction over the last 420, 000 years, Earth Planet. Sci. Lett., 2O<sub>3</sub>, 829-843.</ref>). The moisture source temperature is an integrated quantity that may correspond to either changes in sea surface temperature in the dominant evaporation areas or to geographical changes of the main moisture origin with time.<br />
<br />
Most Antarctic ice cores exhibit comparable magnitudes of glacial-interglacial water stable isotope changes, with any major departure likely to be due to changes in ice sheet elevation. The magnitude of glacial-interglacial temperature change in central Antarctica has been estimated to be 9&deg;C with an uncertainty of 2&deg;C (Stenni et al., 2001<ref name="Stenni et al, 2001">Stenni, B., Masson-Delmotte, V., Johnsen, S., Jouzel, J., Longinelli, A., Monnin, E., Rothlisberger, R. and Selmo, E. 2001. An Oceanic Cold Reversal During the Last Deglaciation, ''Science'', '''293''', 2074-2077.</ref>). In central Antarctica, the coherence between the various temperature records based on stable isotopes is very high (within 1&deg;C) (Watanabe et al., 2003<ref name="Watanabe et al, 2003">Watanabe, O., Jouzel, J., Johnsen, S., Parrenin, F., Shoji, H. and Yoshida, N. 2003. Homogeneous climate variability across East Antarctica over the past three glacial cycles, ''Nature'', '''422''', 509-512.</ref>). Based on stable isotopes, Antarctic temperatures appear very constant during the period from 19 to 23 ka BP without a clear &ldquo;glacial maximum&rdquo; (i.e. temperature minimum). A first Antarctic warming phase associated with Termination 1 is identified as starting at about 19 ka BP in central Antarctica. The warming is interrupted by a 1.5&deg;C cooling from 14 to 12.5 ka BP, the &ldquo;Antarctic Cold Reversal&rdquo; (Jouzel et al., 1995<ref name="Jouzel et al, 1995">Jouzel, J., Vaikmae, R., Petit, J.R., Martin, M., Duclos, Y., Sti&eacute;venard, M., Lorius, C., Toots, M., Burckle, L.H., Barkov, N.I., Kotlyakov, V.M. 1995. The two-step shape and timing of the last deglaciation in Antarctica, ''Climate Dynamics'', '''11''', 151-161.</ref>). The second warming phase into the early Holocene is the fastest temperature rise detected in the 800 ka EPICA Dome C record, with a pacing of ~4&deg;C per 1,000 years (Masson-Delmotte et al., 2006<ref name="Masson-Delmotte et al, 2006">Masson-Delmotte, V., Kageyama, M., Braconnot, P., Charbit, S., Krinner, G., Ritz, C., Guilyardi, E., Jouzel, J., Abe-Ouchi, A., Crucifix, M., Gladstone, R.M., Hewitt, C.D., Kitoh, A., Legrande, A., Marti, O., Merkel, U., Motoi, T., Ohgaito, R., Otto-Bliesner, B., Peltier, W.R., Ross, I., Valdes, P.J., Vettoretti, G., Weber, S.L. and Wolk, F. 2006. Past and future polar amplification of climate change: climate model intercomparisons and ice-core constraints, ''Climate Dynamics'', '''26''', 513-529</ref>). Intergovernmental Panel on Climate Change (IPCC) Assessment Report 4 coupled ocean-atmosphere-sea ice climate models run under LGM and present-day conditions tend to underestimate the range of glacial-interglacial temperature changes in central Antarctica (Masson-Delmotte et al., 2006<ref name="Masson-Delmotte et al, 2006">Masson-Delmotte, V., Kageyama, M., Braconnot, P., Charbit, S., Krinner, G., Ritz, C., Guilyardi, E., Jouzel, J., Abe-Ouchi, A., Crucifix, M., Gladstone, R.M., Hewitt, C.D., Kitoh, A., Legrande, A., Marti, O., Merkel, U., Motoi, T., Ohgaito, R., Otto-Bliesner, B., Peltier, W.R., Ross, I., Valdes, P.J., Vettoretti, G., Weber, S.L. and Wolk, F. 2006. Past and future polar amplification of climate change: climate model intercomparisons and ice-core constraints, ''Climate Dynamics'', '''26''', 513-529</ref>): when considering an unchanged ice sheet elevation, the median simulated glacial-interglacial central Antarctic surface temperature change is 4.5&deg;C. Future climate change in Antarctica, in response to anthropogenic CO<sub>2</sub> emissions, is expected to reach 2&deg;C in 100 years, which is much faster than the fastest change of the last transition (Masson-Delmotte et al., 2006<ref name="Masson-Delmotte et al, 2006">Masson-Delmotte, V., Kageyama, M., Braconnot, P., Charbit, S., Krinner, G., Ritz, C., Guilyardi, E., Jouzel, J., Abe-Ouchi, A., Crucifix, M., Gladstone, R.M., Hewitt, C.D., Kitoh, A., Legrande, A., Marti, O., Merkel, U., Motoi, T., Ohgaito, R., Otto-Bliesner, B., Peltier, W.R., Ross, I., Valdes, P.J., Vettoretti, G., Weber, S.L. and Wolk, F. 2006. Past and future polar amplification of climate change: climate model intercomparisons and ice-core constraints, ''Climate Dynamics'', '''26''', 513-529</ref>). Other IPCC models described later in this volume predict temperature changes of up to 4-6&deg;C<br />
<br />
Over the last transition in Antarctica, warming seems to start first, followed after a few hundred years by an increase in atmospheric CO<sub>2</sub> concentrations (Monnin et al., 2001<ref name="Monnin et al, 2001">Monnin, E., Indermuhle, A., Dallenbach, A., Fluckiger, J., Stauffer, B., Stocker, T.F., Raynaud, D. and Barnola, J-M. 2001. Atmospheric CO<sub>2</sub> Concentrations over the Last Glacial Termination, ''Science'', '''291''', 112-114.</ref>). The rise in CH<sub>4</sub> also appears to lag the initial rise in Antarctic temperature, but while the significant rapid excursions of CH<sub>4</sub> apparent in the Greenland record (which are related to the abrupt warming into the B&oslash;lling-Aller&oslash;d, the cooling to the Younger Dryas and the warming to the pre-Boreal) are mirrored in the Antarctic CH<sub>4</sub> record, these rapid changes seem to have no direct climate parallel in Antarctic temperature. The abrupt B&oslash;lling-Aller&oslash;d warming recorded in the Northern Hemisphere takes place at the beginning of the Antarctic Cold Reversal, but the precise cross-dating of these events remains disputed (Morgan et al., 2002<ref name="Morgan et al, 2002">Morgan, V., Delmotte, M., Van Ommen, T., Jouzel, J., Chappellaz, J., Woon, S., Masson-Delmotte, V. and Raynaud, D. 2002. Relative Timing of Deglacial Climate Events in Antarctica and Greenland, ''Science'', '''297''', 1862-1864.</ref>). Finally, the end of the Younger Dryas occurs when Antarctic temperatures have already reached their early Holocene optimum. The north-south sequence of events is affected by changes in orbital forcing, oceanic and atmospheric circulations, including see-saw effects related to changes in the thermohaline circulation, and biogeochemical cycle feedbacks. The precise role of Northern Hemisphere or Southern Hemisphere insolation changes on the termination onset remains uncertain (Schulz and Zeebe, 2006<ref name="Schulz and Zeebe, 2006">Schulz, K.G. and Zeebe, R.E. 2006. Pleistocene glacial terminations triggered by synchronous changes in Southern and Northern Hemisphere insolation : the insolation canon hypothesis, ''Earth and Planetary Science Letters'', '''249''', 326-336.</ref>). Similarly, and despite the strong correlation between CO<sub>2</sub> variations and Antarctic temperature, the mechanisms involved in glacial-interglacial changes in greenhouse gas concentrations and their lags with Antarctic temperature are still a challenge for modellers (K&ouml;hler et al., 2005<ref name="K&ouml;hler et al, 2005">K&ouml;hler, P., Fischer, H., Munhoven, G. and Zeebe, R.E. 2005. Quantitative interpretation of atmospheric carbon records over the last glacial termination, Glob. Biogeochem. Cycles, 19, Art. No. GB4020.</ref>).<br />
<br />
Across the transition, the concentration of the majority of aerosol species falls from high glacial levels to lower levels in the Holocene. However, the concentration measured in ice is a combination of changes in both the atmospheric aerosol loading and the accumulation rate. While the use of concentration or flux (expressed in &micro;g m<sup>-2</sup> year<sup>-1</sup>) is still debated for high accumulation sites, it is generally agreed that for the low accumulation central Antarctic sites, where dry deposition dominates wet deposition, the ice core flux measurement gives a better indication of the initial atmospheric aerosol concentration (Fischer et al., 2007b). At Dome C, the flux of sea-salt sodium and non-sea-salt calcium decrease by factors of 2 and 30, while in Dronning Maud Land, they decrease by factors of 2.6 and 10. The coherence of the sea-salt records from both sites may indicate the waning of the sea ice cover in the Indian and Atlantic Ocean sectors, with the greater magnitude of changes in Dronning Maud Land attributed to a greater retreat in sea ice cover and an additional role of summer sea-ice extent in the Weddell Sea sector. The non-sea-salt Ca is the fraction that is considered representative of mineral dust of continental origin, and the likely source region for both Dronning Maud Land and Dome C is Patagonia (Smith et al., 2003<ref name="Smith et al, 2003">Smith, J., Vance, D., Kempa, R.A., Archer, C., Tomsa, P., Kinga, M. and Z&aacute;rate, M. 2003. Isotopic constraints on the source of Argentinian loess-with implications for atmospheric circulation and the provenance of Antarctic dust during recent glacial maxima, ''Earth and Planetary Science Letters'', '''212''', 181-196.</ref>). The higher concentration of dust in Dronning Maud Land is indicative of a greater input of aeolian dust to the Weddell Sea region (Fischer et al., 2007a), presumably as a consequence of the closer proximity to the source region. Models have indicated that there is surprisingly little change in transport strength and atmospheric residence time of atmospherically entrained dust. Change in residence time makes no sense in this case between South America and Dome C during glacial and interglacial periods, so the large reduction in flux during the transition seen at both sites, which began at about 18 ka BP, is thought to be primarily resulting from the changes in the aridity and extent of the dust source region (Wolff et al., 2006<ref name="Wolff et al, 2006">Wolff, E.W., Fischer, H., Fundel, F., Ruth, U., Twarloh, B., Littot, G.C., Mulvaney, R., R&ouml;thlisberger, R., De Angelis, M., Boutron, C.F., Hansson, M., Jonsell, U., Hutterli, M.A,, Lambert, F., Kaufmann, P., Stauffer, B., Stocker, T.F., Steffensen, J.P., Bigler, M., Siggaard-Andersen, M.L., Udisti, R., Becagli, S., Castellano, E., Severi, M., Wagenbach, D., Barbante, C., Gabrielli, P. and Gaspari, V. 2006. Southern Ocean sea-ice extent, productivity and iron flux over the past eight glacial cycles, Nature, 440, 491-496 (doi:10.1038/nature04614).</ref>; Fischer et al., 2007a). An alternative hypothesis is that changes in the dust flux may be influenced by changes in the position of the polar front and strength as demonstrated by calibration of non sea-salt Ca and westerlies (Yan et al., 2005<ref name="Yan et al, 2005">Yan, Y., Mayewski, P.A., Kang, S. and Meyerson, E. 2005. An ice core proxy for Antarctic circumpolar wind intensity, ''Annals of Glaciology'', '''41''', 121-130.</ref>).<br />
<br />
Although the broad features of the last termination are now well documented in Antarctica, improvements can still be made in the detailed sequence of events with better resolution both temporally (for instance, in the phasing of changes in Antarctic climate, environmental parameters and atmospheric composition) and spatially, particularly in near coastal regions. This calls for further high resolution records of the last deglaciation and a comparison of records from ice drilling sites reflecting different oceanic basins. In addition, for a better understanding of climate dynamics, new ice cores are also needed to improve the knowledge of Antarctic ice sheet dynamics over the past deglaciation. Recently drilled records at Berkner Island and Talos Dome spanning the last deglaciation and beyond, and future new deep ice cores ([[:File:Figure 3.15 - Map of Antarctica showing some deep ice core sites.png|Figure 3.15]]) will help to progress the understanding of the climate and biogeochemical cycle responses to climate forcings, the climate variability around Antarctica and its source areas, and the reaction of the Antarctic ice sheet to major climate changes of the past.<br />
==References==<br />
<references /><br />
[[Category:The pre-instrumental period]]<br />
[[Category:The last million years]]<br />
[[Category:The ice core record]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_Southern_Ocean_in_the_instrumental_periodThe Southern Ocean in the instrumental period2014-08-06T14:34:21Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Antarctic climate and environment change in the instrumental period]]''<br />
<br />
==Introduction==<br />
<br />
At first sight the Southern Ocean seems to be dominated by circumpolar symmetry mainly assured by the circumpolar current bands of the eastward flowing ACC covering mainly the mid latitudes (Figure 1.8) and the westward flowing Antarctic Coastal Current surrounding the continental margin in high latitudes. Superimposed on the zonal flow there are meridional circulation cells (Figure 1.9), the deep one related to sinking motion along the continental slope to form the Antarctic Bottom Water, and the shallow one forming Mode and Intermediate Waters in the mid latitudes. In spite of the current system linking the Atlantic, Indian and Pacific sectors, significant zonal differences are obvious and require a regional description of the variability and change in the Southern Ocean. Differences are related to the shape of the ocean basins and ridges which strongly affect the ocean currents, the shape of the continent with huge embayments, such as the Weddell and the Ross Seas giving rise to subpolar gyres (Figure 1.10), the distributions of ice shelves, the zonal structure of the forcing (storm tracks) and finally the different hydrographic conditions in the northward adjacent ocean basins.<br />
<br />
[[File:Figure 4.22 - Schematic illustrating the formation areas and pathways of Antarctic Bottom Water.png|thumb|'''4.22''' Schematic presentation of the formation areas and pathways of Antarctic Bottom Water as evidenced by water mass properties, in particular the concentration of the anthropogenic tracer CFC. It is injected into the ocean at the surface and indicates, by concentration maxima, recently ventilated water sinking to the sea bottom and spreading there from the formation areas for different densities here displayed as neutral density where the most dense water with neutral density &gt; 28.4 kg/m<sup>3</sup> is found in the Weddell Sea (Orsi et al., 1999<ref name="Orsi et al, 1999">Orsi, A.H., Johnson, G.C. and Bullister, J.B. 1999. Circulation, mixing and production of Antarctic Bottom Water, ''Prog. Oceanog.'', '''43''', 55-109.</ref>).]]<br />
The meridional circulation cells which can only schematically be represented in a two dimensional way (Figure 1.9), display clear zonal differences as well. They are obvious in the long term mean conditions, e.g. the horizontal distribution of the newly formed bottom water ([[:File:Figure 4.22 - Schematic illustrating the formation areas and pathways of Antarctic Bottom Water.png|Figure 4.22]], Orsi et al., 1999<ref name="Orsi et al, 1999">Orsi, A.H., Johnson, G.C. and Bullister, J.B. 1999. Circulation, mixing and production of Antarctic Bottom Water, ''Prog. Oceanog.'', '''43''', 55-109.</ref>) which result in the variable contributions of different sectors to the bottom water formation. It is estimated that 66% can be contributed to originate in the Weddell Sea, 25% in the Australian Sector and 7% in the Ross Sea (Rintoul, 1998<ref name="Rintoul, 1998">Rintoul, S.R. 1998. On the origin and influence of Adelie Land Bottom Water. In: Ocean, Ice and Atmosphere: Interactions at the Antarctic Continental Margin, S. Jacobs and R. Weiss (eds.), Antarctic Research Series, 75, 151-171, American Geophysical Union, Washington.</ref>). Due to the zonal differences in the hydrographic and forcing conditions, variations differ in the sectors as well, e.g. cooling and freshening of the newly formed bottom water in the Australian sector (Rintoul, 2007<ref name="Rintoul, 2007">Rintoul, S.R. 2007. Rapid freshening of Antarctic Bottom Water formed in the Indian and Pacific Oceans, ''Geophys. Res. Lett.'', '''34''', L06606, doi:10.1029/2006GL028550.</ref>) and warming and increase of salinity of bottom water on the prime meridian (Fahrbach et al., 2004<ref name="Fahrbach et al, 2004">Fahrbach, E., Hoppema, M., Rohardt, G., Schr&ouml;der, M. and Wisotzki, A. 2004. Decadal-scale variations of water mass properties in the deep Weddell Sea, ''Ocean Dynamics'', '''54''', 77-91.</ref>). However, even within the basins different patterns are detected e.g. in the Weddell Sea (Fahrbach et al., 2004<ref name="Fahrbach et al, 2004">Fahrbach, E., Hoppema, M., Rohardt, G., Schr&ouml;der, M. and Wisotzki, A. 2004. Decadal-scale variations of water mass properties in the deep Weddell Sea, ''Ocean Dynamics'', '''54''', 77-91.</ref>). To take into account the circumpolar differences in the observed variations the description of the Southern Ocean is arranged here zonally in different sectors. Since regional differences occur even on smaller scales than the three large ocean sectors, we focus on sub-areas where a particular type of variation is observed.<br />
<br />
There is evidence that changes in the Antarctic Bottom Water propagate into the global ocean. Warming of the northward flow of Antarctic Bottom Water on a decadal time scale is observed in Vema Channel from the Argentine to the Brazil Basin (Zenk and Morozov, 2007<ref name="Zenk and Morozov, 2007">Zenk, W. and Morozov, E. 2007. Decadal warming of the coldest Antarctic Bottom Water flowing through the Vema Channel, ''Geophysical Research Letters'', '''34''', L14607, doi:10.1029/2007/GLR030340</ref>). Large scale abyssal warming over decades is observed in the South Atlantic (Johnson and Doney, 2006<ref name="Johnson and Doney, 2006">Johnson, G.C. and Doney, S.C. 2006. Recent western South Atlantic bottom water warming, ''Geophysical research Letters'', '''33''', L14614, doi:10.1029/2006GL026769.</ref>) and the Pacific (Johnson et al., 2007<ref name="Johnson et al, 2007">Johnson, G.C., Mecking, S., Sloyan, B.M. and Wijffels, S.E. 2007. Recent Bottom Water Warming in the Pacific Ocean, Journal of Climate, 5365-5375, doi:10.1175/2007JCLI1879.1.</ref>).<br />
<br />
Observation of change in surface waters is difficult to detect since there is an intensive seasonal cycle and only few observations. However, around South Georgia observations exist since 1925, which are frequent enough to resolve the annual cycle. They reveal significant trends in the upper 150 m, with 2.3&deg;C of warming over 81 years, which are twice as strong in winter than in summer (Whitehouse et al., 2008<ref name="Whitehouse et al, 2008">Whitehouse, M.J., Meredith, M.P., Rothery, P., Atkinson, A., Ward, P. and Korb, R.E. 2008. Rapid warming of the ocean at South Georgia, Southern Ocean during the 20<sup>th</sup> Century: forcings, characteristics and implications for lower trophic levels, ''Deep-Sea Research I'', '''55''', 1218-1228.</ref>).<br />
<br />
Sub-Antarctic Mode Water is formed by winter cooling and convection just north of the Sub-Antarctic Front (McCartney, 1977<ref name="McCartney, 1977">McCartney, M.S. 1977. Subantarctic Mode Waters. In: Angel, M. (ed) A voyage of discovery, Pergamon, New York, 103-119.</ref>, 1982<ref name="McCartney, 1982">McCartney, M.S. 1982. The subtropical circulation of Mode Waters, Journal of Marine Research, 40 (suppl), 427-464.</ref>). When it is subducted it becomes Antarctic Intermediate Water (Hanawa and Talley, 2001<ref name="Hanawa and Talley, 2001">Hanawa, K. and Talley, L.D. 2001. Mode Waters. In: Eds. G. Siedler, J. Church and J. Gould, Ocean circulation and climate; observing and modelling the global ocean, International Geophysics Series, 77, 373-386, Academic Press.</ref>). This water mass is of particular interest due to its capacity to take up atmospheric CO<sub>2</sub> (Sabine et al., 2004<ref name="Sabine et al, 2004">Sabine, C.L., ET AL. 2004. The Oceanic Sink for Anthropogenic CO<sub>2</sub>, ''Science'', '''305''', 367-371.</ref>). Significant warming (Wong, 2001; Gille, 2002<ref name="Gille, 2002">Gille, S.T. 2002. Warming of the Southern Ocean since the 1950s, ''Science'', '''295'''(5558), 1275-1277, doi:10.1126/science.1065863.</ref>; Aoki et al., 2003<ref name="Aoki et al, 2003">Aoki, S., Nakazawa, T., Machida, T., Sugawara, S., Morimoto, S., Hashida, G., Yamanouchi, T., Kawamura, K. and Honda, H. 2003. Carbon Dioxide Variations in the Stratosphere Over Japan, Scandinavia and Antarctic, ''Tellus'', '''55B''', 178-186.</ref>) and freshening was observed in these water masses (Wong et al., 1999<ref name="Wong et al, 1999">Wong, A.P.S., Bindoff, N.L. and Church, J.A. 1999. Large-scale freshening of intermediate waters in the Pacific and Indian oceans, Nature, 400, 86743), 440-443.</ref>; Curry et al., 2003<ref name="Curry et al, 2003">Curry, R., Dickson, B. and Yashayaev, I. 2003. A change in the freshwater balance of the Atlantic Ocean over the past decades, ''Nature'', '''426'''(6968), 826-829.</ref>; Bindoff et al., 2007<ref name="Bindoff et al, 2007">Bindoff, N., Willebrand, J., Artale, V., Cazenave, A., Gregory, J., Gulev, S., Hanawa, K., Le Qu&eacute;r&eacute;, C., Levitus, S., Nojiri, Y., Shum, C., Talley, L. and Unnikrishnan, A. 2007. Observations: oceanic climate change and sea level. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group 1 to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (eds. S. Solomon, D. Qin, and M. Manning). Cambridge University Press. United Kingdom and New York, NY, USA.</ref>; B&ouml;ning et al., 2008<ref name="B&ouml;ning et al, 2008">B&ouml;ning, C. W., A. Dispert, M. Visbeck, S. R. Rintoul, and F. Schwarzkopf, 2008. Observed multi-decadal ocean warming and density trends across the Antarctic Circumpolar Current, Submitted.</ref>) over the last decades. To understand the changes, the formation processes (e.g. Sall&eacute;e et al., 2006<ref name="Sall&eacute;e et al, 2006">Sall&eacute;e, J.-B., Wienders, N., Speer, K. and Morrow, R. 2006. Formation of subantarctic mode water in the southeastern Indian Ocean, ''Ocean Dynamics'', '''56''', 525-542, DOI 10.1007/S10236-005-0054-X.</ref>) and the turbulent character of the field (Tomczak, 2007<ref name="Tomczak, 2007">Tomczak, M. 2007. Variability of Antarctic intermediate Water properties in the South Pacific Ocean, ''Ocean Science'', '''3''', 363-377.</ref>) need to be understood.<br />
<br />
[[File:Figure 4.23 - ARGO temperature anomalies and ACC fronts.png|thumb|'''4.23''' Temperature differences between measurements from Argo floats deployed after 2001, presented as dots, and the climatological mean from CARS in the density layers of the ACC show an increase of potential temperature of the water masses on the southward flank of the ACC and a freshening on the northward side. On surfaces of constant densities freshening becomes visible as cooling, since the density loss by less salt is compensated by a density gain due to colder temperature. The major fronts representing ACC branches are displayed as dynamic height contours where the 1.4-m-dynamic-height contour is related to the Polar Front and 1.7-m one to the Subantarctic Front (B&ouml;ning et al., 2008<ref name="B&ouml;ning et al, 2008">B&ouml;ning, C. W., A. Dispert, M. Visbeck, S. R. Rintoul, and F. Schwarzkopf, 2008. Observed multi-decadal ocean warming and density trends across the Antarctic Circumpolar Current, Submitted.</ref>)]]<br />
In a circumpolar view the ACC band has warmed in recent decades ([[:File:Figure 4.23 - ARGO temperature anomalies and ACC fronts.png|Figure 4.23]]; Gille, 2002<ref name="Gille, 2002">Gille, S.T. 2002. Warming of the Southern Ocean since the 1950s, ''Science'', '''295'''(5558), 1275-1277, doi:10.1126/science.1065863.</ref>, 2008<ref name="Gille, 2008">Gille, S.T. 2008. Decadal-scale temperature trends in the Southern Hemisphere ocean, ''J. Clim.'', '''21'''(18), 4749-4765.</ref>; Levitus et al., 2000<ref name="Levitus et al, 2000">Levitus, S., Antanov, J.I., Boyer, T.P. and Stephens, C. 2000. Warming of the world ocean, ''Science'', '''287''' (5461), 2225-2229.</ref>, 2005<ref name="Levitus et al, 2005">Levitus, S., Antonov, J. and Boyer, T. 2005. Warming of the world ocean, ''Geophysical Res. Letters'', '''32''', L02604, doi:10.1029/2004GL021592.</ref>; B&ouml;ning et al., 2008<ref name="B&ouml;ning et al, 2008">B&ouml;ning, C. W., A. Dispert, M. Visbeck, S. R. Rintoul, and F. Schwarzkopf, 2008. Observed multi-decadal ocean warming and density trends across the Antarctic Circumpolar Current, Submitted.</ref>). The changes are consistent with a southward shift of the ACC (Gille, 2008<ref name="Gille, 2008">Gille, S.T. 2008. Decadal-scale temperature trends in the Southern Hemisphere ocean, ''J. Clim.'', '''21'''(18), 4749-4765.</ref>). Some climate models suggest that the ACC shifts south in response to a southward shift of the westerly winds driven by enhanced greenhouse forcing (Fyfe and Saenko, 2006<ref name="Fyfe and Saenko, 2006">Fyfe, J.C. and Saenko, O.A. 2006. Simulated changes in extratropical Southern Hemisphere winds and currents, ''Geophys. Res. Letters'', '''33''', L06701, doi:10.1029/2005GL025332.</ref>; Bi et al., 2002<ref name="Bi et al, 2002">Bi, D., Budd, W.F., Hirst, A.C. and Wu, X. 2002. Response of the Antarctic circumpolar current transport to global warming in a coupled model, ''Geophys. Res. Lett.'', '''29''' (24), 2173, doi:10.1029/2002GL015919.</ref>). A significant part of the changes in the wind system can be related to the positive trend in the SAM (e.g. Hughes et al., 2003<ref name="Hughes et al, 2003">Hughes, C.W., Woodworth, P.L., Meredith, M.P., Stepanov, V., Whitworth, T. and Pyne A.R. 2003. Coherence of Antarctic sea levels, Southern Hemisphere Annular Mode, and flow through Drake Passage, ''Geophysical Research Letters'', '''30'''(9), 1464, doi:10.1029/2003GL017240.</ref>). The poleward shift and intensification of winds over the Southern Ocean has been attributed to both changes in ozone in the Antarctic stratosphere (Thompson and Solomon, 2002<ref name="Thompson and Solomon, 2002">Thompson, D. and Solomon, S. 2002. Interpretation of recent southern hemisphere climate change, ''Science'', '''296'''(5569), 895-899.</ref>) and to greenhouse warming (Fyfe et al., 1999<ref name="Fyfe et al, 1999">Fyfe, J.C., Boer, G.J. and Flato, G.M. 1999. The Arctic and Antarctic oscillations and their projected changes under global warming, ''Geophys. Res. Lett.'', '''26''', 1601-1604.</ref>). In addition to driving changes in the ACC, the wind changes have caused a southward expansion of the subtropical gyres (Cai, 2006<ref name="Cai, 2006">Cai, W. 2006. Antarctic ozone depletion causes in intensification of the Southern Ocean super-gyre circulation, Geophys. Res. Lett., 33, doi:10.1029/2005GL024911.</ref>) and an intensification of the Southern Hemisphere &ldquo;supergyre&rdquo; that links the three subtropical gyres (Speich et al., 2002<ref name="Speich et al, 2002">Speich, S., Blanke, B., De Vries, P., ET AL. 2002. Tasman leakage: A new route in the global ocean conveyor belt, ''Geophys. Res. Lett.'', '''29''', 1416.</ref>, 2007<ref name="Speich et al, 2007">Speich, S., Blanke, B. and Cai, W.J. 2007. Atlantic meridional overturning circulation and the Southern Hemisphere supergyre, ''Geophys. Res. Let.'', '''34''', L23614.</ref>). The &ldquo;supergyre&rdquo; provides the mechanism by which Sub-Antarctic Mode Water and Antarctic Intermediate Water is distributed between the ocean basins (Ridgway and Dunn, 2007<ref name="Ridgway and Dunn, 2007">Ridgway, K.R. and Dunn, J.R. 2007. Observational evidence for a Southern Hemisphere oceanic supergyre, ''Geophysical Research Letters'', '''34''', L13612, doi:10.1029/2007GL030392.</ref>). In the area of the formation of Antarctic Intermediate Water north of the Subantarctic Front freshening is observed. Surprisingly the changes are rather similar all along the ACC ([[:File:Figure 4.23 - ARGO temperature anomalies and ACC fronts.png|Figure 4.23]]; B&ouml;ning et al., 2008<ref name="B&ouml;ning et al, 2008">B&ouml;ning, C. W., A. Dispert, M. Visbeck, S. R. Rintoul, and F. Schwarzkopf, 2008. Observed multi-decadal ocean warming and density trends across the Antarctic Circumpolar Current, Submitted.</ref>).<br />
==Pages in this topic==<br />
#[[The Australian sector in the instrumental period]]<br />
#[[The Amundsen and Bellingshausen seas in the instrumental period]]<br />
#[[Ross Sea shelf waters in the instrumental period]]<br />
#[[The Weddell Sea sector in the instrumental period]]<br />
#[[Small-scale processes in the Southern Ocean]]<br />
#[[Modelling of the ACC and polar gyres]]<br />
==References==<br />
<references /><br />
[[Category:The instrumental period]]<br />
[[Category:The Southern Ocean]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_Southern_Ocean_carbon_cycle_response_to_historical_climate_changeThe Southern Ocean carbon cycle response to historical climate change2014-08-06T14:34:20Z<p>Acce: Corrected formatting of Cant (anthropogenic carbon)</p>
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<div>:''This page is part of the topic [[Antarctic climate and environment change in the instrumental period]]''<br />
<br />
==Introduction==<br />
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The Southern Ocean, with its energetic interactions between the atmosphere, ocean and sea ice, plays a critical role in ventilating the global oceans and regulating the climate system though the uptake and storage of heat, freshwater and atmospheric CO<sub>2</sub> (Rintoul et al., 2001<ref name="Rintoul et al, 2001">Rintoul, S.R., Hughes, C.W. and Olbers, D. 2001. The Antarctic Circumpolar Current system. In: Eds. G. Siedler, J. Church and J. Gould, Ocean circulation and climate; observing and modelling the global ocean. International Geophysics Series, 77, 271-302, Academic Press.</ref>; Sarmiento et al., 2004a<ref name="Sarmiento et al, 2004a">Sarmiento, J.L., Gruber, N., Brzezinski, M.A. and Dunne, J.P. 2004a. High-latitude Controls of Thermocline Nutrients and Low Latitude Biological Productivity, ''Nature'', '''427''', 56-60.</ref>). The Southern Ocean is dominated by the eastward flowing ACC. The surface of the ACC is characterized by a northward Ekman flow creating a divergent driven deep upwelling south of the ACC and convergent flow north of the ACC. In the southern part, the upwelling of mid-depth (2-2.5 km) water to the surface provides a unique connection between the deep ocean and the atmosphere while in the northern part the downwelling provides a strong connection to water masses that resurface at lower latitudes e.g. Sarmiento et al. (2004a<ref name="Sarmiento et al, 2004a">Sarmiento, J.L., Gruber, N., Brzezinski, M.A. and Dunne, J.P. 2004a. High-latitude Controls of Thermocline Nutrients and Low Latitude Biological Productivity, ''Nature'', '''427''', 56-60.</ref>). These connections make the Southern Ocean extremely important in controlling the storage of carbon in the ocean and a key driver in setting atmospheric CO<sub>2</sub> levels (Caldeira and Duffy, 2000<ref name="Caldeira and Duffy, 2000">Caldeira, K. and Duffy, P.B. 2000. The role of the Southern Ocean in uptake and storage of anthropogenic carbon dioxide, ''Science'', '''287''', 620-622.</ref>).<br />
<br />
==CO<sub>2</sub> fluxes in the Southern Ocean==<br />
<br />
[[File:Figure 4.55 - The annual cycle atmosphere-ocean CO2 transport in the Southern Ocean.png|thumb|'''4.55''' The annual cycle of epCO<sub>2</sub> (pCO<sub>2</sub><sup>ocean</sup>-pCO<sub>2</sub><sup>atm</sup>; i.e. negative/positive = atmospheric CO<sub>2</sub> sink) in the Southern Ocean for the regions 40&deg;-50&deg;S (black line) and 50&deg;-62&deg;S (dashed line; Takahashi et al., 2009<ref name="Takahashi et al, 2009">Takahashi, T., Sutherland, S.C., Wanninkhof, R., Sweeney, C., Feely, R.A., Chipman, D.W., Hales, B., Friederich, G., Chavez, F., Sabine, C., Watson, W., Bakker, D.C.E., Schuster, U., Metzl, N., Yoshikawa-Inoue, H., Masao Ishii, M., Midorikawa, T., Nojiri, Y., Kortzinger, A., Steinhoff, T., Hoppema, M., Olafsson, J., Thorarinn, S.., Arnarson, T.S., Tilbrook, B., Johannessen, T., Olsen, A., Bellerby, R., Wong, C.S., Delille, B., Bates, N.R. and De Baar, H.J.W. 2009. Climatological mean surface PCO<sub>2</sub> and net CO<sub>2</sub> flux over the global oceans, Deep Sea Research, doi:10.1016/dsr2.2008.12.009.</ref>). The Sub-Antarctic zone (40&deg;-50&deg;S) acts a permanent CO<sub>2</sub> sink but at higher latitudes the ocean acts as an atmospheric sink during summer and a source during winter (Metzl et al., 2006<ref name="Metzl et al, 2006">Metzl, N., Brunet, C., Jabaud-Jan, A., Poisson, A. and Schauer, B. 2006. Summer and Winter Air-Sea CO<sub>2</sub> Fluxes in the Southern Ocean, Deep-Sea Research II, 53.</ref>).]]<br />
The Southern Ocean carbon cycle response uptake can be described as a combination of seasonal and non-seasonal variability. In the Southern Ocean seasonal variability is the dominant mode of variability, and the mechanisms that drive the air-sea CO<sub>2</sub> fluxes are well known but their magnitudes remained poorly constrained (Metzl et al., 2006<ref name="Metzl et al, 2006">Metzl, N., Brunet, C., Jabaud-Jan, A., Poisson, A. and Schauer, B. 2006. Summer and Winter Air-Sea CO<sub>2</sub> Fluxes in the Southern Ocean, Deep-Sea Research II, 53.</ref> and references therein). The seasonal cycle of CO<sub>2</sub> uptake is a complex interplay between the biological and physical pumps and can be described both in terms of its magnitude and phase. A recent climatological synthesis of more than 3 million measurements of surface pCO<sub>2</sub> measurements provides important insights into Southern Ocean behaviour at the seasonal timescales (Takahashi et al., 2009<ref name="Takahashi et al, 2009">Takahashi, T., Sutherland, S.C., Wanninkhof, R., Sweeney, C., Feely, R.A., Chipman, D.W., Hales, B., Friederich, G., Chavez, F., Sabine, C., Watson, W., Bakker, D.C.E., Schuster, U., Metzl, N., Yoshikawa-Inoue, H., Masao Ishii, M., Midorikawa, T., Nojiri, Y., Kortzinger, A., Steinhoff, T., Hoppema, M., Olafsson, J., Thorarinn, S.., Arnarson, T.S., Tilbrook, B., Johannessen, T., Olsen, A., Bellerby, R., Wong, C.S., Delille, B., Bates, N.R. and De Baar, H.J.W. 2009. Climatological mean surface PCO<sub>2</sub> and net CO<sub>2</sub> flux over the global oceans, Deep Sea Research, doi:10.1016/dsr2.2008.12.009.</ref>). During the austral summer biological production reduces surface ocean pCO<sub>2</sub> through photosynthetic activity and then exports part of this organic matter to the deep ocean. This reduction in surface pCO<sub>2</sub> is offset by the changes in the physical pump that reduce the capacity of the surface water to store CO<sub>2</sub> through upper ocean warming, which lowers solubility, thereby increasing the surface ocean pCO<sub>2</sub>. The net result of this competition between the biological and physical pumps is that the Southern Ocean acts a sink of atmospheric CO<sub>2</sub> in the summer in the sub-Antarctic zone (SAZ; nominally 40&deg;S &ndash; 50&deg;S) and south of the Polar Front (PF; ~50&deg;S; [[:File:Figure 4.55 - The annual cycle atmosphere-ocean CO2 transport in the Southern Ocean.png|Figure 4.55]]). During the austral winter, a picture of two distinct zones separated by the PF is evident. South of the PF relatively little biological activity occurs in winter (deep mixing and light limitation); therefore surface pCO<sub>2</sub> values are set by the competition within the physical pump between deep winter mixing bringing up CO<sub>2</sub> water from the carbon rich deep ocean, leading to increased pCO<sub>2</sub> surface levels, and a cooling that increases the ability of the surface waters to store CO<sub>2</sub>. As the deep winter mixing dominates in this region a net out-gassing of CO<sub>2</sub> to the atmosphere occurs. North of the PF during the austral winter the same cooling and mixing processes that occur further south exist, but with the addition of biological activity during the period, albeit at a reduced rate, reducing the pCO<sub>2</sub> in surface waters. The result of the combined responses of the biological and physical pumps means this region acts as a weak sink of atmospheric CO<sub>2</sub> during the austral winter.<br />
<br />
When the observed winter and summer fluxes are integrated, the annual mean uptake is small south of 50&deg;S (about -0.08 PgC/yr); conversely the SAZ (40&deg;S-50&deg;S) behaves as a strong sink (-0.74 PgC/yr) (Takahashi et al, 2009<ref name="Takahashi et al, 2009">Takahashi, T., Sutherland, S.C., Wanninkhof, R., Sweeney, C., Feely, R.A., Chipman, D.W., Hales, B., Friederich, G., Chavez, F., Sabine, C., Watson, W., Bakker, D.C.E., Schuster, U., Metzl, N., Yoshikawa-Inoue, H., Masao Ishii, M., Midorikawa, T., Nojiri, Y., Kortzinger, A., Steinhoff, T., Hoppema, M., Olafsson, J., Thorarinn, S.., Arnarson, T.S., Tilbrook, B., Johannessen, T., Olsen, A., Bellerby, R., Wong, C.S., Delille, B., Bates, N.R. and De Baar, H.J.W. 2009. Climatological mean surface PCO<sub>2</sub> and net CO<sub>2</sub> flux over the global oceans, Deep Sea Research, doi:10.1016/dsr2.2008.12.009.</ref>; see comments below). The response of the SAZ is consistent with other studies that suggest the SAZ is also a strong CO<sub>2</sub> sink approaching -1 PgC/yr (McNeil et al., 2007<ref name="McNeil et al, 2007">McNeil, B.I., Metzl, N., Key, R.M., Matear R.J. and Corviere, A. 2007., An empirical estimate of the Southern Ocean air-sea CO<sub>2</sub> flux, ''Global Biogeochemical Cycles'', '''21'''(3), GB03011, doi:10.1029/2007GB002991</ref>; Metzl et al., 1999<ref name="Metzl et al, 1999">Metzl, N., Tilbrook, B. and Poisson, A. 1999. The annual fCO<sub>2</sub> cycle and the air-sea CO<sub>2</sub> flux in the sub-Antarctic Ocean, ''Tellus'', '''51B''', 849-861.</ref>) and an important region of mode and intermediate water formation and transformation. In comparison to the total uptake of 2 GtC/yr for the global ocean, the Southern Ocean south of 40&deg;S takes up more than 40% of the total uptake. Note in these calculations we have used the gas transfer coefficent of Wanninkhof (1992<ref name="Wanninkhof, 1992">Wanninkhof, R. 1992. Relationship between wind speed and gas exchange over the ocean, ''Journal of Geophysical Research'', '''97''', 7373-7382.</ref>) with the dataset of Takahashi et al., (2008).<br />
<br />
[[File:Figure 4.56 - Annual mean uptake of air-sea CO2 fluxes as calculated from OPA-PISCES 1990-1999.png|thumb|'''4.56''' Annual mean uptake of air-sea CO<sub>2</sub> fluxes as calculated from OPA/PISCES 1990-1999 (Lenton et al., 2006<ref name="Lenton et al, 2006">Lenton, A., Metzl, N., Aumont, O., Lo Monaco, C. and Rodgers, K. 2006. Simulating the Ocean Carbon Cycle: A focus on the Southern Ocean. Presented at Second CARBOOCEAN Annual Meeting, 4-8 Dec 2006, Maspalomas, Spain.</ref>) and that from the new climatology of Takahashi et al. (2009<ref name="Takahashi et al, 2009">Takahashi, T., Sutherland, S.C., Wanninkhof, R., Sweeney, C., Feely, R.A., Chipman, D.W., Hales, B., Friederich, G., Chavez, F., Sabine, C., Watson, W., Bakker, D.C.E., Schuster, U., Metzl, N., Yoshikawa-Inoue, H., Masao Ishii, M., Midorikawa, T., Nojiri, Y., Kortzinger, A., Steinhoff, T., Hoppema, M., Olafsson, J., Thorarinn, S.., Arnarson, T.S., Tilbrook, B., Johannessen, T., Olsen, A., Bellerby, R., Wong, C.S., Delille, B., Bates, N.R. and De Baar, H.J.W. 2009. Climatological mean surface PCO<sub>2</sub> and net CO<sub>2</sub> flux over the global oceans, Deep Sea Research, doi:10.1016/dsr2.2008.12.009.</ref>). The sub-Antarctic region (40-50&deg;S) represents a strong sink (blue colors), whereas south of 50&deg;S, large regions act as a CO<sub>2</sub> source for the atmosphere (red).]]<br />
Significant progress has been made in recent years in simulating the annual mean uptake of CO<sub>2</sub> by the Southern Ocean as precursor to understanding interannual to decadal variability. [[:File:Figure 4.56 - Annual mean uptake of air-sea CO2 fluxes as calculated from OPA-PISCES 1990-1999.png|Figure 4.56]] shows that the spatial pattern of the annual mean uptake is well represented in comparison with that simulated from the current class of ocean biogeochemical models (e.g. OPA/PISCES model; Aumont and Bopp, 2006<ref name="Aumont and Bopp, 2006">Aumont, O. and Bopp, L. 2006. Globalizing results from ocean ''in situ'' iron fertilization studies, Global Biogeochemical Cycles, 20, doi:10,029/2005GB002519.</ref>)). Although different models do contain different representations of the magnitude of the seasonal cycle, the phase between each model and observations shows good agreement. This suggests that seasonal processes that drive this variability are well captured, and that as a result, we are in a better position to explore changes at interannual and longer timescales.<br />
<br />
==Historical Change - Observed Response==<br />
<br />
The Southern Ocean has undergone significant changes in response to climate changes; such as a net increase in heat and freshwater fluxes, and a poleward movement and intensification of winds e.g. Thompson and Solomon (2002<ref name="Thompson and Solomon, 2002">Thompson, D. and Solomon, S. 2002. Interpretation of recent southern hemisphere climate change, ''Science'', '''296'''(5569), 895-899.</ref>). The major driver of these changes has been the SAM that is associated with changes in the Antarctic Vortex, primarily in response to depletion of stratospheric ozone and increasing atmospheric greenhouse gas concentrations (Arblaster and Meehl, 2006<ref name="Arblaster and Meehl, 2006">Arblaster, J.M. and Meehl G.A. 2006. Contributions of external forcings to Southern Annual Mode trends, ''Journal of Climate'', '''19''', 2896-2905.</ref>). While the SAM is a significant driver of variability it cannot explain all the variability present, as it has both linear and non-linear interactions with other climatic modes such as ENSO and Indian Ocean Dipole (IOD) that drives diverse responses in different regions. These physical changes impact directly on the physical pump and to a lesser extent on the biological pumps, therefore on the concentration of CO<sub>2</sub> in surface waters and the magnitude of both uptake and export of CO<sub>2</sub> from the atmosphere to the deep ocean.<br />
<br />
In the Southern Ocean the interannual to decadal changes in biological production, ocean dynamics and thermodynamics that drive oceanic pCO<sub>2</sub> and air-sea CO<sub>2</sub> exchanges remain poorly understood and very undersampled. Decadal and interannual variations have been observed at high latitudes, but at only very few locations and during the austral summer (Jabaud-Jan et al., 2004<ref name="Jabaud-Jan et al, 2004">Jabaud-Jan, A., Metzl, N., Brunet, C., Poisson, A. and Schauer, B. 2004. Interannual varaibility of the carbon dioxide system in the southern Indian Ocean (20˚-60˚S): the impact of a warm anomaly in the austral summer 1998, Global Biogeochemical Cycles, 18, GB1042,1010.1029/2002GB002017.</ref>; Br&eacute;vi&egrave;re et al., 2006<ref name="Br&eacute;vi&egrave;re et al, 2006">Br&eacute;vi&egrave;re, E., Metzl, N., Poisson, A. and Tilbrook, B. 2006. Changes of the oceanic CO<sub>2</sub> sink in the Eastern Indian sector of the Southern Ocean, ''Tellus B'', '''58B''', 438-446.</ref>, Borges et al., 2008<ref name="Borges et al, 2008">Borges, A.V., Tilbrook, B., Metzl, N., Lenton, A. and Delille, B. 2008. Inter-annual variability of the carbon dioxide oceanic sink south of Tasmania, ''Biogeosciences'', '''5''', 141-145.</ref>). Although these analyses provide important information on the response of the ocean to climate variability in the Southern Hemisphere, there is no clear detection of the decadal trends of oceanic CO<sub>2</sub> and associated air-sea CO<sub>2</sub> fluxes, unlike the situation in the north Atlantic or the north and equatorial Pacific where there are long-term time series of such data (e.g. Bates, 2001<ref name="Bates, 2001">Bates, N. 2001. Interannual variability of oceanic CO<sub>2</sub> and biogeochemical properties in the Western North Atlantic subtropical gyre, ''Deep-Sea Research II'', '''48''', 1507-1528.</ref>; Feely et al., 2002<ref name="Feely et al, 2002">Feely, R.A., Boutin, J., Cosca, C.E., Dandonneau, Y., Etcheto, J., Inoue, H.Y., Ishii, M., Le Quere, C., Mackey, D.J., McPhaden, M., Metzl, N., Poisson, A. and Wanninkhof, R. 2002, Seasonal and interannual variability of CO<sub>2</sub> in the equatorial Pacific, Deep-Sea Research Part II - Topical Studies in Oceanography, 49, 2443-2469.</ref>). Takahashi et al. (2009<ref name="Takahashi et al, 2009">Takahashi, T., Sutherland, S.C., Wanninkhof, R., Sweeney, C., Feely, R.A., Chipman, D.W., Hales, B., Friederich, G., Chavez, F., Sabine, C., Watson, W., Bakker, D.C.E., Schuster, U., Metzl, N., Yoshikawa-Inoue, H., Masao Ishii, M., Midorikawa, T., Nojiri, Y., Kortzinger, A., Steinhoff, T., Hoppema, M., Olafsson, J., Thorarinn, S.., Arnarson, T.S., Tilbrook, B., Johannessen, T., Olsen, A., Bellerby, R., Wong, C.S., Delille, B., Bates, N.R. and De Baar, H.J.W. 2009. Climatological mean surface PCO<sub>2</sub> and net CO<sub>2</sub> flux over the global oceans, Deep Sea Research, doi:10.1016/dsr2.2008.12.009.</ref>) have recently constructed a pCO<sub>2</sub> data synthesis, from which a significant increase of oceanic pCO<sub>2</sub> during winter has been calculated, about +2.1&plusmn;0.6 &micro;atm/yr, which is close to or faster than the growth rate in the atmosphere (1.7 ppm/yr) over the period 1986-2007.<br />
<br />
[[File:Figure 4.57 - Annual mean trends of temperature normalized fCO2 for regions of the SW Indian Ocean.png|thumb|'''4.57''' Annual mean trends of temperature normalized fCO<sub>2</sub> in 4 regions of the South-Western Indian Ocean (based on summer and winter observations in 1991-2007). The open bars indicate the growth rates estimated for summer and black bars for winter. Standard errors associated to each trend are also indicated. The dashed line indicates the atmospheric CO<sub>2</sub> annual growth rate (figure reproduced from Metzl, 2009<ref name="Metzl, 2009">Metzl, N. 2009. Decadal Increase of ocean carbon dioxide in the Southwest Indian Ocean Surface Waters (1991-2007), Deep Sea Research II, doi:10.1016/j.dsr2.2008.12.007.</ref>).]]<br />
Repeat underway measurements of surface pCO<sub>2</sub> have been made regularly in the Southern Indian Ocean since the 1990s (e.g. Metzl et al.,1999<ref name="Metzl et al, 1999">Metzl, N., Tilbrook, B. and Poisson, A. 1999. The annual fCO<sub>2</sub> cycle and the air-sea CO<sub>2</sub> flux in the sub-Antarctic Ocean, ''Tellus'', '''51B''', 849-861.</ref>). Although these measurements are quite often confined to regions where ships travel to resupply Antarctic and sub-Antarctic bases, they represent a valuable timeseries for exploring the evolution of the surface ocean. A recent study by Metzl (2009<ref name="Metzl, 2009">Metzl, N. 2009. Decadal Increase of ocean carbon dioxide in the Southwest Indian Ocean Surface Waters (1991-2007), Deep Sea Research II, doi:10.1016/j.dsr2.2008.12.007.</ref>; Figure 3.68) in the South Western Indian Ocean calculated surface trends of pCO<sub>2</sub> between 1991-2007 and showed that oceanic pCO<sub>2</sub> increased at all latitudes south of 20&deg;S (1.5 to 2.4 &micro;atm/yr depending the location and season). More specifically, at latitudes of less than 40&deg;S, they determined that oceanic pCO<sub>2</sub> increased faster than in the atmosphere since 1991, suggesting the strength of the oceanic sink decreased. In addition, when pCO<sub>2</sub> data are normalized to temperature, removing the effect of solubility on CO<sub>2</sub>, this analysis showed that the system is increasing much faster in the winter than in the summer ([[:File:Figure 4.57 - Annual mean trends of temperature normalized fCO2 for regions of the SW Indian Ocean.png|Figure 4.57]]). These results suggest that the increase may be due to changes in ocean dynamics, given that the largest response occurs in the austral winter, when the winds are strongest. In the recent period (since the 1980s) the increase of pCO<sub>2</sub> appeared to be faster compared to the trends based on historical observations from 1969-2002 (Inoue and Ishii, 2005<ref name="Inoue and Ishii, 2005">Inoue, H.Y. and Ishii, M. 2005. Variations and trends of CO<sub>2</sub> in the surface seawater in the Southern Ocean south of Australia between 1969 and 2002, ''Tellus B'', '''57''', 58-69.</ref>), suggesting that the Southern Ocean CO<sub>2</sub> sink has continued to evolve in response to climate change.<br />
<br />
[[File:Figure 4.58 - Sea-air CO2 flux anomalies in the Southern Ocean.png|thumb|'''4.58''' Sea-air CO<sub>2</sub> flux anomalies in the Southern Ocean (PgC/y, &gt;45&deg;S) based on atmospheric CO<sub>2</sub> data and inversed transport model (on top) and a global biogeochemical ocean model (bottom). Compared to experiments that do not take into account the climate variability (in red), both approaches suggest a stabilization or reduction of the ocean CO<sub>2</sub> sink since the 1980s (Le Qu&eacute;r&eacute; et al., 2007<ref name="Qu&eacute;r&eacute; et al, 2007">Le Qu&eacute;r&eacute;, C., Rodenbeck, C., Buitenhuis, E.T., Conway, T.J., Langenfelds, R., Gomez, A., Labuschagne, C., Ramonet, M., Nakazawa, T., Metzl, N., Gillett, N. and Heimann, M. 2007. Saturation of the Southern Ocean CO<sub>2</sub> Sink Due to Recent Climate Change, ''Science'', '''316''', 1735-1738, doi: 10.1126/science.1136188.</ref>).]]<br />
As oceanic pCO<sub>2</sub> in recent years has been observed to be increasing close to, or faster than in the atmosphere, the signature of these changes in atmospheric CO<sub>2</sub> data should be detectable, as has been observed in the Equatorial Pacific during ENSO events (e.g. Peylin et al., 2005<ref name="Peylin et al, 2005">Peylin, P., ET AL. 2005. Multiple constrains on regional CO<sub>2</sub> flux variations over land and oceans, Global Biogeochemical Cycles, 19, doi:10.1029/2003GB002214.</ref>). At latitudes south of 40&deg;S the ocean has a very large surface and it is expected that continental carbon source/sink variability has a low imprint in atmospheric CO<sub>2</sub> records (compared to the tropics and north hemisphere). This is clearly seen in the CO<sub>2</sub> record at La Nouvelle Amsterdam Island (in the South-Indian Ocean), for example, where the seasonality of atmospheric CO<sub>2</sub> is very low. A recent study by Le Qu&eacute;r&eacute; et al. (2007<ref name="Qu&eacute;r&eacute; et al, 2007">Le Qu&eacute;r&eacute;, C., Rodenbeck, C., Buitenhuis, E.T., Conway, T.J., Langenfelds, R., Gomez, A., Labuschagne, C., Ramonet, M., Nakazawa, T., Metzl, N., Gillett, N. and Heimann, M. 2007. Saturation of the Southern Ocean CO<sub>2</sub> Sink Due to Recent Climate Change, ''Science'', '''316''', 1735-1738, doi: 10.1126/science.1136188.</ref>) using a combination of atmospheric observations and inverse methods reported that in the period 1981-2004, the strength of the Southern Ocean CO<sub>2</sub> sink (south of 45&deg;S) was reduced ([[:File:Figure 4.58 - Sea-air CO2 flux anomalies in the Southern Ocean.png|Figure 4.58]]). Although this result remains controversial (e.g. Law et al., 2008<ref name="Law et al, 2008">Law, R.M., Matear, R.J. and Francey, R.J. 2008. Comment on &quot;Saturation of the Southern Ocean CO<sub>2</sub> Sink Due to Recent Climate Change&quot;, ''Science'', '''359''', 570, doi: 10.1126/science.1136188.</ref>), it does suggest that the observed increase in oceanic pCO<sub>2</sub> acts to reduce the strength of the air-sea CO<sub>2</sub> gradient ( pCO<sub>2</sub>) and this in turn translates to reduction in the strength of the Southern Ocean CO<sub>2</sub> sink. This result is significant as it was expected in response to strengthening air-sea CO<sub>2</sub> gradient that the Southern Ocean CO<sub>2</sub> uptake would increase. These results are also consistent with a number of recent modeling studies, that have also suggested a decrease in uptake in the last decades and which attributed the upper ocean pCO<sub>2</sub> values to increases in the wind speed increasing the ventilation of carbon rich deep waters e.g. Lenton and Matear (2007<ref name="Lenton and Matear, 2007">Lenton, A. and Matear, R.J. 2007. The role of the Southern Annular Mode (SAM) in Southern Ocean CO<sub>2</sub> uptake, Global Biogeochemical Cycles, 21, doi: 10:1029/2006GB002714.</ref>).<br />
<br />
==Historical Changes &ndash; Simulated View==<br />
<br />
[[File:Figure 4.59 - Annual-averaged Southern Ocean uptake of total, natural and anthropogenic carbon.png|thumb|'''4.59''' Annual-averaged Southern Ocean uptake of: a) total carbon; b) natural carbon; c) anthropogenic carbon. The different experiments use the following colour coding, total experiment (black line), 1948 (red line), wind stress (tau, green line), heat flux (blue line) and freshwater flux (cyan line).]]<br />
To explore how changes in Southern Ocean air-sea CO<sub>2</sub> fluxes have responsed to historical climate change between 1948-2003. Matear and Lenton (2008<ref name="Matear and Lenton, 2008">Matear, R.J. and Lenton, A. 2008. Impact of Historical Climate Change on the Southern Ocean Carbon Cycle, ''Journal of Climate'', '''21''', 5820-5834, doi: 10.1175/2008JCLI2194.1.</ref>) used a biogeochemical ocean model driven with observed changes (NCEP R1; Kalnay et al., 1996<ref name="Kalnay et al, 1996">Kalnay, E. and coauthors. 1996. The NCEP/NCAR 40-year reanalysis project, ''Bull. Amer. Meteor. Soc.'', '''77''', 437-471.</ref>). They explored how the total carbon cycle as well as the natural and anthropogenic carbon uptake responded to the observed increases in windstress, heat and freshwater fluxes ([[:File:Figure 4.59 - Annual-averaged Southern Ocean uptake of total, natural and anthropogenic carbon.png|Figure 4.59]]). Their results show a complex picture: when only either heat or freshwater fluxes increase, the total uptake is slightly greater than the total CO<sub>2</sub> flux response (with all fields increasing). In contrast, when only the wind speed increased, the total uptake was less than the total response. In addition, the anthropogenic response was always much smaller than the natural carbon cycle response and hence the natural response dominated the total response. The natural carbon sink dominates the total response over the last 50 years for two reasons: (i) because wind stress changes are larger than the corresponding changes in heat and freshwater flux, therefore changes in solubility are dominated by changes in ocean dynamics; and ii) the CO<sub>2</sub> gradient between the atmosphere and the ocean due to anthropogenic emissions is strong enough over this period to counter the increased CO<sub>2</sub> in surface waters caused by winds bringing up water rich in dissolved inorganic carbon from the deep ocean. Although there is some question of the validity of the changes in the pre-satellite era (1948-1979; Marshall, 2003<ref name="Marshall, 2003">Marshall, G. J. 2003. Trends in the Southern Annular Mode from Observations and Reanalyses, ''Journal of Climate'', '''16''', 4134-4143.</ref>), the largest changes occur in the later period 1979-2003, as seen in [[:File:Figure 4.58 - Sea-air CO2 flux anomalies in the Southern Ocean.png|Figure 4.58]].<br />
<br />
As reanalysis products used in Matear and Lenton (2008<ref name="Matear and Lenton, 2008">Matear, R.J. and Lenton, A. 2008. Impact of Historical Climate Change on the Southern Ocean Carbon Cycle, ''Journal of Climate'', '''21''', 5820-5834, doi: 10.1175/2008JCLI2194.1.</ref>) are assimilated products, all climate modes/variability are represented. Other studies using different ocean models and experiments to explore the response of the Southern Ocean air-sea CO<sub>2</sub> flux to the SAM alone e.g. Lenton and Matear (2007<ref name="Lenton and Matear, 2007">Lenton, A. and Matear, R.J. 2007. The role of the Southern Annular Mode (SAM) in Southern Ocean CO<sub>2</sub> uptake, Global Biogeochemical Cycles, 21, doi: 10:1029/2006GB002714.</ref>), Lovenduski et al. (2007<ref name="Lovenduski et al, 2007">Lovenduski, N., Gruber, N., Doney, S.C. and Lima, I.D. 2007. Enhanced CO<sub>2</sub> outagssing in the Southern Ocean from a positive phase of the Southern Annular Mode, ''Global Biogeochemical Cycles'', '''21''', GB2026, doi:10.1029/2006GB002900.</ref>) are very consistent with the view determined using all the superposition of all the climatic modes. This is not surprising given that these studies suggest that more than 40% of the total variance in CO<sub>2</sub> flux is be explained by the SAM in the recent period Lenton and Matear (2007<ref name="Lenton and Matear, 2007">Lenton, A. and Matear, R.J. 2007. The role of the Southern Annular Mode (SAM) in Southern Ocean CO<sub>2</sub> uptake, Global Biogeochemical Cycles, 21, doi: 10:1029/2006GB002714.</ref>).<br />
<br />
Over the period only a small increase in primary production and export production is evident, suggesting only a weak link between atmospheric forcing and export production, despite the increased upwelling of deep waters in response to increased winds (i.e. supplying also macro and micro nutrients). The largest response was on the northern boundary of the High Nutrient Low Chlorophyll (HNLC) area of the Southern Ocean. Over the rest of the Southern Ocean, very little response was seen, demonstrating only a weak link between atmospheric forcing and production.<br />
<br />
The increased ventilation of the Southern Ocean from simulations does not only alter the concentration of upper ocean CO<sub>2</sub> and the air-sea CO<sub>2</sub> fluxes, it also alters the carbonate chemistry of the upper ocean. These changes in carbonate chemistry affect the ability of the ocean to take up atmospheric CO<sub>2</sub>, through changes in the Revelle factor (Revelle and Suess, 1957<ref name="Revelle and Suess, 1957">Revelle, R. and Suess, H.E. 1957. Carbon dioxide exchange between atmosphere and ocean and the question of an increase of atmospheric CO<sub>2</sub> during the past decades, ''Tellus'', '''9''', 18-27.</ref>) and through the lowering of seawater pH. This pH reduction, or ocean acidification, reduces the ability of organisms that use calcite to build shells, potentially adversely impacting the marine ecosytem (Feely et al., 2004<ref name="Feely et al, 2004">Feely, R.A., Sabine, C.L., Lee, K., Berelson, W., Kleypas, J., Fabry, V.J. and Millero, F.J. 2004. Impact of Anthropogenic CO<sub>2</sub> on the CaCO<sub>3</sub> System in the Oceans, ''Science'', '''305''', 362-366.</ref>). In response to ocean acidification, a key carbon parameter is the aragonite saturation state (&Omega;<sub>A</sub>), which influences the rate of calcification of marine organisms (Riebesell et al., 2000<ref name="Riebesell et al, 2000">Riebesell, U., ET AL. 2000. Reduced calcification of marine plankton in response to increased atmospheric CO<sub>2</sub>, ''Nature'', '''407''', 364-367.</ref>; Langdon and Atkinson, 2005<ref name="Langdon and Atkinson, 2005">Langdon, C. and Atkinson, M.J. 2005. Effect of elevated pCO<sub>2</sub> on photosynthesis and calcification of corals and interactions with seasonal change in temperature/irradiance and nutrient enrichment, Journal of Geophysical Research-Oceans, 110, C09S07, doi:10.1029/2004JC002576.</ref>). Simulations show that the observed increases and variability in heat, freshwater fluxes and in particular wind stress in the last 50 years, has moved the saturation horizon closer to the surface (Orr et al, 2005<ref name="Orr et al, 2005">Orr, J.C., Fabry, V.J., Aumont, O., Bopp, L., Doney, S.C., Feely, R. A., Gnanadesikan, A., Gruber, N., Ishida, A., Joos, F., Key, R.M., Lindsay, K., Maier-Reimer, E., Matear, R., Monfray, P., Mouchet, A., Najjar, R.G., Plattner, G.K., Rodgers, K.B., Sabine, C.L., Sarmiento, J.L., Schlitzer, R., Slater, R.D., Totterdell, I.J., Weirig, M.F., Yamanaka, Y. and Yool, A. 2005. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms, ''Nature'', '''437''', 681-686.</ref>), potentially already impacting on the ecosystems in the Southern Ocean, although we do not yet have the observational evidence to support this hypothesis.<br />
<br />
==Changes in CO<sub>2</sub> inventories==<br />
<br />
[[File:Figure 4.60 - Anthropogenic carbon concentrations in the Indian Ocean sector of the Southern Ocean.png|thumb|'''4.60''' Anthropogenic carbon concentrations (colour scale, in umol/kg) derived using a back-calculation method in the Indian Ocean sector of the Southern Ocean between Antarctica (left) and Africa (right) (redrawn from Lo Monaco et al., 2005<ref name="Monaco et al, 2005">Lo Monaco, C., Metzl, N., Poisson, A., Brunet, C. and Schauer, B. 2005. Anthropogenic CO<sub>2</sub> in the Southern Ocean: Distribution and inventory at the Indian-Atlantic boundary (WOCE line I6), ''Journal of Geophysical Research'', '''110''', 18.</ref>).]]<br />
In the previous sections, we focused on the changes of the air-sea CO<sub>2</sub> fluxes as observed and simulated over the last 50 years. When discussing the capacity of the ocean to reduce the impact of climate changes, the change in anthropogenic CO<sub>2</sub> in the water column since the pre-industrial era must be evaluated. This is important not only to estimate the global ocean&rsquo;s capacity to absorb anthropogenic CO<sub>2</sub> emissions, but also to detect the changes in carbonate saturation levels and the potential increase of ocean acidity, especially in the Southern Ocean (Feely et al., 2004<ref name="Feely et al, 2004">Feely, R.A., Sabine, C.L., Lee, K., Berelson, W., Kleypas, J., Fabry, V.J. and Millero, F.J. 2004. Impact of Anthropogenic CO<sub>2</sub> on the CaCO<sub>3</sub> System in the Oceans, ''Science'', '''305''', 362-366.</ref>; Orr et al., 2005<ref name="Orr et al, 2005">Orr, J.C., Fabry, V.J., Aumont, O., Bopp, L., Doney, S.C., Feely, R. A., Gnanadesikan, A., Gruber, N., Ishida, A., Joos, F., Key, R.M., Lindsay, K., Maier-Reimer, E., Matear, R., Monfray, P., Mouchet, A., Najjar, R.G., Plattner, G.K., Rodgers, K.B., Sabine, C.L., Sarmiento, J.L., Schlitzer, R., Slater, R.D., Totterdell, I.J., Weirig, M.F., Yamanaka, Y. and Yool, A. 2005. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms, ''Nature'', '''437''', 681-686.</ref>). It has been estimated that in recent decades 30% of the total uptake since the preindustrial period has been taken up by Southern Ocean mode waters (Sabine et al., 2004<ref name="Sabine et al, 2004">Sabine, C.L., ET AL. 2004. The Oceanic Sink for Anthropogenic CO<sub>2</sub>, ''Science'', '''305''', 367-371.</ref>). The anthropogenic CO<sub>2</sub> in the ocean (C<sub>ant</sub>) cannot be directly measured, but under several assumptions, it can be derived from ''in-situ'' observations. This was first suggested by Brewer (1978<ref name="Brewer, 1978">Brewer, P.G. 1978. Direct observation of oceanic CO<sub>2</sub> increase, ''Geophys. Res. Let.'', '''5''', 997-1000.</ref>) and Chen and Millero (1979<ref name="Chen and Millero, 1979">Chen, C.T. and F.J. Millero. 1979. Gradual increase of oceanic CO<sub>2</sub>, ''Nature'', '''277''', 205-206.</ref>), and in the last ten years several data-based methods have been investigated at regional and global scales (see a review in Wallace 2001<ref name="Wallace, 2001">Wallace, D.W.R. 2001. Introduction to special section: Ocean measurements and models of carbon sources and sinks, ''Global Biogeochemical Cycles'', '''15''', 3-10.</ref>; Sabine et al., 2004<ref name="Sabine et al, 2004">Sabine, C.L., ET AL. 2004. The Oceanic Sink for Anthropogenic CO<sub>2</sub>, ''Science'', '''305''', 367-371.</ref>; Waugh et al., 2006<ref name="Waugh et al, 2006">Waugh, D.W., Hall, T.M., McNeil, B. and Key, R. 2006. Anthropogenic CO<sub>2</sub> in the oceans estimated using transit-time distributions, ''Tellus B'', '''58''', 376-389.</ref>; Lo Monaco et al., 2005<ref name="Monaco et al, 2005">Lo Monaco, C., Metzl, N., Poisson, A., Brunet, C. and Schauer, B. 2005. Anthropogenic CO<sub>2</sub> in the Southern Ocean: Distribution and inventory at the Indian-Atlantic boundary (WOCE line I6), ''Journal of Geophysical Research'', '''110''', 18.</ref>). Comparisons of data-based methods (Lo Monaco et al., 2005<ref name="Monaco et al, 2005">Lo Monaco, C., Metzl, N., Poisson, A., Brunet, C. and Schauer, B. 2005. Anthropogenic CO<sub>2</sub> in the Southern Ocean: Distribution and inventory at the Indian-Atlantic boundary (WOCE line I6), ''Journal of Geophysical Research'', '''110''', 18.</ref>) clearly show that all methods converge to estimate large inventories associated with mode and intermediate waters ([[:File:Figure 4.60 - Anthropogenic carbon concentrations in the Indian Ocean sector of the Southern Ocean.png|Figure 4.60]]). Meanwhile in Southern Ocean uptake south of 50&deg;S uncertainties in C<sub>ant</sub> still exist, but these differences appear to reflect techniques that have been recognized to underestimate anthropogenic carbon in deep and bottom waters along the Antarctic coast (Lo Monaco, personal communication).<br />
<br />
==Concluding Remarks==<br />
<br />
The Southern Ocean plays a critical role in the uptake of atmospheric CO<sub>2</sub>, accounting for more that 40% of the annual mean CO<sub>2</sub> uptake. Modelling and observational studies show that the Southern Ocean has undergone significant changes in the last 50 years; these views appear to be converging towards a coherent view. The largest change has been a reduction in the total CO<sub>2</sub> uptake in recent decades in response to the observed changes in climatic forcings, particularly changes in wind speed. The increased wind speed drives strong changes in the physical carbon pump, specifically through ocean dynamics, rather than through changes in solubility or in the Revelle factor; this view is further reinforced by only a weak response in the biological pump. In the future, in response to climate change, both the biological and physical pumps are expected to be impacted (see Section 5.8 for details).<br />
==References==<br />
<references /><br />
[[Category:The instrumental period]]<br />
[[Category:The Southern Ocean]]<br />
[[Category:The carbon cycle]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_Southern_Ocean_carbon_cycle_response_to_future_climate_changeThe Southern Ocean carbon cycle response to future climate change2014-08-06T14:34:19Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Antarctic climate and environment change over the next 100 years]]''<br />
<br />
==Background==<br />
<br />
[[File:Figure 5.21 - Observed CO2 emissions over the last 25 years compared with IPCC emission scenarios.png|thumb|'''5.21''' Observed CO<sub>2</sub> emissions over the last 25 years as compared with the different IPCC emission scenarios (Raupach et al., 2007<ref name="Raupach et al, 2007">Raupach, M., ET AL. 2007, Global and regional drivers of accelerating CO<sub>2</sub> emissions, Proc. Nat. Acad. Sci., 104, no. 24 10288-10293.</ref>) (Copyright (2007) National Academy of Sciences, U.S.A).]]<br />
Atmospheric CO<sub>2</sub> emissions continue to increase at unprecedented rates. Already emissions have equalled or exceeded the previous worst-case scenarios for the IPCC (Raupach et al., 2007<ref name="Raupach et al, 2007">Raupach, M., ET AL. 2007, Global and regional drivers of accelerating CO<sub>2</sub> emissions, Proc. Nat. Acad. Sci., 104, no. 24 10288-10293.</ref>; [[:File:Figure 5.21 - Observed CO2 emissions over the last 25 years compared with IPCC emission scenarios.png|Figure 5.21]]). Currently it is estimated that about 30% of the CO<sub>2</sub> emitted annually is taken up by the ocean (Sabine et al., 2004<ref name="Sabine et al, 2004">Sabine, C.L., ET AL. 2004. The Oceanic Sink for Anthropogenic CO<sub>2</sub>, ''Science'', '''305''', 367-371.</ref>). The Southern Ocean plays a critical role in taking up this CO<sub>2</sub>, with more than 40% of the annual mean uptake of atmospheric CO<sub>2</sub> being taken up in the region south of 40&deg;S (Takahashi et al., 2009<ref name="Takahashi et al, 2009">Takahashi, T., Sutherland, S.C., Wanninkhof, R., Sweeney, C., Feely, R.A., Chipman, D.W., Hales, B., Friederich, G., Chavez, F., Sabine, C., Watson, W., Bakker, D.C.E., Schuster, U., Metzl, N., Yoshikawa-Inoue, H., Masao Ishii, M., Midorikawa, T., Nojiri, Y., Kortzinger, A., Steinhoff, T., Hoppema, M., Olafsson, J., Thorarinn, S.., Arnarson, T.S., Tilbrook, B., Johannessen, T., Olsen, A., Bellerby, R., Wong, C.S., Delille, B., Bates, N.R. and De Baar, H.J.W. 2009. Climatological mean surface PCO<sub>2</sub> and net CO<sub>2</sub> flux over the global oceans, Deep Sea Research, doi:10.1016/dsr2.2008.12.009.</ref>). Clearly then the way in which the Southern Ocean responds to climate change will directly impact global atmospheric CO<sub>2</sub> levels and hence the rate of the earth&rsquo;s warming. Hence, determining the role of the Southern Ocean in CO<sub>2</sub> exchange must be a priority if we are to formulate effective global policies for stabilizing atmospheric CO<sub>2</sub> levels.<br />
<br />
==Future Southern Ocean carbon response==<br />
<br />
===Response to increased winds===<br />
<br />
As the Southern Ocean becomes windier in response to global warming there will be an associated increase in the upwelling of carbon-rich water from the deep ocean. As this upwelled carbon reaches the surface it will increase the CO<sub>2</sub> content of surface water, reducing the pCO<sub>2</sub> gradient between the atmosphere and the ocean, and hence the uptake of CO<sub>2</sub> by the ocean from the atmosphere (Zickfeld et al., 2007<ref name="Zickfeld et al, 2007">Zickfeld, K., Fyfe, J.C., Saenko, O.A., Eby, M. and Weaver, A.J. 2007. Response of the global carbon cycle to human-induced changes in Southern Hemisphere winds, Geophysical Research Letters, 34, L12712 doi:10.1029/2006GL028797</ref>). At the same time atmospheric CO<sub>2</sub> concentrations will continue to rise in response to human activities. Eventually, as the atmospheric concentration of CO<sub>2</sub> exceeds the maximum deep-water pCO<sub>2</sub> value, estimated to be 430 microatmospheres (&mu;atm) (McNeil et al., 2007<ref name="McNeil et al, 2007">McNeil, B.I., Metzl, N., Key, R.M., Matear R.J. and Corviere, A. 2007., An empirical estimate of the Southern Ocean air-sea CO<sub>2</sub> flux, ''Global Biogeochemical Cycles'', '''21'''(3), GB03011, doi:10.1029/2007GB002991</ref>), the Southern Ocean CO<sub>2</sub> sink will then change from a saturated CO<sub>2</sub> sink (Le Qu&eacute;r&eacute; et al., 2007<ref name="Qu&eacute;r&eacute; et al, 2007">Le Qu&eacute;r&eacute;, C., Rodenbeck, C., Buitenhuis, E.T., Conway, T.J., Langenfelds, R., Gomez, A., Labuschagne, C., Ramonet, M., Nakazawa, T., Metzl, N., Gillett, N. and Heimann, M. 2007. Saturation of the Southern Ocean CO<sub>2</sub> Sink Due to Recent Climate Change, ''Science'', '''316''', 1735-1738, doi: 10.1126/science.1136188.</ref>) to a strengthening CO<sub>2</sub> sink. Model simulations suggest that this change will take place in the period 2020-2030 (under the IPCC A2 emission scenario) (Matear and Lenton, 2008<ref name="Matear and Lenton, 2008">Matear, R.J. and Lenton, A. 2008. Impact of Historical Climate Change on the Southern Ocean Carbon Cycle, ''Journal of Climate'', '''21''', 5820-5834, doi: 10.1175/2008JCLI2194.1.</ref>; Zickfeld et al., 2008<ref name="Zickfeld et al, 2008">Zickfeld, K., Fyfe, J.C., Eby, M. and Weaver, A.J. 2008. Comment on &quot;Saturation of the Southern Ocean CO<sub>2</sub> Sink due to Recent Climate Change&quot;, Nature, 570b.</ref>).<br />
<br />
===Response to ocean warming===<br />
<br />
[[File:Figure 5.22a - Expected impact of increased stratification on ocean dissolved inorganic carbon.png|thumb|'''5.22a''' Schematic of how global warming is expected to impact on dissolved inorganic carbon (DIC) in the ocean and hence air-sea carbon fluxes of CO<sub>2</sub>: sea surface warming decreases CO<sub>2</sub> solubility (solid horizontal arrow) and drives outgassing (Large open arrow).]]<br />
As the Southern Ocean becomes warmer, its ability to store CO<sub>2</sub> through solubility changes is affected. Freshening associated with the warming will lead to a stratification of the upper ocean that will affect ocean carbon uptake through biogeochemical and physical changes. Warming will reduce the solubility of CO<sub>2</sub> in seawater, so reducing the ocean&rsquo;s ability to take up and store CO<sub>2</sub> ([[:File:Figure 5.22a - Expected impact of increased stratification on ocean dissolved inorganic carbon.png|Figure 5.22a]]). A warming of 1% decreases oceanic pCO<sub>2</sub> by 4.23% (Takahashi et al., 1993<ref name="Takahashi et al, 1993">Takahashi, T., Olafsson, J., Goddard, J.G., Chipman, D.W. and Sutherland, S.C. 1993. Seasonal variation of CO<sub>2</sub> and nutrients in the high-latitude surface oceans: a comparative study, ''Global Biogeochemical Cycles'', '''7''', 843-878.</ref>). The result of these solubility changes will be to reduce the pCO<sub>2</sub> gradient between the atmosphere and the ocean, reducing the efficiency of ocean CO<sub>2</sub> uptake. Various studies of future CO<sub>2</sub> uptake suggest that the solubility effect will be significant (Matear and Hirst, 1999<ref name="Matear and Hirst, 1999">Matear, R.J. and Hirst, A.C. 1999. Climate change feedback on the future oceanic CO<sub>2</sub> uptake, ''Tellus'', '''51B''', 722-733.</ref>; Plattner et al., 2001<ref name="Plattner et al, 2001">Plattner, G.K., Joos, F., Stocker, T.F. and Marchal, O. 2001. Feedback Mechanisms and sensitivities of ocean carbon uptake under global warming, ''Tellus B'', '''53''', 564-592.</ref>; Sarmiento et al., 1998<ref name="Sarmiento et al, 1998">Sarmiento, J.L., Hughes, T.M.C., Stouffer, R.J. and Manabe, S. 1998. Simulated response of the ocean carbon cycle to an anthropogenic climate warming, ''Nature'', '''393''', 245-249.</ref>), although its magnitude remains poor quantified.<br />
<br />
[[File:Figure 5.22b - Expected impact of sea surface warming on ocean dissolved inorganic carbon.png|thumb|'''5.22b''' Schematic of how global warming is expected to impact on dissolved inorganic carbon (DIC) in the ocean and hence air-sea carbon fluxes of CO<sub>2</sub>: increased stratification impacts CO<sub>2</sub> through biological production; there is both enhanced biological production in response to warming and light supply, and reduction associated with reduced nutrient supply. The net effect is predicted to be an increase in CO<sub>2</sub> uptake (large downward open arrow).]]<br />
The Southern Ocean is a high nutrient low chlorophyll (HNLC) region rich in the macronutrients (nitrogen, phosphate and silicate) needed by phytoplankton to grow, but poor in phytoplankton, most likely due to a lack of micronutrients, in particular iron. It also suffers from the low levels of light at high latitude, which may inhibit productivity. As the upper ocean warms, freshens and stratifies, conditions will favour an increase in productivity. That in turn will use up CO<sub>2</sub> in surface waters, which will increase the ocean &ndash; atmosphere pCO<sub>2</sub> gradient, thereby encouraging more uptake of CO<sub>2</sub> by the ocean from the atmosphere ([[:File:Figure 5.22b - Expected impact of sea surface warming on ocean dissolved inorganic carbon.png|Figure 5.22b]]). Increasing stratification of the surface ocean will also tend to limit the supply of nutrients from below, hence limiting productivity and increasing the uptake of CO<sub>2</sub>. The amount of nutrients available in the stratified surface waters, hence productivity and CO<sub>2</sub> uptake, will also depend on the efficiency with which organic matter is exported out of the mixed layer by sinking ([[:File:Figure 5.22b - Expected impact of sea surface warming on ocean dissolved inorganic carbon.png|Figure 5.22b]]).<br />
<br />
[[File:Figure 5.23 - Primary productivity changes for 2040-2060 in six coupled climate carbon models.png|thumb|'''5.23''' Primary Productivity (PP) changes PgC/degree calculated for the period averaged 2040-2060 using six different coupled climate carbon models (Sarmiento et al., 2004b<ref name="Sarmiento et al, 2004b">Sarmiento, J.L., Slater, R., Barber, R.T., Bopp, L., Doney, S.C., Hirst, A.C., Kleypas, J., Matear, R.J., Miklolajewicz, U., Monfray, P., Soldatov, V., Spall, S.A. and Stouffer, R., 2004b. Response of Ocean Ecosystems to Climate Warming, Global Biogeochemical Cycles, 18, doi:10.1029/2003GB002134.</ref>). PP changes were calculated using Behrenfield and Falkowski (1997<ref name="Behrenfield and Falkowski, 1997">Behrenfield, M.J. and Falkowski, P.G. 1997. A consumers guide to phytoplankton primary production models, ''Limnol. Oceanogr.'', '''47''', 1479-1491.</ref>), and changes were assessed against control simulations that excluded global warming.]]<br />
Bopp et al. (2001<ref name="Bopp et al, 2001">Bopp, L., Monfray, P., Aumont, O., Dufresne, J.L., Le Truet, H., Madec, G., Terray, L. and Orr, J.C. 2001. Potential impact of climate change on marine export production, ''Global Biogeochemical Cycles'', '''15''', 81-99.</ref>) explored the relationship between the different competing processes in a coupled climate carbon model and showed that because the Southern Ocean was nutrient-limited, the largest effect was from stratification, which increased the interaction between light and nutrients and led to a longer and more efficient growing season with a 30% increase in marine productivity and export production. Consistent with this, Sarmiento et al. (2004b<ref name="Sarmiento et al, 2004b">Sarmiento, J.L., Slater, R., Barber, R.T., Bopp, L., Doney, S.C., Hirst, A.C., Kleypas, J., Matear, R.J., Miklolajewicz, U., Monfray, P., Soldatov, V., Spall, S.A. and Stouffer, R., 2004b. Response of Ocean Ecosystems to Climate Warming, Global Biogeochemical Cycles, 18, doi:10.1029/2003GB002134.</ref>) found that six different coupled climate carbon models showed increased primary production in the Southern Ocean between now and 2060 ([[:File:Figure 5.23 - Primary productivity changes for 2040-2060 in six coupled climate carbon models.png|Figure 5.23]]). The magnitude of the response was highly correlated with the strength of stratification, which in turn was related to changes in sea ice extent forced by continued global warming.<br />
<br />
In addition to inducing biogeochemical changes, increased ocean stratification may also reduce uptake of CO<sub>2</sub> from the atmosphere through Southern Ocean density changes. Mignone et al. (2006<ref name="Mignone et al, 2006">Mignone, B.K., Gnanadeskikan, A., Sarmeinto, J.L. and Slater, R.D. 2006. Central role of the Southern Hemisphere winds and eddies in modulating the oceanic uptake of anthropogenic carbon, Geophysical Research Letters, 33, doi:10.0129/2005GL024464.</ref>) showed that the depth of the pycnocline is highly correlated with CO<sub>2</sub> uptake. As a result it is predicted that as stratification increases the pycnocline will shallow, so further reducing CO<sub>2</sub> uptake (Sarmiento et al., 1998<ref name="Sarmiento et al, 1998">Sarmiento, J.L., Hughes, T.M.C., Stouffer, R.J. and Manabe, S. 1998. Simulated response of the ocean carbon cycle to an anthropogenic climate warming, ''Nature'', '''393''', 245-249.</ref>).<br />
<br />
==Response to increased CO<sub>2</sub> uptake==<br />
<br />
[[File:Figure 5.24 - Revelle factor in the IPSL coupled climate carbon model forced by the A2 scenario.png|thumb|'''5.24''' Surface values of the Revelle factor computed with the IPSL Coupled Climate Carbon Model forced by the A2 scenario and for the recent past and future (1990-1999 and 2090-2099) (Friedlingstein et al, 2006<ref name="Friedlingstein et al, 2006">Friedlingstein, P., Cox, P., Betts, R., Bopp, L., Von Bloh, W., Brovkin, V., Cadule, P., Doney, S., Eby, M., Fung, I., Bala, G., John, J., Jones, C., Joos, F., Kato, T., Kawamiya, M., Knorr, W., Lindsay, K., Matthews, H.D., Raddatz, T., Rayner, P., Reick, C., Roeckner, E., Schnitzler, K. G., Schnur, R., Strassmann, K., Weaver, A.J., Yoshikawa, C. and Zeng, N. 2006. Climate-carbon cycle feedback analysis: Results from the C4MIP model intercomparison, ''Journal of Climate'', '''19''', 3337-3353.</ref>; Reprinted by permission from Macmillan Publishers Ltd: Nature doi:10.1038/nature04095, copyright (2005))]]<br />
The uptake of CO<sub>2</sub> by the Southern Ocean alters ocean carbonate chemistry, causing both a reduction in ocean pH (i.e. acidification) and a reduction in the ocean&rsquo;s ability to take up and store CO<sub>2</sub> through an increase in the Revelle Factor. The Revelle factor (Revelle and Suess, 1957<ref name="Revelle and Suess, 1957">Revelle, R. and Suess, H.E. 1957. Carbon dioxide exchange between atmosphere and ocean and the question of an increase of atmospheric CO<sub>2</sub> during the past decades, ''Tellus'', '''9''', 18-27.</ref>), or buffer capacity, describes by how much the concentration of CO<sub>2</sub> in the ocean will change for a given increase in the partial pressure of CO<sub>2</sub> (pCO<sub>2</sub>). The higher the Revelle (or buffer) Factor, the less able the ocean is to take up atmospheric CO<sub>2</sub>. As shown in [[:File:Figure 5.24 - Revelle factor in the IPSL coupled climate carbon model forced by the A2 scenario.png|Figure 5.24]], the Revelle factor in the Southern Ocean lies between 10 and 15. In the next 100 years it is predicted to increase to 17 or more, which will reduce the efficiency of the ocean to take up atmospheric CO<sub>2</sub>.<br />
<br />
[[File:Figure 5.25 - Predicted aragonite saturation state of the surface ocean in the year 2100.png|thumb|'''5.25''' Predicted aragonite saturation state of the surface ocean in the year 2100 (Orr et al., 2005<ref name="Orr et al, 2005">Orr, J.C., Fabry, V.J., Aumont, O., Bopp, L., Doney, S.C., Feely, R. A., Gnanadesikan, A., Gruber, N., Ishida, A., Joos, F., Key, R.M., Lindsay, K., Maier-Reimer, E., Matear, R., Monfray, P., Mouchet, A., Najjar, R.G., Plattner, G.K., Rodgers, K.B., Sabine, C.L., Sarmiento, J.L., Schlitzer, R., Slater, R.D., Totterdell, I.J., Weirig, M.F., Yamanaka, Y. and Yool, A. 2005. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms, ''Nature'', '''437''', 681-686.</ref>). The dashed line represents the saturation horizon and shows that much of the Southern Ocean (&lt;50&ordm;S) is expected to be under-saturated with respect to aragonite by 2100.]]<br />
As the CO<sub>2</sub> levels in the ocean increase, the associated changes in ocean carbonate chemistry will lower the pH. Currently the upper ocean is supersaturated with respect to aragonite (used by important grazers like pteropods), and calcite (used by coccolithophores). As the ocean becomes more acidic (lower pH), the saturation states of both aragonite and calcite will be reduced until they drop below 1, when they pass through the saturation horizon - the point where the saturation state changes from super- to under-saturated. When the waters become under-saturated with respect to either aragonite or calcite it will no longer be possible for marine organisms to use these compounds to build calcium carbonate shells (Feely et al., 2004<ref name="Feely et al, 2004">Feely, R.A., Sabine, C.L., Lee, K., Berelson, W., Kleypas, J., Fabry, V.J. and Millero, F.J. 2004. Impact of Anthropogenic CO<sub>2</sub> on the CaCO<sub>3</sub> System in the Oceans, ''Science'', '''305''', 362-366.</ref>). Orr et al. (2005<ref name="Orr et al, 2005">Orr, J.C., Fabry, V.J., Aumont, O., Bopp, L., Doney, S.C., Feely, R. A., Gnanadesikan, A., Gruber, N., Ishida, A., Joos, F., Key, R.M., Lindsay, K., Maier-Reimer, E., Matear, R., Monfray, P., Mouchet, A., Najjar, R.G., Plattner, G.K., Rodgers, K.B., Sabine, C.L., Sarmiento, J.L., Schlitzer, R., Slater, R.D., Totterdell, I.J., Weirig, M.F., Yamanaka, Y. and Yool, A. 2005. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms, ''Nature'', '''437''', 681-686.</ref>) used a suite of ocean models to show that by 2100 the saturation horizon will have shallowed significantly, the pH having dropped by an additional 0.3, and hence much of the Southern Ocean will be under-saturated with respect to aragonite ([[:File:Figure 5.25 - Predicted aragonite saturation state of the surface ocean in the year 2100.png|Figure 5.25]]). This acidification is expected to bring about a shift in marine ecosystems (Feely et al., 2004<ref name="Feely et al, 2004">Feely, R.A., Sabine, C.L., Lee, K., Berelson, W., Kleypas, J., Fabry, V.J. and Millero, F.J. 2004. Impact of Anthropogenic CO<sub>2</sub> on the CaCO<sub>3</sub> System in the Oceans, ''Science'', '''305''', 362-366.</ref>), and potentially a reduction in export production (Klaas and Archer, 2002<ref name="Klaas and Archer, 2002">Klaas, C. and Archer, D.E. 2002. Association of sinking organic matter with various types of mineral ballast in the deep sea: Implications for the rain ratio, Global Biogeochemical Cycles, 16, doi:10.1029/2001GB001765.</ref>), although it has been suggested that this effect may be offset by increased CO<sub>2</sub> uptake causing an increase in alkalinity (Heinze, 2004<ref name="Heinze, 2004">Heinze, C. 2004. Simulating oceanic CaCO<sub>3</sub> export production in the greenhouse, ''Geophys. Res. Let.'', '''31''', L16308, doi: 10.1029/2004GL020613.</ref>).<br />
<br />
While mean long-term changes in acidification and the Revelle Factor are significant, they are subject to large interannual variability (Matear and Lenton, 2008<ref name="Matear and Lenton, 2008">Matear, R.J. and Lenton, A. 2008. Impact of Historical Climate Change on the Southern Ocean Carbon Cycle, ''Journal of Climate'', '''21''', 5820-5834, doi: 10.1175/2008JCLI2194.1.</ref>). This variability has the potential to perturb the system significantly, such that far reaching changes in the ocean ecosystem may occur well in advance of those expected by simply increasing CO<sub>2</sub> levels.<br />
<br />
Studies of the global uptake of CO<sub>2</sub> from the ocean by the atmosphere suggest that over time the ocean will release sufficient CO<sub>2</sub> to amplify global warming (a positive feedback). As we have seen here the Southern Ocean is expected to evolve to act as a net sink for CO<sub>2</sub> over the next 100 years (a negative feedback). Nevertheless, the eventual behaviour of the Southern Ocean will depend not only on what happens around Antarctica, but also on what happens in the terrestrial biosphere. Changes in the uptake of CO<sub>2</sub> from the atmosphere by the terrestrial biosphere, particularly at mid-latitudes, could be large enough to change the pCO<sub>2</sub> gradient between the atmosphere and the ocean, thereby affecting the response of the Southern Ocean.<br />
<br />
==Concluding remarks==<br />
<br />
The magnitude of the response of the Southern Ocean to climate change remains uncertain. Simulations from coupled climate carbon models show a large range of responses (e.g. Friedlingstein et al., 2006<ref name="Friedlingstein et al, 2006">Friedlingstein, P., Cox, P., Betts, R., Bopp, L., Von Bloh, W., Brovkin, V., Cadule, P., Doney, S., Eby, M., Fung, I., Bala, G., John, J., Jones, C., Joos, F., Kato, T., Kawamiya, M., Knorr, W., Lindsay, K., Matthews, H.D., Raddatz, T., Rayner, P., Reick, C., Roeckner, E., Schnitzler, K. G., Schnur, R., Strassmann, K., Weaver, A.J., Yoshikawa, C. and Zeng, N. 2006. Climate-carbon cycle feedback analysis: Results from the C4MIP model intercomparison, ''Journal of Climate'', '''19''', 3337-3353.</ref>), but do agree that the Southern Ocean will be an increased sink of atmospheric CO<sub>2</sub> in the future and that the recent reduction in CO<sub>2</sub> uptake will not continue. The magnitude of the total uptake is dependent on how the ocean responds to predicted increases in ocean warming and stratification, which can drive both increases in CO<sub>2</sub> uptake through biological and export changes, and decreases through solubility and density changes. The expected increased ventilation of carbon-rich deep water, combined with uptake of atmospheric CO<sub>2</sub>, will increase the carbon content of the upper ocean, reducing the Southern Ocean&rsquo;s ability to take up more CO<sub>2</sub> in the future (through the Revelle or buffer Factor) and enhancing ocean acidification. This acidification is potentially worrisome because of its potential to impact the entire marine ecosystem. The various projections of future response are based primarily on coupled climate-carbon simulations, but such predictions need to be validated. It is therefore extremely important that strategies be developed to observe and detect change in the ocean carbon system (e.g. Lenton et al., 2006<ref name="Lenton et al, 2006">Lenton, A., Metzl, N., Aumont, O., Lo Monaco, C. and Rodgers, K. 2006. Simulating the Ocean Carbon Cycle: A focus on the Southern Ocean. Presented at Second CARBOOCEAN Annual Meeting, 4-8 Dec 2006, Maspalomas, Spain.</ref>, 2009<ref name="Lenton et al, 2009">Lenton, A., Bopp, L. and Matear R.J. 2009. Strategies for high-latitude northern hemisphere CO<sub>2</sub> sampling now and in the future, Deep Sea Research II, doi:10.1016/j.dsr2.2008.12.008.</ref>) to complement ongoing and planned physical measurements e.g. by Argo floats.<br />
==References==<br />
<references /><br />
[[Category:The next 100 years]]<br />
[[Category:The carbon cycle]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_role_of_the_Antarctic_in_the_global_climate_systemThe role of the Antarctic in the global climate system2014-08-06T14:34:18Z<p>Tonyp: Changed book chapter reference to a page link; moved figure 1.8 to improve layout somewhat</p>
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<div>:''This page is part of the topic [[The Antarctic environment in the global system]]''<br />
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The global climate system is driven by solar radiation, most of which, at any one time, arrives at low latitudes. Over the year as a whole the Equator receives about five times as much radiation as the poles, creating a large Equator-to-pole temperature difference. The atmospheric and oceanic circulations respond to this large horizontal temperature gradient by transporting heat polewards (Trenberth and Caron, 2001<ref name="Trenberth and Caron, 2001">Trenberth, K.E. and Caron, J.M. 2001. Estimates of meridional atmosphere and ocean heat transports, ''Journal of Climate'', '''13''', 4358-4365.</ref>). In fact the climate system can be regarded as an engine, with the low latitude areas being the heat source and the polar regions the heat sink. Although the dynamics of atmosphere-ocean interaction are well known, the complexities of the heat exchange engine and the interactions of ice shelves and land ice with the ocean and atmosphere make predictions about climate change a challenge.<br />
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The combination of tropical heating, poleward moving air and the Coriolis Force imposed by the Earth&rsquo;s rotation, leads to the development of the Hadley Cell, an atmospheric circulation in which air rises at the equator, creating the tropical belt of low pressure, and descends in the subtropics, forming the subtropical high pressure belt. At higher latitudes (60-65&ordm;S) the air ascends again, creating a low-pressure zone. The pressure gradient at the Earth&rsquo;s surface between the high pressure in the subtropics and the low pressure at 60-65&ordm;S forces air to move eastwards under the influence of the Earth&rsquo;s rotation, creating the mid-latitude westerlies that help to drive surface waters east in the ACC. The air ascending at 60-65&ordm;S moves poleward at upper levels and sinks again over the poles, forming a high-pressure system over the Antarctic continent. The pressure gradient from the low pressure at 60-65&ordm;S to the high pressure over the continent gives rise to easterly winds along the Antarctic coast, driving the westward-directed Antarctic Coastal Current over the continental shelf; that shelf current tends to be focused along the fronts of ice shelves (Deacon, 1937<ref name="Deacon, 1937">Deacon, G.E.R. 1937. The hydrology of the Southern Ocean, ''Discovery Reports'', '''15''', 1-124.</ref>), Heywood et al, 2004<ref name="Heywood et al, 2004">Heywood, K. J., Naveira-Garabato, A.C., Stevens, D.P., and Muench, R.D. 2004. On the fate of the Antarctic Slope Front and the origin of the Weddell Front, ''Journal of Geophysical Research'', '''109''' (C06021), 1-13.</ref>, Ismael N&uacute;&ntilde;ez-Riboni and Fahrbach, 2009<ref name="N&uacute;&ntilde;ez-Riboni and Fahrbach, 2009">N&uacute;&ntilde;ez-Riboni, I. and Fahrbach, E. 2009. Seasonal variability of the Antarctic Coastal Current and its driving mechanisms in the Weddell Sea, Deep-Sea Research I, in press.</ref>). The main westward flow is that associated with the Antarctic Slope Front, over the continental slope. The north-to-south distribution of surface pressure around Antarctica is subject to remarkable variability in the intensity of the meridional pressure gradient and its zonal location. Due to the circumpolar character of this variation it is called the Southern Annular Mode (of variability) (SAM) (See Trenberth et al., 2007<ref name="Trenberth et al, 2007">Trenberth, K.E. Et AL. 2007. Observations: Surface and Atmospheric Change. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change,. In: D.Q. S. Solomon, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (Editor), Climate Change 2007: The Physical Science Basis. Cambridge University Press,, Cambridge, U.K., and New York, U.S.A.</ref> for a review). The SAM can also be seen as a measure of the intensity of the westerly winds that propel the ACC. The SAM is the southern hemisphere equivalent of the Arctic Oscillation (also known as the Northern Annular Mode) or the related North Atlantic Oscillation, which is measured from the pressure difference between the Azores and Iceland. Variations in the SAM (which can be thought of as variations in a North - South pressure gradient) drive variability in the Southern Ocean&rsquo;s winds and currents: the steeper the gradient, the stronger the winds. In addition to the SAM, there are other significant modes of variability with meridional or zonal patterns.<br />
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[[File:Figure 1.8 - Schematic of the global ocean circulation.png|thumb|'''1.8''' Model of the global ocean circulation, emphasising the central role played by the Southern Ocean. From Lumpkin and Speer (2007<ref name="Lumpkin and Speer, 2007">Lumpkin, R. and Speer, K. 2007. Global ocean meridional overturning, ''Journal of Physical Oceanography'', '''37''', 2550-2562.</ref>). NADW = North Atlantic Deep Water; CDW = Circumpolar Deep Water; AABW = Antarctic Bottom Water. Units are in Sverdrups (1 Sv = 10<sup>6</sup> m<sup>3</sup> of water per second). The two primary overturning cells are the Upper Cell (red and yellow), and the Lower Cell (blue, green, yellow). The bottom water of the Lower Cell (blue) wells up and joins with the southward flowing deep water (green or yellow), which connects with the upper cell (yellow and red). This demonstrates the global link between Southern Ocean convection and bottom water formation and convective processes in the Northern Hemisphere.]]<br />
Both the atmosphere and ocean play major roles in the poleward transfer of heat (Trenberth and Caron, 2001<ref name="Trenberth and Caron, 2001">Trenberth, K.E. and Caron, J.M. 2001. Estimates of meridional atmosphere and ocean heat transports, ''Journal of Climate'', '''13''', 4358-4365.</ref>), with the atmosphere being responsible for 60% of the heat transport, and the ocean the remaining 40%. In the atmosphere, heat is transported by both low-pressure systems (depressions) and the mean flow. Clockwise-circulating depressions carry warm air poleward on their eastern sides and cold air towards lower latitudes on their western flanks. The atmosphere is able to respond relatively quickly to changes in the high or low latitude heating rates, with storm tracks and the mean flow changing on scales from days to years. The process by which air arrives at the poles also imports pollutants from industrialised areas, though quantities are tiny compared with the Arctic, not least because most industrialised areas are in the north, and the mean flow is zonal around Antarctica rather than more meridional - as in the Arctic.<br />
<br />
[[File:Figure 1.9 - Meridional section through the Southern Ocean overturning circulation.png|thumb|'''1.9''' South (left) to north (right) section through the overturning circulation in the Southern Ocean. South-flowing products of deep convection in the North Atlantic are converted into upper-layer mode and intermediate waters and deeper bottom waters and returned northward. Marked are the positions of the main fronts (PF &ndash; Polar Front; SAF &ndash; Sub-Antarctic Front; and STF &ndash; Subtropical Front), and water masses (AABW &ndash; Antarctic Bottom Water; LCDW and UCDW, Lower and Upper Circumpolar Deep Waters; NADW &ndash; North Atlantic Deep Water; AAIW &ndash; Antarctic Intermediate Water and SAMW &ndash; Sub-Antarctic Mode Water) (from Speer et al., 2000). Note that as well as water moving north to south or ''vice versa'', it is also generally moving eastward (i.e. towards the observer in the case of this cross section), except along the coast where coastal currents move water westward (away from the observer).]]<br />
The oceans carry heat and salt south towards the pole in upper ocean surface currents moving down the western sides of the Atlantic, Pacific and Indian Ocean basins (e.g. Schmitz, 1995<ref name="Schmitz, 1995">Schmitz, W. J. 1995. On the Interbasin-scale Thermohaline Circulation, Reviews of Geophysics, 33, 2, American Geophysical Union, 151-173.</ref>, Lumpkin and Speer, 2007<ref name="Lumpkin and Speer, 2007">Lumpkin, R. and Speer, K. 2007. Global ocean meridional overturning, ''Journal of Physical Oceanography'', '''37''', 2550-2562.</ref>, [[:File:Figure 1.8 - Schematic of the global ocean circulation.png|Figure 1.8]]). In addition, heat and salt move south through the Atlantic Ocean in the sub-surface in North Atlantic Deep Water, which rises to the surface near the Antarctic coast ([[:File:Figure 1.8 - Schematic of the global ocean circulation.png|Figure 1.8]] and [[:File:Figure 1.9 - Meridional section through the Southern Ocean overturning circulation.png|Figure 1.9]]), as a consequence of upwelling driven by the divergence of surface water forced north by the westerly winds of the Southern Ocean (e.g. Rintoul et al, 2001<ref name="Rintoul et al, 2001">Rintoul, S.R., Hughes, C.W. and Olbers, D. 2001. The Antarctic Circumpolar Current system. In: Eds. G. Siedler, J. Church and J. Gould, Ocean circulation and climate; observing and modelling the global ocean. International Geophysics Series, 77, 271-302, Academic Press.</ref>). In addition to their transport by the large-scale currents, heat and salt are transported by mesoscale eddies (with diameters of tens to hundreds of kilometers). In the Southern Ocean, where the meridional currents are normally small, meridional transport by eddies is significant (e.g. de Szoeke and Levine, 1981<ref name="Szoeke and Levine, 1981">De Szoeke, R.A. and Levine, M.D. 1981. The advective flux of heat by mean geostrophic motions in the Southern Ocean, ''Deep-Sea Research'', '''28''', 1057-1085.</ref>; Bryden, 1979<ref name="Bryden, 1979">Bryden, H.L. 1979. Poleward heat flux and conversion of available potential energy in Drake Passage, ''Journal of Marine Research'', '''37''', 1-22.</ref>; Hughes and Ash, 2001<ref name="Hughes and Ash, 2001">Hughes, C.W. and Ash, E.R. 2001. Eddy forcing of the mean flow in the Southern Ocean, ''Journal of Geophysical Research'', '''106''', 2713-2722.</ref>; Rintoul et al, 2001<ref name="Rintoul et al, 2001">Rintoul, S.R., Hughes, C.W. and Olbers, D. 2001. The Antarctic Circumpolar Current system. In: Eds. G. Siedler, J. Church and J. Gould, Ocean circulation and climate; observing and modelling the global ocean. International Geophysics Series, 77, 271-302, Academic Press.</ref>; Hogg et al., 2008<ref name="Hogg et al, 2008">Hogg, A.M., Meredith, M.P., Blundell, J.R. and Wilson, C. 2008. Eddy heat flux in the Southern Ocean: Response to variable wind forcing, ''Journal of Climate'', '''21'''(4), 608-620.</ref>).<br />
<br />
Eastward circulation is focused in the ACC, which lies broadly between the Southern Boundary Front (SB) and the Sub-Antarctic Front (SAF) (Rintoul et al, 2001<ref name="Rintoul et al, 2001">Rintoul, S.R., Hughes, C.W. and Olbers, D. 2001. The Antarctic Circumpolar Current system. In: Eds. G. Siedler, J. Church and J. Gould, Ocean circulation and climate; observing and modelling the global ocean. International Geophysics Series, 77, 271-302, Academic Press.</ref>, compare Figures 1.2 and 1.9). The core of the ACC lies roughly beneath the core of the predominant westerly winds. The ACC is an outstanding feature in the global ocean&rsquo;s circulation. Stretching over a length of around 20,000 km, it is the only current to completely encircle the globe. It transports around 140 &times; 10<sup>6</sup> m<sup>3</sup> of water per second (140 Sverdrups), making it the world&rsquo;s largest ocean current. And it links the three main ocean basins (Atlantic, Pacific and Indian) into one global system by transporting heat and salt from one ocean to another.<br />
<br />
The westerly winds of the Southern Ocean act on the surface waters, which are forced north under the influence of the Coriolis Force of the Earth&rsquo;s rotation. Deep water from below wells up to replace these surface waters, as is evident from [[:File:Figure 1.9 - Meridional section through the Southern Ocean overturning circulation.png|Figure 1.9]]. This ascent (e.g. Schmitz, 1995<ref name="Schmitz, 1995">Schmitz, W. J. 1995. On the Interbasin-scale Thermohaline Circulation, Reviews of Geophysics, 33, 2, American Geophysical Union, 151-173.</ref>; Lumpkin and Speer, 2007<ref name="Lumpkin and Speer, 2007">Lumpkin, R. and Speer, K. 2007. Global ocean meridional overturning, ''Journal of Physical Oceanography'', '''37''', 2550-2562.</ref>) from the deep in the ACC forms the upward part of the global overturning circulation. The northward moving surface water is cold and dense and sinks at the Polar Front to form Antarctic Intermediate Water (W&uuml;st, 1935<ref name="W&uuml;st, 1935">W&uuml;st, G. 1935. The Stratosphere of the Atlantic Ocean. Scientific Results of the German Atlantic Expedition of the Research Vessel &ldquo;Meteor&rdquo; 1925-1927 (English translation, E.J. Emery (ed.), Amerind, new Delhi, 1978), 6, 109-288.</ref>), which sinks northward at intermediate depths to permeate the world&rsquo;s oceans especially in the Southern Hemisphere (McCartney, 1982<ref name="McCartney, 1982">McCartney, M.S. 1982. The subtropical circulation of Mode Waters, Journal of Marine Research, 40 (suppl), 427-464.</ref>). Some of the surface water reaches the SAF, where it sinks to form Sub-Antarctic Mode Water (Hanawa and Talley, 2001<ref name="Hanawa and Talley, 2001">Hanawa, K. and Talley, L.D. 2001. Mode Waters. In: Eds. G. Siedler, J. Church and J. Gould, Ocean circulation and climate; observing and modelling the global ocean, International Geophysics Series, 77, 373-386, Academic Press.</ref>). Due to the excess of precipitation over evaporation at these latitudes, the surface water gets lighter as is moves north (gain of buoyancy in [[:File:Figure 1.9 - Meridional section through the Southern Ocean overturning circulation.png|Figure 1.9]]), which explains how the Mode Water (lighter) comes to overlie the Intermediate Water (heavier), despite their having the same source. South of the divergence zone, where upwelling takes place, the surface waters reaching coastal seas are cooled by contact with ice, and pick up salt rejected when surface waters freeze to form sea ice, so losing buoyancy ([[:File:Figure 1.9 - Meridional section through the Southern Ocean overturning circulation.png|Figure 1.9]]) and becoming dense enough to sink. The sinking waters cascade down the continental shelf (Baines and Condie, 1998<ref name="Baines and Condie, 1998">Baines, P.G. and Condie, S. 1998. Observations and modelling of Antarctic downslope flows: a review. In Ocean, Ice and Atmosphere: Interactions at the Antarctic Continental Margin, AGU Antarctic Research Series Vol. 75, S.S. Jacobs and R. Weiss editors, 29-49.</ref>; Foldvik et al. 2004<ref name="Foldvik et al, 2004">Foldvik, A., Gammelsrod, T., Osterhus, S., Fahrbach, E., Rohardt, G., Schroder, M., Nicholls, K.W., Padman, L. and Woodgate, R.A. 2004. Ice shelf water overflow and bottom water formation in the southern Weddell Sea, ''J. Geophys. Res.'', '''109''', C02015, doi:10.1029/2003JC002008.</ref>) and slope to form Antarctic Bottom Water, which spreads around Antarctica (Orsi et al, 1999<ref name="Orsi et al, 1999">Orsi, A.H., Johnson, G.C. and Bullister, J.B. 1999. Circulation, mixing and production of Antarctic Bottom Water, ''Prog. Oceanog.'', '''43''', 55-109.</ref>) and aerates most of the global deep ocean floor (e.g. W&uuml;st, 1935<ref name="W&uuml;st, 1935">W&uuml;st, G. 1935. The Stratosphere of the Atlantic Ocean. Scientific Results of the German Atlantic Expedition of the Research Vessel &ldquo;Meteor&rdquo; 1925-1927 (English translation, E.J. Emery (ed.), Amerind, new Delhi, 1978), 6, 109-288.</ref>; Hogg, 2001<ref name="Hogg, 2001">Hogg, N.G, 2001. Quantification of the deep Circulation. In: Eds. G. Siedler, J. Church and J. Gould, Ocean circulation and climate; observing and modelling the global ocean, International Geophysics Series, 77, 259-270, Academic Press.</ref>). These sinking cold waters provide a fairly uniform cold environment for bottom dwelling (benthic) organisms.<br />
<br />
[[File:Figure 1.10 - Major currents south of 20S.png|thumb|'''1.10''' Schematic map of major currents south of 20&ordm;S (F = Front; C = Current; G = Gyre) (Rintoul et al, 2001<ref name="Rintoul et al, 2001">Rintoul, S.R., Hughes, C.W. and Olbers, D. 2001. The Antarctic Circumpolar Current system. In: Eds. G. Siedler, J. Church and J. Gould, Ocean circulation and climate; observing and modelling the global ocean. International Geophysics Series, 77, 271-302, Academic Press.</ref>); showing (i) the Polar Front and Sub-Antarctic Front, which are the major fronts of the Antarctic Circumpolar Current; (ii) Other regional currents; (iii) the Weddell and Ross Sea Gyres; and (iv) depths shallower than 3,500m shaded (all from Rintoul et al, 2001<ref name="Rintoul et al, 2001">Rintoul, S.R., Hughes, C.W. and Olbers, D. 2001. The Antarctic Circumpolar Current system. In: Eds. G. Siedler, J. Church and J. Gould, Ocean circulation and climate; observing and modelling the global ocean. International Geophysics Series, 77, 271-302, Academic Press.</ref>). In orange are shown (a) the cyclonic circulation west of the Kerguelen Plateau, (b) the Australian-Antarctic Gyre (south of Australia), (c) the slope current, and the (d) cyclonic circulation in the Bellingshausen Sea, as suggested by recent modelling studies (Wang and Meredith, 2008<ref name="Wang and Meredith, 2008">Wang, Z. and Meredith, M.P. 2008. Density-driven Southern Hemisphere subpolar gyres in coupled climate models, Geophysical Research Letters, 35(14) 5, pp. 10.1029/2008GL034344.</ref>), and observations &ndash; e.g. eastern Weddell Gyre - Prydz Bay Gyre (Smith et al, 1984<ref name="Smith et al, 1984">Smith, N.R., Dong, Z. Kerry, K.R. and Wright, S. 1984. Water masses and circulation in the region of PrydzBay, Antarctica, ''Deep-sea Research'', '''31''', 1121-1147.</ref>), westward flow through Princess Elizabeth Trough (Heywood et al, 1999<ref name="Heywood et al, 1999">Heywood, K.J., Sparrow, M.D., Brown, J. and Dickson, R.R. 1999. Frontal structure and Antarctic Bottom Water flow through the Princess Elizabeth Trough, Antarctica, ''Deep-Sea Research I'', '''46''', 1181-1200.</ref>), and circulation east of Kerguelen Plateau (McCartney and Donohue, 2007<ref name="McCartney and Donohue, 2007">McCartney, M.S. and Donohue, K.A. 2007. A deep cyclonic gyre in the Australian-Antarctic Basin, ''Progress in Oceanography'', '''75''', 675-750.</ref>).]]<br />
Jets along the SAF and the PF typically carry a large fraction of the transport of the ACC ([[:File:Figure 1.9 - Meridional section through the Southern Ocean overturning circulation.png|Figure 1.9]] and [[:File:Figure 1.10 - Major currents south of 20S.png|Figure 1.10]]; and Cunningham et al., 2003<ref name="Cunningham et al, 2003">Cunningham, S.A., Alderson, S.G., King, B.A. and Brandon, M.A. 2003. Transport and variability of the Antarctic Circumpolar Current in Drake Passage. J. Geophys. Res., 108 (C5), 8084, doi:10.1029/2001JC001147.</ref>), but other fronts can have comparable transports across individual hydrographic sections. In fact, the ACC is not a single front but a complex system of fronts (e.g. Sokolov and Rintoul, 2002<ref name="Sokolov and Rintoul, 2002">Sokolov, S. and Rintoul, S.R. 2002. The structure of Southern Ocean fronts at 140E, ''Journal of Marine Systems'', '''37''', 151-184.</ref>), several of which are thought to be of circumpolar extent (Orsi et al., 1995<ref name="Orsi et al, 1995">Orsi, A.H., Whitworth III, T.W. and Nowlin Jr.,W.D. 1995. On the meridional extent and fronts of the Antarctic Circumpolar Current, ''Deep-Sea Res.'', '''42''', 641-673.</ref>). This complex system approach provides a new paradigm for considering the response of the ACC to climate change.<br />
<br />
Superimposed on the circumpolar circulation system are regional clockwise gyres, principally the Weddell Gyre and Ross Gyre ([[:File:Figure 1.10 - Major currents south of 20S.png|Figure 1.10]]), whose southern boundaries are the west-moving Slope Current, and whose outer limits reach to the east-moving ACC. These gyres constitute the detailed pathway for transport to, from, and along the continental margin. They are visible in traditional hydrographic surveys as dome-shaped structures surrounded by downward sloping masses of equal density that extend towards the coast in the south and towards the ACC in the north. They are also clearly revealed in Southern Ocean numerical model outputs. Estimates of the extent of transport in the gyres are few. The Weddell Gyre carries 30 +/- 10 Sverdrups (Sv) in the Weddell Sea (Fahrbach et al., 1994<ref name="Fahrbach et al, 1994">Fahrbach, E., Rohardt, G., Schroder, M. and Strass, V. 1994. Transport and structure of the Weddell Gyre, ''Ann. Geophysicae'', '''12''', 840-855.</ref>) and 56 +/- 8 Sv across the Greenwich Meridian (Klatt et al, 2005<ref name="Klatt et al, 2005">Klatt, O., Fahrbach, E., Hoppema, M. and Rohardt, G. 2005. The transport of the Weddell Gyre across the Prime Meridian, ''Deep-Sea Research II'', '''52''', 513-528.</ref>). The Ross Gyre transports 40 Sv across longitude 150&ordm;W, and the Australian-Antarctic Gyre 76 +/- 26 Sv across longitude 110&ordm;E (McCartney and Donohue, 2007<ref name="McCartney and Donohue, 2007">McCartney, M.S. and Donohue, K.A. 2007. A deep cyclonic gyre in the Australian-Antarctic Basin, ''Progress in Oceanography'', '''75''', 675-750.</ref>).<br />
<br />
[[File:Figure 1.2 - Antarctic topography and bathymetry.png|thumb|'''1.2''' The land topography and the bathymetry of the seabed around Antarctica (in metres). Four major oceanic fronts are shown (not labelled), which are (north-to-south): the Sub-Antarctic Front (SAF), the Polar Front (PF), the Southern ACC Front (SACCF) and the Southern Boundary (SB). The Antarctic Circumpolar Current runs between the SAF and the SB. Figure produced by M.P. Meredith (BAS; pers. comm.) using frontal locations adapted from Orsi et al., 1995<ref name="Orsi et al, 1995">Orsi, A.H., Whitworth III, T.W. and Nowlin Jr.,W.D. 1995. On the meridional extent and fronts of the Antarctic Circumpolar Current, ''Deep-Sea Res.'', '''42''', 641-673.</ref>) and bathymetry from the General Bathymetric Chart of the Oceans (GEBCO) Centennial digital topography data (www.gebco.net).]]<br />
The distribution of land and sea in the two polar regions is responsible for the very different atmospheric and oceanic circulations observed in each. Antarctica is a continent surrounded by ocean, while the Arctic is an ocean surrounded by land. The Antarctic continent is a sub-circular dome lying over the pole and surrounded by a broad swath of deep ocean, apart from the slight constriction of Drake Passage between the Antarctic Peninsula and South America ([[:File:Figure 1.10 - Major currents south of 20S.png|Fig. 1.10]]). As a result, the mean atmospheric flow and surface ocean currents are zonal in nature (parallel to latitude). The ACC, which is the major oceanographic feature of the Southern Ocean, flows unrestricted around the continent ([[:File:Figure 1.2 - Antarctic topography and bathymetry.png|Figure 1.2]] and [[:File:Figure 1.8 - Schematic of the global ocean circulation.png|Figure 1.8]]), isolating the cold high latitude land and sea areas from warm temperate mid- and low-latitude surface waters. This has only been the case since about 30 million years ago when the Drake Passage opened up, as discussed in greater detail in [[The Greenhouse world - from the breakup of Gondwana to 34 million years ago|The Greenhouse world]]. Before then the ocean currents had more of a meridional component (parallel to longitude), which allowed greater poleward penetration of warm waters from temperate latitudes. In contrast, as mentioned earlier, surface ocean circulation in the Arctic tends to be meridional, especially the northward flow of warm water at the surface in the North Atlantic Current.<br />
<br />
[[File:Figure 1.11 - Column inventories of anthropogenic CO2 in the ocean.png|thumb|'''1.11''' Column inventories of anthropogenic CO<sub>2</sub> in the ocean. Dissolved old CO<sub>2</sub> is lost to the atmosphere south of the Polar Front, where NADW wells up to the surface close to the coast (purplish colours); most of this old CO<sub>2</sub> is not anthropogenic. The cold sinking Mode and Intermediate Waters north of the polar front (green colours) have extracted considerable anthropogenic CO<sub>2</sub> from the atmosphere (from Sabine et al., 2004<ref name="Sabine et al, 2004">Sabine, C.L., ET AL. 2004. The Oceanic Sink for Anthropogenic CO<sub>2</sub>, ''Science'', '''305''', 367-371.</ref>).]]<br />
The Southern Ocean plays a key role in the global carbon cycle. The upwelling deep water south of the Polar Front ([[:File:Figure 1.8 - Schematic of the global ocean circulation.png|Figure 1.8]] and [[:File:Figure 1.9 - Meridional section through the Southern Ocean overturning circulation.png|Figure 1.9]]) brings to the surface dissolved nutrients and carbon dioxide (CO<sub>2</sub>), and releases this gas to the atmosphere. In contrast, the Intermediate Water and Mode Water masses sinking north of the Polar Front ([[:File:Figure 1.9 - Meridional section through the Southern Ocean overturning circulation.png|Figure 1.9]]) take up CO<sub>2</sub> from the atmosphere. These complementary processes make the Southern Ocean both a source and a sink for atmospheric CO<sub>2</sub> (see [[:File:Figure 1.11 - Column inventories of anthropogenic CO2 in the ocean.png|Figure 1.11]]; from Sabine et al., 2004<ref name="Sabine et al, 2004">Sabine, C.L., ET AL. 2004. The Oceanic Sink for Anthropogenic CO<sub>2</sub>, ''Science'', '''305''', 367-371.</ref>).<br />
<br />
Because of its upwelling nutrients, the Southern Ocean is the world&rsquo;s most biologically productive ocean. Nonetheless, its productivity is limited by the low availability of micronutrients such as iron, except around the islands that are scattered through the ACC. As a result the Southern Ocean is classified as High Nutrient Low Chlorophyll (HNLC). Through photosynthesis, the growth of phytoplankton extracts CO<sub>2</sub> from the atmosphere and pumps it to the seabed or into subsurface waters through the sinking of decaying organic matter. Without this process, and without the solution of carbon dioxide in cold dense sinking water near the coast, the build up of carbon dioxide in the atmosphere would be much faster.<br />
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CO<sub>2</sub> is exchanged between the ocean and the atmosphere primarily through air-sea fluxes, which are highly variable in space and time (Mahadevan et al., 2004<ref name="Mahadevan et al, 2004">Mahadevan, A., Levy, M. and Memery, L. 2004. Mesoscale variability of sea surface pCO<sub>2</sub>: What does it respond to?, Global Biogeochemical Cycles, 18 (1), GB1017/10.1029/2003GB002102.</ref>; Volk and Hoffert, 1985<ref name="Volk and Hoffert, 1985">Volk, T. and Hoffert, M.I. 1985. Ocean Carbon Pumps: Analysis of relative strenghts and efficencies in ocean-driven atmospheric CO<sub>2</sub> changes, ''Geophys. Monogr.'', '''32''', 99-110.</ref>) balancing to within 2% (net) when integrated globally (Watson and Orr, 2003<ref name="Watson and Orr, 2003">Watson, A.J. and Orr, J.C. 2003. Carbon Dioxide Fluxes in the Global Ocean, in Ocean Biogeochemistry: The Role of the Ocean carbon Cycle in Global Change, edited by M. J. R. Fasham, Springer, Heidelberg.</ref>).<br />
<br />
[[File:Figure 1.12 - Schematic of the biological and physical pump.png|thumb|'''1.12''' A simple schematic representation of the biological pump (left) and physical pump (right) (Chisholm, 2000<ref name="Chisholm, 2000">Chisholm, S.W. 2000. Stirring times in the Southern Ocean, ''Nature'', '''407''', 685-687.</ref>). Reprinted by permission from Macmillan Publishers Ltd: Nature doi: 10.1038/35037696, copyright 2000.]]<br />
Surface ocean CO<sub>2</sub> levels (pCO<sub>2</sub>) and their related atmospheric CO<sub>2</sub> levels can be described as being controlled by the combination of physical and biological processes that move CO<sub>2</sub> from the upper ocean into the deep ocean. These processes can be separated into the &ldquo;physical pump&rdquo; and the &ldquo;biological pump&rdquo; ([[:File:Figure 1.12 - Schematic of the biological and physical pump.png|Figure 1.12]]). The biological pump is the most effective at removing carbon from the system through burial in sediments. Both pumps are effective at transferring CO<sub>2</sub> to deep water.<br />
<br />
The biological pump refers to the biological cycling of ocean carbon into the ocean interior. It is a complex process operating over timescales from hours to months, and is dependent on the physical processes of ocean mixing and transport (Anderson and Totterdell, 2004<ref name="Anderson and Totterdell, 2004">Anderson, T.R. and Totterdell, I.J. 2004. Modelling the response to the biological pump to climate change., in The Ocean Carbon Cycle and Climate, edited by M. Folllows and T. Oguz, Kluwer Academic Press, Netherlands, 65-96.</ref>). The first stage is fixing dissolved inorganic carbon (DIC) into dissolved organic carbon (DOC) by photosynthesis in the euphotic zone; the amount of carbon fixed is crudely proportional to the nutrients available. Most of this DOC is then processed, consumed and recycled by the marine ecosystem within the euphotic zone. Some of the organic material in the euphotic zone is exported, sinking under gravity to be either remineralised by the benthos (converted back into DIC) or buried in the seafloor as sediment. As a rule the biological pump always acts to reduce surface seawater pCO<sub>2</sub> levels.<br />
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The physical pump describes the role of ocean dynamic and thermodynamic processes in changing the distribution of dissolved carbon between the upper ocean and the deep ocean. Dynamical processes that can impact the distribution of CO<sub>2</sub> in the upper ocean include changes in the rate or volume of upwelling or downwelling waters, and buoyancy or wind driven mixing. The thermodynamic response of the carbonate system is such that at a fixed atmospheric pCO<sub>2</sub>, the upper ocean (in contact with the air) can hold more dissolved inorganic carbon at cooler ocean temperatures than at warmer temperatures.<br />
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The carbon in the ocean is a mixture from both natural and anthropogenic sources. The anthropogenic fraction comes from human-induced emissions of CO<sub>2</sub> into the atmosphere that have continued at an increasing rate since the start of the industrial revolution and in 2007 were approaching 10 PgC/yr (where 1 petagram (Pg) = 10<sup>15</sup> grams CO<sub>2</sub> equivalent, which is equal to 10<sup>9</sup> metric tons C) (Canadell et al., 2007<ref name="Canadell et al, 2007">Canadell, J.G., and 9 other authors. 2007. Contributions to accelerating atmospheric CO<sub>2</sub> growth from economic activity, carbon intensity, and efficiency of natural sinks, PNAS, doi:10.1073/pnas.0700609104.</ref>). It is estimated that 30% of total anthropogenic emissions annually are taken up from the atmosphere and sequestered by the ocean (Sabine et al., 2004<ref name="Sabine et al, 2004">Sabine, C.L., ET AL. 2004. The Oceanic Sink for Anthropogenic CO<sub>2</sub>, ''Science'', '''305''', 367-371.</ref>). Regardless of source (natural or anthropogenic) the carbon in the ocean follows the carbon cycle described above.<br />
==References==<br />
<references /><br />
[[Category:The Antarctic environment in the global system]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_physical_setting_of_AntarcticaThe physical setting of Antarctica2014-08-06T14:34:16Z<p>Acce: Removed initial paragraph which belongs on the page The Antarctic environment in the global system</p>
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<div>:''This page is part of the topic [[The Antarctic environment in the global system]]''<br />
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[[File:Figure 1.1a - Map of Antarctica.png|thumb|'''1.1a''' A map of Antarctica showing selected topographic features and locations.]]<br />
[[File:Figure 1.2 - Antarctic topography and bathymetry.png|thumb|'''1.2''' The land topography and the bathymetry of the seabed around Antarctica (in metres). Four major oceanic fronts are shown (not labelled), which are (north-to-south): the Sub-Antarctic Front (SAF), the Polar Front (PF), the Southern ACC Front (SACCF) and the Southern Boundary (SB). The Antarctic Circumpolar Current runs between the SAF and the SB. Figure produced by M.P. Meredith (BAS; pers. comm.) using frontal locations adapted from Orsi et al., 1995<ref name="Orsi et al, 1995">Orsi, A.H., Whitworth III, T.W. and Nowlin Jr.,W.D. 1995. On the meridional extent and fronts of the Antarctic Circumpolar Current, ''Deep-Sea Res.'', '''42''', 641-673.</ref>) and bathymetry from the General Bathymetric Chart of the Oceans (GEBCO) Centennial digital topography data (www.gebco.net).]]<br />
The Scientific Committee on Antarctic Research (SCAR) considers the Antarctic region to include the continent, its offshore islands, and the surrounding Southern Ocean ([[:File:Figure 1.1a - Map of Antarctica.png|Figure 1.1a]]) including the Antarctic Circumpolar Current, the northern boundary of which is the Sub-Antarctic Front (SAF) ([[:File:Figure 1.2 - Antarctic topography and bathymetry.png|Figure 1.2]]), and we will do the same here. Southern Ocean islands that lie north of the SAF and yet fall into SCAR's area of interest include Ile Amsterdam, Ile St. Paul, Macquarie Island and Gough Island.<br />
<br />
[[File:Figure 1.1b - Map of Antarctic stations.png|thumb|'''1.1b''' A map of Antarctica showing selected stations.]]<br />
Antarctica is renowned as being the highest, driest, windiest and coldest continent, boasting the lowest recorded temperature on Earth, -89.2&ordm;C, at Russia&rsquo;s Vostok Station ([[:File:Figure 1.1b - Map of Antarctic stations.png|Figure 1.1b]]), which is located on the Polar Plateau (the white area of East Antarctica in [[:File:Figure 1.2 - Antarctic topography and bathymetry.png|Figure 1.2]]). The continent covers an area of 14 &times; 10<sup>6</sup> km<sup>2</sup>, which is about 10% of the land surface of the Earth. That area includes the ice sheet, the floating ice shelves (offshore continuations of the continental ice sheet) and the areas of fast ice (sea-ice that has become frozen to ice shelves or to the land and does not drift with wind and currents). Most of the continent apart from the northern part of the Antarctic Peninsula lies south of the Antarctic Circle (at latitude 66&ordm; 33&rsquo; 39&rdquo;S), beyond which there are 24 hours of continuous daylight at the austral summer solstice in December, and 24 hours continuous darkness at the austral winter solstice in June. The sun rises at the South Pole on 22 September, the austral vernal equinox, and sets on 23 March, the austral autumn equinox.<br />
<br />
The land surface rises rapidly away from the coast ([[:File:Figure 1.2 - Antarctic topography and bathymetry.png|Figure 1.2]]), and the continent has the highest mean elevation of any continent on Earth, at around 2,200 m. Much of East Antarctica comprises a high, domed Polar Plateau, with a crest at 4,093 m at Dome A (80&ordm; 22&rsquo;S, 77&ordm; 32&rsquo;E). The South Pole is much lower, at 2,835 m, reflecting the fact that although it lies on the Plateau it is not the geographic centre of the continent, which lies instead at the Pole of Inaccessibility at 85&ordm; 50&rsquo;S, 65&ordm; 47&rsquo;E. West Antarctica, with a mean elevation of 850 m, is much lower than East Antarctica, though it includes the highest Antarctic mountain - Mt Vinson in the Ellsworth Mountains, at 4,892 m (78&ordm; 35&rsquo;S, 85&ordm; 25&rsquo;W). Rocky outcrops like Mt Vinson rise above the surrounding ice sheet in several places, notably in the Transantarctic Mountains, which separate East and West Antarctica along a line from the Ross Sea to the Weddell Sea ([[:File:Figure 1.1a - Map of Antarctica.png|Figure 1.1a]]) and rise to a maximum height of 4,528 m. Exposed rock and soil total only about 46,000 km<sup>2</sup> of continental Antarctica (approximately 0.33% of the land area) (Fox and Cooper, 1994<ref name="Fox and Cooper, 1994">Fox, A.J. and Cooper, A.P.R. 1994. Measured properties of the Antarctic Ice Sheet derived from the SCAR Antarctic Digital Database, ''Pol. Rec.'', '''30'''(174), 201-206.</ref>).<br />
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The surrounding seabed ([[:File:Figure 1.2 - Antarctic topography and bathymetry.png|Figure 1.2]]) comprises a number of features, including the continental shelf, the deeper parts of which are typically grooved by furrows carved by glaciers during the ice ages, when sea level was lower and the ice margin extended out towards the edge of the continental shelf. The shallower parts of the shelf are still being furrowed by modern icebergs, whose keels may reach depths of several hundred metres. The continental slope (red and yellow in [[:File:Figure 1.2 - Antarctic topography and bathymetry.png|Figure 1.2]]) is cut by submarine canyons, and merges with the adjacent deep sea floor (&gt;3,500 m below sea level, and blue in [[:File:Figure 1.2 - Antarctic topography and bathymetry.png|Figure 1.2]]), which includes abyssal plains built by sediment deposited from episodic turbidity currents that flowed down the canyons. Other features of note are abyssal hills mantled with pelagic clays, and mountainous ridges of various kinds, like the Scotia Arc (red and yellow in [[:File:Figure 1.2 - Antarctic topography and bathymetry.png|Figure 1.2]]), which connects the tip of the Antarctic Peninsula to South America); and a surrounding deep-sea mountain range consisting of the southern parts of the global Mid-Ocean Ridge System (pale green with yellow crests in [[:File:Figure 1.2 - Antarctic topography and bathymetry.png|Figure 1.2]]). The crest of the Mid-Ocean Ridge System outlines the borders of the Antarctic Plate, one of the Earth&rsquo;s great tectonic plates. These various topographic features form key elements of the habitats of marine organisms, and constrain ocean circulation.<br />
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[[File:Figure 1.3 - Antarctic surface elevation.png|thumb|'''1.3''' Surface elevation illuminated from directly overhead shows the general shape of the continent as well as the smaller scale roughness. Topographic divides between major catchments are bright (white) sinuous ridges. Fringing ice shelves are extremely flat (shown as pale grey matt). Smaller scale roughness is often associated with subglacial relief. The smoother surface surrounding the South Pole is the result of sparser and less accurate elevation data south of 86&ordm;S. (from Bamber et al., 2008<ref name="Bamber et al, 2008">Bamber, J.L., Gomez-Dans, J. L. and Griggs, J. A. 2008., A new 1 km digital elevation model of the Antarctic derived from combined satellite radar and laser data. Part I: Data and methods, The Cryosphere Discuss., 2(6).</ref>).]]<br />
Many benthic (bottom-dwelling) organisms live on the Antarctic continental shelf, which comprises almost 15% of the global continental shelf area (Clarke and Johnston, 2003<ref name="Clarke and Johnston, 2003">Clarke, A. and Johnston, N.M. 2003. Antarctic marine benthic diversity, ''Oceanogr. Mar. Biol. Ann. Rev.'', '''41''', 47-114.</ref>) &ndash; in total around 4.6 &times; 10<sup>6</sup> km<sup>2</sup>. The shelf is unusually deep, reaching 800 m in places, as a side effect of the continent being depressed by the weight of its massive ice sheet. More than 95% of the shelf lies at depths beyond the reach of the scouring effects of sea ice or wave action, is below the reach of sunlight (i.e. outside the euphotic zone), and is inaccessible to scuba divers. Floating ice shelves ([[:File:Figure 1.3 - Antarctic surface elevation.png|Figure 1.3]]) cover about one third of the continental shelf; the rest is covered by sea ice for around half the year. Both the sea and the seabed below the ice shelves are among the least known habitats on Earth.<br />
==References==<br />
<references /><br />
[[Category:The Antarctic environment in the global system]]<br />
[[Category:The geography of Antarctica]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_open_ocean_system_in_the_instrumental_periodThe open ocean system in the instrumental period2014-08-06T14:34:16Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Marine biology in the instrumental period]]''<br />
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[[File:Figure 4.46 - Circumpolar distribution of Antarctic krill based on standardised data from KRILLBASE.png|thumb|'''4.46''' Circumpolar distribution of Antarctic krill, ''Euphausia superba''. based on standardised data from KRILLBASE (8,789 stations). Black lines (from north to south) show Antarctic Polar Front and Southern Boundary of the Antarctic Circumpolar Current. Population centres drawn by eye, relative to the Commission for the Conservation of Antarctic Living Resources (CCAMLR) Survey (from Atkinson et al., 2008<ref name="Atkinson et al, 2008">Atkinson, A., Siegel, V., Pakhomov, E.A., Rothery, P., Loeb, V., Ross, R.M., Quetin, L.B., Schmidt, K., Fretwell, P., Murphy, E.J., Tarling, G.A. and Fleming, A.H. 2008. Oceanic circumpolar habitats of Antarctic krill, ''Mar. Ecol. Prog. Ser'', '''362''', 1-23.</ref>, &copy; Atkinson et al., and InterResearch).]]<br />
The area covered by winter sea ice in the Southern Ocean has not changed significantly over the past decades suggesting that the impact of global warming on Antarctic ecosystems is not as severe as it is in the Arctic. There the sea ice cover is declining in both thickness and extent at a rapid rate, profoundly affecting the structure and functioning of Arctic marine ecosystems, particularly mammal and bird populations. For a comprehensive and most recent review with additional relevant literature see Nichol (2008). In the Antarctic, comparable shrinking of the winter ice cover has occurred only along the western side of the Peninsula and adjoining seas. This is a relatively small region but home to the well-known whale-krill-diatom food chain. Based on data shown in [[:File:Figure 4.46 - Circumpolar distribution of Antarctic krill based on standardised data from KRILLBASE.png|Figure 4.46]], Atkinson et al. (2008<ref name="Atkinson et al, 2008">Atkinson, A., Siegel, V., Pakhomov, E.A., Rothery, P., Loeb, V., Ross, R.M., Quetin, L.B., Schmidt, K., Fretwell, P., Murphy, E.J., Tarling, G.A. and Fleming, A.H. 2008. Oceanic circumpolar habitats of Antarctic krill, ''Mar. Ecol. Prog. Ser'', '''362''', 1-23.</ref>) calculated that 70% of the total krill stock resides in the sector 0&deg;-90&deg;W, a region characterised by rapid regional changes both in water temperature (Meredith and King, 2005<ref name="Meredith and King, 2005">Meredith, M.P. and King, J.C. 2005. Rapid climate change in the ocean west of the Antarctic Peninsula during the second half of the 20<sup>th</sup> century, Geophys. Res. Lett., 32, L19604. (doi: 10.1029/2005GL024042)</ref>; Whitehouse et al., 2008<ref name="Whitehouse et al, 2008">Whitehouse, M.J., Meredith, M.P., Rothery, P., Atkinson, A., Ward, P. and Korb, R.E. 2008. Rapid warming of the ocean at South Georgia, Southern Ocean during the 20<sup>th</sup> Century: forcings, characteristics and implications for lower trophic levels, ''Deep-Sea Research I'', '''55''', 1218-1228.</ref>) and winter sea ice cover (Parkinson, 2004<ref name="Parkinson, 2004">Parkinson, C.L. 2004. Southern Ocean sea ice and its wider linkages: insights revealed from models and observations, ''Antarctic Science'', '''16''', 387-400.</ref>).<br />
<br />
Following near-extinction of the whale populations, the krill stock was expected to increase as a result of release from grazing pressure. Although predation pressure by seals and birds increased, the total biomass remained only a few percent of that of the former whale population. About 300,000 blue whales alone were killed within the span of a few decades equivalent to more than 30 million tonnes of biomass. Most of these whales were killed on their feeding grounds in the southwest Atlantic in an area of at most 2 million km<sup>2</sup> (10 % of the entire winter sea ice cover), which translates to a density of one blue whale per 6 km<sup>2</sup>. Today&rsquo;s whale watchers would be thrilled. A 100 tonne blue whale (adults weigh 150 tonnes) contains about 10 tonnes of carbon, so the biomass of the whales on their feeding grounds would have amounted to 1.5 g C m<sup>-2</sup> which is equivalent to the average coastal zooplankton biomass. Adding the biomass of krill estimated to have been annually eaten by the whales (150 million tonnes) to the m<sup>2</sup> calculation, we get 12 g C m<sup>-2</sup> just for blue whales and their annual food intake. This number is equivalent to the biomass of an average phytoplankton bloom or, to take an example of more familiar grazers, to 240 cows of 500 kg each grazing on one km<sup>2</sup> of meadow.<br />
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The actual krill stock, from which the 150 million tonnes were being eaten, will have been at least three times higher prior to whaling. The magnitude of primary production required them to fulfil their food demands at the trophic transfer rule of thumb (10:1) would be around 300 g C m<sup>-2</sup> yr<sup>-1</sup> which is about that estimated for the North Sea, hence this does not leave much scope for other grazers such as protozoa and copepods. What percentage of the production was exported then from the surface through the mesopelagial habitat and ultimately to the deep-sea benthos, hence also sequestered as carbon, is an interesting but open question.<br />
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[[File:Figure 4.47 - Temporal change of krill and salps.png|thumb|'''4.47''' Temporal change of krill and salps. a, Krill density in the southwest Atlantic sector, 30&deg; &ndash; 70&deg;W. Illustrated temporal trends include b, post 1976 krill data from scientific trawls; c, 1926-2003 circumpolar salp data. Green spots denote grid cells useable in a spatio-temporal model, with data subdivided according to sampling method (Atkinson et al., 2004<ref name="Atkinson et al, 2004">Atkinson, A., Pakhomov, E., Rothery, P., Siegel, V. 2004. Long-term decline in krill stocks and increase in salps within the Southern Ocean, ''Nature'', '''432''' (7013), 100-103.</ref>). This model revealed significant decreases in krill density within the southwest Atlantic sector since 1976 and a significant increase in salp densities at high latitudes since 1926. Reprinted by permission from Macmillan Publishers Ltd: Nature (Atkinson et al., 2004<ref name="Atkinson et al, 2004">Atkinson, A., Pakhomov, E., Rothery, P., Siegel, V. 2004. Long-term decline in krill stocks and increase in salps within the Southern Ocean, ''Nature'', '''432''' (7013), 100-103.</ref>), &copy; 2004]]<br />
The above calculations indicate that krill stocks in the whale feeding grounds were close to the carrying capacity of the ecosystem prior to whaling. That being the case, when less krill was eaten by whales, enabling more krill to survive, many of the survivors would have starved. This would explain why a krill surplus, at least equivalent to the amount annually eaten by whales, was not recorded. At their former high stock sizes, and given the tendency of krill schools to appear at the very surface and discolour the water, they were commonly observed from ship decks, as noted for example by the scientists of the Discovery cruises (Hardy, 1967<ref name="Hardy, 1967">Hardy, A. 1967. Great waters. Harper and Row, New York, 542 pp.</ref>). Since that time, despite a significant increase in the numbers of observers, from cruise ships to research vessels, krill swarms are now rarely seen from ships decks. A thorough statistical assessment of all net catches has been carried out for different sectors of the Southern Ocean. The analysis suggests a 38 - 81% decline in krill stocks of the southwest Atlantic accompanied by an increase in salp populations ([[:File:Figure 4.47 - Temporal change of krill and salps.png|Figure 4.47]], Atkinson et al., 2004<ref name="Atkinson et al, 2004">Atkinson, A., Pakhomov, E., Rothery, P., Siegel, V. 2004. Long-term decline in krill stocks and increase in salps within the Southern Ocean, ''Nature'', '''432''' (7013), 100-103.</ref>; Ross et al., 2008<ref name="Ross et al, 2008">Ross, R.M., Quetin, L.B., Martinson, D.G., Iannuzzi, R.A., Stammerjohn, S.E. and Smith, R.C. 2008. Palmer LTER: Patterns of distribution of five dominant zooplankton species in the epipelagic zone west of the Antarctic Peninsula, 1993-2004, ''Deep-Sea Res. II'', '''55''', 2086-2105.</ref>). The extent of the krill decline and the underlying factors are under vigorous debate (Ainley et al., 2007<ref name="Ainley et al, 2007">Ainley, D.G., Ballard, G., Ackley, S., Blight, L., Eastman, J.T., Emslie, S.D., Lescroel, A., Olmastroni, S., Townsend, S.E., Tynan, C.T., Wilson, P. and Woehler, E. 2007. Paradigm lost, or is top-down forcing no longer significant in the Antarctic marine ecosystem? Antarctic Sci., 19, 283-290.</ref>; Nicol et al., 2007<ref name="Nicol et al, 2007">Nicol, S., Croxall, J., Trathan, P., Gales, N. and Murphy, E. 2007. Paradigm misplaced ? Antarctic marine ecosystems are affected by climate change as well as biological processes and harvesting, ''Antarctic Sci.'', '''19''', 291-295 .</ref>), because of difficulties in unravelling the effects of industrial whaling from those of sea ice retreat; there are also discrepancies between the abundances of krill as measured by net and acoustic methods, and enormous intra-annual as well as spatial variability has to be considered (Hewitt et al., 2003<ref name="Hewitt et al, 2003">Hewitt, R.P., Demer, D.A. and Emery, J.H. 2003. An 8-year cycle in krill biomass density inferred from acoustic surveys conducted in the vicinity of the South Shetland Islands during the austral summers of 1991-1992 through 2001-2002, ''Aquatic Living Resources'', '''16''', 205-213.</ref>; Saunders et al., 2007<ref name="Saunders et al, 2007">Saunders, R.A., Brierley, A.S., Watkins, J.L., Reis, K., Murphy, E.J., Enderlein, P. and Bone, D.G. 2007. Intra-Annual variability in the density of Antarctic krill (''Euphausia superba'') at South Georgia, 2002-2005: within-year variation provides a new framework for interpreting previous 'annual' estimates of krill densitiy, ''CCAMLR Science'', '''14''', 27-41.</ref>). However, a significant negative correlation between krill density (30&deg;W to 70&deg;W) and mean sea surface temperature at South Georgia has been found for the period 1928-2003, which implies a large-scale response not only of krill but of the entire open ocean ecosystem to climate change (Whitehouse et al., 2008<ref name="Whitehouse et al, 2008">Whitehouse, M.J., Meredith, M.P., Rothery, P., Atkinson, A., Ward, P. and Korb, R.E. 2008. Rapid warming of the ocean at South Georgia, Southern Ocean during the 20<sup>th</sup> Century: forcings, characteristics and implications for lower trophic levels, ''Deep-Sea Research I'', '''55''', 1218-1228.</ref>).<br />
<br />
If former krill stocks were close to the carrying capacity provided by primary production, then a decrease in grazing pressure should have resulted in a &ldquo;phytoplankton surplus&rdquo;, but there is also little evidence for that. Unfortunately, a comparison with phytoplankton stocks recorded during the Discovery era is not possible because the methods used at that time soon became obsolete. Nevertheless, the impression gained by the Discovery scientists is one of large diatom stocks: &ldquo;...extremely rich production, which will probably be found to exceed that of any other large area in the world &hellip;&rdquo; (Hart, 1934<ref name="Hart, 1934">Hart, T.J. 1934. On the phytoplankton of the South-West Atlantic and the Bellinghausen Sea, 1929-31, ''Discovery Reports'', '''8''', 1-268.</ref>). The Discovery scientists were familiar with North Sea phytoplankton, which today has much higher biomass levels than those recorded for the Scotia Sea in recent decades. That comparison makes it likely that phytoplankton production has indeed decreased with that of krill stocks, a conclusion supported by the increase in the salp population. In contrast to krill, which are equipped to deal with the characteristically spiny and heavily silicified diatoms of the Southern Ocean, salps are adapted to feed on the lower biomass concentrations typical of the iron-limited microbial food webs. Their encroachment into the former krill habitat is an indication of declining phytoplankton, in particular diatom stocks.<br />
<br />
A decline in phytoplankton concentrations can be explained by a corresponding decline in the supply of iron. There is reason to believe that the reduction in sea ice formation has resulted in a decrease in iron input from the continental margins of the Western Peninsula. In contrast, the simultaneous retreat of glaciers should have increased run-off, and possibly also iron input, from the land along the coasts of the Peninsula and adjacent islands. Comparisons of the chlorophyll concentrations recorded by the CZCS satellite of the 1980s with those from the current SeaWifs satellite indicate a decline along the Antarctic ice edge and particularly in the Scotia Sea, the only region of the globe where production has declined, but no major change along the coast (Gregg and Conkright, 2002<ref name="Gregg and Conkright, 2002">Gregg, W.W. and Conkright, M.E. 2002. Decadal changes in global ocean chlorophyll, ''Geophys. Res. Lett.'', '''29''', 21-24.</ref>). Production was found to have increased by 50%, off the Patagonian shelf, so it is also possible that the wind field transporting Patagonian dust from mud fields laid bare by retreating glaciers has changed, reducing the aeolian iron supply to the Scotia Sea. Also, the westerlies have intensified and would carry more dust. Whatever the mechanism, a reduction in phytoplankton biomass can only be explained by a corresponding reduction in iron supply combined with light limitation by deep mixed layers and heavy grazing pressure on phytoplankton stocks. Recently, five mesoscale, ''in situ'' iron fertilization experiments, carried out in the Pacific and southeast Atlantic Sectors of the Southern Ocean, have unambiguously demonstrated that plankton biomass is limited by iron availability. It follows that the higher productivity of coastal regions, including the southwest Atlantic, is maintained by input of iron supplied from land-masses, and from the sediments by deep mixing and upwelling along the continental margin. The presence of excess nutrients in these regions allows the assumption that iron supply limits productivity over most of the year throughout the Southern Ocean. The ramifications of this finding for the structure and functioning of Antarctic ecosystems have yet to be adequately explored, particularly because ongoing global change will affect coastal hydrography and hence the supply of iron.<br />
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An alternative but not mutually exclusive explanation for the phytoplankton decline can be a decrease in the rate of recycling of the iron entering the system. It is now well established that primary production in microbial food webs is based on recycling by grazers feeding on pico- and nano-phytoplankton, which are in turn eaten by predators such as ciliates and copepod larvae. The latter are the preferred prey of copepods, whereas filter-feeding salps consume all the components ''en masse''. In the Southern Ocean the microbial community is characteristic of the iron-limited HNLC (high nutrient, low chlorophyll) area, where chlorophyll concentrations remain below 0.5 mg-Chl/m<sup>2</sup> throughout the year. These regions support a surprisingly high zooplankton biomass, comprising slow growing copepods and fast-growing salps, throughout the year, suggesting that they are an integral part of a recycling system that also regenerates iron in addition to ammonium (Barbeau et al., 1996<ref name="Barbeau et al, 1996">Barbeau, K., Moffett, J.W., Caron, D.A., Croot, P.L. and Erdner, D.L. 1996. Role of protozoan grazing in relieving iron limitation of phytoplankton, ''Nature'', '''380''', 61-64.</ref>).<br />
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Local increases in production above this level are invariably due to accumulation of diatoms and ''Phaeocystis'' colonies, and will be caused by input of new iron, whether from above or from below. The fate of these diatom blooms is under debate: are they consumed and their nutrients recycled in the surface or sub-surface layer, or does a significant portion sink to greater depths or to the sea floor? The latter fate is of particular interest in the light of proposals for large-scale ocean fertilization to sequester atmospheric CO<sub>2</sub>.<br />
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Given the high densities of the former krill stocks, their rate of recycling will have been as effective as that in the microbial food web today, but on a much larger scale. Predation by whales will have contributed significantly to the iron recycling pool. That is because, except in the case of pregnant and lactating females, whales convert krill protein into blubber (hydrocarbons), so retaining energy but excreting nutrients, including iron. Whale faeces are liquid and rise to the surface where they are likely to have released iron, thereby increasing the efficiency of recycling. Krill also have a high (50%) lipid content, and krill excretion releases large amounts of iron (Tovar-Sanchez et al., 2007<ref name="Tovar-Sanchez et al, 2007">Tovar-Sanchez, A., Duarte, C.M., Hernandez-Leon, S. and Sanudo-Wilhelmy, S.A. 2007. Krill as a central node for iron cycling in the Southern Ocean, Geophys. Res. Lett., 34, (L11601, doi:10.1029/2006GL029096).</ref>). It follows that the exceptionally productive ecosystem characterised by the food chain of the giants was maintained by the recycling of iron by krill and whale feeding. An alternative, but mutually inclusive hypothesis in which large whale stocks promoted the development of large stocks of their prey (krill) by dispersing them over a larger area has been recently suggested.<br />
<br />
It is now acknowledged that large terrestrial herbivores (megafauna) condition ecosystems by promoting a vegetation cover conducive to their demands, e.g. grassland instead of forest by elephants. The removal of those herbivores leads to profound changes in landscape. Similarly, the top predators in lakes can determine the structure of the ecosystem down to the composition and biomass of the phytoplankton. Predation pressure on upper trophic levels is propagated down the food web by mechanisms known as trophic cascades. Although the effects of top-down control have been demonstrated for shallow benthic environments from many coastal regions, comparable mechanisms are only now coming to light from planktonic ecosystems, such as the reported worldwide increase in gelatinous plankton after removal of dominant fish stocks. Whether such changes can cascade down to the level of phytoplankton is not known. We simply do not know what effect the removal of whales (and seals) through hunting had on the ocean ecosystem around Antarctica. The belief that marine phytoplankton productivity is determined primarily by bottom-up driving forces is entrenched in the marine biological literature, but why the interaction between marine plankton and whales around Antarctica should be fundamentally different from that between their non-polar lake counterparts has yet to be addressed. The fact is that the processes driving annual cycles of phytoplankton production, biomass and species composition in the marine environment remain largely unknown. It is time to explore new approaches.<br />
<br />
The linkages between phytoplankton and bacterioplankton in the Southern Ocean are not well understood. The timing of spring phytoplankton blooms and bacterioplankton activities are not necessarily linked, as they are in other pelagic systems, though there may just be a lag in this linkage that is longer than in lower latitude systems (Ducklow et al., 2007<ref name="Ducklow et al, 2007">Ducklow, H.W., Baker, K., Martinson, D.G., Quentin, L.B., Ross, R.M., Smith, R.C., Stammerjohn, S.E., Vernet, M. and Fraser, W. 2007. Marine pelagic ecosystems: the West Antarctic Peninsula, ''Phil. Trans. R. Soc. B'', '''362''', 67-94.</ref>). Even less is known of the relationships between phytoplankton species and bacterioplankton species composition (e.g. whether particular bacteria are associated with diatom vs. cryptophyte phytoplankton). Considerations of phytoplankton primary productivity, linkages to CO<sub>2</sub> drawdown, and grazer populations are also linked to phytoplankton species composition. Phytoplankton species are susceptible to changes in sea ice duration and position of the ice edge, and shifts in water column properties such as the depth of the mixed layer. Shifts in phytoplankton species composition from diatom-dominated communities to more diverse communities dominated by cryptomonads and flagellates occurs following the ice edge retreat and spring diatom bloom in the Western Antarctic Peninsula (Moline and Prezelin, 1996<ref name="Moline and Prezelin, 1996">Moline, M.A. and Prezelin, B.B. 1996. Palmer LTER 1991-1994: Long-term monitoring and analyses of physical factors regulating variability in coastal Antarctic phytoplankton biomass, ''in situ'' productivity and taxonomic composition over subseasonal, seasonal and interannual time scales phytoplankton dynamics, ''Mar. Ecol. Prog. Ser.'', '''145''', 143-160.</ref>), and will be (or already are) potentially more common occurrences as a result of warming in this region (Clarke et al., 2007<ref name="Clarke et al, 2007">Clarke, A., Murphy, E.J., Meredith, M.P., King, J.C., Peck, L.S., Barnes, D.K.A. and Smith, R.C. 2007. Climate change and the marine ecosystem of the western Antarctic Peninsula, ''Phil. Trans. R. Soc. B'', '''362''', 149-166.</ref>). The linkages between phytoplankton and zooplankton populations are tight, as krill tend to dominate the zooplankton assemblages when diatoms are abundant, and zooplankton dominance can shift to salp dominance when the community is cryptomonad or flagellate dominated. One study in the Western Antarctic Peninsula recently reported that there has been a shift from krill-dominated waters to salp dominance since 1999 This trend may also be potentially representative of the longer term trends referred to above.<br />
<br />
Acquiring a mechanistic understanding of the structure and functioning of the ecosystems surrounding Antarctica is a prerequisite for predicting their performance under the influence of global warming. Hypothetical conceptual frameworks of relevant mechanisms need to be developed that can be tested by comparing intact ecosystems with those where top-predators have been depleted both regionally and, where baseline data are available, temporally. Satellite data have vastly extended the scales accessible to such regional studies. Larger scale ''in situ'' iron fertilization experiments open up an exciting new avenue to study the effects of bottom-up versus top-down factors on higher trophic levels, and if carried out over several years, also on krill populations and on the underlying deep sea and benthos. Such experiments provide an ideal background to study the relationship between ecology and biogeochemistry at the species level, which in turn will improve interpretation of sedimentary proxies, in particular microfossils, for reconstruction of past climate change. Conceptual frameworks emerging from field studies and experiments can be explored, tested and refined with new generations of 4D mathematical models.<br />
<br />
==Iron fertilization experiments==<br />
<br />
It has been suggested that one way in which the rise of carbon dioxide in the atmosphere may be mitigated is to fertilise the ocean with iron so as to stimulate the production of plankton and hence the draw-down of carbon dioxide from the atmosphere into the ocean (Boyd et al., 2007). These ideas are based on the results of a limited number of experiments in which different parts of the ocean, including the Southern Ocean, were seeded with iron (Boyd et al., 2007b<ref name="Boyd et al, 2007b">Boyd, P.W., Jickells, T., Law, C.S., Blain, S., Boyle, E.A., Buesseler, K.O., Coale, K.H., Cullen, J.J., De Baar, H.J.W., Follows, M., Harvey, M., Lancelot, C., Levasseur, M., Owens, N.P.J., Pollard, R., Rivkin, R.B., Sarmiento, J., Schoemann, V., Smetacek, V., Takeda, S., Tsuda, A., Turner, S. and Watson, A.J. 2007b. Mesoscale iron enrichment experiments 1993-2005: Synthesis and future directions, ''Science'', '''315''', 612-617.</ref>). This current debate (e.g. see Oceanus, 24 June 2009, http://www.whoi.edu/oceanus/viewArticle.do?id=34167) may at some time shift its focus to exploring how to maximise the efficiency of the process and minimise harmful side effects. The hypothesis could be tested by a new generation of iron fertilization experiments carried out at larger scales and longer periods on the former whale feeding grounds. Given the apparent high rates of krill decline and a steady southward encroaching ocean warming there is a pressing need to develop an integrated understanding of how this ecosystem functioned not only in the recent past but also in the glacial ocean, in order to predict future changes in the pelagic and underlying benthic ecosystems around Antarctica. In situ iron fertilization experiments provide one methodology for testing ecosystem models, by enabling the study of interactions within ecosystems with a full complement of grazers and pathogens. The effect of iron fertilisation on higher trophic levels will depend on the locality and duration of the experiment. A regional survey of the underlying benthos prior to fertilization would yield a baseline to monitor possible changes in this ecosystem. Preliminary surveys of the deep-sea benthos of the Peninsula region have shown the presence of communities with high biomass and species numbers (Brandt et al., 2007<ref name="Brandt et al, 2007">Brandt, A., Gooday, A.J., ET AL. 2007. First insights into the biodiversity and biogeography of the Southern Ocean deep sea, ''Nature'', '''447'''(7142), 307-311.</ref>) but their areal extent is not known. Deep carbon export flux has been shown to be above global average and to have a high regional variability in the Southern Ocean (Boyd and Trull, 2007<ref name="Boyd and Trull, 2007">Boyd, P.W. and Trull, T.W. 2007. Understanding the export of biogenic particles in oceanic waters: Is there consensus? Progress in Oceanography, 72, 276-312.</ref>; Sachs, 2008<ref name="Sachs, 2008">Sachs, O. 2008. Benthic organic carbon fluxes in the Southern Ocean: regional differences and links to surface primary production and carbon export, ''Ber. Polarforsch. Meeresforsch.'', '''578''', 1-143.</ref>). Since fertilization will be carried out offshore, it is quite unlikely that shelf and coastal benthos are significantly affected.<br />
<br />
An added incentive to carrying out such experiments is that they would offer an ideal training ground for the kind of large-scale international, interdisciplinary research taking a whole Earth System Science approach to investigating global change. Such large-scale experiments would not only provide a wealth of new insights into the structure and functioning of pelagic and underlying benthic ecosystems. They could also provide more reliable data for parameterising current and new coupled ecological-biogeochemical ocean-circulation models for use in assessing the Southern Ocean as a sink for anthropogenic CO<sub>2</sub>. There is a concern that the incentive offered by the carbon credit market could result in excessive fertilization which could lead to unacceptable harm to Southern Ocean ecosystems (Chisholm et al., 2001<ref name="Chisholm et al, 2001">Chisholm, S., Falkowski, P. and Cullen, J. 2001. Dis-crediting ocean fertilization, ''Science'', '''294''', 309-310</ref>). Three United Nation bodies, the Intergovernmental Oceanographic Commission (IOC), the International Maritime Organization (IMO), and the Convention on Biological Diversity (CBD) agreed that proposals to use ocean fertilization to sequester carbon in the ocean give cause for concern due to unknown negative impact to the ecosystems (http://ioc3.unesco.org/oanet/OAdocs/INF1247-1.pdf). The two latter organizations recently argued that large operations are currently not justified and should not be allowed (http://www.cbd.int/decisions/?m=COP-09&amp;id=11659&amp;lg=0, http://www.maritime-connector.com/NewsDetails/2203/lang/.wshtml, http://www.ioccp.org). Scientists emphasized the necessity for independent research on small scale fertilisation studies (Buesseler et al., 2008<ref name="Buesseler et al, 2008">Buesseler, K.O., Doney, S.C., Karl, D.M., Boyd, P.W., Caldeira, K., Chai, F., Coale, K.H., De Baar, H.J.W., Falkowski, P.G., Johnson, K.S., Lampitt, R.S., Michaels, A.F., Naqvi, S.W.A., Smetacek, V., Takeda, S. and Watson, A.J. 2008. Ocean Iron Fertilization--Moving Forward in a Sea of Uncertainty, ''Science'', '''319''', 162.</ref>). In addition it must be considered that once done such large global experiments with unknown outcomes would be very difficult or impossible to reverse.<br />
==References==<br />
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[[Category:The instrumental period]]<br />
[[Category:Antarctic biology]]<br />
[[Category:Marine biology]]<br />
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<div>:''This page is part of the topic [[Antarctic climate and environment history in the pre-instrumental period]]''<br />
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This section describes the Antarctic climate and environmental history of the last 1 Ma, including the development of the current pattern of glacial-interglacial cycles. This is followed by a description of the transition from the height of the last ice age &ndash; the Last Glacial Maximum (LGM, c. 21 ka BP), to our present interglacial state, a period known as Termination 1. It includes an account of the deglaciation of the continental shelf, coastal margin and continental interior and the impact of this loss of ice on global sea level. Finally we review the influence of changing sea ice distributions on climate.<br />
==Pages in this topic==<br />
#[[Glacial interglacial cycles]]<br />
#[[The transition to Holocene interglacial conditions]]<br />
#[[Deglaciation of the continental shelf, coastal margin and continental interior]]<br />
#[[Antarctic deglaciation and its impact on global sea level]]<br />
#[[Sea ice over the last million years]]<br />
[[Category:The pre-instrumental period]]<br />
[[Category:The last million years]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_IPCC_fourth_assessment_reportThe IPCC fourth assessment report2014-08-06T14:34:14Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Antarctic climate and environment change over the next 100 years]]''<br />
<br />
==IPCC scenarios==<br />
<br />
===Introduction===<br />
<br />
Given the information about climate contained in the reports of the IPCC (IPCC, 2007<ref name="IPCC, 2007">IPCC 2007. Climate Change 2007: The Physical Science Basis. Contribution of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge.</ref>) the degree to which the climate of the Earth will change over the next century will be heavily dependent on the success of efforts to reduce the rate of greenhouse gas (GHG) emissions. The Antarctic is a long way from the main centres of population, but greenhouse gases are well mixed through the atmosphere. Even with sharp reductions now in GHG production, it will take a long time for the levels of GHGs in the atmosphere to decrease. For instance, even if anthropogenic emissions of CO<sub>2</sub> were halted now, recent studies indicate that 25% of CO<sub>2</sub> from fossil fuels will persist in the atmosphere indefinitely (Archer, 2005<ref name="Archer, 2005">Archer, D. 2005. Fate of fossil fuel CO<sub>2</sub> in geologic time, J. Geophys. Res., DOI: 10.1029/2004JC002625.</ref>; Matthews and Caldeira, 2008<ref name="Matthews and Caldeira, 2008">Matthews, H.D. and Caldeira, K. 2008. Stabilizing climate requires near-zero emissions, ''Geophysical Research Letters'', '''35''', L04705, doi:10.1029/2007GL032388.</ref>), and Solomon et al. (2009<ref name="Solomon et al, 2009">Solomon, S., Plattner, G.-K., Knutti, R. and Friedlingstein, P. 2009. Irreversible climate change due to carbon dioxide emissions. Proceedings of the National Academy of Sciences, 106, 1704-9.</ref>) show that the climate change resulting from increases in CO<sub>2</sub> concentration in the atmosphere is largely irreversible for 1,000 years after emissions stop.<br />
<br />
Future levels of GHG emissions will be determined by the complex interactions between many factors, such as changes in the energy mix of oil, gas, nuclear and renewables, the drive to greater fuel efficiency, the rise in population (which will increase energy demand and is likely to negate efficiency gains), the development and commercial application of new technologies (the so-called hydrogen economy; fuel cells etc), the extent of de- or re-forestation, and regionally varying social and economic developments - notably the growth of major economies in China, India and Brazil, and the gradual industrialisation and urbanisation of the developing world. The path of future evolution of GHGs and aerosols is therefore uncertain. Nevertheless, some assumptions have to be made for the purposes of determining how the Antarctic may be affected by climate change. To simulate the climate of the Twenty First Century through mathematical models requires first the selection of likely GHG emissions from a range of possible emission scenarios.<br />
<br />
===The IPCC greenhouse gas and aerosol emission scenarios===<br />
<br />
In the year 2000, the IPCC refined the emission scenarios that it had used until then as contributions to models of future climate change (Nakicenovic et al., 2000<ref name="Nakicenovic et al, 2000">Nakicenovic, N., Alcamo, J., Davis, G., De Vries, B., Fenhann, J., Gaffin, S., Gregory, K., Yong Jung, T., Kram, T., Lebre, L.A., Rovere, E., Michaelis, E., Mori, S., Morita, T., Pepper, W., Pitcher, H., Price, L., Riahi, K., Roehrl, A., Rogner, H-H., Schlesinger, A.M., Shukla, P., Smith, S., Swart, R., Van Rooijen, S., Victor, N. and, Zhou Dadi, L. 2000. Special Report on Emissions Scenarios: A Special Report of Working Group III of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, U.K., 599 pp. Available online at: http://www.grida.no/climate/ipcc/emission/index.htm).</ref>). The new scenarios were based, in IPCC terminology, on four storylines, which are narrative descriptions that highlight the main characteristics and dynamics of future economic and social development. The storylines (Nakicenovic et al., 2000<ref name="Nakicenovic et al, 2000">Nakicenovic, N., Alcamo, J., Davis, G., De Vries, B., Fenhann, J., Gaffin, S., Gregory, K., Yong Jung, T., Kram, T., Lebre, L.A., Rovere, E., Michaelis, E., Mori, S., Morita, T., Pepper, W., Pitcher, H., Price, L., Riahi, K., Roehrl, A., Rogner, H-H., Schlesinger, A.M., Shukla, P., Smith, S., Swart, R., Van Rooijen, S., Victor, N. and, Zhou Dadi, L. 2000. Special Report on Emissions Scenarios: A Special Report of Working Group III of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, U.K., 599 pp. Available online at: http://www.grida.no/climate/ipcc/emission/index.htm).</ref>) are:<br />
* A1: a world of very rapid economic growth, global population that peaks in the middle of the Twenty First Century and declines thereafter, and rapid introduction of new and more efficient technologies.<br />
* A2: a heterogeneous world with continuously increasing global population and regionally oriented economic growth that is more fragmented and slower than in other storylines.<br />
* B1: a convergent world with the same global population as in the A1 storyline, but with rapid changes in economic structures toward a service and information economy, with reductions in material and energy intensity, and the introduction of clean and resource-efficient technologies.<br />
* B2: a world in which the emphasis is on local solutions to economic, social, and environmental sustainability, with continuously increasing population (lower than A2) and intermediate economic development.<br />
<br />
For each of the above storylines a &lsquo;family&rsquo; of scenarios was developed, which include quantitative projections of major driving variables, such as possible population change and economic development. Six groups of scenarios were drawn from the four families: one group each in the A2, B1 and B2 families, and three groups in the A1 family. These characterised alternative developments of energy technology as A1FI (fossil intensive), A1T (predominantly non-fossil) and A1B (balanced across energy sources). Altogether IPCC developed 40 different scenarios, each considered equally valid and equally probable.<br />
<br />
In the present report we use outputs from Scenario A1B, because climate model projections of the Twenty First Century using this scenario produce global mean near-surface warming that is about halfway between the output of simulations based on the warmest (A1F1) and coolest (B1) scenarios. Under this scenario emissions increase from the 1990 value of about 8 Gt/yr to a peak around the middle of the century and then decrease to about 14 Gt/yr by 2100. Concentrations of CO<sub>2</sub> are projected to rise from the current concentration of around 380 ppm to reach 720 ppm in the year 2100 (i.e. approximately a doubling of CO<sub>2</sub>).<br />
<br />
===The emission scenarios in the 2007 IPCC AR4 models===<br />
<br />
In the IPCC&rsquo;s AR4 in 2007, climate model simulations were run using prescribed concentrations of GHGs (CO<sub>2</sub>, CH<sub>4</sub> and N<sub>2</sub>O) following the scenarios of Nakicenovic et al. (2000<ref name="Nakicenovic et al, 2000">Nakicenovic, N., Alcamo, J., Davis, G., De Vries, B., Fenhann, J., Gaffin, S., Gregory, K., Yong Jung, T., Kram, T., Lebre, L.A., Rovere, E., Michaelis, E., Mori, S., Morita, T., Pepper, W., Pitcher, H., Price, L., Riahi, K., Roehrl, A., Rogner, H-H., Schlesinger, A.M., Shukla, P., Smith, S., Swart, R., Van Rooijen, S., Victor, N. and, Zhou Dadi, L. 2000. Special Report on Emissions Scenarios: A Special Report of Working Group III of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, U.K., 599 pp. Available online at: http://www.grida.no/climate/ipcc/emission/index.htm).</ref>). As well, the scenarios for the future, two other emissions experiments were conducted: (i) a &ldquo;climate of the Twentieth Century&rdquo; (20C3M) modelling experiment, which was forced by observed GHG concentrations from the mid Nineteenth Century to the end of the Twentieth Century; and (ii) a &ldquo;pre-industrial control&rdquo; (PICNTRL) modelling experiment for which GHG and aerosol concentrations were kept at constant pre-industrial levels. For both the 20C3M and the &ldquo;pre-industrial control&rdquo; scenario runs the sulfate aerosol concentrations were calculated from prescribed sulfur dioxide (SO<sub>2</sub>) emissions, but varied between models due to different methods of calculation. Both sets of runs also included inter-model differences of forcing by stratospheric ozone and volcanic aerosols, which are important for the Southern Hemisphere circulation due their role in forcing long-term changes of the Southern Annular Mode (SAM) (Shindell and Schmidt, 2004<ref name="Shindell and Schmidt, 2004">Shindell, D.T. and Schmidt, G.A. 2004. Southern hemisphere climate response to ozone changes and greenhouse gas increases, ''Geophys. Res. Lett.'', '''31''', L18209, doi:10.1029/2004GL020724.</ref>; Miller et al., 2006<ref name="Miller et al, 2006">Miller, R.L., Schmidt, G.A. and Shindell, D.T. 2006. Forced annular variations in the 20<sup>th</sup> century Intergovernmental Panel on Climate Change Fourth Assessment Report models, J. Geophys. Res., 111, doi:10.1029/2005JD006323.</ref>). For the A1B scenario, most models were run with a gradual recovery of stratospheric ozone to pre-industrial levels over the Twenty First Century, but some did not include any stratospheric ozone forcing (see Miller et al., 2006<ref name="Miller et al, 2006">Miller, R.L., Schmidt, G.A. and Shindell, D.T. 2006. Forced annular variations in the 20<sup>th</sup> century Intergovernmental Panel on Climate Change Fourth Assessment Report models, J. Geophys. Res., 111, doi:10.1029/2005JD006323.</ref>).<br />
[[File:Figure 5.1 - Projected global annual CO2 emissions, 1990-2100, used to run models for the IPCC AR4.png|thumb|'''5.1''' Total global annual CO<sub>2</sub> emissions from all sources (energy, industry, and land-use change) from 1990 to 2100 in Gigatonnes of carbon/year (GtC/yr) for the four emission families (A1, A2, B1 and B2) and the six scenario groups (A1F1, A1T, A1B and A2, B1 and B2) (1Gt = 10<sup>9</sup> metric tonnes). The 40 scenarios are represented by the four families (A1, A2, B1, and B2) and the six scenario groups. The fossil-fuel-intensive A1FI (comprising the high-coal and high-oil-and-gas scenarios) has the greatest emissions. Each coloured emission band shows the range of scenarios within each group. The solid and dashed lines within each scenario represent the paths of illustrative marker scenarios.]]<br />
<br />
<br />
In this present volume (as explained below) we have weighted the model outputs to reflect the skill of each model at reproducing modern conditions in the 20C3M experiment. Although the weighted results include models that do not include stratospheric ozone recovery, four out of five of the most strongly weighted models do include representation of stratospheric ozone recovery. The consequences of neglecting stratospheric ozone recovery are significant only in the summer months when ozone concentrations are lowest.<br />
<br />
We used 19 of the 25 AR4 coupled AOGCMs, which had the data required for weighting. The models used here were as follows: BCCR BCM2, CCCMA CGCM3, CNRM CM3, CSIRO Mk3, GFDL CM2.0, GFDL CM2.1, GISS EH, GISS ER, IAP FGOALS1, INM CM3, IPSL CM4, MIROC (hires), MIROC (medres), MPI ECHAM5, MRI CGCM2, NCAR CCSM3, NCAR PCM1, UKMO HadCM3, UKMO HadGEM1.<br />
<br />
==The AR4 climate models==<br />
<br />
Climate models are now being developed within the framework of Earth System Science, where the aim is to build a comprehensive picture of the feedbacks between the atmosphere, hydrosphere, cryosphere, biosphere and geosphere in the Earth System. Given the likely effects on global sea levels of melting ice at the poles, considerable attention is now being paid to modelling high latitude climate processes accurately. In that context a particular difficulty lies in our ability to validate models and model predictions, for which we require specific field and ocean observations that are commonly missing at these latitudes. Without those observations it is difficult to comprehensively verify whether or not the internal variability displayed by the different runs of the climate models is similar to that seen in reality. We face the further problem that because of the broad scale of climate model grids, we are not yet able to model with confidence variation at the regional to local scale, for example over the Antarctic Peninsula. Nevertheless, what models do provide, especially when tested against their ability to reproduce (or rather, simulate) modern conditions, is a reasonable, albeit crude, projection of what may happen given certain external forcings (e.g. changing carbon dioxide in the atmosphere).<br />
<br />
The results from 24 climate models contributed by many modelling centres around the world were compiled into a central database for the IPCC AR4. This repository of model data forms the Coupled Model Inter-comparison Project phase 3 (CMIP3) multi-model dataset. Much of the discussion in this chapter is based on research that makes use of the CMIP3 dataset. This section therefore provides some background to these models and their strengths and weaknesses.<br />
<br />
The CMIP3 climate models are coupled atmosphere/ocean/sea-ice/land-surface models. Each model is made up of sub-models that simulate either the atmosphere, ocean, sea ice, or land surface. These sub-models are coupled together to allow interaction between the systems. These interactions are complex and in some ways still not well understood.<br />
<br />
===Atmosphere models===<br />
<br />
Models of the atmosphere are founded on the fundamental laws of motion, thermodynamics and chemistry. The models use equations representing those laws to step forward in time from some initial state. The result is a representation of the evolution of the atmosphere in both space and time.<br />
<br />
For the CMIP3 models the horizontal spacing of grid points is around 250 km and the vertical spacing is around 500m. This is sufficient to resolve features such as the typical atmospheric eddy &ndash; the extra-tropical cyclone &ndash; which can be around 1,000 km across. However, many smaller-scale phenomena, such as atmospheric convection and heat and momentum exchange at the lower boundary, are not resolved in these models. The effects of these small-scale processes on the larger scale must be parameterized (i.e. represented in the models statistically). Many of the parameterizations are developed and optimised for mid and low latitudes and therefore may not always be appropriate for the polar regions.<br />
<br />
One example is parameterizations of the atmospheric boundary layer. These parameterizations perform poorly in the polar regions, where a very stable boundary layer is often present over snow and ice covered surfaces. Parameterizations based on observations of more weakly stable boundary layers at lower latitudes are often not suitable for such conditions. As a result, where conditions are very stable and stratified, the computed fluxes of momentum, heat and water vapour are often too small.<br />
<br />
Climate models also do not include parameterizations for polar stratospheric clouds (PSC), which are the thin, tenuous clouds found in the stratosphere. Yet PSCs are known to be important for the radiation balance and temperature of the atmosphere between the mid/upper troposphere and the lower stratosphere.<br />
<br />
Some of the CMIP3 models do not include the effects of stratospheric ozone, which has a significant impact on their representation of Southern Hemisphere wind patterns (Miller et al., 2006<ref name="Miller et al, 2006">Miller, R.L., Schmidt, G.A. and Shindell, D.T. 2006. Forced annular variations in the 20<sup>th</sup> century Intergovernmental Panel on Climate Change Fourth Assessment Report models, J. Geophys. Res., 111, doi:10.1029/2005JD006323.</ref>). Incorporating stratospheric ozone affects the strength of the circumpolar westerlies in climate models as expected, given that we now understand that the creation of the ozone hole helped to increase the strength of the circumpolar westerlies.<br />
<br />
===Ocean models===<br />
<br />
Ocean models also use a set of equations to represent the evolution of the ocean in response to climate change at specific time and space intervals. Baroclinic eddies in the ocean are smaller than those in the atmosphere, averaging around 100 km across, and to capture their variability the resolution of these models has to be higher than for atmosphere models. The grid spacing for the CMIP3 ocean models is around 100 km in the horizontal; as a result they do not simulate ocean eddy behaviour well. This is an important constraint given the key role of ocean eddies in north-south heat transport. This grid spacing is also too coarse to accurately represent the effect of narrow topographic ridges on the seabed in steering currents.<br />
<br />
A particularly challenging phenomenon at high latitudes is the calculation of heat and moisture fluxes from the ocean to the atmosphere during cold air outbreaks from the continent. During such events very cold polar continental air can come into contact with relatively warm ocean surfaces. The flux parameterizations are difficult to verify due to the challenge of making observations in such conditions.<br />
<br />
===Sea ice===<br />
<br />
The formation and melting of sea ice is a complex process and has important feedbacks onto the ocean and atmosphere. Most sea ice models include simplistic representation of the thermodynamic energy transfer between ocean and atmosphere. The most notable difference between the sea ice models in the CMIP3 set is in their treatment of deformation and flow (rheology). The rheology used ranges from simple ocean drift models to more advanced Elastic Viscous Plastic (EVP) schemes. For more details see page 606 of the AR4 Working Group 1 report (IPCC, 2007<ref name="IPCC, 2007">IPCC 2007. Climate Change 2007: The Physical Science Basis. Contribution of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge.</ref>).<br />
<br />
===Terrestrial ice and snow===<br />
<br />
Most global climate models (including the CMIP3 models) represent non-interactively the dynamics of the large ice sheets that cover much of Antarctica. Their presence is implicit in the land surface orography and surface albedo. There is also no explicit representation of glaciers. Some modelling studies covered by IPCC used ice sheet models run offline and forced by output from climate models to assess future mass balance evolution (e.g. Gregory and Huybrechts, 2006<ref name="Gregory and Huybrechts, 2006">Gregory, J. and Huybrechts, P. 2006. Ice-sheet contributions to future sea-level change, Philosophical Transactions of the Royal Society A, 1709-1731.</ref>). However, these ice sheet models do not currently include the effects of moving ice sheets and glaciers. The discharge of ice into the ocean due to dynamical effects (mechanical breakup of ice sheets and glaciers plus lubrication of the bed by subsurface hydrological systems) is a large uncertainty at present, but must be addressed through models in due course because of its potential impact on sea-level rise (Rignot et al., 2005<ref name="Rignot et al, 2005">Rignot, E., Casassa, G., Gogineni, P., Kanagaratnam, P., Krabill, W., Pritchard, H., Rivera, A., Thomas, R., Turner, J. and Vaughan, D.G. 2005. Recent ice loss from the Fleming and other glaciers,Wordie Bay, West Antarctic Peninsula, Geophys. Res. Let., 32, doi:10.1029/2004GL021947.</ref>; Pfeffer et al., 2008<ref name="Pfeffer et al, 2008">Pfeffer, W.T, Harper, J.T. and O&rsquo;Neill, S. 2008. Kinematic Constraints on Glacier Contributions to 21<sup>st</sup>-Century Sea-Level Rise, ''Science'', '''321''', 1340-3.</ref>).<br />
<br />
===Regional climate models===<br />
<br />
In order to examine regional climate, stretched-grid global climate models and nested regional models can be used to provide the benefits of high resolution modelling in a region of interest without the computational expense of running a climate model with high resolution globally. One key improvement that can be gained from a regional climate model is the representation of the effects of steep and high orography, especially in mountainous areas like the Antarctic Peninsula, or along steeply rising coasts. This can give improved regional detail of precipitation (Berg and Avery, 1995<ref name="Berg and Avery, 1995">Berg, W. and Avery, S.K. 1995. Evaluation of monthly rainfall estimates derived from the special sensor microwave/imager (SSM/I) over the tropical Pacific, ''J. Geophys. Res.'', '''100''', 1295-1315.</ref>) and winds (van Lipzig et al., 2004b<ref name="Lipzig et al, 2004b">Van Lipzig, N.P.M., Turner, J., Colwell, S.R. and. Van Den Broeke, M.R. 2004b. The near-surface wind field over the Antarctic continent, ''International Journal of Climatology'', '''24''', 1973-1982.</ref>). Due to the sparse observation network over Antarctica it is difficult to determine whether or not regional climate models might improve Antarctic-wide precipitation projections.<br />
<br />
===Model evaluation===<br />
<br />
It is difficult to evaluate the performance of a climate model, because long-term projections cannot be checked against observations. As a substitute, the performance of climate models is assessed by having them simulate the climate of the Twentieth Century and comparing their output with observations (see chapter 8 of the AR4 report, IPCC, 2007<ref name="IPCC, 2007">IPCC 2007. Climate Change 2007: The Physical Science Basis. Contribution of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge.</ref>). Even so, there is no guarantee that a climate model that does a good job of replicating the observed mean climate will be realistic in terms of its sensitivity to future forcing. At high latitudes differences in surface temperature between simulations and observations are significantly correlated with projected temperature change under future scenarios (R&auml;is&auml;nen, 2007<ref name="R&auml;is&auml;nen, 2007">R&auml;is&auml;nen, J. 2007. How reliable are climate models? Tellus, 59A, 2-29.</ref>). This may be caused, at least in part, by sea ice biases (differences in sea ice concentration between simulations and observations of the present day), which have a strong impact on projected regional changes in the sea ice zone. This implies that for reliable projections of future changes an accurate simulation of current climate is particularly important at high latitudes.<br />
<br />
====Sea ice in the CMIP3 models====<br />
<br />
All models produce a seasonal cycle of sea ice with a peak in approximately the right season, though HadCM3 is a month late and NCAR CCSM two months early. IAP FGOALS has vastly over extensive ice, so that even the summer minimum would be off the scale used for the other plots. A more effective way to rank the relative success of the different models against observations is to use a measure based on the pointwise root mean square difference from the statistical average (the climatology). This shows that the MRI, CSIRO, HADGEM and MIROC_hires models are the best, although even the best scores are low. Clearly, a good simulation of Antarctic sea ice is a difficult challenge for a GCM. Most of the models use a viscous-plastic (VP) or EVP rheology; CSIRO uses cavitating fluid; and HADCM3 and MRI implement &quot;ocean drift&quot;. Only the INM model has no ice advection. The best performing model is MRI, which has the most primitive &quot;rheology&quot;. However, the MRI model is flux-corrected globally, and this is likely to strongly affect the sea ice simulation. The next best, CSIRO, uses the relatively simple cavitating fluid rheology. This illustrates the fact that many aspects of the model simulation besides sea ice model quality go into making up the simulation of the sea ice. If all else is equal, a more sophisticated and physically plausible scheme would be preferable. Parkinson et al. (2006<ref name="Parkinson et al, 2006">Parkinson, C.L., Vinnikov, K.Y. and Cavalieri, D.J. 2006. Evaluation of the simulation of the annual cycle of Arctic and Antarctic sea ice coverages by 11 major global climate models, J. Geophys. Res., 111, doi:10.1029/2005JC003408.</ref>) note that some of these models, especially CSIRO, show rather lower skills in the Northern Hemisphere, and suggest that there may be some tuning to one hemisphere or the other; we have only examined the Southern Hemisphere in this report.<br />
<br />
The unweighted model average displays significantly higher skill (0.42) than any of the individual models (Connolley and Bracegirdle, 2007<ref name="Connolley and Bracegirdle, 2007">Connolley, W.M. and Bracegirdle, T.J. 2007. An Antarctic assessment of IPCC AR4 coupled models, ''Geophys. Res. Lett.'', '''34''', L22505, doi:10.1029/2007GL031648.</ref>), presumably due to cancellation of errors (Parkinson et al., 2006<ref name="Parkinson et al, 2006">Parkinson, C.L., Vinnikov, K.Y. and Cavalieri, D.J. 2006. Evaluation of the simulation of the annual cycle of Arctic and Antarctic sea ice coverages by 11 major global climate models, J. Geophys. Res., 111, doi:10.1029/2005JC003408.</ref>). The implication is that it is better to use a multi-model average (even un-weighted) than to use just the single best model. This is akin to using the ensemble approach in weather forecasting. It is accepted that the models have some severe limitations (not least because of their coarse resolution), and that the process of averaging may not produce a more accurate result. Despite these caveats the models are likely to be improvements on simple projections of current trends, as they take a large number of parameters into consideration in an integrated way.<br />
<br />
====Temperature====<br />
<br />
[[File:Figure 5.2 - Temperature trends for 1960-2000 for winter (JJA).png|thumb|'''5.2''' Temperature trends in &deg;C/decade from 1960&ndash;2000 for winter (JJA). (a) Unweighted average of 19 models. (b) Weighted average, using weightings accorded to the individual models on the basis of their skill at reproducing modern conditions. Blue dots in (b) are the locations of the maximum trends from the individual models; four models position the maximum trend west of the Peninsula; two to the east of the Peninsula; four in the Weddell Sea; three in the seas around East Antarctica; three over the continent itself (although the magnitudes of these three trends are small); one on the Ross ice shelf and two in the Ross Sea (adapted from Connolley and Bracegirdle, 2007<ref name="Connolley and Bracegirdle, 2007">Connolley, W.M. and Bracegirdle, T.J. 2007. An Antarctic assessment of IPCC AR4 coupled models, ''Geophys. Res. Lett.'', '''34''', L22505, doi:10.1029/2007GL031648.</ref>).]]<br />
Station observations (Turner et al, 2005a<ref name="Turner et al, 2005a">Turner, J., Colwell, S.R., Marshall, G.J., Lachlan-Cope, T.A., Carleton, A.M., Jones, P.D., Lagun, V., Reid, P.A. and Iagovkina, S. 2005a. Antarctic climate change during the last 50 years, ''International Journal of Climatology'', '''25''', 279-294.</ref>) show a maximum increasing temperature trend since 1958 on the west side of the Antarctic Peninsula, with smaller and generally non-significant changes around East Antarctica. Much of West Antarctica has no station observations and therefore satellite data have been interpolated to assess temperature changes in the region. Ongoing studies show significant warming trends that extend beyond the Peninsula region across most of West Antarctica (Monaghan et al., 2008<ref name="Monaghan et al, 2008">Monaghan, A.J., Bromwich, D.H., Chapman, W. and Comiso, J.C. 2008. Recent variability and trends of Antarctic near-surface temperature, J. Geophys. Res., 113D04105, doi:10.1029/2007JD009094.</ref>; Steig et al., 2009<ref name="Steig et al, 2009">Steig, E.J., Schneider, D.P., Rutherford, S.D., Mann, M.E., Comiso, J. C. and Shindell, D.T. 2009. Warming of the Antarctic ice-sheet surface since the 1957 International Geophysical Year, ''Nature'', '''457''', 459-462.</ref>). Since long-term observations are available for temperature trends, and because trends are less stable than means over shorter periods, we use data from 1960 onwards to evaluate the temperature trends. The un-weighted model average, for June &ndash; August (JJA) of 1960-1999, reproduces a maximum warming in winter over the Peninsula, but not extending over West Antarctica ([[:File:Figure 5.2 - Temperature trends for 1960-2000 for winter (JJA).png|Figure 5.2]]).<br />
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The weighted average maximum winter trend is most similar to observations and reconstructions, with generally more warming over West Antarctica and the Peninsula compared to East Antarctica ([[:File:Figure 5.2 - Temperature trends for 1960-2000 for winter (JJA).png|Figure 5.2]]). The trend over the Peninsula (at 65&ordm;S, 70&ordm;W) is 0.38eC/decade (unweighted) or 0.45&ordm;C/decade (weighted), both of which are smaller than the observed value at Faraday/Vernadsky of approximately 1/C/decade. In this context, the observed winter temperature at Faraday/Vernadsky is highly variable and the trend depends strongly on the exact start date chosen. In the weighted average (Figure 5.2b) the trends around East Antarctica increase somewhat but remain fairly small; warming over the continent itself increases somewhat. Observations show small and insignificant cooling at the pole, and smaller and insignificant warming at Vostok.<br />
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[[File:Figure 5.3 - Surface temperature trends for 1960-2000 for winter (JJA) for the MPI ECHAM5 model.png|thumb|'''5.3''' Surface temperature trends in &deg;C/decade from 1960&ndash;2000 for winter (JJA) for the MPI ECHAM5 model. The four different runs all follow the IPCC climate of the Twentieth Century (20C3M) experiment and use the same anthropogenic forcings.]]<br />
There is a large scatter in the regional trends seen in different climate models (see Figure 2 of Connolley and Bracegirdle, 2007<ref name="Connolley and Bracegirdle, 2007">Connolley, W.M. and Bracegirdle, T.J. 2007. An Antarctic assessment of IPCC AR4 coupled models, ''Geophys. Res. Lett.'', '''34''', L22505, doi:10.1029/2007GL031648.</ref>). However, there is also a large scatter within different runs of a single model, e.g. MPI ECHAM5 with the same external forcing; [[:File:Figure 5.3 - Surface temperature trends for 1960-2000 for winter (JJA) for the MPI ECHAM5 model.png|Figure 5.3]] shows winter (JJA) surface temperature trends from four different ensemble runs of ECHAM5/MPI-OM all using the same Twentieth Century anthropogenic forcings. The differences are due to the internal variability of the model. Two runs (run 1 and run 3) show large winter warming to the west of the Peninsula as seen in observations. However, run 2 shows maximum trends over the Ross Sea and relatively small trends to the west of the Peninsula. There are also differences over the high interior of Antarctica, with some runs showing a slight warming and others a slight cooling. This indicates that the internal variability of both climate models and the real world may contribute to differences between observations and climate models on a regional scale. That makes it difficult to verify climate model simulations of regional-scale variability such as Antarctic Peninsula temperature changes. All the large model trends are over the sea ice rather than the continent, and are closely related to sea ice changes. In this they are behaving realistically, in that the observed winter trends around the Peninsula are believed to be reinforced by sea ice feedbacks (King, 1994<ref name="King, 1994">King, J.C. 1994. Recent variability in the Antarctic Peninsula, ''International Journal of Climatology'', '''14'''(4), 357-369.</ref>).<br />
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====Surface Mass Balance====<br />
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Estimates of Antarctic surface mass balance (SMB) vary (Uotila et al., 2007<ref name="Uotila et al, 2007">Uotila P., Lynch, A.H., Cassano, J.J. and Cullather, R.I. 2007. Changes in Antarctic net precipitation in the 21<sup>st</sup> century based on Intergovernmental Panel on Climate Change (IPCC) model scenarios, ''J. Geophys. Res.'', '''112''', D10107, doi:10.1029/2006JD007482.</ref>). For this report we use a central value of 167 mm/yr water equivalent from Vaughan et al. (1999<ref name="Vaughan et al, 1999">Vaughan D.G., Bamber, J.L., Giovinetto, M.B., Russell, J. and Cooper, A.P.R. 1999. Reassessment of net surface mass balance in Antarctica, Journal of Climate, 12, No. 4, 933-946.</ref>) with a spread of 30 mm/yr, which recognises the considerable variability in estimates from observations and models, and also allows for interannual variation. Typically, models indicate that this SMB is made up mostly of precipitation, with sublimation removing approximately 10-20%. Other studies show that blowing snow and melt (which are ignored here) are small on a continental scale. Nine models have SMB within 15 mm/yr of 167. IAP_FGOALS greatly overestimates (500 mm/yr final value); the GISS models (despite having a large value for sublimation) and MRI overestimate by about 100 mm/yr, but for different reasons: GISS&rsquo;s have a &quot;central desert&quot; area that is too small; whereas MRI does not simulate the very low values of SMB in the interior. Only the MIROC_medres model (116) and HADGEM model (131) substantially underestimate SMB. Of those models that do well on overall totals, two (BCCR and CNRM) produce SMB simulations that are implausible. They fail to produce large (&gt; 500 mm/yr) SMB on and around the coast of East Antarctica.<br />
==References==<br />
<references /><br />
[[Category:The next 100 years]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_impact_of_global_climate_change_in_polar_marine_environmentsThe impact of global climate change in polar marine environments2014-08-06T14:34:13Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Marine biology over the next 100 years]]''<br />
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Given the differences in topography, substrates, freshwater input and glaciation history, the Antarctic and Arctic Oceans and their organisms are likely to respond differently to climate change. Laboratory experimental work has suggested that some of the more common species in the shallows are highly stenothermal (Peck, 2005<ref name="Peck, 2005">Peck, L.S. 2005. Prospects for survival in the Southern Ocean: vulnerability of benthic species to temperature change, ''Antarctic Sci.'', '''17''', 497-507.</ref>). Small temperature differences (just a degree or two) may have great impacts on the physiology of stenothermal organisms as well as on the extent of sea ice, hence on the life history and biology of many species (but see Barnes and Peck, 2008<ref name="Barnes and Peck, 2008">Barnes, D.K.A. and Peck, L.S. 2008. Vulnerability of Antarctic shelf biodiversity to predicted climate change, ''Climate Research'', '''37''', 149-163.</ref>). Although projected climate change should alter the situation, the polar regions currently offer an important opportunity to study species biodiversity and ecosystem functioning in environments largely undisturbed by humans. This is mainly the case in the Antarctic, where national territorial claims are still not applied, and international initiatives and organisations, e.g. the Antarctic Treaty System and the Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR), prevent, or at least limit, commercial activities (exploitation of natural resources, industry, fishery, etc) with their consequent anthropogenic impacts. The main direct influences on the Antarctic marine ecosystem are likely to come from global climate change in the mid- to long term. It has to be recognised from the outset that the ecosystem has been radically disturbed by the effects of historic whaling and sealing, and that while much of the fur seal population may have recovered since then, whale populations are still severely depleted compared with former times. After a short phase of overfishing demersal fish stocks (rock cod and icefish) around the Antarctic Peninsula and Scotia Arc, the populations collapsed, but legal and illegal fishing continues, especially for Patagonian and Antarctic Toothfish as well as for krill.<br />
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Climate change is already having significant impacts on global marine and terrestrial systems (Hughes, 2000<ref name="Hughes, 2000">Hughes, L. 2000. Biological consequences of global warming: is the signal already apparent?, ''Trends Ecol. Evol.'', '''15''', 56-61.</ref>; Walther et al., 2002<ref name="Walther et al, 2002">Walther, G-R., Post, E., Convey, P., Menzel, A., Parmesan, C., Beebee, T.J.C, Fromentin, J-M., Hoegh-Guldberg, O. and Bairlein, F. 2002. Ecological responses to recent climate change, ''Nature'', '''416''', 389-395.</ref>), and will continue to influence biological diversity. Many species are susceptible to climate change, and those of the marine environment are particularly vulnerable, even though warming is more evident in the air than in the sea as is evident from IPCC reports. The polar regions are undergoing more rapid environmental changes than elsewhere, in many instances due to the combined effects of natural climate change and human activity. However, these changes are much more evident in the Arctic than in the Antarctic except west of the Antarctic Peninsula.<br />
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Present patterns of biodiversity and distribution are a consequence of processes working on both evolutionary and ecological timescales. Among the physical and chemical factors controlling distribution and biodiversity of the modern polar marine fauna, the most important are ice scour, topography, substrate, temperature, currents, ice cover, oxygen, light, UVB, wind, and nutrients. Besides being largely interconnected, some of these factors are not constant, and vary over a range of temporal scales from less than daily through seasonal to inter-annual. Variability is of fundamental importance to ecosystem dynamics. The system may be disrupted if the pattern of environmental variability is upset.<br />
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The most important anthropogenic changes currently affecting the Antarctic are accelerated global warming and increased UV-B levels resulting from the ozone hole that develops in spring. Illegal and unregulated fishing and the introduction of alien species constitute further threats, although they are more limited in geographic scope. Pollution associated with scientific activities and ships, and visitor pressure from the growing tourism industry have very localised effects on community structure and diversity. Many of these changes have complex and interacting effects. For example, an impact on the lowest or highest level in a food web can propagate through to affect other taxa indirectly. Thus UV-B impact on primary producers may affect consumers and higher levels in the food web, while the extraction of the great whales undoubtedly had an effect that has cascaded down through lower levels.<br />
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Around the western side of the Antarctic Peninsula, which is currently subject to one of the fastest rates of climate change anywhere on the planet (Cook et al., 2005<ref name="Cook et al, 2005">Cook, A., Fox, A., Vaughan, D. and Ferrigno, J. 2005, Retreating glacier fronts on the Antarctic Peninsula over the past half-century, ''Science'', '''308''', 541-544.</ref>), there has been a considerable reduction of annual mean sea ice extent (reviewed in Clarke et al., 2007<ref name="Clarke et al, 2007">Clarke, A., Murphy, E.J., Meredith, M.P., King, J.C., Peck, L.S., Barnes, D.K.A. and Smith, R.C. 2007. Climate change and the marine ecosystem of the western Antarctic Peninsula, ''Phil. Trans. R. Soc. B'', '''362''', 149-166.</ref>). There are indications that populations of ''Pleuragramma antarcticum'', a key fish species of the trophic web, and whose reproduction is closely associated to sea ice, declined locally, to be replaced by myctophids, a new food item for predators (M. Vacchi, pers. com.; W.R. Fraser, Regional loss of Antarctic Silverfish from the western Antarctic Peninsula food web, in preparation). This change is thought to have been caused by seasonal changes in sea ice dynamics compromising reproduction processes.<br />
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Temperature trends elsewhere in Antarctica show little change or, in some places, a cooling that may be accompanied by local impacts, as in the lakes and soils of the Dry Valleys. There is no evidence of a continent-wide &ldquo;polar amplification&rdquo; similar to that predicted in the Arctic (Dyurgerov and Meier, 2000<ref name="Dyurgerov and Meier, 2000">Dyurgerov, M.B. and Meier, M.F. 2000. Twentieth century climate change: evidence from small glaciers, ''Proc. Natl. Acad. Sci. USA'', '''97''', 1406-1411.</ref>; Oechel et al., 2000<ref name="Oechel et al, 2000">Oechel, W.C., Vourlitis, G.L., Hastings, S.J., Zulueta, R.C., Hinzman, L. and Kane, D. 2000. Acclimation of ecosystem CO<sub>2</sub> exchange in the Alaskan Arctic in response to decadal climate warming, ''Nature'', '''406''', 978-981.</ref>; Romanovsky et al., 2002<ref name="Romanovsky et al, 2002">Romanovsky, V.E., Burgess, M., Smith, S., Yoshikawa, K. and Brown, J. 2002. Permafrost temperature records: indicators of climate change, ''EOS Transactions'', '''83''', 589-594.</ref>; Lemke et al., 2007<ref name="Lemke et al, 2007">Lemke, P., Ren, J., Alley, R., Allison, I., Carrasco, J., Flato, G., Fujii, Y., Kaser, G., Mote, P., Thomas, R. and Zhang, T. 2007. Observations: change in snow, ice and frozen ground. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 337-384 (Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M, Miller HL, Eds), Cambridge University Press, Cambridge, UK.</ref>). On balance this overall lack of change would not be expected to result in significant biological change, even in the open Southern Ocean, which has warmed by some 0.2&ordm;C. Sea ice, which has a significant relation to the ecosystem, has increased in the Ross Sea, and decreased in the Bellingshausen and Amundsen Sea, with local effects on the ecosystem. Acidification is beginning to occur in the ocean, slightly changing the chemistry, which on the basis of laboratory experiments is expected to first affect organisms with aragonite skeletons, such as pteropods, and ultimately to reduce the uptake of carbon dioxide from the atmosphere. As yet there is no evidence for any large-scale change in the Antarctic ecosystem associated with this effect. For many species, uncertainty in climate predictions leads to uncertainty in projecting impacts; however, continued warming and winter sea ice decrease are likely to affect reproduction cycles and the growth of fish, krill and benthos, possibly leading to declines in some populations and changes in their distributions.<br />
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In areas experiencing warming, increases have been recorded in sponge species (with extreme natural variability in recruitment and exceptionally fast growth) and their predators, and decreases in krill, Ad&eacute;lie and Emperor penguins and Weddell seals (Ainley et al., 2005<ref name="Ainley et al, 2005">Ainley, D.G., Clarke E.D., Arrigo, K., Fraser, W.R., Kato, A., Barton, K.J. and Wilson, P.R. 2005. Decadal-scale changes in the climate and biota of the Pacific sector of the Southern Ocean, 1950s to the 1990s. Antarctic Science, 17, 171-182.</ref>). The reduction in krill biomass and the increase in abundance of salps (gelatinous pelagic organisms) have been suggested to be linked to regional decreases in sea ice (Loeb et al., 1997<ref name="Loeb et al, 1997">Loeb, V., Siegel, V. Holm-Hansen, O., Hewitt, R., Fraser, W., Trivelpiece, W. and Trivelpiece, S. 1997. Effects of sea-ice extent and krill or salp dominance on the Antarctic food web, ''Nature'', '''387''', 897-900.</ref>) that may also underlie recent changes in the demography of krill predators, e.g. mammals and birds (Fraser and Hofmann, 2003<ref name="Fraser and Hofmann, 2003">Fraser, W.R. and Hofmann, E.E. 2003. A predator&rsquo;s perspective on causal links between climate change, physical forcing and ecosystem response, ''Marine Ecology Progress Series'', '''265''', 1-15.</ref>). Examination of growth of some seabed suspension feeders over the last two decades has revealed recent increases in annual growth rates in one species but little change or decreases in other similar species (see Barnes et al., 2007<ref name="Barnes et al, 2007">Barnes, D.K.A., Webb, K.E. and Linse, K. 2007. Growth rate and its variability in erect Antarctic bryozoans, ''Polar Biol.'', '''30''', 1069-1081.</ref>). In all of these cases it is hard to state that any change is definitively due to climate change, but evidence is mounting. There are signs that warming air temperatures have had negative impacts on the local biota on some sub-Antarctic islands. Signy Island and some sites at the West Antarctic Peninsula have witnessed an explosion of the fur-seal numbers that may be related to decreased ice cover resulting in increasing areas available for resting and moulting, but which may also be related to population increases on South Georgia; the growing seal population has had deleterious impacts on the local terrestrial vegetation.<br />
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The large reductions in the extent of cover and thickness of sea ice on the western side of the Antarctic Peninsula are potentially devastating to some species. The warming of the sea surface has been accompanied by an increase in phytoplankton in cooler regions (which might actually be beneficial to commercial fish stocks) and a decrease in warmer regions (Richardson and Schoeman, 2004<ref name="Richardson and Schoeman, 2004">Richardson, A.J. and Schoeman, D.S. 2004. Climate impact on ecosystems in the northeast Atlantic, ''Science'', '''305''', 1609-1612.</ref>; Montes-Hugo et al., 2009<ref name="Montes-Hugo et al, 2009">Montes-Hugo, M., Doney, S.C., Ducklow, H.W., Fraser, W., Martinson, D., Stammerjohn, S.E. and Schofield, O. 2009. Recent Changes in Phytoplankton Communities Associated with Rapid Regional Climate Change Along the Western Antarctic Peninsula, ''Science'', '''323''' (5920), 1470-1473.</ref>). If the sea ice cover continues to decrease, as models suggest, such responses to changes will widen, impairing predation processes and affecting community composition and levels of primary and secondary producers. For instance, marine ice algae would disappear due to loss of habitat. That may cause a cascade through higher trophic levels in the food web, diminishing the zooplankton that feed on algae, the fish that feed on zooplankton, and the sea birds and mammals that feed on the fish.<br />
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The responsiveness of species elsewhere to recent and future climate change raises the possibility that human influence may cause a major extinction event for some vulnerable species; we must consider such a possibility, though with due care. Thomas C D et al. (2004) analysed the global extinction risk from climate warming and concluded that many of today&rsquo;s species could be driven to extinction by climate change over the next 50 years. Such analyses provide compelling arguments for the development of policies aimed at reducing the impact of warming due to human activity. Greater and more rapid warmings have occurred around Antarctica before, during interglacial periods. Extinctions and radiations of species occur continuously, and most current species have probably survived through the climate changes of one or more glacial cycles (we can&rsquo;t be certain how many because the fossil record of many areas around Antarctica is very poor). However, projected warming and rates exceed those of the last eight interglacial warm periods. Given complete disappearance of sea ice we would expect extinction of those species that currently depend on it for survival. Climate models suggest that in the Antarctic such a reduction is unlikely within the next 100 years, when instead a 33% reduction in sea ice cover is projected.<br />
==References==<br />
<references /><br />
[[Category:The next 100 years]]<br />
[[Category:Antarctic biology]]<br />
[[Category:Marine biology]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_ice_sheet_in_the_instrumental_periodThe ice sheet in the instrumental period2014-08-06T14:34:13Z<p>Acce: Removed link to Changes in Antarctic permafrost, which now belongs under the main Instrumental period page</p>
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<div>:''This page is part of the topic [[Antarctic climate and environment change in the instrumental period]]''<br />
<br />
==Introduction==<br />
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Ice sheets are to some degree self-regulating systems. Increasing snowfall over the ice sheet will act to increase the ice thickness, but that will then increase the rate of ice-flow towards the coasts and thus remove the extra snowfall. Thus the ice sheet will evolve towards a shape and pattern of flow specific to the current climate, where flow exactly compensates the spatial pattern of ice accumulation (snowfall and frost deposition) and ice ablation (melting, wind erosion and calving).<br />
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This equilibrium state is a useful concept, but it is rarely realised; the driving environmental parameters of climate are themselves constantly changing, which continually modifies the equilibrium state sought by the ice sheet. At any given time, the changes in the ice sheet reflect responses to both recent and long-term changes in climate, and for this reason, areal extent, magnitude and duration of changes, as well as time scale must be carefully considered in any discussion of ice-sheet change.<br />
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Until the era of satellites, large-scale changes in the ice sheet were considered likely to unfold over thousands of years (Payne et al., 2006<ref name="Payne et al, 2006">Payne, A.J., Sammonds, P. and Hunt, J.C.R. 2006, Evolution of the Antarctic ice sheet: new understanding and new challenges, ''Philosophical Transactions of the Royal Society A'', '''364''', 1844.</ref>). Now, significant accelerations and decelerations of large outlet glaciers and ice streams have been observed on much shorter timescales (Bindschadler et al., 2003<ref name="Bindschadler et al, 2003">Bindschadler, R. A., King, M.A., Alley, R.B., Anandakrishnan, S. and Padman, L. 2003, Tidally controlled stick-slip discharge of a West Antarctic ice stream, ''Science'', '''301''', 1087-1089.</ref>; Rignot and Kanagaratnam, 2006<ref name="Rignot and Kanagaratnam, 2006">Rignot, E. and Kanagaratnam, P. 2006. Changes in the Velocity Structure of the Greenland Ice Sheet, Science, 311, no. 5763, 986-990.</ref>; Truffer and Fahnestock, 2007<ref name="Truffer and Fahnestock, 2007">Truffer M. and Fahnestock, M. 2007. Rethinking Ice Sheet Time Scales, ''Science'', '''315''', 1508.</ref>). In Antarctica, these changes affect a complex drainage system of ice streams and tributaries whose full extent has only become appreciated through satellite observations (Joughin et al., 1999<ref name="Joughin et al, 1999">Joughin, I., Gray, L., Bindschadler, R., Price, S., Morse, D., Hulbe, C., Mattar, K. and Werner, C. 1999. Tributaries of West Antarctic ice streams revealed by Radarsat interferometry, ''Science'', '''286''', 283-286, (doi:10.1126/science.286.5438.283).</ref>; Bamber et al., 2000<ref name="Bamber et al, 2000">Bamber, J., Vaughan, D.G. and Joughin, I. 2000. Widespread complex flow in the interior of the Antarctic ice sheet, ''Science'', '''287''', 1248-1250, (doi:10.1126/science.287.5456.1248).</ref>). Since the 1970s, there have been competing hypotheses concerning the influence of ice shelves on the flow rates of upstream glaciers. Satellites have confronted these hypotheses with empirical measurements for the first time. Another important contribution of satellites has been to identify two key regions of change in Antarctica; one near the northern tip of the Antarctic Peninsula, another within the rarely visited Amundsen Sea sector of West Antarctica.<br />
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Despite a good understanding of the complexity of the issues, its importance to understanding sea level rise has meant that measurement of the ice sheet&rsquo;s mass balance has been a primary goal of Antarctic science since the early efforts following the IGY (1957/58). Many of these were based on accounting methods involving calculations of the imbalance between net snow accumulation and outflow of ice over particular domains. Such efforts have always been hampered by the intrinsic uncertainty in measuring these parameters, and very few have produced measurements of ice-sheet imbalance that do not plausibly allow changes in a particular domain to be either positive or negative. So while there have been a few notable exceptions (e.g. Joughin and Tulaczyk, 2002<ref name="Joughin and Tulaczyk, 2002">Joughin, I. and Tulaczyk, S. 2002. Positive mass balance of the Ross Ice Streams, West Antarctica, Science, 295 (Jan 18), No. 5554, 476-480.</ref>; Rignot and Thomas, 2002<ref name="Rignot and Thomas, 2002">Rignot, E. and Thomas, R.H. 2002. Mass balance of polar ice sheets, Science, 297 (5586), 1502-1506 AUG 30 2002.</ref>; Rignot, 2008<ref name="Rignot, 2008">Rignot, E. 2008. Changes in West Antarctic ice stream dynamics observed with ALOS PALSAR data, ''Geophysical Research Letters'', '''35''', L12505, doi:10.1029/2008GL033365.</ref>), and future efforts based on satellite data may prove to be valuable, our best measurements of change across the majority of the Antarctic ice sheet come not from accounting methods, but rather from those techniques that seek to measure the changing volume of the ice-sheet directly.<br />
<br />
[[File:Figure 4.34 - Antarctic ice sheet elevation change measured by satellite altimetry.png|thumb|'''4.34''' Elevation change Zwally (left) and Davis (right).]]<br />
The most successful of these techniques of direct measurement has been the use of satellite altimetry ([[:File:Figure 4.34 - Antarctic ice sheet elevation change measured by satellite altimetry.png|Figure 4.34]]). A number of research groups have evaluated data beginning in the early 1980s. Spanning data from multiple satellite altimeters, they have produced broadly consistent results (e.g., Wingham et al., 1998<ref name="Wingham et al, 1998">Wingham, D.J., Ridout, A.L., Scharroo, R., Arthern, R.J. and Schum, C.K. 1998, Antarctic elevation change from 1990 to 1996, ''Science'', '''282''', 456-458, (doi:10.1126/science.282.5388.456).</ref>, 2006a<ref name="Wingham et al, 2006a">Wingham, D.J., Shepherd, A., Muir, A. and Marshall, G.J. 2006a. Mass balance of the Antarctic ice sheet, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, DOI: 10.1098/rsta.2006.1792.</ref>; Davis et al., 2005<ref name="Davis et al, 2005">Davis, C.H., Li Y., McConnell, J.R., Frey, M.M. and Hanna, E. 2005. Snowfall-driven growth in East Antarctic Ice Sheet mitigates recent sea-level rise, ''Science'', '''308''', 1898-1901.</ref>; Zwally et al., 2005<ref name="Zwally et al, 2005">Zwally, H.J., Giovinetto, M., Li, J., Cornejo, H., Beckley, M., Brenner, A., Saba, J. and Yi, D. 2005. Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002, ''Journal of Glaciology'', '''51'''(175), 509-527.</ref>). These results illustrate that separate catchment basins within the ice sheet behave somewhat independently. What altimetry often fails to capture are the largest changes at the ice sheet margins, where steep slopes lead to large errors of measurement, and the large thickness changes on the floating ice shelves at the perimeter, where elevation changes represent only 1/8 of the full thickness change.<br />
<br />
More recently, measurements of changes in the Earth&rsquo;s gravity field have been made using GRACE (Gravity Recovery and Climate Experiment). These satellites have confirmed that the Amundsen Sea sector is losing mass to the ocean (Velicogna and Wahr, 2006<ref name="Velicogna and Wahr, 2006">Velicogna, I. and Wahr, J. 2006, Measurements of Time-Variable Gravity Show Mass Loss in Antarctica, Science, 311, (5768), 1754 DOI:10.1126/science.1123785.</ref>; Ramillien et al., 2006<ref name="Ramillien et al, 2006">Ramillien, G., Lombard, A., Cazenave, A., Ivins, E.R., Llubes, M., Remy, F. and Biancale, R. 2006. Interannual variations of the mass balance of the Antarctica and Greenland ice sheets from GRACE, ''Global and Planetary Change'', '''53''', 198-208.</ref>; Chen et al., 2006<ref name="Chen et al, 2006">Chen, J. L., Wilson, C.R., Blankenship, D.D. and Tapley, B.D. 2006. Antarctic mass rates from GRACE, ''Geophys. Res. Lett.'', '''33''', L11502, doi:10.1029/2006GL026369.</ref>). The GRACE system works by tracking the range between two orbiting satellites: their differential accelerations provide a sensitive measure of how the distribution of mass across the Earth&rsquo;s surface is changing. The system is unable to distinguish separately the changes in ice, rock, and air; so the flows within the Earth&rsquo;s mantle and atmosphere must be removed to reveal changes in the ice sheets. So far this correction has been derived using models of the atmospheric and lithospheric mass flows, but observations of isostatic uplift measured in the field using GPS receivers mounted on exposed rock outcrops should provide a better constraint, especially if GRACE observations are combined with altimetric observations (Velicogna and Wahr, 2002<ref name="Velicogna and Wahr, 2002">Velicogna, I. and Wahr, J. 2002. A method for separating Antarctic postglacial rebound and ice mass balance using future ICESat Geoscience Laser Altimeter System, Gravity Recovery and Climate Experiment, and GPS satellite data, J. Geophys. Res., 107, 10.1029/2001JB000708.</ref>). Chen et al. (2006<ref name="Chen et al, 2006">Chen, J. L., Wilson, C.R., Blankenship, D.D. and Tapley, B.D. 2006. Antarctic mass rates from GRACE, ''Geophys. Res. Lett.'', '''33''', L11502, doi:10.1029/2006GL026369.</ref>) report a localised region of mass increase in East Antarctica. This could either be anomalously high snowfall, leading to growth of ice in this region, or an artefact of un-modelled post-glacial rebound. The system has only been in operation for a few years, and snowfall is variable from year to year, so the long-term significance of the available results can be questioned. However there is little doubt that longer records from these satellites will eventually provide extremely valuable information on the changes in the mass of ice sheets.<br />
<br />
Antarctica&rsquo;s ice shelves and ice tongues (&lsquo;ice tongues&rsquo; are narrowly-confined ice shelves bounded by fjord walls) are particularly sensitive to climate change because both the upper and lower surface of the ice plate are exposed to different systems, each with a potential to cause rapid change: these are the atmosphere and the ocean. The ice-atmosphere system affects the ice shelf through changing surface accumulation, dust and soot deposition, and surface melt; the ice-ocean system controls basal melting or freezing, tidal flexure, and wave action. Moreover, it is now recognized that sea ice, which is not part of the ice sheet, but is viewed more properly as a component of the atmosphere-ocean system, has a large impact on the local climate and dynamics of the ice shelf front, controlling regional surface energy balance, moisture flux, and the presence or absence of ocean swell at the front.<br />
<br />
Antarctica&rsquo;s ice shelves have provided the most dramatic evidence to date that at least some regions of the Antarctic are warming significantly, and have shown, what has been suspected for long time (e.g. Mercer, 1978<ref name="Mercer, 1978">Mercer, J.H. 1978. West Antarctic ice sheet and CO<sub>2</sub> greenhouse effect: a threat of disaster, ''Nature'', '''271''', 321-325.</ref>), namely that changes in floating ice shelves can cause significant changes in the grounded ice sheet. This evidence has led to an ongoing re-assessment of how quickly greenhouse-driven climate changes could translate into sea level rise (Meehl et al, 2007<ref name="Meehl et al, 2007">Meehl, G.A., T.F. Stocker, W.D. Collins, P. Friedlingstein, A.T. Gaye, J.M. Gregory, A. Kitoh, R. Knutti, J.M. Murphy, A. Noda, S.C.B. Raper, I.G. Watterson, A.J. Weaver, and Z.-C. Zhao, 2007: Global Climate Projections. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 749-845.</ref>).<br />
<br />
Ice shelves in two regions of the Antarctic Ice Sheet have shown rapid changes in recent decades: the Antarctic Peninsula and the northern region of West Antarctica draining into the Amundsen Sea. Because there is such spatial variety in the observed changes, and because the causes of the changes likely vary regionally, changes over the past 50 years are discussed below according to province. A review of events in these two regions will introduce two main mechanisms by which climate change is thought to lead to changes in ice sheet mass balance: surface summer melt increase, leading to ponding, fracturing and disintegration: and basal melting of the ice shelves, leading to shelf thinning and grounding line retreat.<br />
<br />
==Calving==<br />
<br />
Aside from the catastrophic ice shelf disintegration events discussed below, available data suggest that major rift-driven calving events have neither increased nor decreased on the major ice shelves (the Ross, Flichner-Ronne, or Amery). Rather there is ample evidence that their calving patterns continue to follow quasi-repetitive patterns extending back to the Nineteenth Century, when the ice fronts were first mapped (e.g. Jacobs et al., 1986<ref name="Jacobs et al, 1986">Jacobs, S.S., Macayeal, D.R. and Ardai, J.L., Jr., 1986. The recent advance of the Ross Ice Shelf, Antarctica, ''Journal of Glaciology'', '''32'''(112), 464-474.</ref>; Keys et al, 1990<ref name="Keys et al, 1990">Keys, H., Jacobs, S. and Barnett, D. 1990. The calving and drift of iceberg B-9 in the Ross Sea, Antarctica, ''Antarctic Science'', '''2'''(3), 243-257.</ref>; Lazzara et al., 1999<ref name="Lazzara et al, 1999">Lazzara, M.A., Jezek, K.C., Scambos, T.A., Macayeal, D.R. and Van Der Veen, C.J. 1999. On the recent calving of icebergs from the Ross Ice Shelf, ''Polar Geography'', '''23'''(3), 201-212.</ref>; Budd, 1966<ref name="Budd, 1966">Budd, W. 1966. The dynamics of the Amery Ice Shelf, ''Journal of Glaciology'', '''6''', 335-358.</ref>; Fricker et al., 2005<ref name="Fricker et al, 2005">Fricker, H.A., Young, N.W., Coleman, R., Bassis, J.N. and Minster, J.-B. 2005. Multi-year monitoring of rift propagation on the Amery Ice Shelf, East Antarcitca, ''Geophysical Research Letters'', '''32''', L02502, doi:10.1029/2004GL021036.</ref>; see also Frezzotti and Polizzi, 2002<ref name="Frezzotti and Polizzi, 2002">Frezzotti, M. and Polizzi, M. 2002. 50 years of ice-front changes between the Ad&eacute;lie and Banzare Coasts, East Antarctica, ''Annals of Glaciology'', '''34''', 235-240.</ref>, and Kim et al., 2007<ref name="Kim et al, 2007">Kim, K., Jezek, K.C. and Liu, H. 2007. Orthorectifield image mosaic of Antarctica from 1963 Argon satellite photography: image processing and glaciological applications, ''International Journal of Remote Sensing'', '''28'''(23), 5357-5373, doi: 10.1080/01431160601105850.</ref>). So while the periodic calving of massive icebergs that appear to represent, in some cases, many decades of ice shelf advance, may appear dramatic, there is no reason to believe that they are not part of the normal fluctuations in a ice sheet that is, in the long-term, close to equilibrium.<br />
<br />
==Sub-glacial Water Movement==<br />
<br />
[[File:Figure 4.42 - Subglacial elevation and locations of Antarctic sub-glacial lakes.png|thumb|'''4.42''' Locations of Antarctic sub-glacial lakes are indicated by the yellow circles. Over 145 subglacial lakes have been discovered with the majority clustered in the Dome C area of East Antarctica. Colours represent subglacial elevation. There is no clear correspondence between the subglacial topography and subglacial lake occurrence.]]<br />
Regional surface elevation changes confined to areas of a few kilometres have been interpreted as manifestations of subglacial water movements (Gray et al., 2005<ref name="Gray et al, 2005">Gray, L., Joughin, I., Tulazcyk, S., Spikes, V.B., Bindschadler, R. and Jezek, K. 2005. Evidence for subglacial water transport in the West Antarctica Ice Sheet through three-dimensional satellite radar interferometry, Geophysical Research Letters, 32, L03501, doi:10.1029/2004GL021387,.</ref>; Wingham et al., 2006b<ref name="Wingham et al, 2006b">Wingham, D.J., Siegert, M.J., Shepherd, A. and Muir, A.S. 2006b. Rapid discharge connects Antarctic subglacial lakes, Nature, 440, pp. 1033-1036 , doi:10.1038/nature04660.</ref>; Fricker et al., 2007<ref name="Fricker et al, 2007">Fricker, H.A., Scambos, T.A., Bindschadler, R. and Padman, L. 2007. An Active Subglacial Water System in West Antarctica Mapped from Space, ''Science'', '''315''', 1544, doi: 10.1126/science.1136897.</ref>). These observations are interpreted to reflect the activity of a subglacial hydrologic system permitting faster ice flow that is more active than previously thought ([[:File:Figure 4.42 - Subglacial elevation and locations of Antarctic sub-glacial lakes.png|Figure 4.42]]). While those earlier studies lacked simultaneous measurements of ice flow to accompany these likely shifts in water mass, a recent study of Byrd Glacier identified a period of 10% faster flow that fell within a period where the nearby subglacial lakes discharged water that probably exited the system by traveling underneath Byrd Glacier (Stearns et al., 2008<ref name="Stearns et al, 2008">Stearns, L.A., Smith, B.E. and Hamilton, G.S. 2008. Increased flow speed on a large East Antarctic outlet glacier caused by subglacial floods, ''Nature Geoscience'', '''1''', 827-831.</ref>). Improvements to numerical ice flow models are including subglacial water, but the specific nature of its role in ice sheet dynamics remains undetermined.<br />
<br />
==Effects of sedimentation on ice sheet stability==<br />
<br />
Much attention is focused on accelerating changes and instability, yet sediment deposited beneath fast moving outlet glaciers might provide some stability to ice sheet retreat driven by rising sea level. Recent observations and modeling suggest that wedges of sediment deposited near the grounding line may be important in stabilizing the ice sheet against sea level rise (Anandakrishnan et al. 2007<ref name="Anandakrishnan et al, 2007">Anandakrishnan, S., Catania, G.A., Alley, R.B. and Horgan, H.J. 2007. Discovery of till deposition at the grounding line of Whillans Ice Stream, ''Science'', '''315''', 1835-1838.</ref>; Alley et al, 2007<ref name="Alley et al, 2007">Alley, R.B., Anandakrishnan, S., Dupont, T.K., Parizek, B.R. and Pollard, D., 2007, Effect of sedimentation on ice-sheet grounding-line stability. Science, 315, 1838-1841.</ref>). This stabilization is conditional on the position of the grounding line with respect to the crest of the wedge: should the grounding line retreat from the sediment wedge an unstable retreat analogous to that seen in tidewater glaciers could occur (Weertman, 1974<ref name="Weertman, 1974">Weertman, J. 1974. Stability of the junction of an ice sheet and an ice shelf, ''J. Glaciol.'', '''13''', 3-11.</ref>; Schoof, 2007<ref name="Schoof, 2007">Schoof, C. 2007. Marine ice sheet dynamics. Part I: The case of rapid sliding, ''J. Fluid. Mech.'', '''573''', 27-55.</ref>). It is becoming clear that rates of erosion and sediment transport can be large, as shown by the appearance of drumlin-scale sedimentary features underneath the ice sheet within just a few years (Smith et al., 2007a<ref name="Smith et al, 2007a">Smith, A.M., Murray, T., Nicholls, K.W., Makinson, K., A&eth;algeirsd&oacute;ttir, G., Behar, A.E. and Vaughan, D.G. 2007a. Rapid erosion, drumlin formation, and changing hydrology beneath an Antarctic ice stream, ''Geology'', '''35''', 127-130.</ref>). Predictive models of the ice sheet will need to include sediment transport, as well as forces imposed by ice shelves, since these effects may compete to determine the stability or instability of the ice sheet margin.<br />
==Pages in this topic==<br />
#[[The Antarctic Peninsula cryosphere in the instrumental period]]<br />
#[[The West Antarctic cryosphere in the instrumental period]]<br />
#[[The East Antarctic cryosphere in the instrumental period]]<br />
#[[Attribution of ice sheet changes in the instrumental period]]<br />
#[[Conclusions on the ice sheet in the instrumental period]]<br />
==References==<br />
<references /><br />
[[Category:The instrumental period]]<br />
[[Category:The Antarctic ice sheet]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_Icehouse_world_-_the_last_34_million_yearsThe Icehouse world - the last 34 million years2014-08-06T14:34:12Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Deep time]]''<br />
<br />
[[File:Figure 3.5 - Change in average global temperature over the last 80 million years.png|thumb|'''3.5''' Change in average global temperature over the last 80 Ma. The temperature curve of Crowley and Kim (1995<ref name="Crowley and Kim, 1995">Crowley, T.J. and Kim, K. 1995. Comparison of long-term greenhouse projections with the Geologic Record, ''Geophysical Research Letters'', '''22''', 933-936.</ref>) is modified to show the effect of the methane discharge at 55 Ma (Zachos et al., 2003<ref name="Zachos et al, 2003">Zachos, J.C., Wara, M.W., Bohaty, S., Delaney, M.L., Petrizzo, M.R., Brill, A., Bralower, T.J. and Premoli-Silva, I. 2003. A Transient Rise in Tropical Sea Surface Temperature During the Paleocene-Eocene Thermal Maximum, ''Science'', '''302''', 1551-1554.</ref>; Zachos et al., 2005<ref name="Zachos et al, 2005">Zachos, J.C., Rohl, U., Schellenberg, S.A., Sluijs, A., Hodell, D.A., Kelly, D.C., Thomas, E., Nicolo, M., Raffi, I., Lourens, L.J., McCarren, H. and Kroon, D. 2005. Rapid Acidification of the Ocean During the Paleocene-Eocene Thermal Maximum, ''Science'', '''308''', 1611-1615.</ref>). The future rise in temperature expected from energy use projections shows the Earth warming back into the &lsquo;greenhouse world&rsquo;(from Mayewski et al., 2009<ref name="Mayewski et al, 2009">Mayewski, P.A., Meredith, M.P., Summerhayes, C.P., Turner, J., Worby, A.P., Barrett, P.J., Casassa, G., Bertler, N.A.N., Bracegirdle, T.J., Naveira-Garabato, A.C., Bromwich, D.H., Campbell, H., Hamilton, G.H., Lyons, W.B., Maasch, K.A., Aoki, S., and Xiao, C. 2009. State of the Antarctic and Southern Ocean Climate System (SASOCS), ''Reviews of Geophysics'', '''47''', RG1003, doi:10.1029/2007RG000231.</ref>).]]<br />
[[File:Figure 3.6 - Main climatic events of the last 65 million years in the Antarctic context.png|thumb|'''3.6''' Main climatic events of the last 65 million years in the Antarctic context (adapted from Mayewski et al., 2009<ref name="Mayewski et al, 2009">Mayewski, P.A., Meredith, M.P., Summerhayes, C.P., Turner, J., Worby, A.P., Barrett, P.J., Casassa, G., Bertler, N.A.N., Bracegirdle, T.J., Naveira-Garabato, A.C., Bromwich, D.H., Campbell, H., Hamilton, G.H., Lyons, W.B., Maasch, K.A., Aoki, S., and Xiao, C. 2009. State of the Antarctic and Southern Ocean Climate System (SASOCS), ''Reviews of Geophysics'', '''47''', RG1003, doi:10.1029/2007RG000231.</ref>).]]<br />
The first continental-scale ice sheets formed on Antarctica close to the Eocene-Oligocene boundary around 34 Ma), and are physically recorded in strata from Prydz Bay, McMurdo Sound and Seymour Island (Francis et al., 2008b<ref name="Francis et al, 2008b">Francis, J.E., Marenssi, S., Levy, R., Hambrey, M., Thorn, V.T., Mohr, B., Brinkhuis, H., Warnaar, J., Zachos, J., Bohaty, S. and Deconto, R. 2008b. From Greenhouse to Icehouse - The Eocene/Oligocene in Antarctica. In: Florindo F, Siegert M (eds) Antarctic Climate Evolution, Developments in Earth and Environmental Science, Vol 8. Elsevier, Amsterdam.</ref>). Deep-sea isotope data suggest they were similar in size to that of today (Miller et al., 2008<ref name="Miller et al, 2008">Miller, K.G., Wright, J.D., Katz, M.E., Browning, J.V., Cramer, B.S., Wade, B.S. and Mizintseva, S.F. 2008. A View of Antarctic Ice-Sheet Evolution from Sea-Level and Deep-Sea Isotope Changes During the Late Cretaceous-Cenozoic. In: Cooper AK, Barrett P, Stagg H, Storey B, Stump E, Wise W (eds) Antarctica: A Keystone in a Changing World. Proceedings of the 10th International Symposium on Antarctic Earth Sciences, Santa Barbara, California, August 26 to September 1, 2007. Polar Research Board, National Research Council, U.S. Geological Survey.</ref>). According to DeConto and Pollard (2003<ref name="DeConto and Pollard, 2003">Deconto, R.M. and Pollard, D. 2003. Rapid Cenozoic glaciation of Antarctica induced by declining atmospheric CO<sub>2</sub>, ''Nature'', '''421''', 245-249.</ref>) the development of these first Antarctic ice sheets was triggered in an interval when the Earth&rsquo;s orbit of the Sun favoured cool summers as atmospheric CO<sub>2</sub> levels declined below a critical threshold (~2.8 times pre-industrial). This decline has been documented (Pagani et al., 2005<ref name="Pagani et al, 2005">Pagani, M., Zachos, J.C., Freeman, K.H., Tipple, B. and Bohaty, S. 2005. Marked Decline in Atmospheric Carbon Dioxide Concentrations During the Paleogene, ''Science'', '''309''', 600-603.</ref>; Siegert et al., 2008<ref name="Siegert et al, 2008">Siegert, M.J., Barrett, P., Deconto, R., Dunbar, R., Cofaigh, C.O., Passchier, S. and Naish, T.R. 2008. Recent Advances in Understanding Antarctic Climate Evolution, ''Antarctic Sci.'', '''1''', 1-13.</ref>) and has been ascribed to reduced CO<sub>2</sub> outgassing from ocean ridges, volcanoes and metamorphic belts and increased carbon burial (Pearson and Palmer, 2000<ref name="Pearson and Palmer, 2000">Pearson, P.N. and Palmer, M.R. 2000. Atmospheric carbon dioxide concentrations over the past 60 million years, ''Nature'', '''406''', 695-699.</ref>), dropping global temperatures from at least 6 to around 4&ordm;C higher than today ([[:File:Figure 3.5 - Change in average global temperature over the last 80 million years.png|Figure 3.5]] and [[:File:Figure 3.6 - Main climatic events of the last 65 million years in the Antarctic context.png|Figure 3.6]]). The fall in CO<sub>2</sub> levels at this time is also reflected in a 1-km drop in the calcium compensation depth in the tropical Pacific Ocean (Coxall et al., 2005<ref name="Coxall et al, 2005">Coxall, H., Wilson, P.A., Palike, H., Lear, C.H. and Backman, J. 2005. Rapid stepwise onset of Antarctic glaciation and deeper calcite compensation in the Pacific Ocean, ''Nature'', '''433''', 53-57.</ref>).<br />
<br />
[[File:Figure 3.1 - Map of Antarctica showing locations of selected deep ice and sediment cores.png|thumb|'''3.1''' Map of Antarctica showing main geographic features (EAIS &ndash; East Antarctic Ice Sheet; WAIS &ndash; West Antarctic Ice Sheet) and locations of selected deep ice and sediment cores. Note: The McMurdo Sound area (box) includes 23 sediment cores, 14 on land and 9 from floating ice and ranging in length from ~ 50 to 1,285 m (see Naish et al., 2008a<ref name="Naish et al, 2008a">Naish, T.R., Carter, L., Wolff, E. and Powel, R.D. 2008a. Chapter 11 - Late Pliocene-Pleistocene Antarctic climate variability at orbital and suborbital scale: Ice sheet, ocean and atmospheric interactions. In: Florindo F, Siegert M (eds) Antarctic Climate Evolution, Developments in Earth and Environmental Science, 8, Elsevier, Amsterdam.</ref>; Barrett, 2009 for details). Further records of Antarctic geological history are found at Seymour Island (SI), James Ross Island (JRI), Alexander Island (AI) and the Beardmore Glacier region (BG).]]<br />
[[File:Figure 3.7 - View of the Victoria Land coast off Cape Roberts during Oligocene and early Miocene times.png|thumb|'''3.7''' View of the Victoria Land coast off Cape Roberts during Oligocene and early Miocene times (from Barrett, 2007<ref name="Barrett, 2007">Barrett, P.J. 2007. Cenozoic climate and sea-level history of glacimarine strata off the Victoria Land coast, Cape Roberts Project, Antarctica. In: Hambrey, al. e (eds) Glacial Sedimentary Processes and Products. International Association of Sedimentologists, Special Publication.</ref>). (A) Glacial period, with an expanded inland ice sheet feeding thin temperate piedmont glaciers depositing sediment to a shallow shelf to be reworked by waves and currents nearshore with mud settling out offshore. (B) Interglacial period, with higher sea level and a much reduced ice sheet. Sediment carried to the coast largely by proglacial rivers. Low woodland beech forest at lower elevations. Insets for A and B show examples of the modelled extent of an ice sheet that might have existed during glacial and interglacial periods in early Oligocene times, representing 21 &times; 10<sup>6</sup> km<sup>3</sup> of ice (50 m of sea level equivalent) in A, and 10 &times; 10<sup>6</sup> km<sup>3</sup> of ice (24 m of sea level equivalent) in B (DeConto et al., 2007<ref name="DeConto et al, 2007">Deconto, R., Pollard, D. and Harwood, D. 2007. Sea ice feedback and Cenozoic evolution of Antarctic climate and ice sheets, Paleoceanography, 22, doi: PA3214, doi:10.1029/2006PA001350.</ref>). The location of Cape Roberts is shown as a white filled circle. TAM = Transantarctic Mountains, EAIS = East Antarctic Ice Sheet.]]<br />
The pulsating style of Antarctic glaciation in Oligocene-early Miocene times is best recorded from 1,500 m of nearshore marine sediments drilled off Cape Roberts in the southwest Ross Sea ([[:File:Figure 3.1 - Map of Antarctica showing locations of selected deep ice and sediment cores.png|Figure 3.1]] and [[:File:Figure 3.7 - View of the Victoria Land coast off Cape Roberts during Oligocene and early Miocene times.png|Figure 3.7]]) and in the middle to late Oligocene Polonez Cove Formation, exposed on south-eastern King George Island, South Shetland Islands (Troedson and Smellie, 2002<ref name="Troedson and Smellie, 2002">Troedson, A.L. and Smellie, J.L. 2002. The Polonez Cove formation of King George Island, Antarctica: stratigraphy, facies and implications for mid-Cenozoic cryosphere development, ''Sedimentology'', '''49''', 277-301.</ref>). At Cape Roberts, sediments resulting from 55 glacial-interglacial cycles have accumulated close to sea level on the subsiding margin of the Victoria Land Basin, spanning the period from 33 to 17 Ma (Barrett, 2007<ref name="Barrett, 2007">Barrett, P.J. 2007. Cenozoic climate and sea-level history of glacimarine strata off the Victoria Land coast, Cape Roberts Project, Antarctica. In: Hambrey, al. e (eds) Glacial Sedimentary Processes and Products. International Association of Sedimentologists, Special Publication.</ref>). The site was close to the edge of the continental ice sheet and recorded its cyclic expansion and contraction on Milankovitch frequencies (41 ka and 100 ka, Naish et al., 2001<ref name="Naish et al, 2001">Naish, T.R., Woolfe, K.J., Barrett, P.J., Wilson, G.S., Atkins, C., Bohaty, S.M., Bucker, C.J., Claps, M., Davey, F.J., Dunbar, G.B., Dunn, A.G., Fielding, C.R., Florindo, F., Hannah, M.J., Harwood, D.M., Henrys, S.A., Krissek, L.A., Lavelle, M., Van Der Meer, J., McIntosh, W.C., Niessen, F., Passchier, S., Powell, R.D., Roberts, A.P., Sagnotti, L., Scherer, R.P., Strong, C.P., Talarico, F., Verosub, K.L., Villa, G., Watkins, D.K., Webb, P-N. and Wonik, T. 2001. Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary, ''Nature'', '''413''', 719-723.</ref>; Naish et al., 2008b<ref name="Naish et al, 2008b">Naish, T.R., Powell, R.D., Barrett, P.J., Levy, R.H., Henrys, S., Wilson, G.S., Krissek, L.A., Niessen, F., Pompilio, M., Scherer, R. and Talarico, F. 2008b. Late Neogene climate history of the Ross Embayment from the AND-1B drill core: culmination of three decades of Antarctic margin drilling. In: Cooper AK, Barrett PJ, Stagg H, Storey B, Stump E, Wise W (eds) Antarctica: A Keystone in a Changing World. Proceedings of the 10th International Symposium on Antarctic Earth Sciences. The National Academies Press, Washington, DC, 71-82.</ref>; Huybrechts, 2009<ref name="Huybrechts, 2009">Huybrechts, P. 2009. West-side story of Antarctic ice, ''Nature'', '''458''', 295-296.</ref>; Naish et al., 2009<ref name="Naish et al, 2009">Naish, T., Powell, R., Levy, R., Wilson, G., Scherer, R., Talarico, F., Krissek, L., Niessen, F., Pompilio, M., Wilson, T., Carter, L., Deconto, R., Huybers, P., McKay, R., Pollard, D., Ross, J., Winter, D., Barrett, P., Browne, G., Cody, R., Cowan, E., Crampton, J., Dunbar, G., Dunbar, N., Florindo, F., Gebhardt, C., Graham, I., Hannah, M., Hansaraj, D., Harwood, D., Helling, D., Henrys, S., Hinnov, L., Kuhn, G., Kyle, P., L&auml;ufer, A., Maffioli, P., Magens, D., Mandernack, K., McIntosh, W., Millan, C., Morin, R., Ohneiser, C., Paulsen, T., Persico, D., Raine, I., Reed, J., Riesselman, C., Sagnotti, L., Schmitt, D., Sjunneskog, C., Strong, P., Taviani, M., Vogel, S., Wilch, T. and Williams, T. 2009. Obliquity-paced Pliocene West Antarctic ice sheet oscillations, ''Nature'', '''458''', 322-328.</ref>; Pollard and DeConto, 2009<ref name="Pollard and DeConto, 2009">Pollard, D. and Deconto, R. 2009. Modelling West Antarctic ice sheet growth and collapse through the past five million years, ''Nature'', '''458''', 329-332.</ref>). The changes in sediment type that characterize the cycles - from glacial deposits through nearshore sand and offshore mud to sand again - indicate sea level changes on a scale of tens of metres (Dunbar et al., 2008<ref name="Dunbar et al, 2008">Dunbar, G.B., Naish, T.R., Barrett, P.J., Fielding, C.R. and Powell, R.D. 2008. Constraining the amplitude of late Oligocene bathymetric changes in western Ross Sea during orbitally-induced oscillations in the East Antarctic Ice Sheet: (1) Implications for glacimarine sequence stratigraphic models, Palaeogeography, Palaeoclimatology, Palaeoecology, In Press.</ref>). Despite episodes of extensive ice cover palynological studies indicate a coastal vegetation ranging from low woodland (''Nothofagus'') to tundra persisting through the Oligocene with a slight cooling in early Miocene times (Prebble et al., 2006<ref name="Prebble et al, 2006">Prebble, J.G., Hannah, M.J. and Barrett, P.J. 2006. Changing Oligocene climate recorded by palynomorphs from two glacio-eustatic sedimentary cycles, Cape Roberts Project, Victoria Land Basin, Antarctica, Palaeogeography, Palaeoclimatology, ''Palaeoecology'', '''231''', 58-70.</ref>).<br />
<br />
Recent studies of an ancient glacial landscape in the Olympus Range on the inland edge of the McMurdo Dry Valleys have revealed warm-based glacial deposits overlain by ridges of cold-based gravelly debris, each bearing volcanic ash beds ~14 and ~13.6 Ma respectively. The landscape has changed little since that time, the lack of alteration being ascribed to a persistent frozen state from that time on, recording a sharp Middle Miocene cooling (Lewis et al., 2007<ref name="Lewis et al, 2007">Lewis, A.R., Marchant, D.R., Ashworth, A.C., Hemming, S.R. and Machlus, M.L. 2007. Major middle Miocene global climate change: Evidence from East Antarctica and the Transantarctic Mountains, ''GSA Bulletin'', '''119''', 1449-1461.</ref>). Ash-bearing proglacial lake beds, also dated at ~14 Ma, include an ostracod fauna, ''Nothofagus'' pollen and beds of moss, and are regarded as possibly the last vestiges of this fauna and flora in the region (Ashworth et al., 2007<ref name="Ashworth et al, 2007">Ashworth, A.C., Lewis, A.R., Marchant, D.R., Askin, R.A., Cantrill, D.J., Francis, J.E., Leng, M.J., Newton, A.E., Raine, J.I., Williams, M. and A.P. W. 2007. The Neogene biota of the Transantarctic Mountains - Online Proceedings of the ISAES X, edited by A.K. Cooper and C.R. Raymond et al., USGS Open-File Report 2007-1047, Extended Abstract 071, 4 p.</ref>; Lewis et al.; 2008). A well preserved flora of ''in situ'' ''Nothofagus'' dwarf shrubs, mosses and cushion plants, along with the remains of beetles, molluscs, fish and parts of flies, from the Sirius Group, Oliver Bluffs in the Dominion Range, Transantarctic Mountains also provide good evidence for tundra conditions in this region only 300 miles from the South Pole. The fossils are preserved in a glacio-fluvial-lacustrine-palaeosol layer that represents a warmer interval that prompted glacial retreat between colder intervals during which glaciers were present at that site (Francis and Hill, 1996<ref name="Francis and Hill, 1996">Francis, J.E. and Hill, R.S. 1996. Fossil plants from the Pliocene Sirius Group, Transantarctic Mountains: evidence for climate from growth rings and fossil leaves. Palaios, 11, 389-396.</ref>; Ashworth and Cantrill, 2004<ref name="Ashworth and Cantrill, 2004">Ashworth, A.C. and Cantrill, D.J. 2004. Neogene vegetation of the Meyer Desert Formation (Sirius Group), Transantarctic Mountains, Antarctica, Palaeogeography, Palaeoclimatology, ''Palaeoecology'', '''213''', 65-82.</ref>). Unfortunately an undisputed age for these deposits is not available.<br />
<br />
The sharp cooling in the Middle Miocene has long been known from deep-sea isotopic studies (Shackleton and Kennett, 1975<ref name="Shackleton and Kennett, 1975">Shackleton, N.J. and Kennett, J.P. 1975. Paleotemperature history of the Cenozoic and the initiation of Antarctic glaciation: oxygen and carbon isotope analysis in DSDP sites 277, 279 and 281. In: Kennett JP, Houtz RE (eds) Initial Reports of the Deep Sea Drilling Project, 29. Washington (U.S. Government Printing Office), 743-755.</ref>), and was probably caused by the growing thermal isolation of Antarctica and related intensification of the Antarctic Circumpolar Current described above, accompanied by a drop in atmospheric CO<sub>2</sub> (Shevenell et al., 1996<ref name="Shevenell et al, 1996">Shevenell, A.E., Domack, E.W. and Kernan, G. 1996. Record of Holocene paleoclimate change along the Antarctic Peninsula: Evidence from glacial marine sediments, Lallemand Fjord, Papers and Proceedings of the Royal Society of Tasmania, 130, 55-64.</ref>). This thickened the ice sheet to more or less its modern configuration, which is thought to have persisted through the early Pliocene warming from 5 Ma to 3 Ma (Kennett and Hodell, 1993<ref name="Kennett and Hodell, 1993">Kennett, J.P. and Hodell, D.A. 1993. Evidence for Relative Climatic Stability of Antarctica during the Early Pliocene: A Marine Perspective, Geografiska Annaler. Series A, ''Physical Geography'', '''75''', 205-220.</ref>; Barrett, 1996<ref name="Barrett, 1996">Barrett, P.J. 1996. Antarctic palaeoenvironment through Cenzoic times - a review, ''Terra Antartica'', '''3''', 103-119.</ref>; McKay et al., 2008<ref name="McKay et al, 2008">McKay, R.M., Barrett, P.J., Hannah, M.H. and Harper, M.A. 2008. Atmospheric transport and concentration of diatoms in surficial and glacial sediments of the Allan Hills, Transantarctic Mountains. Palaeogeography, Palaeoclimatology, ''Palaeoecology'', '''260''', 168-183.</ref>).<br />
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During the Pliocene, mean global temperatures were 2-3&ordm;C above pre-industrial values ([[:File:Figure 3.6 - Main climatic events of the last 65 million years in the Antarctic context.png|Figure 3.6]]) with CO<sub>2</sub> values less than 400 ppm and sea levels 15-25m above modern levels (Raymo et al., 1996<ref name="Raymo et al, 1996">Raymo, M.E., Grant, B., Horowitz, M. and Rau, G.H. 1996. Mid-Pliocene warmth: stronger greenhouse and stronger conveyor, ''Marine Micropaleontology'', '''27''', 313-326.</ref>; Jansen et al., 2007<ref name="Jansen et al, 2007">Jansen, E.J., Overpeck, K.R., Briffa, J-C., Duplessy, F., Joos, V., Masson-Delmotte, D., Olago, B., Otto-Bliesner, W.R., Peltier, S., Rahmstorf, R., Ramesh, D., Raynaud, D., Rind, O., Solomina, R., Villalba, R. and Zhang, D. 2007. Palaeoclimate. In: Solomon SD, Qin M, Manning Z, Chen M, Marquis KB, Averyt M, Tignor, Miller HL (eds) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 433-497.</ref>, and references therein). The Antarctic margin also records Pliocene temperatures several degrees warmer than today in diatom-bearing coastal sediments (Harwood et al., 2000<ref name="Harwood et al, 2000">Harwood, D.M., McMinn, A. and Quilty, P.G. 2000. Diatom biostratigraphy and age of the Pliocene Sorsdal Formation, Vestfold Hills, East Antarctica, ''Antarctic Science'', '''12''', 443-462.</ref>) and offshore cores (Whitehead et al., 2005<ref name="Whitehead et al, 2005">Whitehead, J.M., Wotherspoon, S. and Bohaty, S.M. 2005. Minimal Antarctic sea ice during the Pliocene, ''Geology'', '''33''', 137-140.</ref>).<br />
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[[File:Figure 3.8 - Lithological column for the upper 600m of the AND-1B drillcore.png|thumb|'''3.8''' Lithological column and inferred glacial proximity curve for the upper 600 m of the AND-1B drillcore recovered by the ANDRILL McMurdo Ice Shelf Project (Naish et al., 2009<ref name="Naish et al, 2009">Naish, T., Powell, R., Levy, R., Wilson, G., Scherer, R., Talarico, F., Krissek, L., Niessen, F., Pompilio, M., Wilson, T., Carter, L., Deconto, R., Huybers, P., McKay, R., Pollard, D., Ross, J., Winter, D., Barrett, P., Browne, G., Cody, R., Cowan, E., Crampton, J., Dunbar, G., Dunbar, N., Florindo, F., Gebhardt, C., Graham, I., Hannah, M., Hansaraj, D., Harwood, D., Helling, D., Henrys, S., Hinnov, L., Kuhn, G., Kyle, P., L&auml;ufer, A., Maffioli, P., Magens, D., Mandernack, K., McIntosh, W., Millan, C., Morin, R., Ohneiser, C., Paulsen, T., Persico, D., Raine, I., Reed, J., Riesselman, C., Sagnotti, L., Schmitt, D., Sjunneskog, C., Strong, P., Taviani, M., Vogel, S., Wilch, T. and Williams, T. 2009. Obliquity-paced Pliocene West Antarctic ice sheet oscillations, ''Nature'', '''458''', 322-328.</ref>), compared with the continuous deep-sea oxygen-isotope record (Lisiecki and Raymo, 2005<ref name="Lisiecki and Raymo, 2005">Lisiecki, L.E. and Raymo, M.E. 2005. A Pliocene-Pleistocene stack of 57 globally distributed benthic d<sup>18</sup>O records, ''Paleoceanography'', '''20''', PA1003, doi:1010.1029/2004PA001071.</ref>) and the modelled contribution of Antarctic Ice Sheet to sea level change for the last 5 million years (Pollard and DeConto, 2009<ref name="Pollard and DeConto, 2009">Pollard, D. and Deconto, R. 2009. Modelling West Antarctic ice sheet growth and collapse through the past five million years, ''Nature'', '''458''', 329-332.</ref>). The &ldquo;glacial proximity&rdquo; column shows glacial surfaces of erosion, which mark boundaries of orbital-scale, glacimarine sedimentary cycles, and the prospect of cycles missing on account of erosion. The glacial proximity curve tracks the cycles themselves from inferred depositional environment - I = ice contact, P = proximal glacimarine, D = distal glacimarine, M = open marine. Pairs of images of the Antarctic Ice Sheet on the right show hypothesized glacial and interglacial states for the last million years and the period between 1 and ~5 Ma from Pollard and DeConto (2009<ref name="Pollard and DeConto, 2009">Pollard, D. and Deconto, R. 2009. Modelling West Antarctic ice sheet growth and collapse through the past five million years, ''Nature'', '''458''', 329-332.</ref>). Figure adapted by Tim Naish from Naish et al. (2009<ref name="Naish et al, 2009">Naish, T., Powell, R., Levy, R., Wilson, G., Scherer, R., Talarico, F., Krissek, L., Niessen, F., Pompilio, M., Wilson, T., Carter, L., Deconto, R., Huybers, P., McKay, R., Pollard, D., Ross, J., Winter, D., Barrett, P., Browne, G., Cody, R., Cowan, E., Crampton, J., Dunbar, G., Dunbar, N., Florindo, F., Gebhardt, C., Graham, I., Hannah, M., Hansaraj, D., Harwood, D., Helling, D., Henrys, S., Hinnov, L., Kuhn, G., Kyle, P., L&auml;ufer, A., Maffioli, P., Magens, D., Mandernack, K., McIntosh, W., Millan, C., Morin, R., Ohneiser, C., Paulsen, T., Persico, D., Raine, I., Reed, J., Riesselman, C., Sagnotti, L., Schmitt, D., Sjunneskog, C., Strong, P., Taviani, M., Vogel, S., Wilch, T. and Williams, T. 2009. Obliquity-paced Pliocene West Antarctic ice sheet oscillations, ''Nature'', '''458''', 322-328.</ref>).]]<br />
A particularly instructive record comes from the ANDRILL McMurdo Ice Shelf site, where over 1,200 m of strata dating back to 13 Ma were cored from a deep-water basin south of Ross Island (Naish et al., 2007<ref name="Naish et al, 2007">Naish, T.R., Powell, R.D., Henrys, S., Wilson, G.S., Krissek, L.A., Niessen, F., Pompilio, M., Scherer, R., Talarico, F., Levy, R.H. and Pyne, A. 2007. New insights into Late Neogene (13-0Ma) climate history of Antarctica: Initial results from the ANDRILL McMurdo Ice Shelf (MIS) Project (AND-1B). In: A.K. Cooper, C.R. Raymond, al e (eds) A Keystone in a Changing World - Online Proceedings of the 10th ISAES, USGS Open-File Report 2007, 1-11.</ref>; 2008a,b, 2009). The record comprises many cycles of sedimentation, alternating between deposition beneath grounded ice (diamictite) and ice shelf ice (mudstone) in the last million years. However, from 1 to ~5 Ma depositional environments ranging from grounded ice (diamictite) to open water (diatomite) ([[:File:Figure 3.8 - Lithological column for the upper 600m of the AND-1B drillcore.png|Figure 3.8]]). The diatomite beds show the drill site to have been essentially ice-free at this time, i.e. no McMurdo Ice Shelf and hence no Ross Ice Shelf. The lack of buttressing from the loss of the Ross Ice Shelf in turn implies a much reduced West Antarctic Ice Sheet (Mercer, 1978<ref name="Mercer, 1978">Mercer, J.H. 1978. West Antarctic ice sheet and CO<sub>2</sub> greenhouse effect: a threat of disaster, ''Nature'', '''271''', 321-325.</ref>; Dupont and Alley, 2005a<ref name="Dupont and Alley, 2005a">Dupont, T., and Alley, R.B. 2005a. Assessment of the Importance of Ice-Shelf Buttressing to Ice-Sheet Flow, ''Geophysical Research Letters'', '''32'''(4), LO4503.</ref>). However geomorphological evidence, and the antiquity of high level surfaces in the Transantarctic Mountains (Sugden et al., 1993<ref name="Sugden et al, 1993">Sugden, D.E., Marchant, D.R. and Denton, G.H. 1993. The case for a stable East Antarctic Ice Sheet: a special volume arising from the Vega Symposium, 26 April 1993 Stockholm, Geografiska Annaler, 75A, No. 4.</ref>), along with modelling Pliocene ice sheets (Hill et al., In Press), supports the persistence of an ice sheet in the East Antarctic interior through this period. A new ice sheet model by Pollard and DeConto (2009<ref name="Pollard and DeConto, 2009">Pollard, D. and Deconto, R. 2009. Modelling West Antarctic ice sheet growth and collapse through the past five million years, ''Nature'', '''458''', 329-332.</ref>) confirms the persistence of East Antarctic ice and the disappearance of the West Antarctic ice sheet during warm Pliocene times and even as recently as the MIS 31 interglacial stage a little over a million years ago ([[:File:Figure 3.8 - Lithological column for the upper 600m of the AND-1B drillcore.png|Figure 3.8]]).<br />
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Global cooling from around 3 Ma onwards (Ravelo et al., 2004<ref name="Ravelo et al, 2004">Ravelo, A.C., Andreasen, D.H., Lyle, M., Olivarez Lyle, A. and Wara, M.W. 2004. Regional climate shifts caused by gradual global cooling in the Pliocene epoch, ''Nature'', '''429''', 263-267.</ref>) led to the first big ice sheets on North America and NW Europe around 2.6 Ma (Shackleton et al., 1984<ref name="Shackleton et al, 1984">Shackleton, N.J., Backman, J., Zimmerman, H., Kent, D.V., Hall, M.A., Roberts, D.G., Schnitker, D., Baldauf, J.G., Desprairies, A., Homrighausen, R., Huddlestun, P., Keene, J.B., Kaltenback, A.J., Krumsiek, K.A.O., Morton, A.C., Murray, J.W. and Westberg-Smith, J. 1984. Oxygen isotope calibration of the onset of ice-rafting and history of glaciation in the North Atlantic region, ''Nature'', '''307''', 620-623.</ref>), enhancing the Earth&rsquo;s climate response to orbital forcing with a 40,000 year cyclicity, and taking us to the Earth&rsquo;s present intense &ldquo;ice house&rdquo; state. For the last million years ([[:File:Figure 3.8 - Lithological column for the upper 600m of the AND-1B drillcore.png|Figure 3.8]]) this has alternated between (i) longer (100,000 years) glacial cycles, when much of the Northern Hemisphere was ice-covered, global average temperature was around 5&ordm;C colder, and sea level was approximately 120 m lower than today, and (ii) much shorter warm interglacial cycles like that of the last ~10,000 years, with sea levels near or slightly above those of the present.<br />
==References==<br />
<references /><br />
[[Category:The pre-instrumental period]]<br />
[[Category:Deep time]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_HoloceneThe Holocene2014-08-06T14:34:11Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Antarctic climate and environment history in the pre-instrumental period]]''<br />
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In this section we review the climate changes that have occurred in the present Holocene interglacial. In many long ice core records this period may superficially appear to be one of relative climate and environmental stability. However, closer inspection of these and other records reveal a complex of changes resulting from the interplay between the ice sheet &ndash; ocean &ndash; sea ice &ndash; atmosphere system to past and present climate forcing. In this section we first examine evidence from the ice core record looking particularly at the phasing of these climate and environmental changes on regional to hemispheric timescales. Second we examine changes in sea ice extent through the Holocene and how these have interacted with changing climate and ocean circulation. Finally we look at the regional patterns of Holocene climate and environmental change experienced in the major regions of Antarctica which are the result of both continental and local forcing mechanisms.<br />
==Pages in this topic==<br />
#[[Holocene climate changes]]<br />
#[[Changes in sea ice extent through the Holocene]]<br />
#[[Regional patterns of holocene climate change in Antarctica]]<br />
[[Category:The pre-instrumental period]]<br />
[[Category:The Holocene]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_Greenhouse_world_-_from_the_breakup_of_Gondwana_to_34_million_years_agoThe Greenhouse world - from the breakup of Gondwana to 34 million years ago2014-08-06T14:34:11Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Deep time]]''<br />
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[[File:Figure 3.2 - Apparent polar wander path for East Antarctica over the last 120 million years.png|thumb|'''3.2''' Apparent polar wander path for East Antarctica over the last 120 Ma (modified from DiVenere et al., 1994<ref name="DiVenere et al, 1994">Divenere, V.J., Kent, D.V. and Dalziel, I.W.D. 1994. Mid-Cretaceous paleomagnetic results from Marie Byrd Land, WestAntarctica: a test of post-100 Ma relative motion between East and West Antarctica, ''Journal of Geophysical Research'', '''99''', 15115-15139.</ref>). The shaded area represents the modelled error envelope.]]<br />
[[File:Figure 3.3 - Reconstruction of mid-Cretaceous forests of Alexander Island, Antarctica.png|thumb|'''3.3''' Reconstruction of mid-Cretaceous forests of Alexander Island, Antarctica. Based on the work of J. Howe, J.E. Francis, and numerous British Antarctic Survey geologists. Painted by Robert Nicholls (Francis et al., 2008a<ref name="Francis et al, 2008a">Francis, J.E., Ashworth, A., Cantrill, D.J., Crame, J.A., Howe, J., Stephens, R., Tosolini, A-M. and Thorn, V. 2008a. 100 million years of Antarctic climate evolution: evidence from fossil plants. In: Cooper AK, Barrett PJ, Stagg H, Storey B, Stump E, Wise W (eds) Antarctica: A Keystone in a Changing World. Proceedings of the 10th International Symposium on Antarctic Earth Sciences. The National Academies Press, Washington, DC, 19-27.</ref>).]]<br />
Some 200 million years ago (Ma) Antarctica was the centrepiece of the Gondwana super-continent, which began to break up around 180 Ma in the Jurassic Period of the Mesozoic Era. The Gondwanan fragments separated by sea-floor spreading largely between around 100 to 65 Ma, during the Cretaceous Period, although by this time Antarctica had already moved into a position over the South Pole ([[:File:Figure 3.2 - Apparent polar wander path for East Antarctica over the last 120 million years.png|Figure 3.2]]). A rich record of plant and animal fossils from Late Cretaceous and early Tertiary times has been found on Seymour Island and Alexander Island in the Antarctic Peninsula. This demonstrates that, despite its polar position, Antarctica had a warm temperate climate that allowed the growth of lush forests ([[:File:Figure 3.3 - Reconstruction of mid-Cretaceous forests of Alexander Island, Antarctica.png|Figure 3.3]]). These were inhabited by dinosaurs in the Cretaceous and mammals during the early Tertiary (Francis and Poole, 2002<ref name="Francis and Poole, 2002">Francis, J.E. and Poole, I. 2002. Cretaceous and early Tertiary climates of Antarctica : evidence from fossil wood, Palaeogeography, Palaeoclimatology, ''Palaeoecology'', '''182''', 47-64.</ref>).<br />
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Analysis of changing biodiversity in the forests, and of the climate record preserved in the foliage, indicates that climate cooling signalled the decline of Antarctic warmth during the Middle Eocene, about 45 Ma, when heat-loving plants were lost from Antarctica and replaced by types such as species of ''Nothofagus'' trees that could tolerate cold climates (Francis et al., 2008a<ref name="Francis et al, 2008a">Francis, J.E., Ashworth, A., Cantrill, D.J., Crame, J.A., Howe, J., Stephens, R., Tosolini, A-M. and Thorn, V. 2008a. 100 million years of Antarctic climate evolution: evidence from fossil plants. In: Cooper AK, Barrett PJ, Stagg H, Storey B, Stump E, Wise W (eds) Antarctica: A Keystone in a Changing World. Proceedings of the 10th International Symposium on Antarctic Earth Sciences. The National Academies Press, Washington, DC, 19-27.</ref>).<br />
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[[File:Figure 3.4 - Maps showing the progressive separation of Antarctica and the other continents.png|thumb|'''3.4''' Three maps showing the progressive separation of Antarctica and the other Southern Hemisphere continents, leading to the opening of &ldquo;ocean gateways&rdquo; that allowed the development of the Antarctic Circum-polar Current sometime after 34 Ma. Modified from Kennett (1978<ref name="Kennett, 1978">Kennett, J.P. 1978. The development of planktonic biostratigraphy in the Southern Ocean during the Cenozoic, ''Marine Micropaleontology'', '''3''', 301-345.</ref>).]]<br />
Over time, South America, Africa, India, Australia and New Zealand moved away from Antarctica, opening the South Atlantic, Indian and Southern Oceans ([[:File:Figure 3.4 - Maps showing the progressive separation of Antarctica and the other continents.png|Figure 3.4]]). As the proto-Pacific Plate was subducted beneath the Antarctic Plate an active volcanic arc grew, the eroded remnants of which now form the Antarctica Peninsula. The western margin of Antarctica was thus characterised by volcanism and lateral movement of fragments of the formerly continuous Gondwana terrain (for example see McCarron and Larter, 1998<ref name="McCarron and Larter, 1998">McCarron, J.J. and Larter, R.D. 1998. Late Cretaceous to Early Tertiary Subduction History of the Antarctic Peninsula, ''J. Geol. Soc. Lond.'', '''155''', 255-268.</ref>). This western arc broke apart first at around 85 Ma south of New Zealand, but much later between South America and the Antarctic Peninsula to create the Drake Passage.<br />
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Over the last 40 million years, the Antarctic shelf experienced a series of tectonic, climatic and oceanographic events that lead to isolation from other oceans, establishment of colder conditions and complete replacement and impoverishment of the fish fauna. The deepening of the separations from other continental masses and the removal of the last barriers to circumpolar flow, allowed the establishment of the Antarctic Circumpolar Current (ACC) and, at its northern border, of the Antarctic Polar Front (APF), a roughly circular oceanic system running between 50&deg;S and 60&deg;S and extending to 2,000 m in depth.<br />
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Two key events allowed the establishment of the ACC: (1) the opening of the Tasman Seaway between Antarctica and Australia, which according to tectonics and marine geology occurred approximately 35.5-30 Ma (Kennett, 1977<ref name="Kennett, 1977">Kennett, J.P. 1977. Cenozoic evolution of the Antarctic glaciation, the circum-Antarctic ocean and their impact on global paleoceanography, ''J. Geophys. Res.'', '''82''', 3843-3876.</ref>; Lawver and Gahagan, 2003<ref name="Lawver and Gahagan, 2003">Lawver, L.A. and Gahagan, L.M. 2003. Evolution of Cenozoic seaways in the circum-Antarctic region, Palaeogeography, Palaeoclimatology, ''Palaeoecology'', '''198''', 11-37.</ref>; Stickley et al., 2004<ref name="Stickley et al, 2004">Stickley, C.E., Brinkhuis, H., Schellenberg, S.A., Sluijs, A., R&ouml;hl, U., Fuller, M., Grauert, M., Huber, M., Warnaar, J. and Williams, G.L. 2004. Timing and nature of the deepening of the Tasmanian Gateway, ''Paleoceanography'', '''19''', PA4027, doi:4010.1029/2004PA001022.</ref>; Wei, 2004<ref name="Wei, 2004">Wei, W. 2004. Opening of the Australia-Antarctica Gateway as dated by nannofossils, ''Marine Micropaleontology'', '''52''', 133-152.</ref>); (2) the opening of the Drake Passage between southern South America and the Antarctic Peninsula. The two openings allowed the development of the ACC. The timing of the opening of the Passage is controversial, with estimates ranging between 40 and 17 Ma (Barker and Burrell, 1977<ref name="Barker and Burrell, 1977">Barker, P.F. and Burrell, J. 1977. The opening of Drake passage, Marine Geology, 25.</ref>; Livermore et al., 2004<ref name="Livermore et al, 2004">Livermore, R., Nankivell, A., Eagles, G. and Morris, P. 2004. Paleogene opening of Drake Passage, ''Earth and Planetary Science Letters'', '''236''', 459-470.</ref>, Scher and Martin, 2006<ref name="Scher and Martin, 2006">Scher, H.D. and Martin, E.E. 2006. Timing and Climatic Consequences of the Opening of Drake Passage, ''Science'', '''312''', 428-430.</ref>). Such a relatively wide time window is due to contrasting views on the palaeo-elevation of key parts of the Scotia Arc and the Shackleton Fracture Zone (reviewed in Barker et al., 2007<ref name="Barker et al, 2007">Barker, P.F., Diekmann, B. and Escutia, C. 2007. Onset of Cenozoic Antarctic Glaciation, ''Deep Sea. Research'', '''54''', 2293-2307.</ref>). Recently, using neodymium isotopes to detect the presence of Pacific seawater in the Atlantic sector Scher and Martin (2006<ref name="Scher and Martin, 2006">Scher, H.D. and Martin, E.E. 2006. Timing and Climatic Consequences of the Opening of Drake Passage, ''Science'', '''312''', 428-430.</ref>) proposed that the opening of the Drake passage occurred as early as 41 Ma, and was thus completed before the establishment of the Tasman Seaway.<br />
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The timing of the final onset of the ACC is also uncertain (Barker et al., 2007<ref name="Barker et al, 2007">Barker, P.F., Diekmann, B. and Escutia, C. 2007. Onset of Cenozoic Antarctic Glaciation, ''Deep Sea. Research'', '''54''', 2293-2307.</ref>). Estimates from sediment records range from late Eocene (40 Ma) to latest Miocene (8-10 Ma) (Wei and Wise, 1992<ref name="Wei and Wise, 1992">Wei, W. and Wise, S.W. 1992. Eocene-Oligocene calcareous nannofossil magnetobiochronology of the Southern Ocean, ''Newsl. Stratigr.'', '''26''', 119-132.</ref>; Scher and Martin, 2006<ref name="Scher and Martin, 2006">Scher, H.D. and Martin, E.E. 2006. Timing and Climatic Consequences of the Opening of Drake Passage, ''Science'', '''312''', 428-430.</ref>). Concerns have been raised that some of these estimates could rather be climate-related and reflect a change in either temperature or productivity independent of ocean circulation (Barker et al., 2007<ref name="Barker et al, 2007">Barker, P.F., Diekmann, B. and Escutia, C. 2007. Onset of Cenozoic Antarctic Glaciation, ''Deep Sea. Research'', '''54''', 2293-2307.</ref>). The older view that the ACC developed around the Eocene-Oligocene boundary leading to the first continental ice sheets on Antarctica by reducing heat transport into the region (Kennett, 1977<ref name="Kennett, 1977">Kennett, J.P. 1977. Cenozoic evolution of the Antarctic glaciation, the circum-Antarctic ocean and their impact on global paleoceanography, ''J. Geophys. Res.'', '''82''', 3843-3876.</ref>) has been questioned by DeConto and Pollard (2003<ref name="DeConto and Pollard, 2003">Deconto, R.M. and Pollard, D. 2003. Rapid Cenozoic glaciation of Antarctica induced by declining atmospheric CO<sub>2</sub>, ''Nature'', '''421''', 245-249.</ref>). Their modelling study shows the first ice sheets could have formed from a drop in atmospheric CO<sub>2</sub> levels from 3 to 2 times pre-industrial levels and that the opening of Drake Passage could provide only 20% of the cooling. The formation of the circum-polar deep water current may have subsequently helped the intensification of glaciation (Francis et al., 2008b<ref name="Francis et al, 2008b">Francis, J.E., Marenssi, S., Levy, R., Hambrey, M., Thorn, V.T., Mohr, B., Brinkhuis, H., Warnaar, J., Zachos, J., Bohaty, S. and Deconto, R. 2008b. From Greenhouse to Icehouse - The Eocene/Oligocene in Antarctica. In: Florindo F, Siegert M (eds) Antarctic Climate Evolution, Developments in Earth and Environmental Science, Vol 8. Elsevier, Amsterdam.</ref>).<br />
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Despite the uncertainty on both the timing of final onset of the ACC and its importance as a driver of climatic change, palaeoclimatic reconstructions leave little doubt about the fact that oceanographic and climatic processes responsible for current day glacial conditions in the Antarctic started in the same period of time. In fact, switching of climate conditions from &ldquo;greenhouse&rdquo; to &ldquo;icehouse&rdquo; in the Antarctic started in the late Eocene, 42 Ma, with an initial phase of strong but short-lived glaciations that match with the most recently published time estimates of formation of the Drake passage (Scher and Martin, 2006<ref name="Scher and Martin, 2006">Scher, H.D. and Martin, E.E. 2006. Timing and Climatic Consequences of the Opening of Drake Passage, ''Science'', '''312''', 428-430.</ref>). A further drastic change occurred at the Eocene-Oligocene boundary ~34 Ma, (Tripati et al., 2005<ref name="Tripati et al, 2005">Tripati, A., Backman, J., Elderfield, H. and Ferretti, P. 2005. Eocene bipolar glaciation associated with global carbon cycle changes, ''Nature'', '''436''', 341-346.</ref>; Barker et al., 2007<ref name="Barker et al, 2007">Barker, P.F., Diekmann, B. and Escutia, C. 2007. Onset of Cenozoic Antarctic Glaciation, ''Deep Sea. Research'', '''54''', 2293-2307.</ref>), leading to ice-sheet coverage similar in extent to the present ice sheet and contemporaneous with the opening of the Tasman Seaway.<br />
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The palaeontological record suggests these climatic events also influenced the Southern Ocean and its fish fauna. The best studied Antarctic fish fossils have been discovered in La Meseta Formation on Seymour Island (at the tip of the Antarctic Peninsula). The deposit from the late Eocene showed a fauna still &ldquo;cool and temperate in character&rdquo; that possibly lived in waters such as those found today around Tasmania, New Zealand and southern South America (Eastman, 2005<ref name="Eastman, 2005">Eastman, J.T. 2005. The nature of the diversity of Antarctic fishes, ''Polar Biology'', '''28''', 1432-2056.</ref>). Accordingly, the earliest cold-climate marine Antarctic faunas are thought to date back to the latest Eocene-Oligocene (35 Ma).<br />
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With the ACC, the Southern Ocean became the cold isolated habitat that we know today. However, ice sheet coverage oscillated throughout the Cenozoic and probably produced repeated shifts of species distribution in Antarctic coastal waters, allowing allopatric speciation and diversification of Antarctic taxa (Rogers, 2007<ref name="Rogers, 2007">Rogers, A.D. 2007. Evolution and biodiversity of Antarctic organisms: a molecular perspective, Philosophical Transactions of the Royal Society of London, Series B, ''Biological Sciences'', '''362''', 2191-2214.</ref>). In addition, the present APF is now suspected to be &ldquo;leaky&rdquo; and to allow transport of plankton north in mesoscale eddies with cold cores (Clarke et al., 2005<ref name="Clarke et al, 2005">Clarke, A., Barnes, D.K.A. and Hodgson, D.A. 2005. How isolated is Antarctica? Trends in Ecology and Evolution, 20, 1-3.</ref>).<br />
<br />
[[File:Figure 3.5 - Change in average global temperature over the last 80 million years.png|thumb|'''3.5''' Change in average global temperature over the last 80 Ma. The temperature curve of Crowley and Kim (1995<ref name="Crowley and Kim, 1995">Crowley, T.J. and Kim, K. 1995. Comparison of long-term greenhouse projections with the Geologic Record, ''Geophysical Research Letters'', '''22''', 933-936.</ref>) is modified to show the effect of the methane discharge at 55 Ma (Zachos et al., 2003<ref name="Zachos et al, 2003">Zachos, J.C., Wara, M.W., Bohaty, S., Delaney, M.L., Petrizzo, M.R., Brill, A., Bralower, T.J. and Premoli-Silva, I. 2003. A Transient Rise in Tropical Sea Surface Temperature During the Paleocene-Eocene Thermal Maximum, ''Science'', '''302''', 1551-1554.</ref>; Zachos et al., 2005<ref name="Zachos et al, 2005">Zachos, J.C., Rohl, U., Schellenberg, S.A., Sluijs, A., Hodell, D.A., Kelly, D.C., Thomas, E., Nicolo, M., Raffi, I., Lourens, L.J., McCarren, H. and Kroon, D. 2005. Rapid Acidification of the Ocean During the Paleocene-Eocene Thermal Maximum, ''Science'', '''308''', 1611-1615.</ref>). The future rise in temperature expected from energy use projections shows the Earth warming back into the &lsquo;greenhouse world&rsquo;(from Mayewski et al., 2009<ref name="Mayewski et al, 2009">Mayewski, P.A., Meredith, M.P., Summerhayes, C.P., Turner, J., Worby, A.P., Barrett, P.J., Casassa, G., Bertler, N.A.N., Bracegirdle, T.J., Naveira-Garabato, A.C., Bromwich, D.H., Campbell, H., Hamilton, G.H., Lyons, W.B., Maasch, K.A., Aoki, S., and Xiao, C. 2009. State of the Antarctic and Southern Ocean Climate System (SASOCS), ''Reviews of Geophysics'', '''47''', RG1003, doi:10.1029/2007RG000231.</ref>).]]<br />
Atmospheric CO<sub>2</sub> levels ranged from ~3,000 ppm in the Early Cretaceous (at 130 Ma) to around 1,000 ppm in the Late Cretaceous (at 70 Ma) and Early Cenozoic (at 45 Ma) (e.g. Pagani et al., 2005<ref name="Pagani et al, 2005">Pagani, M., Zachos, J.C., Freeman, K.H., Tipple, B. and Bohaty, S. 2005. Marked Decline in Atmospheric Carbon Dioxide Concentrations During the Paleogene, ''Science'', '''309''', 600-603.</ref>; Royer, 2006<ref name="Royer, 2006">Royer, D.L. 2006. CO<sub>2</sub>-forced climate thresholds during the Phanerozoic, ''Geochim. Cosmochim. Acta'', '''70''', 5665-5675.</ref>), leading to global temperatures at least 6 or 7&ordm;C warmer than present at times. These high CO<sub>2</sub> levels were probably the consequence of volcanic outgassing. Temperatures peaked at ~85 Ma during the mid-Late Cretaceous when sub-tropical climates prevailed over the pole (Francis et al., 2008a<ref name="Francis et al, 2008a">Francis, J.E., Ashworth, A., Cantrill, D.J., Crame, J.A., Howe, J., Stephens, R., Tosolini, A-M. and Thorn, V. 2008a. 100 million years of Antarctic climate evolution: evidence from fossil plants. In: Cooper AK, Barrett PJ, Stagg H, Storey B, Stump E, Wise W (eds) Antarctica: A Keystone in a Changing World. Proceedings of the 10th International Symposium on Antarctic Earth Sciences. The National Academies Press, Washington, DC, 19-27.</ref>). Even in this Cretaceous greenhouse world from oxygen isotope and backstripping data some researchers have inferred sea level changes in the order of tens of metres and intermittent polar ice sheets (Miller et al., 2008<ref name="Miller et al, 2008">Miller, K.G., Wright, J.D., Katz, M.E., Browning, J.V., Cramer, B.S., Wade, B.S. and Mizintseva, S.F. 2008. A View of Antarctic Ice-Sheet Evolution from Sea-Level and Deep-Sea Isotope Changes During the Late Cretaceous-Cenozoic. In: Cooper AK, Barrett P, Stagg H, Storey B, Stump E, Wise W (eds) Antarctica: A Keystone in a Changing World. Proceedings of the 10th International Symposium on Antarctic Earth Sciences, Santa Barbara, California, August 26 to September 1, 2007. Polar Research Board, National Research Council, U.S. Geological Survey.</ref>), although there is as yet no geological evidence (Thorn et al., 2007<ref name="Thorn et al, 2007">Thorn, V.C., Francis, J.E., Riding, J.B., Raiswell, R.W., Pirrie, D., Haywood, A.M., Crame, J.A. and Marshall, J.M. 2007. Terminal Cretaceous climate change and biotic response in Antarctica In: Online Proceedings of the 10th ISAES X, USGS Open-File Report 2007-1047, Extended Abstract 096, 4 p.</ref>). Temperatures peaked again at 50 Ma ([[:File:Figure 3.5 - Change in average global temperature over the last 80 million years.png|Figure 3.5]]) and subsequently declined, as did atmospheric CO<sub>2</sub> levels (Pearson and Palmer, 2000<ref name="Pearson and Palmer, 2000">Pearson, P.N. and Palmer, M.R. 2000. Atmospheric carbon dioxide concentrations over the past 60 million years, ''Nature'', '''406''', 695-699.</ref>; Pagani et al., 2005<ref name="Pagani et al, 2005">Pagani, M., Zachos, J.C., Freeman, K.H., Tipple, B. and Bohaty, S. 2005. Marked Decline in Atmospheric Carbon Dioxide Concentrations During the Paleogene, ''Science'', '''309''', 600-603.</ref>; Royer, 2006<ref name="Royer, 2006">Royer, D.L. 2006. CO<sub>2</sub>-forced climate thresholds during the Phanerozoic, ''Geochim. Cosmochim. Acta'', '''70''', 5665-5675.</ref>). Superimposed on the high CO<sub>2</sub> world of the Early Cenozoic, deep-sea sediments provide evidence of the catastrophic release of more than 2,000 gigatonnes of carbon into the atmosphere from methane hydrates around 55 Ma ago, at the Paleocene-Eocene boundary, raising global temperatures by a further ~4-5&deg;C, although they recovered after about 100,000 years ([[:File:Figure 3.5 - Change in average global temperature over the last 80 million years.png|Figure 3.5]]) (Zachos et al., 2003<ref name="Zachos et al, 2003">Zachos, J.C., Wara, M.W., Bohaty, S., Delaney, M.L., Petrizzo, M.R., Brill, A., Bralower, T.J. and Premoli-Silva, I. 2003. A Transient Rise in Tropical Sea Surface Temperature During the Paleocene-Eocene Thermal Maximum, ''Science'', '''302''', 1551-1554.</ref>; Zachos et al., 2005<ref name="Zachos et al, 2005">Zachos, J.C., Rohl, U., Schellenberg, S.A., Sluijs, A., Hodell, D.A., Kelly, D.C., Thomas, E., Nicolo, M., Raffi, I., Lourens, L.J., McCarren, H. and Kroon, D. 2005. Rapid Acidification of the Ocean During the Paleocene-Eocene Thermal Maximum, ''Science'', '''308''', 1611-1615.</ref>).<br />
==References==<br />
<references /><br />
[[Category:The pre-instrumental period]]<br />
[[Category:Deep time]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_East_Antarctic_cryosphere_in_the_instrumental_periodThe East Antarctic cryosphere in the instrumental period2014-08-06T14:34:10Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[The ice sheet in the instrumental period]]''<br />
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Changes are less dramatic across most of the East Antarctic ice sheet with the most significant changes concentrated close to the coast. Increasing coastal melt is suggested by some recent passive microwave data (Tedesco, 2008<ref name="Tedesco, 2008">Tedesco, M. 2008. Updated 2008 surface snowmelt trends in Antarctica, Eos. Trans., 89 (13).</ref>). Satellite altimetry data indicate recent thickening in the interior that has been attributed to increased snowfall likely because of year-to-year and decade-to-decade fluctuations in snowfall (Davis et al., 2005<ref name="Davis et al, 2005">Davis, C.H., Li Y., McConnell, J.R., Frey, M.M. and Hanna, E. 2005. Snowfall-driven growth in East Antarctic Ice Sheet mitigates recent sea-level rise, ''Science'', '''308''', 1898-1901.</ref>), but ice core data do not show recent accumulation changes as significantly higher than during the past 50 years (Monaghan et al., 2006a<ref name="Monaghan et al, 2006a">Monaghan, A.J., Bromwich, D.H. and Wang, S-H. 2006a. Recent trends in Antarctic snow accumulation from Polar MM5, ''Philosophical Trans. Royal. Soc. A'', '''364''', 1683-1708.</ref>). A resolution of this apparently conflicting evidence may be that there is a long-term imbalance in this area, which could possibly reflect a response to much more ancient climate changes. An alternate suggestion, based on direct accumulation measurements at South Pole, is that this thickening represents a short period of increased snowfall between 1992 and 2000 (Thompson and Solomon, 2002<ref name="Thompson and Solomon, 2002">Thompson, D. and Solomon, S. 2002. Interpretation of recent southern hemisphere climate change, ''Science'', '''296'''(5569), 895-899.</ref>). The absence of significant atmospheric warming inland, distinct from the global trend of warming atmospheric temperatures, may have forestalled an anticipated increase in snowfall associated with the global trend.<br />
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The only significant exceptions to this broad-scale quiescence of the East Antarctic ice sheet occur on the Cook Ice Shelf and in the mouth of the Totten Glacier where thinning rates in excess of 25 cm/year have been measured (Shepherd and Wingham, 2007<ref name="Shepherd and Wingham, 2007">Shepherd, A. and Wingham, D. 2007. Recent sea-level contributions of the Antarctic and Greenland ice sheets, ''Science'', '''315''' (5818), 1529-1532.</ref>). It remains unknown whether these events are recent, or indeed, whether they are related to changing adjacent ocean conditions, as in the case of the Amundsen Sea outlets, or whether they are just longer-term responses of a regional dynamic origin. Both these areas are the outlets of the ice sheet occupying the two major marine basins lying beneath the ice sheet (Lythe et al., 2001<ref name="Lythe et al, 2001">Lythe, M.B., Vaughan, D.G. and BEDMAP Consortium, 2001. BEDMAP: A new ice thickness and subglacial topographic model of Antarctica, ''Journal of Geophysical Research'', '''106'''(B6), 11335-11351.</ref>).<br />
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The mass balance of the East Antarctic ice sheet has been calculated by many research teams with various sensors and methodologies: +22 &plusmn; 23 Gt/yr (Rignot and Thomas, 2002<ref name="Rignot and Thomas, 2002">Rignot, E. and Thomas, R.H. 2002. Mass balance of polar ice sheets, Science, 297 (5586), 1502-1506 AUG 30 2002.</ref>); -4 &plusmn; 61 (Rignot et al., 2008<ref name="Rignot et al, 2008">Rignot, E., Bamber, J.L., Van Den Broeke, M.R., Davis, C., Yonghong, L., Van Deberg, W.J. and Van Meijgaard, E. 2008. Recent Antarctic ice mass loss from radar interferometry and regional climate modeling, Nature Geoscience, 13 January 2008; doi:10.1038/ngeo102.</ref>); 0 &plusmn; 56 (Velicogna and Wahr, 2006<ref name="Velicogna and Wahr, 2006">Velicogna, I. and Wahr, J. 2006, Measurements of Time-Variable Gravity Show Mass Loss in Antarctica, Science, 311, (5768), 1754 DOI:10.1126/science.1123785.</ref>); and +15.1 &plusmn; 10.7 (Zwally et al., 2005<ref name="Zwally et al, 2005">Zwally, H.J., Giovinetto, M., Li, J., Cornejo, H., Beckley, M., Brenner, A., Saba, J. and Yi, D. 2005. Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002, ''Journal of Glaciology'', '''51'''(175), 509-527.</ref>). The results range from near zero to slightly positive with some of the variations dependent on the time interval investigated. One of the most significant factors giving rise to this uncertainty is that, at present, an ''ad hoc'' interpretation of the thickness changes must be made to determine whether they represent changes in snow surface accumulation, and thus changes in low-density snow and firn, or whether they are dynamic in origin and represent a change in ice, which has a much higher density.<br />
==References==<br />
<references /><br />
[[Category:The instrumental period]]<br />
[[Category:The Antarctic ice sheet]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_climate_of_the_Antarctic_and_its_variabilityThe climate of the Antarctic and its variability2014-08-06T14:34:09Z<p>Tonyp: Changed references to book chapters to page links</p>
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<div>:''This page is part of the topic [[The Antarctic environment in the global system]]''<br />
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In the summer, when the sun is above the horizon for long periods, the Antarctic receives more solar radiation than the tropics, but the highly reflective ice- and snow-covered surfaces reflect much of this radiation back to space, aided by a relatively cloud-free atmosphere that contains little water vapour. This reflection is one of the important feedback mechanisms found in the ice-covered polar regions, because it enhances cooling. Where snow melts, exposing large patches of bare (dark) ground, or where sea ice melts exposing ice free (dark) ocean, solar radiation will be absorbed rather than reflected, and the environment will warm.<br />
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The Polar Plateau of East Antarctica experiences very low temperatures because of its high elevation, the lack of cloud and water vapour in the atmosphere, and the isolation of the region from the relatively warm maritime air masses found over the Southern Ocean. The very cold temperatures in the interior of Antarctica year-round, coupled with its isolation from warm, moist air masses, mean that precipitation there is very low, with only about 5 cm water equivalent falling per year (King and Turner, 1997<ref name="King and Turner, 1997">King, J.C. and Turner, J. 1997. Antarctic meteorology and climatology, Cambridge University Press, Cambridge, UK, 409 pp.</ref>). That makes much of Antarctica a desert and the driest continent on Earth. The low temperatures mean that there is very little evaporation and sublimation, so that although the amount of precipitation is small, it builds up year by year to form the ice sheet. Many blizzards tend to be fallen snow resuspended, rather than new snow.<br />
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Temperatures are much less extreme in the Antarctic coastal region than on the plateau. In general, at most of the coastal stations the monthly mean summer temperatures never rise above freezing point, although daily temperatures may show excursions above freezing in places during summer. There are exceptions. The highest temperatures on the continent are found on the western side of the Antarctic Peninsula where there is a prevailing northwesterly wind; there temperatures can rise to several degrees above freezing during the summer, and monthly means are positive for 2-4 months of the year. Temperatures also tend to be above freezing at times in places like the Schirmacher Oasis near the coast in Dronning Maud Land, where there are ice-free lakes in the austral summer.<br />
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[[File:Figure 1.13 - The Antarctic polar vortex in mid-winter.png|thumb|'''1.13''' The polar vortex above Antarctica (indicated by the red colours) in mid-winter (August) as seen via the temperatures in degrees C on the 50 hPa pressure surface (roughly 20 km elevation above mean sea level). Generated from the NCEP reanalysis using their online, interactive chart drawing system at: www.cdc.noaa.gov/data/reanalysis/reanalysis.shtml.]]<br />
Because of the lack of incoming solar radiation, the Antarctic stratosphere in winter is extremely cold. A strong temperature gradient develops between the continent and mid-latitudes ([[:File:Figure 1.13 - The Antarctic polar vortex in mid-winter.png|Figure 1.13]]), isolating a pool of very cold air above Antarctica. Very strong winds develop along this thermal gradient. They are stronger than the equivalent winds found in the Arctic, because the Equator-to-pole temperature difference is larger in the south. The pool of cold air and its strong surrounding winds together form the polar vortex. It plays an important part in determining the atmospheric circulation of the high southern latitudes, as well as in the formation of the ozone hole, where the polar vortex acts as a &lsquo;containment vessel&rsquo; allowing chlorofluorocarbon compounds (CFCs) to build up during the winter.<br />
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The Antarctic climate system varies on time scales from the sub-annual to the millennial and is closely coupled to other parts of the global climate system. On the longest time scales it fluctuates on Milankovitch frequencies (20 ka, 41 ka, 100 ka) in response to variations in the Earth&rsquo;s orbit around the sun that cause regular variations in the Earth&rsquo;s climate on these recurrent time scales. We discuss the variability of the Antarctic climate system on these longer timescales in [[Antarctic climate and environment history in the pre-instrumental period]].<br />
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Proxy data from ice cores show that since the Last Glacial Maximum (LGM) at about 21 ka before present (BP) there have been a number of climatic fluctuations across the continent. One of the most marked was the Mid Holocene warm period, which is present in various records from Antarctica. Ice cores also reveal shifts in the intensity of the atmospheric circulation, with the westerlies weakening at 5,200-5,400 years ago and strengthening around 1,200 years ago.<br />
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Because long time-series of observations did not begin until the IGY (1957-58), the instrumental period in Antarctic is only about 50 years long (measurements started before then but were not continuous). Since proxy data show oscillations on longer time scales than 50 years, it is accepted that the instrumental period only provides a snapshot of change in the Antarctic. Nevertheless, it shows the complexity of change and a mix of natural climate variability and anthropogenic influence, as discussed in [[Observations, data accuracy and tools]] and [[Antarctic climate and environment change in the instrumental period]].<br />
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Reliable weather charts for high southern latitudes have only been available since the late 1970s. They reveal a great deal about the patterns in the atmospheric circulation of high southern latitudes. The most pronounced climate variability over this period is evident in changes in the SAM. Over the last few decades there has been a marked drop in pressure around the Antarctic coast and an increase in mid-latitudes. The increase in the pressure gradient across the Southern Ocean has strengthened the surface winds, which in turn has affected ocean currents and the distribution of sea ice. Studies of coupled ocean-atmosphere general circulation models show that the strength of the SAM should increase as the Earth warms, confirming what is known from observations.<br />
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But the strength of the SAM is also modulated by the ozone hole over Antarctica. Stratospheric ozone is an important constituent of the upper atmosphere above the Antarctic, where ozone levels began to decline in the late 1970s following widespread releases of CFCs and halons into the atmosphere. We now know that the presence of CFCs in the Antarctic stratosphere results in a complex chemical reaction during the spring that destroys virtually all ozone at altitudes between 14 and 22 km, especially within the polar vortex where temperatures are coldest. The depletion in ozone maintains the cooling within the polar vortex, which accentuates the winds around the vortex. This strengthening of the winds at high altitude in spring then propagates downwards in the atmosphere through time, strengthening surface winds during the summer and autumn. Thus one effect of the ozone hole has been to accentuate the SAM signal, further strengthening the westerly winds around the Antarctic during the summer and autumn.<br />
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In the strengthened SAM, with its stronger winds, we see two examples of changes from outside the region having a profound impact on the Antarctic environment. One impact comes from the CFCs responsible for the &lsquo;ozone hole&rsquo;, and the other comes from greenhouse gas emissions responsible for global warming. Both CFCs and greenhouse gases have mainly been released in the Northern Hemisphere during the industrial era. Both have had, and continue to have, a profound effect on the radiation balance of the Antarctic atmosphere.<br />
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Although the Antarctic is far removed from where most of the solar energy enters the system at tropical latitudes, it is still influenced by variability in tropical conditions, and signals of low latitude climate variability can be identified in Antarctica and the Southern Ocean (Turner, 2004<ref name="Turner, 2004">Turner, J. 2004. The El Ni&ntilde;o-Southern Oscillation and Antarctica, ''International Journal of climatology'', '''24''', 1-31.</ref>). In addition, there is increasing evidence that signals can also be transmitted in the opposite direction from high to low latitudes.<br />
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Statistically significant long-range linkages between high and low latitudes go by the name of teleconnections. They can take place via the atmosphere and/or the ocean, although the timescales are usually rather different. The most rapid teleconnections generally occur via the atmosphere, with storm track changes occurring on the scale of days or weeks. The El Ni&ntilde;o-Southern Oscillation (ENSO) is one of the largest climatic cycles on Earth, functioning on the scale of years to decades. It has its origins in the tropical Pacific Ocean, but its effects can be felt across the world. As discussed in a number of places in this volume, ENSO signals can be identified in the physical and biological environment of the Antarctic, although some of the links are not robust and there can be large differences in the extra-tropical response to near-identical events in the tropics.<br />
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The circulation of the upper layers of the ocean can change over months to years, but the deep ocean and the global thermohaline circulation (THC) require decades to centuries to respond. At the other extreme, fast wave propagation in the ocean takes place on timescales of just a few days.<br />
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About 30 years of reliable atmospheric reanalysis outputs are available for the high southern latitudes, so it has been possible to establish the nature of the broad-scale teleconnections typical of the Southern Hemisphere. There is evidence of decadal timescale variability in some of these linkages, but with such a short data set it is not possible at present to gain insight into how the teleconnections may vary on longer timescales. High resolution ice core records collected from areas where the rate of precipitation of snow is high shed some light on teleconnections over the century timescale, and where the rate of accumulation is high enough they can even give seasonal data, which is important since some teleconnections are only present in individual seasons.<br />
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Links between the climates of the northern and southern hemispheres can be found, but they vary with time as discussed in [[The last million years]] and [[Holocene climate changes]]. Through most of the Holocene (roughly the past 12,000 years) northern hemisphere events have lagged behind southern hemisphere ones by several hundred years. That has changed in recent decades, and the northern hemisphere signal of rising temperature since about 1850 AD has paralleled the southern hemisphere one. Temperature change in the two hemispheres now appears to be synchronous - a radical departure from former times, which suggests a new and different forcing, most likely related to anthropogenic activity in the form of enhanced greenhouse gases. Comparing data on winds and temperatures from northern and southern hemisphere ice cores confirms that the wind/temperature fields of today differ from those of the recent past indicating that the modern day atmosphere is not an analogue for that of the so-called Medieval Warm Period.<br />
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Changes associated with the changing climate are now plainly evident in the Antarctic, and will be discussed in detail in both the [[Antarctic climate and environment history in the pre-instrumental period|pre-instrumental]] and [[Antarctic climate and environment change in the instrumental period|instrumental]] periods. They range from the glaringly obvious, like the collapse of the Larsen B ice shelf in February-March 2002, and the warming of various parts of the Antarctic Peninsula over the past 50 years (King, 1994<ref name="King, 1994">King, J.C. 1994. Recent variability in the Antarctic Peninsula, ''International Journal of Climatology'', '''14'''(4), 357-369.</ref>), to the rather more subtle, like the 0.2&ordm;C warming of the Southern Ocean, which - though a small amount - represents a major transfer of heat when summed over a vast area.<br />
==References==<br />
<references /><br />
[[Category:The Antarctic environment in the global system]]<br />
[[Category:Antarctic climate]]<br />
[[Category:Ozone]]<br />
[[Category:Teleconnections]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_Australian_sector_in_the_instrumental_periodThe Australian sector in the instrumental period2014-08-06T14:34:09Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[The Southern Ocean in the instrumental period]]''<br />
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[[File:Figure 4.24 - Map of repeat section locations and deep potential temperature-salinity curves.png|thumb|'''4.24''' Top) Map indicating location of repeat sections along which changes in bottom water properties have been assessed. The Australian Antarctic Basin is supplied by two sources of Antarctic Bottom Water: the Ross Sea and Ad&eacute;lie Land. Lower) Changes in the deep potential temperature &ndash; salinity curves along 115&deg;E, over the continental rise (61-63.3&deg;S, left) and further offshore (56.5 &ndash; 61&deg;S, right). From Rintoul (2007<ref name="Rintoul, 2007">Rintoul, S.R. 2007. Rapid freshening of Antarctic Bottom Water formed in the Indian and Pacific Oceans, ''Geophys. Res. Lett.'', '''34''', L06606, doi:10.1029/2006GL028550.</ref>).]]<br />
Knowledge of the circulation in the Australian sector of the Southern Ocean has increased significantly in the last decade. Repeat hydrographic sections ([[:File:Figure 4.24 - Map of repeat section locations and deep potential temperature-salinity curves.png|Figure 4.24]]), moorings and satellite altimeter measurements have provided new insights into the structure, variability and dynamics of the ACC, water mass formation and the overturning circulation. The mean baroclinic transport of the ACC south of Australia is 147&plusmn;10 Sv (Rintoul and Sokolov, 2001<ref name="Rintoul and Sokolov, 2001">Rintoul, S.R. and Sokolov, S. 2001. Baroclinic transport variability of the Antarctic Circumpolar Current south of Australia (WOCE repeat section SR3), ''Journal of Geophysical Research'', '''106''', 2795-2814.</ref>), consistent with recent estimates of the flow leaving the Pacific basin through Drake Passage (136&plusmn;8 Sv, Cunningham et al., 2003<ref name="Cunningham et al, 2003">Cunningham, S.A., Alderson, S.G., King, B.A. and Brandon, M.A. 2003. Transport and variability of the Antarctic Circumpolar Current in Drake Passage. J. Geophys. Res., 108 (C5), 8084, doi:10.1029/2001JC001147.</ref>) and the Indonesian Throughflow (Meyers et al., 1995<ref name="Meyers et al, 1995">Meyers, G., Bailey, R.J. and Worby, A.P. 1995. Geostrophic transport of Indonesian Throughflow, Deep-Sea Res., ''I'', '''42''', 1163-1174.</ref>). A multi-year time series derived from XBT sections and altimetry shows significant interannual variability (with a standard deviation of 4.3 Sv) but no trend in transport (Rintoul et al., 2002<ref name="Rintoul et al, 2002">Rintoul, S.R., Sokolov, S. and Church, J. 2002. A six year record of baroclinic transport variability of the Antarctic Circumpolar Current at 140CE, derived from XBT and altimeter measurements, Journal of Geophysical Research &ndash; Oceans, 107 (C10): art.no. 3155.</ref>). High resolution hydrographic sections reveal that the ACC fronts consist of multiple jets, aligned with streamlines that can be traced using maps of absolute sea surface height (Sokolov and Rintoul, 2002<ref name="Sokolov and Rintoul, 2002">Sokolov, S. and Rintoul, S.R. 2002. The structure of Southern Ocean fronts at 140E, ''Journal of Marine Systems'', '''37''', 151-184.</ref>, 2007<ref name="Sokolov and Rintoul, 2007">Sokolov, S. and Rintoul, S.R. 2007. Multiple jets of the Antarctic Circumpolar Current south of Australia, ''Journal of Physical Oceanography'', '''37''', 1394-1412.</ref>). Eddy fluxes estimated from current meter moorings confirm that the eddies transport heat poleward and zonal momentum downward (Phillips and Rintoul, 2000<ref name="Phillips and Rintoul, 2000">Phillips, H.E. and Rintoul, S.R. 2000. Eddy variability and energetics from direct current measurements in the Antarctic Circumpolar Current south of Australia, ''Journal of Physical Oceanography'', '''30''', 3050-3076.</ref>). A cyclonic gyre lies between the ACC and the Antarctic continent, closed in the west by a northward boundary current along the edge of the Kerguelen Plateau (McCartney and Donohue, 2007<ref name="McCartney and Donohue, 2007">McCartney, M.S. and Donohue, K.A. 2007. A deep cyclonic gyre in the Australian-Antarctic Basin, ''Progress in Oceanography'', '''75''', 675-750.</ref>).<br />
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The ACC belt in the Australian sector has warmed in recent decades, as found elsewhere in the Southern Ocean (Gille, 2002<ref name="Gille, 2002">Gille, S.T. 2002. Warming of the Southern Ocean since the 1950s, ''Science'', '''295'''(5558), 1275-1277, doi:10.1126/science.1065863.</ref>, 2008<ref name="Gille, 2008">Gille, S.T. 2008. Decadal-scale temperature trends in the Southern Hemisphere ocean, ''J. Clim.'', '''21'''(18), 4749-4765.</ref>; Levitus et al., 2000<ref name="Levitus et al, 2000">Levitus, S., Antanov, J.I., Boyer, T.P. and Stephens, C. 2000. Warming of the world ocean, ''Science'', '''287''' (5461), 2225-2229.</ref>, 2005<ref name="Levitus et al, 2005">Levitus, S., Antonov, J. and Boyer, T. 2005. Warming of the world ocean, ''Geophysical Res. Letters'', '''32''', L02604, doi:10.1029/2004GL021592.</ref>; Willis et al., 2004<ref name="Willis et al, 2004">Willis, J.K., Roemmich, D. and Cornuelle, B. 2004. Interannual variability in upper ocean heat content, temperature, and thermosteric expansion on global scales, J. Geophys. Res., 109, doi:10.1029/2003JC002260.</ref>; B&ouml;ning et al., 2008<ref name="B&ouml;ning et al, 2008">B&ouml;ning, C. W., A. Dispert, M. Visbeck, S. R. Rintoul, and F. Schwarzkopf, 2008. Observed multi-decadal ocean warming and density trends across the Antarctic Circumpolar Current, Submitted.</ref>). The southward shift of the ACC fronts has caused warming through much of the water column, resulting in a strong increase in sea level south of Australia between 1992 and 2005 (Sokolov and Rintoul, 2003<ref name="Sokolov and Rintoul, 2003">Sokolov, S. and Rintoul, S.R. 2003. The subsurface structure of interannual temperature anomalies in the Australian sector of the Southern Ocean, Journal of Geophysical Research, 108 (C9): Art. No. 3285.</ref>; Morrow et al., 2008<ref name="Morrow et al, 2008">Morrow, R., Valladeau, G., and Sallee, J. B. 2008. Observed subsurface signature of Southern Ocean sea level rise. Progress in Oceanography, 77(4), 351-366.</ref>). However, there is no observational evidence of the increase in ACC transport also predicted by the models (B&ouml;ning et al., 2008<ref name="B&ouml;ning et al, 2008">B&ouml;ning, C. W., A. Dispert, M. Visbeck, S. R. Rintoul, and F. Schwarzkopf, 2008. Observed multi-decadal ocean warming and density trends across the Antarctic Circumpolar Current, Submitted.</ref>). Recent studies suggest the ACC transport is insensitive to wind changes because the ACC is in an &ldquo;eddy-saturated&rdquo; state, in which an increase in wind forcing causes an increase in eddy activity rather than a change in transport of the current (Hallberg and Gnanadesikan, 2006<ref name="Hallberg and Gnanadesikan, 2006">Hallberg, R. and Gnanadesikan, A. 2006. The role of eddies in determining the structure and response of the wind-driven Southern Hemisphere overturning: Initial results from the Modelling Eddies in the Southern Ocean project, ''J. Phys. Oceanogr.'', '''36''', 3312-3330.</ref>; Meredith and Hogg, 2006<ref name="Meredith and Hogg, 2006">Meredith, M.P. and Hogg, A.M. 2006. Circumpolar response of Southern Ocean eddy activity to a change in the Southern Annular Mode, Geophys. Res. Letters, 33, doi: 10.1029/2006GL026499.</ref>).<br />
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Changes have been observed in several water masses in the Australian sector between the 1960s and the present (Aoki et al., 2005<ref name="Aoki et al, 2005">Aoki, S., Rintoul, S.R., Ushio, S., Watanabe, S. and Bindoff, N.L. 2005. Freshening of the Adelie Land Bottom Water near 140&deg;E, ''Geophys. Res. Lett.'', '''32''', L23601, doi10.1029/2005GL024246.</ref>). Waters north of the ACC have cooled and freshened on density surfaces corresponding to intermediate waters (neutral densities between 26.8 and 27.2 kg m<sup>-3</sup>). South of the ACC, waters have warmed and become higher in salinity and lower in oxygen on neutral density surfaces between 27.7 and 28.0 kg m<sup>-3</sup> (the Upper Circumpolar Deep Water &ndash;UCDW). The changes south of the ACC are consistent with a shoaling of the interface between the warm, salty, low oxygen UCDW and the cold, fresh, high oxygen surface water that overlies it. The pattern of water mass change observed in the Australian sector is consistent with the &ldquo;fingerprint&rdquo; of anthropogenic climate change in a coupled climate model (Banks and Bindoff, 2003<ref name="Banks and Bindoff, 2003">Banks, H. T. and Bindoff, N.L. 2003. Comparison of observed temperature and salinity changes in the Indo-Pacific with results from the coupled climate model HadCM3: processes and mechanisms, ''J. Climate'', '''16''', 156-166.</ref>).<br />
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The Antarctic Bottom Water (AABW) in the Australian Antarctic basin has freshened significantly since the early 1970s. Whitworth (2002<ref name="Whitworth, 2002">Whitworth, T. 2002. Two modes of bottom water in the Australian-Antarctic Basin, ''Geophys. Res. Lett.'', '''29'''(5), 1073, doi:10.1029/2001GL014282.</ref>) detected a shift toward fresher AABW after 1993, concluding that &ldquo;two modes&rdquo; of AABW were present in the basin, with the fresher mode becoming more prominent in the 1990s. More recent studies have documented a monotonic trend toward fresher, and in most cases warmer, bottom water between the late 1960s and the present rather than a bi-modal distribution. Aoki et al. (2005<ref name="Aoki et al, 2005">Aoki, S., Rintoul, S.R., Ushio, S., Watanabe, S. and Bindoff, N.L. 2005. Freshening of the Adelie Land Bottom Water near 140&deg;E, ''Geophys. Res. Lett.'', '''32''', L23601, doi10.1029/2005GL024246.</ref>) used a hydrographic time series with nearly annual resolution between 1993 and 2002 to show a steady decline in salinity of the bottom water at 140&ordm;E. By using repeat observations from the same location and same season, they could demonstrate that the trend was not the result of aliasing of spatial or temporal variability. Rintoul (2007<ref name="Rintoul, 2007">Rintoul, S.R. 2007. Rapid freshening of Antarctic Bottom Water formed in the Indian and Pacific Oceans, ''Geophys. Res. Lett.'', '''34''', L06606, doi:10.1029/2006GL028550.</ref>) showed that the deep potential temperature &ndash; salinity relationship of the entire basin had shifted towards lower salinity between the early 1970s and 2005. The average rate of freshening at 115 E is 7 ppm/decade, which can be compared to a mean freshening rate of 12 ppm/decade in the North Atlantic, at similar distances downstream of the source of dense water (Dickson et al., 2002<ref name="Dickson et al, 2002">Dickson, B., I. Yashayaev, J. Meincke, B. Turrell, S. Dye and J. Holfort. 2002. Rapid freshening of the deep North Atlantic Ocean over the past four decades, ''Nature'', '''416''', 832-837.</ref>). These results suggest that the sources of dense water in both hemispheres have been responding to changes in high latitude climate.<br />
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The abyssal layers of the Australian Antarctic Basin are supplied by the two primary sources of bottom water that lie outside of the Weddell Sea: a fresh variety formed along the Ad&eacute;lie Land coast (144&ordm; E) and a salty variety produced in the Ross Sea (Rintoul, 1998<ref name="Rintoul, 1998">Rintoul, S.R. 1998. On the origin and influence of Adelie Land Bottom Water. In: Ocean, Ice and Atmosphere: Interactions at the Antarctic Continental Margin, S. Jacobs and R. Weiss (eds.), Antarctic Research Series, 75, 151-171, American Geophysical Union, Washington.</ref>). The changes observed in the Australian Antarctic Basin therefore reflect freshening of the AABW formed in the Indian and Pacific sectors of the Southern Ocean, which accounts for about 40% of the total production of AABW (Orsi et al., 1999<ref name="Orsi et al, 1999">Orsi, A.H., Johnson, G.C. and Bullister, J.B. 1999. Circulation, mixing and production of Antarctic Bottom Water, ''Prog. Oceanog.'', '''43''', 55-109.</ref>).<br />
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The cause of the freshening of AABW in the Australian sector is not yet fully understood. Changes in precipitation, sea ice formation and melt, ocean circulation patterns, and melt of floating glacial ice around the Antarctic margin could all influence the salinity where dense water is formed. Oxygen isotope measurements in the Ross Sea indicate that an increase in the supply of glacial melt-water has contributed to the large freshening of shelf waters observed there in recent decades (Jacobs et al., 2002<ref name="Jacobs et al, 2002">Jacobs, S.S., Giulivi, C.F. and Mele, P.A. 2002. Freshening of the Ross Sea during the late 20<sup>th</sup> century. Science, 297(5580), 386-389, doi:10.1126/science.1069574.</ref>). The most likely source of the increased supply of melt-water is the rapidly thinning glaciers and ice shelves of the Amundsen Sea, including the Pine Island Glacier (Jacobs et al., 2002<ref name="Jacobs et al, 2002">Jacobs, S.S., Giulivi, C.F. and Mele, P.A. 2002. Freshening of the Ross Sea during the late 20<sup>th</sup> century. Science, 297(5580), 386-389, doi:10.1126/science.1069574.</ref>), where enhanced basal melt has been linked to warmer ocean temperatures (Rignot and Jacobs, 2002<ref name="Rignot and Jacobs, 2002">Rignot, E.J., and Jacobs, S.S. 2002. Rapid Bottom Melting Widespread near Antarctic Ice Sheet Grounding Lines, ''Science'', '''296''', 2020-2023.</ref>; Shepherd et al., 2004<ref name="Shepherd et al, 2004">Shepherd, A., Wingham, D. and Rignot, E. 2004. Warm ocean is eroding West Antarctic ice sheet, ''Geophys. Res. Lett.'', '''31''', L23402, doi:10.1029/2004GL021106.</ref>). While most of the ice sheet in East Antarctica appears to be gaining mass, glaciers draining parts of the Wilkes Land coast where the Ad&eacute;lie Land bottom water is formed have decreased in elevation (Shepherd and Wingham, 2007<ref name="Shepherd and Wingham, 2007">Shepherd, A. and Wingham, D. 2007. Recent sea-level contributions of the Antarctic and Greenland ice sheets, ''Science'', '''315''' (5818), 1529-1532.</ref>), and the floating ice in this sector thinned between 1992 and 2002 (Zwally et al., 2005<ref name="Zwally et al, 2005">Zwally, H.J., Giovinetto, M., Li, J., Cornejo, H., Beckley, M., Brenner, A., Saba, J. and Yi, D. 2005. Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002, ''Journal of Glaciology'', '''51'''(175), 509-527.</ref>). Therefore increased supply of glacial melt-water may have played a role in the freshening of both the Ad&eacute;lie Land and Ross Sea Bottom Water.<br />
==References==<br />
<references /><br />
[[Category:The instrumental period]]<br />
[[Category:The Southern Ocean]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_Antarctic_Peninsula_cryosphere_in_the_instrumental_periodThe Antarctic Peninsula cryosphere in the instrumental period2014-08-06T14:34:08Z<p>Tonyp: Changed book section reference to page link</p>
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<div>:''This page is part of the topic [[The ice sheet in the instrumental period]]''<br />
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The Antarctic Peninsula north of 70&deg;S represents less than 1% of the area of the entire grounded Antarctic ice sheet, but receives nearly 10% of its snowfall (van Lipzig et al., 2004a<ref name="Lipzig et al, 2004a">Van Lipzig, N.P.M., King, J.C., Lachlan-Cope, T.A. and Van Den Broeke, M.R. 2004a. Precipitation, Sublimation and Snow Drift in the Antarctic Peninsula Region from a Regional Atmospheric Model, Journal of Geophysical Research, 109, doi 10.1029/2004JD004701.</ref>). A third of this area lies close to the coast and below 200 m elevation, where summer temperatures are frequently above 0&deg;C, so that this is the only part of continental Antarctica that experiences substantial summer melt. About 80% of its area is classed as a percolation zone (Rau and Braun, 2002<ref name="Rau and Braun, 2002">Rau, F. and Braun, M. 2002. The regional distribution of the dry-snow zone on the Antarctic Peninsula north of 70&deg; S, ''Annals of Glaciology'', '''34''', 95-100.</ref>), and melt water run-off is a significant component in its mass balance (Vaughan, 2006<ref name="Vaughan, 2006">Vaughan, D.G. 2006. Recent trends in melting conditions on the Antarctic Peninsula and their implications for ice-sheet mass balance, Arctic, ''Antarctic and Alpine Research'', '''38''', 147-152.</ref>).<br />
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Beginning in the early 1990s, climatologists noted a pronounced warming trend present in the instrumental record from the Antarctic Peninsula stations (King, 1994<ref name="King, 1994">King, J.C. 1994. Recent variability in the Antarctic Peninsula, ''International Journal of Climatology'', '''14'''(4), 357-369.</ref>; Vaughan and Doake, 1996<ref name="Vaughan and Doake, 1996">Vaughan, D.G. and Doake, C.S.M. 1996. Recent atmospheric warming and retreat of ice shelves on the Antarctic Peninsula, ''Nature'', '''379''', 328-330.</ref>; Skvarca et al., 1998<ref name="Skvarca et al, 1998">Skvarca, P., Rack, W., Rott, H., Ibarz&aacute;bal, T. and Don&aacute;ngelo, Y. 1998. Evidence of recent climatic warming on the eastern Antarctic Peninsula, ''Annals of Glaciology'', '''27''', 628-635.</ref>). This region has the highest density of long-term weather observations in the Antarctic, dating back to 1903 for Orcadas Station. Rates of warming on the Antarctic Peninsula are some of the fastest measured in the Southern Hemisphere (~3&ordm;C in the last 50 years) (King, 2003<ref name="King, 2003">King, J.C. 2003. Antarctic Peninsula climate variability and its causes as revealed by analysis of instrumental records, Antarctic Peninsula Climate Variability: Historical and Paleoenvironmental Perspectives. Antarctic Research Series, 79, edited by E. Domack, et al., pp. 17-30, AGU, Washington, DC.</ref>; Vaughan et al., 2003<ref name="Vaughan et al, 2003">Vaughan, D.G., Marshall, G., Connolley, W.M., Parkinson, C., Mulvaney, R., Hodgson, D.A., King, J.C., Pudsey, C.J., Turner, J. and Wolff, E. 2003. Recent rapid regional climate warming on the Antarctic Peninsula, ''Climatic Change'', '''60''', 243-274.</ref>) and there has been a clear increase in the duration and intensity of summer melting conditions by up to 74% since 1950 (Vaughan, 2006<ref name="Vaughan, 2006">Vaughan, D.G. 2006. Recent trends in melting conditions on the Antarctic Peninsula and their implications for ice-sheet mass balance, Arctic, ''Antarctic and Alpine Research'', '''38''', 147-152.</ref>).<br />
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A recent study has shown that circa 2005, the Antarctic Peninsula was contributing to global sea level rise through enhanced melt and glacier acceleration at a rate of 0.16 &plusmn; 0.06 mm/yr (which can be compared to an estimated total Antarctic Peninsula ice volume of 95,200 km<sup>3</sup>, equivalent to 242 mm of sea-level) (Pritchard and Vaughan, 2007<ref name="Pritchard and Vaughan, 2007">Pritchard, H.D. and Vaughan, D.G. 2007. Widespread acceleration of tidewater glaciers on the Antarctic Peninsula, J. Geophys. Res., 112, F03S29, doi:10.1029/2006JF000597.</ref>). Although it is known that Antarctic Peninsula glaciers drain a large volume of ice, it is not yet certain how much of the increased outflow is balanced by increased snow accumulation. One estimate of mass change due primarily to temperature-dependent increases in snowfall on the peninsula suggested a contribution to sea level of approximately -0.003 mm/yr (Morris and Mulvaney, 2004<ref name="Morris and Mulvaney, 2004">Morris, E.M. and Mulvaney, R. 2004. Recent variations in surface mass balance of the Antarctic Peninsula Ice Sheet, ''J. Glaciol.'', '''50''', 257-267.</ref>).<br />
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==Glaciers==<br />
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The ice-cover on the Antarctic Peninsula is a complex alpine system of more than 400 individual glaciers that drain a high and narrow mountain plateau. The tidewater/marine glacier systems in this region (excluding ice shelves and the former tributary glaciers of Larsen A, B and Wordie ice shelves) have an area of 95,000 km<sup>2</sup> and a mean net annual accumulation of 143 &plusmn; 29 Gt/yr (after van Lipzig et al., 2004a<ref name="Lipzig et al, 2004a">Van Lipzig, N.P.M., King, J.C., Lachlan-Cope, T.A. and Van Den Broeke, M.R. 2004a. Precipitation, Sublimation and Snow Drift in the Antarctic Peninsula Region from a Regional Atmospheric Model, Journal of Geophysical Research, 109, doi 10.1029/2004JD004701.</ref>). Changes in the ice margin around the Antarctic Peninsula based on data from 1940 to 2001 have been compiled (Ferrigno et al., 2002<ref name="Ferrigno et al, 2002">Ferrigno, J.G., Williams, R.S. Jr. and Thomson, J.W. 2002. Joint U.S. Geological Survey &ndash; British Antarctic Survey Fact Sheet FS-017-02, 2p.</ref>, 2006<ref name="Ferrigno et al, 2006">Ferrigno, J.G., Cook, A.J., Foley, K.M., Williams, R.S. Jr., Swithinbank, C., Fox, A.J., Thomson, J.W. and Sievers, J. 2006. Coastal-Change and Glaciological Maps of the Trinity Peninsula area, Antarctica: 1843-2002 (USGS map number I-2600-A).</ref>; Cook et al. 2005<ref name="Cook et al, 2005">Cook, A., Fox, A., Vaughan, D. and Ferrigno, J. 2005, Retreating glacier fronts on the Antarctic Peninsula over the past half-century, ''Science'', '''308''', 541-544.</ref>). Analysis of the results revealed that of the 244 marine glaciers that drain the ice sheet and associated islands, 212 (87%) have shown overall retreat since their earliest known position (which, on average, was 1953). The other 32 glaciers have shown overall advance, but these advances are generally small in comparison with the scale of retreats observed.<br />
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[[File:Figure 4.35 - Overall change observed in glacier fronts since earliest records.png|thumb|'''4.35''' Overall change observed in glacier fronts since earliest records. (From Cook et al., 2005<ref name="Cook et al, 2005">Cook, A., Fox, A., Vaughan, D. and Ferrigno, J. 2005, Retreating glacier fronts on the Antarctic Peninsula over the past half-century, ''Science'', '''308''', 541-544.</ref>).]]<br />
[[File:Figure 4.36 - Change in Antarctic Peninsula glaciers over time and by latitude.png|thumb|'''4.36''' Change in Antarctic Peninsula glaciers over time and by latitude. Prior to 1945 the limit of glacier retreat was north of 64&deg;S; in 1955 it was in the interval 64-66&deg;S; in 1960 between 66-68&deg;S and in 1965 between 68-70&deg;S. (from Cook et al., 2005<ref name="Cook et al, 2005">Cook, A., Fox, A., Vaughan, D. and Ferrigno, J. 2005, Retreating glacier fronts on the Antarctic Peninsula over the past half-century, ''Science'', '''308''', 541-544.</ref>).]]<br />
The glaciers that have advanced are not clustered in any pattern, but are evenly scattered down the coast ([[:File:Figure 4.35 - Overall change observed in glacier fronts since earliest records.png|Figure 4.35]]). Examination of the timing of changes along the peninsula indicates that from 1945 until 1954 there were more glaciers advancing (62%) than retreating (38%). After that time, the number retreating has risen, with 75% in retreat in the period 2000-2004. The results indicate a transition between mean advance and mean retreat; a southerly migration of that transition at a time of ice shelf retreat and progressive atmospheric warming; and a clear regime of retreat which now exists across the Antarctic Peninsula ([[:File:Figure 4.36 - Change in Antarctic Peninsula glaciers over time and by latitude.png|Figure 4.36]]). The rapidity of the migration suggests that atmospheric warming may not be the sole driver of glacier retreat in this region. Glaciers with fully grounded marine termini exhibit unusually complex responses to changing mass balance because in addition to the normal forcings they are also subject to oceanographic forcing and subglacial topography. Future analysis of changes in all boundary conditions may reveal why the glaciers have responded in this way.<br />
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A recent study of flow rates of tidewater glaciers has revealed a widespread acceleration of ice flow across the Peninsula (Pritchard and Vaughan, 2007<ref name="Pritchard and Vaughan, 2007">Pritchard, H.D. and Vaughan, D.G. 2007. Widespread acceleration of tidewater glaciers on the Antarctic Peninsula, J. Geophys. Res., 112, F03S29, doi:10.1029/2006JF000597.</ref>). This widespread acceleration trend was attributed not to meltwater-enhanced lubrication or increased snowfall but to a dynamic response to frontal retreat and thinning. Measurements were taken from over 300 glaciers on the west coast through nine summers from 1992 to 2005. They showed that overall flow rate increased by 10% and that this trend is greater than the seasonal variability in flow rate. A comparison of measurements between the years 1993 and 2003 only (with profiles tailored to optimize coverage in just these years) revealed a slightly greater overall acceleration of 12.4 &plusmn; 0.9%.<br />
<br />
The loss of ice shelves (described [[The Antarctic Peninsula cryosphere in the instrumental period#Ice shelves|below]]) has caused acceleration of the glaciers that fed them (Rignot et al., 2004a<ref name="Rignot et al, 2004a">Rignot, E., Casassa, G., Gogineni, P., Krabill, W., Rivera, A. and Thomas, R. 2004a. Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B ice shelf, Geophys. Res. Let., 31, doi:10.1029/2004GL020679.</ref>, 2005<ref name="Rignot et al, 2005">Rignot, E., Casassa, G., Gogineni, P., Kanagaratnam, P., Krabill, W., Pritchard, H., Rivera, A., Thomas, R., Turner, J. and Vaughan, D.G. 2005. Recent ice loss from the Fleming and other glaciers,Wordie Bay, West Antarctic Peninsula, Geophys. Res. Let., 32, doi:10.1029/2004GL021947.</ref>; Rott et al., 1996<ref name="Rott et al, 1996">Rott, H., Skvarca, P. and Nagler, T. 1996. Rapid collapse of Northern Larsen Ice Shelf, Antarctica, ''Science'', '''271''', 788-792.</ref>; Scambos et al., 2004<ref name="Scambos et al, 2004">Scambos, T.A., Bohlander, J., Shuman, C. and Skvarca, P. 2004. Glacier acceleration and thinner after ice shelf collapse in the Larsen B embayment, Antarctica, Geophys. Res. Lett., 31, doi:10.1029/2004GL020670.</ref>) creating locally high imbalances in ice mass. Immediately after break-up, glaciers flowing into the now-collapsed sections of the Larsen Ice Shelf accelerated to speeds of 2 to 8 times the pre-disintegration flow rate (Rignot et al., 2004a<ref name="Rignot et al, 2004a">Rignot, E., Casassa, G., Gogineni, P., Krabill, W., Rivera, A. and Thomas, R. 2004a. Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B ice shelf, Geophys. Res. Let., 31, doi:10.1029/2004GL020679.</ref>; Scambos et al., 2004<ref name="Scambos et al, 2004">Scambos, T.A., Bohlander, J., Shuman, C. and Skvarca, P. 2004. Glacier acceleration and thinner after ice shelf collapse in the Larsen B embayment, Antarctica, Geophys. Res. Lett., 31, doi:10.1029/2004GL020670.</ref>). The glaciers flowing into the Wordie Ice Shelf also accelerated following ice shelf loss, and have been losing mass to the ocean over the last decade (Rignot et al., 2005<ref name="Rignot et al, 2005">Rignot, E., Casassa, G., Gogineni, P., Kanagaratnam, P., Krabill, W., Pritchard, H., Rivera, A., Thomas, R., Turner, J. and Vaughan, D.G. 2005. Recent ice loss from the Fleming and other glaciers,Wordie Bay, West Antarctic Peninsula, Geophys. Res. Let., 32, doi:10.1029/2004GL021947.</ref>). One of these, Fleming Glacier, accelerated by about 50% during the period 1974-2003, and the region was losing mass at 18 &plusmn; 5.4 Gt/yr. A field campaign carried out in December 2008 using GPS measurements and an airborne laser survey confirmed that the glacier maintains these high flow rates and experiences a pronounced ice thinning (Wendt et al., In Press). The ice flux increase may be partially offset by increased precipitation in the western Peninsula (Turner et al., 2005b<ref name="Turner et al, 2005b">Turner, J., Lachlan-Cope, T.A., Colwell, S. and Marshall, G.J. 2005b. A positive trend in western Antarctic Peninsula precipitation over the last 50 years reflecting regional and Antarctic-wide atmospheric circulation changes, ''Ann. Glaciol.'', '''41''', 85-91.</ref>), but both ice shelves (Fox and Vaughan, 2005<ref name="Fox and Vaughan, 2005">Fox, A.J. and Vaughan, D.G. 2005. The retreat of Jones Ice Shelf, Antarctic Peninsula, ''J. Glaciol.'', '''51'''(175), 555-560.</ref>) and glaciers in the west (Pritchard and Vaughan, 2007<ref name="Pritchard and Vaughan, 2007">Pritchard, H.D. and Vaughan, D.G. 2007. Widespread acceleration of tidewater glaciers on the Antarctic Peninsula, J. Geophys. Res., 112, F03S29, doi:10.1029/2006JF000597.</ref>) continue to retreat. The combined estimate of mass loss (as of 2005) was 43 &plusmn; 7 Gt/yr, but a more recent assessment of the region suggests this rate has slowed (28&plusmn;45 Gt/yr, Rignot et al., 2008<ref name="Rignot et al, 2008">Rignot, E., Bamber, J.L., Van Den Broeke, M.R., Davis, C., Yonghong, L., Van Deberg, W.J. and Van Meijgaard, E. 2008. Recent Antarctic ice mass loss from radar interferometry and regional climate modeling, Nature Geoscience, 13 January 2008; doi:10.1038/ngeo102.</ref>). In addition to the increase in flow rates, a recent study has revealed profound dynamic thinning of collapsed-ice shelf tributary glaciers flowing from the Antarctic Peninsula plateau to both east and west coasts (Pritchard et al., submitted). Analysis of ICESat laser altimeter data, processed along-track for the period 2003-2007, showed how surface elevation has changed over the whole of the Antarctic Peninsula. The high, central plateau and slow flowing ice caps thickened at rates as high as 1 m/yr. In contrast, some of the highest rates of thinning recorded either in Antarctica or Greenland (up to tens of metres per year) are occurring on glaciers that flowed into ice shelves that have now disappeared. Glacier tributaries feeding the intact but thinning ice shelves of Larsen C and remnants of Larsen B, plus George VI Ice Shelf and the little-studied Larsen D also thinned at rates up to several metres per year. This behaviour confirms that glaciers are very sensitive to ice shelf thinning as well as collapse, and that shelf collapse leads not just to short-term and localized adjustment but to sustained, widespread and substantial loss of grounded ice from tributary glaciers (Pritchard et al., submitted).<br />
<br />
==Ice shelves==<br />
<br />
[[File:Figure 4.37 - Rapid disintegration of Larsen B ice shelf.png|thumb|'''4.37''' Rapid disintegration of Larsen B ice shelf. Image on left collected on January 31, 2002 and on right collected on March 7, 2002.]]<br />
Retreat of several ice shelves on either side of the Peninsula was already occurring when scientific observations began in 1903. Since that time, ice shelves on both the east and west coasts have suffered progressive retreat and some abrupt collapse (Morris and Vaughan, 2003<ref name="Morris and Vaughan, 2003">Morris, E.M. and Vaughan, D.G. 2003. Spatial and temporal variation of surface temperature on the Antarctic Peninsula and the limit of viability of ice shelves, In Antarctic Peninsula Climate Variability: Historical and Paleoenvironmental Perspectives. Antarctic Research Series, 79, edited by E. Domack, et al., 61-68, AGU, Washington, DC.</ref>; Scambos et al., 2000<ref name="Scambos et al, 2000">Scambos, T., Huble, M., Fahnestock, M. and Bohlander, J. 2000. The link between climate warming and break-up of ice shelves in the Antarctic Peninsula, ''Journal of Glaciology'', '''46''', 516-530.</ref>). Ten ice shelves have undergone retreat during the latter part of the 20<sup>th</sup> Century (Cooper, 1997<ref name="Cooper, 1997">Cooper, A.P.R. 1997. Historical observations of Prince Gustav Ice Shelf, ''Polar Record'', '''33'''(187), 285-294.</ref>; Doake and Vaughan, 1991<ref name="Doake and Vaughan, 1991">Doake, C.S.M. and Vaughan, D.G. 1991. Rapid disintegration of the Wordie Ice shelf in response to atmospheric warming, ''Nature'', '''350''', 328-330.</ref>; Fox and Vaughan, 2005<ref name="Fox and Vaughan, 2005">Fox, A.J. and Vaughan, D.G. 2005. The retreat of Jones Ice Shelf, Antarctic Peninsula, ''J. Glaciol.'', '''51'''(175), 555-560.</ref>; Luchitta and Rosanova, 1998<ref name="Luchitta and Rosanova, 1998">Luchitta, B.K. and Rosanova, C.E. 1998. Retreat of northern margins of George VI and Wilkins ice shelves, Antarctic Peninsula, ''Ann. Glaciol.'', '''27''', 41.</ref>; Rott et al., 1996<ref name="Rott et al, 1996">Rott, H., Skvarca, P. and Nagler, T. 1996. Rapid collapse of Northern Larsen Ice Shelf, Antarctica, ''Science'', '''271''', 788-792.</ref>, 2002<ref name="Rott et al, 2002">Rott, H., Rack, W., Skvarca, P. and De Angelis, H. 2002. Northern Larsen Ice Shelf, Antarctica: further retreat after collapse, ''Annals of Glaciology'', '''34''', 277-282.</ref>; Scambos et al., 2000<ref name="Scambos et al, 2000">Scambos, T., Huble, M., Fahnestock, M. and Bohlander, J. 2000. The link between climate warming and break-up of ice shelves in the Antarctic Peninsula, ''Journal of Glaciology'', '''46''', 516-530.</ref>, 2004<ref name="Scambos et al, 2004">Scambos, T.A., Bohlander, J., Shuman, C. and Skvarca, P. 2004. Glacier acceleration and thinner after ice shelf collapse in the Larsen B embayment, Antarctica, Geophys. Res. Lett., 31, doi:10.1029/2004GL020670.</ref>; Skvarca, 1994<ref name="Skvarca, 1994">Skvarca, P. 1994. Changes and surface features of the Larsen Ice Shelf, Antarctica, derived from Landsat and Kosmos mosaics, ''Ann. Glaciol.'', '''20''', 6.</ref>; Ward, 1995<ref name="Ward, 1995">Ward, C.G. 1995. Mapping ice front changes of M&uuml;ller Ice Shelf, Antarctic Peninsula, ''Antarct. Science'', '''7''', 197.</ref>) (Table 4.1). Wordie Ice Shelf, the northernmost large (&gt;1,000 km<sup>2</sup>) shelf on the western Peninsula, disintegrated in a series of fragmentations through the 1970s and 1980s, and was almost completely absent by the early 1990s. The Wordie break-up was followed in 1995 and 2002 by spectacular retreats of the two northernmost sections of the Larsen Ice Shelf (termed Larsen &lsquo;A&rsquo; and Larsen &lsquo;B&rsquo; by nomenclature proposed by Vaughan and Doake, 1996<ref name="Vaughan and Doake, 1996">Vaughan, D.G. and Doake, C.S.M. 1996. Recent atmospheric warming and retreat of ice shelves on the Antarctic Peninsula, ''Nature'', '''379''', 328-330.</ref>) and the last remnant of the Prince Gustav Ice Shelf ([[:File:Figure 4.37 - Rapid disintegration of Larsen B ice shelf.png|Figure 4.37]]). A similar &lsquo;disintegration&rsquo; event was observed in 1998 on the Wilkins Ice Shelf (Scambos et al., 2000<ref name="Scambos et al, 2000">Scambos, T., Huble, M., Fahnestock, M. and Bohlander, J. 2000. The link between climate warming and break-up of ice shelves in the Antarctic Peninsula, ''Journal of Glaciology'', '''46''', 516-530.</ref>), but much of the calved ice remained until 2008 when dramatic calving removed about 1,400 km<sup>2</sup> of ice. The ice bridge connecting the Wilkins Ice Shelf to Charcot Island disintegrated in early April 2009. In all these cases, persistent seasonal retreats by calving (Cooper, 1997<ref name="Cooper, 1997">Cooper, A.P.R. 1997. Historical observations of Prince Gustav Ice Shelf, ''Polar Record'', '''33'''(187), 285-294.</ref>; Skvarca, 1993<ref name="Skvarca, 1993">Skvarca, P. 1993. Fast recession of the northern Larsen Ice Shelf monitored by space images, ''Annals of Glaciology'', '''17''', 317-321.</ref>; Vaughan, 1993<ref name="Vaughan, 1993">Vaughan, D.G. 1993. Implications of the break-up of the Wordie Ice Shelf, Antarctica for sea level rise, ''Antarctic Science'', '''5'''(4), 403-408.</ref>) culminated in catastrophic disintegrations, especially for the Larsen A (Rott et al., 1996<ref name="Rott et al, 1996">Rott, H., Skvarca, P. and Nagler, T. 1996. Rapid collapse of Northern Larsen Ice Shelf, Antarctica, ''Science'', '''271''', 788-792.</ref>; Scambos et al., 2000<ref name="Scambos et al, 2000">Scambos, T., Huble, M., Fahnestock, M. and Bohlander, J. 2000. The link between climate warming and break-up of ice shelves in the Antarctic Peninsula, ''Journal of Glaciology'', '''46''', 516-530.</ref>) and Larsen B (Scambos et al., 2003<ref name="Scambos et al, 2003">Scambos, T.A., Hulbe, C. and Fahnestock, M. 2003. Climate-induced ice shelf disintegration in the Antarcitc Peninsula. In: Antarctic Peninsula Climate Variability; Historical and Paleoenvironmental Perspectives (E. Domack, A. Burnett, A. Leventer, P. Conley, M. Kirby, and R. Bindschadler, editors), ''Antarctic Research Series'', '''79''', 79-92.</ref>).<br />
<br />
The sequence of events leading up to the collapse of the Larsen B ice shelf suggests the processes responsible for the ultimate disintegration. In the 35-day period from 31 January 2002, satellite images recorded by the Moderate Resolution Imaging Spectroradiometer (MODIS) revealed a disintegration of a 5,700 km<sup>2</sup> section of the Larsen B ice shelf. The January MODIS images showed that prior to its disintegration, the Larsen B ice shelf was subject to more extensive ponding of meltwater than in previous years (Scambos et al., 2004<ref name="Scambos et al, 2004">Scambos, T.A., Bohlander, J., Shuman, C. and Skvarca, P. 2004. Glacier acceleration and thinner after ice shelf collapse in the Larsen B embayment, Antarctica, Geophys. Res. Lett., 31, doi:10.1029/2004GL020670.</ref>). As this water drained into pre-existing crevasses, and filled them, the water pressure would have been sufficient to propagate the cracks through the entire thickness of the ice shelf (Weertman, 1973<ref name="Weertman, 1973">Weertman, J., 1973, Can a water-filled crevasse reach the bottom surface of a glacier?, ''International Association of Scientific Hydrology Publication'', '''95''', 139-145.</ref>; Scambos et al. 2000<ref name="Scambos et al, 2000">Scambos, T., Huble, M., Fahnestock, M. and Bohlander, J. 2000. The link between climate warming and break-up of ice shelves in the Antarctic Peninsula, ''Journal of Glaciology'', '''46''', 516-530.</ref>). Satellite radar interferometry has been used with ice flow models to show that the ice shelf sped up considerably in the period before its final collapse because of weakening within its margins, perhaps as a consequence of this mechanism (Vieli et al., 2007<ref name="Vieli et al, 2007">Vieli, A., Payne, A.J., Shepherd, A. and Du, Z. 2007. Causes of pre-collapse o fthe Larsen B ice shelf: numerical modeling and assimilation of satellite observations, ''Earth and Planetary Science Letters'', '''259''', 297-306, doi:10.1016/j.epsl.2007.04.50.</ref>). Once the Larsen B ice shelf had disintegrated into icebergs, the forces set up as they toppled against one another drove them rapidly apart (MacAyeal et al., 2003<ref name="MacAyeal et al, 2003">Macayeal, D.R., Scambos, T.A., Hulbe, C.L. and Fahnestock, M.A., 2003. Catastrophic Ice-Shelf Break-Up by an Ice-Shelf-Fragment-Capsize Mechanism, J. Glaciol., 49(164) 22-36.</ref>). A MODIS image taken on 7 March 2002 ([[:File:Figure 4.37 - Rapid disintegration of Larsen B ice shelf.png|Figure 4.37]]) shows a plume of icebergs being ejected, clearing the bay that was previously occupied by the ice shelf.<br />
<br />
The latest results reveal an overall reduction in total ice shelf area on the Antarctic Peninsula by over 27,000 km<sup>2</sup> in the last 50 years. As discussed in the previous section, recent findings (and studies of similar events in the southern Greenland ice sheet; see Howat et al., 2008<ref name="Howat et al, 2008">Howat, I.M., Smith, B.E., Joughin, I. and Scambos, T.A. 2008. Rates of southeast Greenland ice volume loss from combined ICESat and ASTER observations, ''Geophys. Res. Lett.'', '''35''', L17505, doi:10.1029/2008GL034496.</ref>) have fostered new appreciation of the importance of floating ice on controlling ice flow, and the rapidity with which loss of floating ice could cause an acceleration in the contribution to sea level rise.<br />
<br />
The direct cause of the Peninsula ice shelf retreats is thought by many to be a result of increased surface melting, attributed to atmospheric warming. Increased fracturing via melt-water infilling of pre-existing crevasses explains many of the observed characteristics of the break-up events (Scambos et al., 2000<ref name="Scambos et al, 2000">Scambos, T., Huble, M., Fahnestock, M. and Bohlander, J. 2000. The link between climate warming and break-up of ice shelves in the Antarctic Peninsula, ''Journal of Glaciology'', '''46''', 516-530.</ref>; 2003), and melting in 2002 on the Larsen B was extreme (van den Broeke, 2005<ref name="Broeke, 2005">Van Den Broeke, M. 2005. Strong surface melting preceded collapse of Antarctic Peninsula ice shelf, ''Geophysical Research Letters'', '''32''', L12815, doi:10.1029/2005GL023247.</ref>).<br />
<br />
Observations of northward-drifting icebergs support the theory that surface melt ponds or surface firn saturated with melt-water can rapidly culminate in disintegration of either ice shelves or icebergs (Scambos et al., 2004<ref name="Scambos et al, 2004">Scambos, T.A., Bohlander, J., Shuman, C. and Skvarca, P. 2004. Glacier acceleration and thinner after ice shelf collapse in the Larsen B embayment, Antarctica, Geophys. Res. Lett., 31, doi:10.1029/2004GL020670.</ref>).<br />
<br />
{| class="wikitable"<br />
!Ice Shelf!!First recorded date!!Last recorded date!!Area on first<br />recorded date (km<sup>2</sup>)!!Area on last<br />recorded date (km<sup>2</sup>)!!Change (km<sup>2</sup>)!!% of original<br />area remaining!!Reference<br />
|-<br />
|M&uuml;ller||1956||1993||80||49||-31||61||Ward (1995<ref name="Ward, 1995">Ward, C.G. 1995. Mapping ice front changes of M&uuml;ller Ice Shelf, Antarctic Peninsula, ''Antarct. Science'', '''7''', 197.</ref>)<br />
|-<br />
|Wordie||1966||1989||2,000||700||-1,300||35||Doake and Vaughan (1991<ref name="Doake and Vaughan, 1991">Doake, C.S.M. and Vaughan, D.G. 1991. Rapid disintegration of the Wordie Ice shelf in response to atmospheric warming, ''Nature'', '''350''', 328-330.</ref>)<br />
|-<br />
|Wordie||1989||2009||700||96||-600||5||Wendt et al. (In Press)<br />
|-<br />
|Northern George VI||1974||1995||~ 26,000||~ 25,000||-993||96||Luchitta and Rosanova (1998<ref name="Luchitta and Rosanova, 1998">Luchitta, B.K. and Rosanova, C.E. 1998. Retreat of northern margins of George VI and Wilkins ice shelves, Antarctic Peninsula, ''Ann. Glaciol.'', '''27''', 41.</ref>)<br />
|-<br />
|Northern Wilkins||1990||1995||~ 17,400||~ 16,000||-1,360||92||Luchitta and Rosanova (1998<ref name="Luchitta and Rosanova, 1998">Luchitta, B.K. and Rosanova, C.E. 1998. Retreat of northern margins of George VI and Wilkins ice shelves, Antarctic Peninsula, ''Ann. Glaciol.'', '''27''', 41.</ref>)<br />
|-<br />
| ||1995||1998|| || ||-1,098||85||Scambos et al. (2000<ref name="Scambos et al, 2000">Scambos, T., Huble, M., Fahnestock, M. and Bohlander, J. 2000. The link between climate warming and break-up of ice shelves in the Antarctic Peninsula, ''Journal of Glaciology'', '''46''', 516-530.</ref>)<br />
|-<br />
|Jones||1947||2003||25||0||-25||0||Fox and Vaughan (2005<ref name="Fox and Vaughan, 2005">Fox, A.J. and Vaughan, D.G. 2005. The retreat of Jones Ice Shelf, Antarctic Peninsula, ''J. Glaciol.'', '''51'''(175), 555-560.</ref>)<br />
|-<br />
|Prince Gustav||1945||1995||2,100||~ 100||-2,000||5||Cooper (1997<ref name="Cooper, 1997">Cooper, A.P.R. 1997. Historical observations of Prince Gustav Ice Shelf, ''Polar Record'', '''33'''(187), 285-294.</ref>)<br />
|-<br />
| ||1995||2000|| ||47|| ||2||Rott et al. (2002<ref name="Rott et al, 2002">Rott, H., Rack, W., Skvarca, P. and De Angelis, H. 2002. Northern Larsen Ice Shelf, Antarctica: further retreat after collapse, ''Annals of Glaciology'', '''34''', 277-282.</ref>)<br />
|-<br />
|Larsen Inlet||1986||1989||407||0||-407||0||Rott et al. (2002<ref name="Rott et al, 2002">Rott, H., Rack, W., Skvarca, P. and De Angelis, H. 2002. Northern Larsen Ice Shelf, Antarctica: further retreat after collapse, ''Annals of Glaciology'', '''34''', 277-282.</ref>)<br />
|-<br />
|Larsen A||1986||1995||2,488||320||-2,168||13||Rott et al. (1996<ref name="Rott et al, 1996">Rott, H., Skvarca, P. and Nagler, T. 1996. Rapid collapse of Northern Larsen Ice Shelf, Antarctica, ''Science'', '''271''', 788-792.</ref>)<br />
|-<br />
|Larsen B||1986||2000||11,500||6,831||-4,669||59||Rott et al. (2002<ref name="Rott et al, 2002">Rott, H., Rack, W., Skvarca, P. and DE Angelis, H. 2002. Northern Larsen Ice Shelf, Antarctica: further retreat after collapse, ''Annals of Glaciology'', '''34''', 277-282.</ref>)<br />
|-<br />
| ||2000||2002||||3,631||-3,200||32||Scambos et al. (2004<ref name="Scambos et al, 2004">Scambos, T.A., Bohlander, J., Shuman, C. and Skvarca, P. 2004. Glacier acceleration and thinner after ice shelf collapse in the Larsen B embayment, Antarctica, Geophys. Res. Lett., 31, doi:10.1029/2004GL020670.</ref>)<br />
|-<br />
|Larsen C||1976||1986||~ 60,000||~ 50,000||-9,200||82||Skvarca (1994<ref name="Skvarca, 1994">Skvarca, P. 1994. Changes and surface features of the Larsen Ice Shelf, Antarctica, derived from Landsat and Kosmos mosaics, ''Ann. Glaciol.'', '''20''', 6.</ref>) and<br />Vaughan and Doake (1996<ref name="Vaughan and Doake, 1996">Vaughan, D.G. and Doake, C.S.M. 1996. Recent atmospheric warming and retreat of ice shelves on the Antarctic Peninsula, ''Nature'', '''379''', 328-330.</ref>)<br />
|}<br />
<br />
'''Table 4.1''' Summary of changes observed in ten ice shelves located on the Antarctic Peninsula. The figures were obtained from references that recorded the measured area of a particular ice shelf on both the earliest and most recent dates available.<br />
<br />
Specific mechanisms of ice shelf break-up are still debated. The role of subsurface waters circulating beneath the shelves in thinning and/or warming the ice remains undetermined. Others have suggested that a change to negative surface mass balance (Rott et al., 1998<ref name="Rott et al, 1998">Rott, H., Rack, W., Nagler, T. and Skvarca, P. 1998. Climatically induced retreat and collapse of northern Larsen Ice Shelf, Antarctic Peninsula, ''Annals of Glaciology'', '''27''', 86-92.</ref>), or reduced fracture toughness due to a thickening temperate ice layer (Vaughan and Doake, 1996<ref name="Vaughan and Doake, 1996">Vaughan, D.G. and Doake, C.S.M. 1996. Recent atmospheric warming and retreat of ice shelves on the Antarctic Peninsula, ''Nature'', '''379''', 328-330.</ref>), or basal melting (Shepherd et al., 2002<ref name="Shepherd et al, 2002">Shepherd, A.P., Wingham, D.J. and Mansley, J.A.D. 2002. Inland thinning of the Amundsen Sea sector, West Antarctica, ''Geophys. Res. Lett.'', '''29''', 1364, doi:10.1029/2001GL014183.</ref>) caused the break-up. Recent modeling and observational studies have shown that the Larsen B, at least, was pre-conditioned to a retreat and breakup by faster flow, increased rifting, and detachment from the coast (Vieli et al., 2007<ref name="Vieli et al, 2007">Vieli, A., Payne, A.J., Shepherd, A. and Du, Z. 2007. Causes of pre-collapse o fthe Larsen B ice shelf: numerical modeling and assimilation of satellite observations, ''Earth and Planetary Science Letters'', '''259''', 297-306, doi:10.1016/j.epsl.2007.04.50.</ref>; Glasser and Scambos, 2008<ref name="Glasser and Scambos, 2008">Glasser, N. and Scambos, T.A. 2008. A structural glaciological analysis of the 2002 Larsen B ice shelf collapse, ''Journal of Glaciology'', '''54''' (184), 3-16.</ref>); all these are consistent with a thinning shelf in the years leading up to disintegration.<br />
<br />
[[File:Figure 4.38 - Interpolated mean annual temperature and ice shelf loss due to climate-driven retreat.png|thumb|'''4.38''' Contours of interpolated mean annual temperature. Currently existing ice shelves are shown in grey. Portions of ice shelves that have been lost through climate-driven retreat are shown in red (From Morris and Vaughan, 2003<ref name="Morris and Vaughan, 2003">Morris, E.M. and Vaughan, D.G. 2003. Spatial and temporal variation of surface temperature on the Antarctic Peninsula and the limit of viability of ice shelves, In Antarctic Peninsula Climate Variability: Historical and Paleoenvironmental Perspectives. Antarctic Research Series, 79, edited by E. Domack, et al., 61-68, AGU, Washington, DC.</ref>).]]<br />
The pattern of ice shelf retreat on the Antarctic Peninsula appears to be consistent with the existence of a thermal limit on ice-shelf viability (Morris and Vaughan, 2003<ref name="Morris and Vaughan, 2003">Morris, E.M. and Vaughan, D.G. 2003. Spatial and temporal variation of surface temperature on the Antarctic Peninsula and the limit of viability of ice shelves, In Antarctic Peninsula Climate Variability: Historical and Paleoenvironmental Perspectives. Antarctic Research Series, 79, edited by E. Domack, et al., 61-68, AGU, Washington, DC.</ref>; Vaughan and Doake, 1996<ref name="Vaughan and Doake, 1996">Vaughan, D.G. and Doake, C.S.M. 1996. Recent atmospheric warming and retreat of ice shelves on the Antarctic Peninsula, ''Nature'', '''379''', 328-330.</ref>) ([[:File:Figure 4.38 - Interpolated mean annual temperature and ice shelf loss due to climate-driven retreat.png|Figure 4.38]]). The limit of ice shelves known to have retreated during the last 100 years is bounded by the &ndash;5&deg;C and &ndash;9&deg;C isotherms (calculated for 2000 A.D.) suggesting that the retreat of ice shelves in this region is consistent with the observed warming trend of 3.5 &plusmn; 1.0&deg;C/century (Morris and Vaughan, 2003<ref name="Morris and Vaughan, 2003">Morris, E.M. and Vaughan, D.G. 2003. Spatial and temporal variation of surface temperature on the Antarctic Peninsula and the limit of viability of ice shelves, In Antarctic Peninsula Climate Variability: Historical and Paleoenvironmental Perspectives. Antarctic Research Series, 79, edited by E. Domack, et al., 61-68, AGU, Washington, DC.</ref>).<br />
<br />
==Sub-Antarctic Islands==<br />
<br />
Glaciers on the sub-Antarctic islands of Heard Island (53&deg;S, 73&deg;30&rsquo;E), Kerguelen Islands (49&deg;15&rsquo;S, 69&deg;35&rsquo;E) and South Georgia (54&deg;30&rsquo;S, 36&deg;30&rsquo;W) have shown accelerating rates of retreat over the past half-century. The glaciers on Heard Island have shown extensive retreat since the 1940s (Allison and Keage, 1986<ref name="Allison and Keage, 1986">Allison, I.F., Keage, P.L., 1986. Recent changes in the glaciers of Heard Island. Polar Record, 23, 255-271.</ref>; Kiernan and McConnell, 1999<ref name="Kiernan and McConnell, 1999">Kiernan, K. and McConnell, A. 1999. Geomorphology of the sub- Antarctic Australian territory of Heard Island&ndash;McDonald Island, ''Australian Geographer'', '''30''', 159-195.</ref>, 2002<ref name="Kiernan and McConnell, 2002">Kiernan, K. and McConnell, A. 2002. Glacier retreat and melt-lake expansion at Stephenson Glacier, Heard Island World Heritage Area, ''Polar Record'', '''38''', 297-308.</ref>; Budd, 2000<ref name="Budd, 2000">Budd, G.M. 2000. Changes in Heard Island glaciers, king penguins and fur seals since 1947. Papers and Proceedings of the Royal Society of Tasmania, 133, 47-60.</ref>). After a period of advance between 1963-71, most of the recession occurred since 1970 (Allison and Keage, 1986<ref name="Allison and Keage, 1986">Allison, I.F., Keage, P.L., 1986. Recent changes in the glaciers of Heard Island. Polar Record, 23, 255-271.</ref>). The total glacierized area has reduced by 11%, and several coastal lagoons have been formed as a result. The rapid glacier recession reflects a temperature rise on the island of about 1.3&deg;C during the last 50 years (Budd, 2000<ref name="Budd, 2000">Budd, G.M. 2000. Changes in Heard Island glaciers, king penguins and fur seals since 1947. Papers and Proceedings of the Royal Society of Tasmania, 133, 47-60.</ref>). Of the twelve major glaciers and several minor glaciers on the island, current research includes two specific examples: Stephenson Glacier and Brown Glacier. Historical records, recent observations, and geomorphological evidence indicate that rates of retreat and downwasting of the tidewater Stephenson Glacier, and concurrent expansion of ice-marginal melt-lakes, has increased by an order of magnitude since 1987 (Kiernan and McConnell, 2002<ref name="Kiernan and McConnell, 2002">Kiernan, K. and McConnell, A. 2002. Glacier retreat and melt-lake expansion at Stephenson Glacier, Heard Island World Heritage Area, ''Polar Record'', '''38''', 297-308.</ref>). In addition, Brown Glacier retreated 50 metres since 2000/01, contributing to a retreat of approximately 1.1 km since 1950 (a decrease in total volume of about 38%) (Australian Antarctic Division 2005: http://www.heardisland.aq/). Similarly at Kerguelen, glacier recession has accelerated since the early 1970s (Frenot et al., 1993<ref name="Frenot et al, 1993">Frenot, Y., Gloaguen, J.-C., Picot, G., Boug&egrave;re, J. and Benjamin, D. 1993. Azorella selago Hook used to estimate glacier fluctuations and climatic history in the Kerguelen Islands over the last two centuries, ''Oecologia'', '''95''', 140-144.</ref>, 1997<ref name="Frenot et al, 1997">Frenot, Y., Gloaguen, J.-C. and Tr&eacute;hen, P. 1997. Climate change in Kerguelen Islands and colonization of recently deglaciated areas by Poa kerguelensis and P. annua. In: Battaglia, B., Valencia, J., Walton, D.W.H. (Eds.), Antarctic Communities. Species, Structure and Survival. Cambridge University Press, Cambridge, 358-366.</ref>).<br />
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Glaciers, ice caps and snowfields cover over 50% of the island of South Georgia. In a recent study, the changing positions of the 103 coastal glacier fronts on South Georgia were mapped using archival aerial photographs and satellite imagery dating from the 1950s to the present (Cook et al., in submission). Of these, 97% have retreated since their earliest recorded position (which, on average, was 1961). The majority (64%) of the glaciers retreated by between 0 and 500 m since their first observations. Two glaciers stand out as having retreated the most: Neumayer Glacier by 4.4 km since 1957, and the ice front fed by Ross and Hindle Glaciers, by 2.14 km since 1960. The rate of retreat for all 103 glaciers has increased from (on average) 8 m/yr in the late 1950s, to 35 m/yr at present, revealing an accelerating rate of retreat since the 1990s. The recent rapid increase in the average rate is largely due to large increases in retreat rates of glaciers in the north-east of the island, which are currently showing an average of 60 m/yr retreat. The glaciers along the south-west coast of the island, however, are significantly different in their rate of change, due to dissimilar weather patterns caused by orographic effects (Gordon et. al., 2008). They have been in retreat slowly since the 1950s, but this has remained at a constant rate of approximately 8 m/yr. This retreat rate may now be gradually increasing, although on a much smaller scale (currently 12 m/yr). The climate records from South Georgia (recorded at Grytviken from 1905 until 1988, and subsequently from 2001 until 2008) show that in the early 1900s the summer temperatures were relatively high, lower between the 1920s to the 1940s, and higher from the 1950s to the present (Gordon et al., 2008<ref name="Gordon et al, 2008">Gordon, J.E., Haynes, V.M. and Hubbard, A. 2008. Recent glacier changes and climate trends on South Georgia, ''Global and Planetary Change'', '''60''', 72-84.</ref>). The retreat of South Georgia glaciers over the past half-century coincides with the recent period of climate warming that began in the 1950s. Acceleration in retreat rates of glaciers on the north-east coast has occurred in the past decade as the climate has continued to warm, and although the glaciers on the south-west side have been slow to respond, their retreat rates may now also be on the increase (Cook et al., in submission).<br />
==References==<br />
<references /><br />
[[Category:The instrumental period]]<br />
[[Category:The Antarctic ice sheet]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_Antarctic_marine_ecosystem_in_the_year_2100The Antarctic marine ecosystem in the year 21002014-08-06T14:34:07Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Biological responses to 21st climate climate change]]''<br />
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&ldquo;The complexity of the Southern Ocean food web and the non-linear nature of many interactions mean that predictions based on short-term studies of a small number of species are likely to be misleading&rdquo; (Clarke et al., 2007<ref name="Clarke et al, 2007">Clarke, A., Murphy, E.J., Meredith, M.P., King, J.C., Peck, L.S., Barnes, D.K.A. and Smith, R.C. 2007. Climate change and the marine ecosystem of the western Antarctic Peninsula, ''Phil. Trans. R. Soc. B'', '''362''', 149-166.</ref>). The most critical points for biological predictions are (i) the enormous complexity of living communities, including hundreds to thousands of parameters (species and their life traits; for their general sensitivity see Barnes and Peck, 2008<ref name="Barnes and Peck, 2008">Barnes, D.K.A. and Peck, L.S. 2008. Vulnerability of Antarctic shelf biodiversity to predicted climate change, ''Climate Research'', '''37''', 149-163.</ref>), (ii) our limited knowledge of these, and (iii) the lack of fine-scale detail in predictions of change in the physical environment, e.g. of the likelihood and extent of extreme events, and of fine scale spatial resolution. Nevertheless, physical predictions have improved considerably compared to a few years ago, because they now include some measurements of regional as well as global climate change.<br />
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Polar ecosystems are experiencing significant environmental changes. In the Antarctic the retreat of glaciers and the disintegration of ice shelves, with waters underneath being the least explored marine systems on Earth, provide the most obvious impacts on coastal marine areas. In offshore systems, a shift of pelagic communities towards the south is interpreted as a consequence of regional changes in sea ice dynamics, especially West of the Antarctic Peninsula, although the average sea ice extent has changed little. There are first signs of increased water temperature also along the coast. Because of an assumed high sensitivity of such biological systems to changes, inshore and offshore waters around the Antarctic Peninsula and at the sea ice margin as well as along the Antarctic Convergence (Polar Front) are the main foci of ecological climate research.<br />
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The attempt made here to develop a scenario of how the Antarctic ecosystem might look in 2100 should be considered as a tool for identifying future research needs and recommendations to decision makers rather than as a true prediction. The most important constraint that militates against numerical forecasts is the reliability of a number of assumptions about the biological system and its environment. To make this &ldquo;pre-stage&rdquo; of a prediction as robust as possible, the concept is based only on coarse patterns. Air and ocean temperature are assumed to be the main climate-driven environmental parameters for the Southern Ocean ecosystem. In addition CO<sub>2</sub>-triggered acidification and increased UV-radiation are likely to alter the ecosystem.<br />
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Substantially damage of the sea-floor ecosystem by grounding icebergs has been observed locally, following the climate-induced disintegration of the Larsen B ice shelf, and in very shallow water with a naturally high intensity of disturbance. By 2100 the Larsen C ice shelf and much smaller ice shelves west of the Antarctic Peninsula might also have collapsed, at least partly (Morris and Vaughan, 2003<ref name="Morris and Vaughan, 2003">Morris, E.M. and Vaughan, D.G. 2003. Spatial and temporal variation of surface temperature on the Antarctic Peninsula and the limit of viability of ice shelves, In Antarctic Peninsula Climate Variability: Historical and Paleoenvironmental Perspectives. Antarctic Research Series, 79, edited by E. Domack, et al., 61-68, AGU, Washington, DC.</ref>), and the northeast coast of the Antarctic Peninsula will have experienced a significantly elevated disturbance regime preventing the benthos from reaching a climax stage. After 2100, icebergs from the Filchner-Ronne ice shelf will continue to scour their way along the east coast of the Antarctic Peninsula, contributing to maintaining a patchwork of recolonization stages and, consequently, high benthic biodiversity. Iceberg calving events along the west coast of the Peninsula and around East Antarctica are also likely to increase as ice shelves collapse, so increasing benthic biodiversity, though not in so dramatic a manner as in the western Weddell Sea. Between now and 2100 elevated air and water temperature will not have risen to levels at which the large Filchner-Ronne and Ross Ice Shelves might disintegrate and cause large-scale circumpolar damage. A regional lessening or cessation of production of icebergs might cause local development of a benthic community in which competitive displacement leads to increased productivity but reduced biodiversity. An irreversible negative effect will be the loss of the unique marine biological system below the former ice shelves. These areas will serve as habitats of retreat for some vertebrate and invertebrate species that escape from warmed areas in the north and west of the Antarctic Peninsula, but these newly available areas will not completely compensate for habitat-loss elsewhere (J. Gutt, unpublished results).<br />
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The development of the sea ice plays a key role in the development of the pelagic as well as the open ocean ecosystem, and in intertidal communities; it has indirect effects on the sub-tidal benthos especially on the shelf but also in the deep-sea. Between now and 2100 all components of the ecosystem closely related to the sea ice will show a significant change in their ecological performance in response to the predicted 33% reduction of sea ice extent. A decline of krill by 38-80% in the Atlantic sector was already noticeable by 2004, having begun in the late 1970s (for most recent results and further references see Atkinson et al., 2008<ref name="Atkinson et al, 2008">Atkinson, A., Siegel, V., Pakhomov, E.A., Rothery, P., Loeb, V., Ross, R.M., Quetin, L.B., Schmidt, K., Fretwell, P., Murphy, E.J., Tarling, G.A. and Fleming, A.H. 2008. Oceanic circumpolar habitats of Antarctic krill, ''Mar. Ecol. Prog. Ser'', '''362''', 1-23.</ref>). Our &lsquo;forecast&rsquo; is that this decline will not be compensated by increasing near-shore populations in the less affected &ldquo;refuges&rdquo; in East Antarctica between approximately 90&deg;W and 105&deg;W. In areas with an originally high krill population size the population is likely to stabilize at a low level. The implication is that over the long term all main krill consumers will experience a serious food limitation. The Minke Whale will lose 5-30% of its ice-associated habitat, while for Blue, Humpback, Fin and Sperm Whales a compressed foraging habitat along Southern Ocean fronts is suggested. This negative effect will be superimposed on the recently observed 9.6%/yr rate of increase in Humpback Whales and similar developments in other large species in recent decades, which runs counter to the observed decrease in krill. Due to the trophic complexity of this ecosystem it is impossible to foresee which animal group will suffer most, but some may suffer less than others while other may even become extinct, at least regionally. Colonies of Emperor and Ad&eacute;lie penguins, which are most closely adapted to a complex sea ice regime, but which are also affected by precipitation, will become extinct locally where their pack ice dominated habitat shifts to an open ocean system. They will be locally displaced by king, gentoo and chinstrap penguins. Long-term field data from an Emperor penguin colony in Terre Ad&eacute;lie (East Antarctica) showed very sensitive response to natural climate changes in both directions, when sea ice increased and decreased (Barbraud and Weimerskirch, 2001<ref name="Barbraud and Weimerskirch, 2001">Barbraud, C. and Weimerskirch, H. 2001. Emperor Penguins and climate change, ''Nature'', '''411''', 183-186.</ref>). A projection using predictions of warm events from IPCC climate scenarios and combined with data on the population dynamics of this East Antarctic colony forecast a 36% probability of a quasi-extinction by 2100, in an area with rather minor climate change impact (Jenouvrier et al., 2009<ref name="Jenouvrier et al, 2009">Jenouvrier, S., Caswell, H., Barbraud, C., Holland, M., Str&oelig;ve, J. and Weimerskirch, H. 2009. Demographic models and IPCC climate projections predict the decline of an emperor penguin population, Proc. Natl. Acad. Sci. U.S.A., January 26, doi 10.1073/pnas.0806638106.</ref>). Since it will be difficult for this species to find sites for new colonies, the total net population will decrease significantly and the zoographic range will be compacted southward. Ad&eacute;lies will find new colonies where pack-ice becomes more divergent and at newly exposed coastlines when ice shelves disintegrate.<br />
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As in the case of penguins, the ice-bound Crabeater, Weddell, Leopard and Ross Seals will regionally become extinct due to changes in both, habitat and food web dynamics. The ice-tolerant Fur and Southern Elephant Seal will shift their geographical extent further south and regionally increase their population size &ndash; unless their food resources decline. The Antarctic toothfish also has an ice-related behaviour. A prediction of the effect of a 1.3&deg;C temperature increase that could take place between the next 10-90 years shows a circumpolar decrease of population size. This species would only likely become extinct with an extreme warming coupled with a sea ice retreat of 2 km per year (Cheung et al., 2009<ref name="Cheung et al, 2009">Cheung, W.W.L., Lam, V.W.Y., Sarmiento, J.L., Kearney, K., Watson, R. and Pauly, D. 2009. Projecting global marine biodiversity impacts under climate change scenarios, Fish and Fisheries, DOI: 10.1111/j.1467-2979.2008.00315.x.</ref>). In essence most Antarctic species are adapted to changes in sea ice concentration and its extent. Species of any systematic group with a sufficient initial population size and circumpolar distribution are expected to survive at least in the Pacific sector south of Australia and New Zealand, where according to the predictions the sea ice is likely to remain relatively stable.<br />
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The effect of the future reductions of sea ice on primary production is likely to be more complex. Southernmost near-shore waters, which, under &lsquo;normal&rsquo; conditions, have a low yearly primary production compared to areas like the Antarctic Peninsula, will become more productive as important components of the open ocean system move farther south (Whitehouse et al., 2008<ref name="Whitehouse et al, 2008">Whitehouse, M.J., Meredith, M.P., Rothery, P., Atkinson, A., Ward, P. and Korb, R.E. 2008. Rapid warming of the ocean at South Georgia, Southern Ocean during the 20<sup>th</sup> Century: forcings, characteristics and implications for lower trophic levels, ''Deep-Sea Research I'', '''55''', 1218-1228.</ref>). This shift will coincide with a shift among unicellular algae from larger to smaller diatoms and generally from diatoms to ''Phaeocystis'' aggregations, both being less favourable food for krill. One side effect will be more emissions of precursors of dimethyl sufide (DMS) causing extra cloud cover. Using results from the &ldquo;Climate System Model 1.4-carbon&rdquo; and experiments on the ecology of phytoplankton, Boyd et al. (2007a<ref name="Boyd et al, 2007a">Boyd, P.W., Doney, S.C., Strzepek, R., Dusenberry, J., Lindsay, K. and Fung, I. 2007a. Climate-mediated changes to mixed-layer properties in the Southern Ocean: assessing the phytoplankton response, ''Biogeosciences Discussions'', '''4'''(6), 4283-4322.</ref>) assume that &ldquo;the rate of secular climate change will not exceed background variability, on seasonal to interannual time-scales, for at least several decades&hellip;&rdquo;. These results might underline the high relevance of the sea ice and its biota to the already observed and expected changes in the Antarctic ecosystem. At the sea floor, increased primary production may present a problem for filter feeders that are principally adapted to low food supply rather than decreased production. However, a few opportunists, e.g. among mobile deposit feeders, are likely to benefit and to extend their distribution range. This process will change the functioning of the benthic system, with significant consequences for cycles of organic and inorganic carbon as well as silicate remineralization. The increase in production will also act to reduce diversity in the deep-sea and on the continental slope, where a few species will benefit above the average, coming to dominate the entire assemblage. The extent of extinction of species between now and 2100 will depend very much on the number of at present largely undiscovered &ldquo;cryptic&rdquo; species, those that are visually not discernible from very close relatives, if they have only a limited range of occurrence. Populations of pelagic invertebrates and fish may collapse at the regional level, but due to their good dispersal capacity will survive in refuge areas. In contrast to the above scenario, in which phytoplankton growth increases in coastal waters, we may see in offshore waters especially close to the Antarctic Convergence (Polar Front) a spatially and temporarily hardly predictable decrease of primary productivity resulting from decreased stratification, vertical mixing, and increased microbial oxygen consumption. Such a development may negatively affect higher trophic levels of the food limited pelagic ecosystem.<br />
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Between now and 2100 water temperatures will not rise by much more than 1&ordm;C at the sea surface and down to 200 m depth in most areas. This is likely to have less of an effect on major components of the entire ecosystem than will the reduction in sea ice extent. It seems likely that only few species will become extinct as a direct effect of temperature increase, either because they proved unable to cope ecologically and physiologically with such an increase, or because they were restricted in their occurrence to an area with an above average temperature increase. Such biological changes are most likely at water depths down to 200 m, where a relatively high proportion of species is naturally used to environmental variability. Adaptation could result from a mobile life style, from effective dispersal of pelagic eggs and, or through ecological and physiological tolerance. The area with the highest benthic biodiversity and ecosystem functioning, at 150 &ndash; 300 m depth will remain much less affected since the predicted warming there is very low.<br />
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Most of the above projections do not consider the flexibility of organisms to &ldquo;compensate&rdquo; e.g. physiologically and ecologically for climate change impact. For example, two Emperor penguin colonies are known to live and reproduce even on land in contrast to their usual habitat, the sea ice. It can also not be excluded that adaptation through microevolution will become effective and have similar positive effects for a variety of species. While the potential for both mechanisms - flexibility and adaptation - might not be very high in long-lived and specialized species, it has to be recognized that the overwhelming majority of the species survived the last interglacial, when there was more warming than today. Nevertheless, some models of the future of the physical environment predict, at least regionally, temperatures that exceed those of the past interglacial. In this report we have chosen to use the average temperatures thrown up by IPCC 19 models, rather than any of the extremes.<br />
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The projected temperature changes are supposed to be insufficient to disrupt the thermal barriers that create the marine biological isolation of Antarctica, except locally where temperatures reach values significantly above average. As a consequence the invasion of species will likely remain restricted to areas where invaders can survive at their physiological limits. Under these circumstances it seems unlikely that invaders will have the potential to outcompete or diminish Antarctic species by predation pressure on a broad scale. In effect the Antarctic marine ecosystem will remain buffered from the direct effects of a global temperature-increase by the continued existence of the large and high ice sheet, which keeps the Antarctic cool and quite different from the Arctic. This assumption is confirmed by a newly developed bioclimate envelope model (Cheung et al., 2009<ref name="Cheung et al, 2009">Cheung, W.W.L., Lam, V.W.Y., Sarmiento, J.L., Kearney, K., Watson, R. and Pauly, D. 2009. Projecting global marine biodiversity impacts under climate change scenarios, Fish and Fisheries, DOI: 10.1111/j.1467-2979.2008.00315.x.</ref>), which predicts for 2050 an extremely sharp but non-continuous gradient between highly affected areas north of and close to the ACC and relatively stable conditions in high latitude Antarctic waters in terms of species invasion, local extinction, and turnover of metazoan species between. If, however, ocean temperatures increase by more than 2&ordm;C from today&rsquo;s, and ecologically important species are less temperature tolerant in nature than in ecophysiological laboratory experiments, assemblages that cannot escape to colder regions are likely to become extinct by 2100 (Barnes and Peck, 2008<ref name="Barnes and Peck, 2008">Barnes, D.K.A. and Peck, L.S. 2008. Vulnerability of Antarctic shelf biodiversity to predicted climate change, ''Climate Research'', '''37''', 149-163.</ref>). These may comprise thousands of invertebrate and vertebrate, pelagic and benthic species.<br />
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Our knowledge of the response of Antarctic calcifying organisms to acidification is extremely poor. Such species (coccolithorphorids and pteropods) can reach locally or regionally dense concentrations offshore, with pteropods also reaching high concentrations inshore at high latitudes. At the seabed, calcifying echinoderms, hydrocorals, bryozoans, and molluscs can shape local biodiversity or abundance hot-spots. Undersaturated conditions might become the most severe climate-induced negative impact on Antarctic benthic communities if key species cannot cope with increase pH, as in the case of the common Antarctic sea-urchin ''Sterechinus neumayeri''; however, some species may even be winners in a more acid ocean, e.g. tunicates (Dupont and Thorndyke, 2009<ref name="Dupont and Thorndyke, 2009">Dupont, S. and Thorndyke, M.C. 2009. Impact of CO<sub>2</sub>-driven ocean acidification on invertebrates early life-history &ndash; What we know, what we need to know and what we can do, ''Biogeosciences Discuss'', '''6''', 3109-3131.</ref>).<br />
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One of the most unpredictable parameters that shapes the Antarctic ecosystem is human behaviour. It has ranged from the enormous exploitation of natural resources in the past to the later protection not only of most of these but also of mineral resources. It still ranges on the one hand from more or less continuous global emissions of CO<sub>2</sub> to the atmosphere to proposed geoengineering projects for the Southern Ocean, both of which will or may cause irreversible ecological damage (Smith et al., 2008<ref name="Smith et al, 2008">Smith, P.J., Steinke, D., McVeagh, S.M., Stewart, A.L., Struthers, C.D. and Roberts, C.D. 2008. Molecular analysis of Southern Ocean skates (Bathyraja) reaveals a new species of Antarctic skate, ''Journal of Fish Biology'', '''73''', 1170-1182.</ref>), to on the other hand the Madrid Protocol, one of the most strict international laws for protecting our global environment and to plans to identify, propose and declare High Seas Marine Protected Areas (HSMPAs) and Vulnerable Marine Ecosystems (VMEs).<br />
==References==<br />
<references /><br />
[[Category:The next 100 years]]<br />
[[Category:Antarctic biology]]<br />
[[Category:Marine biology]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_Antarctic_environment_in_the_global_systemThe Antarctic environment in the global system2014-08-06T14:34:07Z<p>Acce: </p>
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<div>'''Chapter Editor:''' Colin Summerhayes<br />
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'''Contributing Authors:''' David Ainsley, Peter Barrett, Robert Bindschadler, Andrew Clarke, Pete Convey, Eberhard Fahrbach, Julian Gutt, Dominic Hodgson, Mike Meredith, Alison Murray, Hans-Otto P&ouml;rtner, Guido di Prisco, Sigrid Schiel, Kevin Speer, Colin Summerhayes, John Turner, Cinzia Verde and Ann Willems<br />
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This first chapter sets out the nature of the physical-chemical environment of the Antarctic region and briefly describes key features of life there today.<br />
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==Pages in this topic==<br />
#[[The physical setting of Antarctica]]<br />
#[[The Antarctic cryosphere]]<br />
#[[The role of the Antarctic in the global climate system]]<br />
#[[Observations for studies of environmental change in the Antarctic]]<br />
#[[The climate of the Antarctic and its variability]]<br />
#[[Biota of the Antarctic]]<br />
[[Category:The Antarctic environment in the global system]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_Antarctic_cryosphereThe Antarctic cryosphere2014-08-06T14:34:07Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[The Antarctic environment in the global system]]''<br />
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[[File:Figure 1.2 - Antarctic topography and bathymetry.png|thumb|'''1.2''' The land topography and the bathymetry of the seabed around Antarctica (in metres). Four major oceanic fronts are shown (not labelled), which are (north-to-south): the Sub-Antarctic Front (SAF), the Polar Front (PF), the Southern ACC Front (SACCF) and the Southern Boundary (SB). The Antarctic Circumpolar Current runs between the SAF and the SB. Figure produced by M.P. Meredith (BAS; pers. comm.) using frontal locations adapted from Orsi et al., 1995<ref name="Orsi et al, 1995">Orsi, A.H., Whitworth III, T.W. and Nowlin Jr.,W.D. 1995. On the meridional extent and fronts of the Antarctic Circumpolar Current, ''Deep-Sea Res.'', '''42''', 641-673.</ref>) and bathymetry from the General Bathymetric Chart of the Oceans (GEBCO) Centennial digital topography data (www.gebco.net).]]<br />
[[File:Figure 1.3 - Antarctic surface elevation.png|thumb|'''1.3''' Surface elevation illuminated from directly overhead shows the general shape of the continent as well as the smaller scale roughness. Topographic divides between major catchments are bright (white) sinuous ridges. Fringing ice shelves are extremely flat (shown as pale grey matt). Smaller scale roughness is often associated with subglacial relief. The smoother surface surrounding the South Pole is the result of sparser and less accurate elevation data south of 86&ordm;S. (from Bamber et al., 2008<ref name="Bamber et al, 2008">Bamber, J.L., Gomez-Dans, J. L. and Griggs, J. A. 2008., A new 1 km digital elevation model of the Antarctic derived from combined satellite radar and laser data. Part I: Data and methods, The Cryosphere Discuss., 2(6).</ref>).]]<br />
[[File:Figure 1.1a - Map of Antarctica.png|thumb|'''1.1a''' A map of Antarctica showing selected topographic features and locations.]]<br />
The continent is dominated by the Antarctic Ice Sheet ([[:File:Figure 1.2 - Antarctic topography and bathymetry.png|Figure 1.2]] and [[:File:Figure 1.3 - Antarctic surface elevation.png|Figure 1.3]]), a vast contiguous mass of glacial ice that covers the Antarctic continent and surrounding seas. It is the single largest solid object on the surface of the planet, containing around 30 &times; 10<sup>6</sup> km<sup>3</sup> of ice or 70% of the Earth&rsquo;s freshwater, and covering around 99.6 % of what we generally consider to be the Antarctic continent (Fox and Cooper, 1994<ref name="Fox and Cooper, 1994">Fox, A.J. and Cooper, A.P.R. 1994. Measured properties of the Antarctic Ice Sheet derived from the SCAR Antarctic Digital Database, ''Pol. Rec.'', '''30'''(174), 201-206.</ref>). The ice sheet is made up of three distinct morphological zones, consisting of East Antarctica (covering an area of 10.35 &times; 10<sup>6</sup> km<sup>2</sup>), West Antarctica (1.97 &times; 10<sup>6</sup> km<sup>2</sup>) and the Antarctic Peninsula (0.52 &times; 10<sup>6</sup> km<sup>2</sup>) ([[:File:Figure 1.1a - Map of Antarctica.png|Figure 1.1a]]). East and West Antarctica are separated by the Transantarctic Mountains, which extend from Victoria Land to the Ronne Ice Shelf. The Antarctic Peninsula is the only part of the continent that extends a significant way northwards from the main ice sheet, reaching latitude 63&ordm;S. It is a narrow mountainous region with an average width of 70 km and a mean height of 1,500 m. This north-south trending mountain barrier has a major influence on the west-east trending oceanic and atmospheric circulations of the high southern latitudes.<br />
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The East Antarctic ice sheet (EAIS) comprises by far the largest part of the ice sheet. Lying primarily in the Eastern Hemisphere, it is bounded clockwise by the Ronne Ice Shelf near 30&ordm;W, the Transantarctic Mountains, and the coast of Victoria Land, which lies along the 165&ordm;E meridian on the western side of the Ross Sea) ([[:File:Figure 1.1a - Map of Antarctica.png|Figure 1.1a]], [[:File:Figure 1.2 - Antarctic topography and bathymetry.png|Figure 1.2]] and [[:File:Figure 1.3 - Antarctic surface elevation.png|Figure 1.3]]). The EAIS contains the coldest ice and is frozen to the bed over much of its area, which restricts its rate of flow. However, the bed in many places is at the pressure melting point as a consequence of the thick blanket of ice and heat flow from the Earth beneath, giving rise to some 145 subglacial lakes (Siegert et al., 2005b<ref name="Siegert et al, 2005b">Siegert, M.J., Carter, S. Tabacco, I. and Popov, S. 2005b. A revised inventory of Antarctic subglacial lakes, ''Antarctic Science'', '''17''', 453-460.</ref>).<br />
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[[File:Figure 1.4 - Antarctic bed elevation.png|thumb|'''1.4''' Bed elevation illustrating major regions (in blue and green) below sea level, the major subglacial continent underneath East Antarctica, and the mountain ranges separating East and West Antarctica and along the Antarctic Peninsula. Note the topography shown here has not been corrected for isostatic rebound. Taken from Lythe, Vaughan and BEDMAP Consortium, 2001.]]<br />
The EAIS lies on a landmass predominantly above sea level with a few broad basins depressed below sea level largely due to the weight of the ice sheet ([[:File:Figure 1.4 - Antarctic bed elevation.png|Figure 1.4]]).<br />
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The West Antarctic ice sheet (WAIS) comprises the ice that lies mostly in the Western Hemisphere and occupies the region from the Transantarctic Mountains counter clockwise through Marie Byrd Land to 150&deg;W (but not including the Antarctic Peninsula) ([[:File:Figure 1.1a - Map of Antarctica.png|Figure 1.1a]]). In contrast to the EAIS, most of the WAIS rests on a bed that is substantially below sea level ([[:File:Figure 1.4 - Antarctic bed elevation.png|Figure 1.4]]), and would remain so even if the ice sheet were removed. For this reason the WAIS is described as a &ldquo;marine ice sheet&rdquo; and is considered inherently unstable, being vulnerable to collapse if it lost its fringing ice shelves (e.g. see Bamber et al., 2009<ref name="Bamber et al, 2009">Bamber, J. L., Riva, R. E. M., Vermeersen, B. L. A. and Lebrocq A. M. 2009. Reassessment of the Potential Sea-Level Rise from a Collapse of the West Antarctic Ice Sheet, Science, 324 (5929), 901. [DOI: 10.1126/science.1169335].</ref>). In many places, the bed beneath the WAIS is over one thousand metres below sea level &ndash; far deeper than most of the world&rsquo;s continental shelves. A few of its buried archipelagos (Ellsworth Mountains, Executive Committee and Flood Ranges, Whitmore Mountains and the Ellsworth Land coast &ndash; not shown here) are sufficiently high to rise above the ice sheet. The WAIS is generally warmer, both at the surface and at the bed, than its East Antarctic neighbour, with basal ice close to the melting point in most areas.<br />
<br />
The ice sheet that covers the Antarctic Peninsula is quite different from either the EAIS or the WAIS. It consists of much smaller and much thinner ice caps that cover the central mountainous spine and some of the larger outlying islands. These ice caps drain into the sea through relatively narrow, but steep and fast-moving, alpine-type glaciers. In contrast to the WAIS and the EAIS, which lose mass primarily through iceberg calving and melt from the base of ice shelves, the Antarctic Peninsula experiences much higher summer temperatures making runoff from surface melt a significant component in the budget of its ice sheet.<br />
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The ice sheets are nourished at their surface by deposition of snow and frost, which remains frozen because of the year-round cold, and accumulates year-on-year. As the surface snows are buried by new snowfall, they are compressed and eventually transformed into solid ice, a process that captures a chemical record of past climates and environments. From this record in East Antarctica it is possible to reconstruct Antarctica&rsquo;s contribution to planetary climate change over the past 800,000 years.<br />
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[[File:Figure 1.5 - Antarctic ice thickness.png|thumb|'''1.5''' Ice thickness (difference between surface and bed elevations) in metres, showing the much thicker East Antarctic ice sheet, gradual thinning from the interior to the coast and the many deep outlet glaciers. Seawards of the continental outline, the dark blue areas are the permanent ice shelves that represent floating extensions of the continental ice sheet. From Lythe, Vaughan and BEDMAP Consortium, 2001.]]<br />
The ice sheet is up to 4,776 m thick in Terre Ad&eacute;lie (at around 140&ordm;E in East Antarctica), averaging 1,829 m ([[:File:Figure 1.5 - Antarctic ice thickness.png|Figure 1.5]]). In places the deepest ice may be more than one million years old; older ice is likely to have been so squeezed out by compression from above that it is in practical terms undateable as well as being very thin. Remnants of much older glaciers survive in parts of the McMurdo Dry Valleys, with ice at one location dated from volcanic ash at over 8 million years (Sugden et al., 1995<ref name="Sugden et al, 1995">Sugden, D.E., Marchant, D.R., Potter, N., Souchez, R.A., Denton, G.H., Swisher, C.C. and Tison, J.-L. 1995. Preservation of Miocene glacier ice in East Antarctica, ''Nature'', '''376''', 412-414.</ref>). However, that age is in dispute (Hindmarsh et al., 1998<ref name="Hindmarsh et al, 1998">Hindmarsh, R.C.C., Van Der Wateren, F.M. and Verbers, A.L.L.M. 1998. Sublimation of ice through sediments in Beacon Valley, Antarctica, ''Geogr. Am'', '''80A''' (3-4), 209-219.</ref>).<br />
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[[File:Figure 1.6 - Antarctic ice sheet balance velocity.png|thumb|'''1.6''' Balance velocity, calculated at any point as the velocity (averaged over the thickness) required to balance upstream accumulation, to illustrate the spatial pattern of ice flow that is required to maintain the ice sheet shape in the present climate (Rignot and Thomas, 2002<ref name="Rignot and Thomas, 2002">Rignot, E. and Thomas, R.H. 2002. Mass balance of polar ice sheets, Science, 297 (5586), 1502-1506 AUG 30 2002.</ref>).]]<br />
All ice flows out from the central ridges and domes to the edge of the continent, converging as ice streams and outlet glaciers that move at speeds of up to 500 m per year ([[:File:Figure 1.6 - Antarctic ice sheet balance velocity.png|Figure 1.6]]). The entire Antarctic ice sheet transports ice from the interior to the coast at a rate of around 2000 billion tonnes per year. Once the ice streams reach the edge of the continent they either calve into icebergs or float on the ocean as ice shelves. The ice shelves constitute 11% of the total area of the Antarctic, with the two largest being the Ronne-Filchner Ice Shelf in the Weddell Sea and the Ross Ice Shelf in the Ross Sea, which have areas of 0.53 &times; 10<sup>6</sup> km<sup>2</sup> and 0.54 &times; 10<sup>6</sup> km<sup>2</sup> respectively ([[:File:Figure 1.6 - Antarctic ice sheet balance velocity.png|Figure 1.6]]). The ice shelves are several hundreds of metres thick and the ocean areas under them are among the most isolated and unvarying environments on Earth. Nevertheless, ocean currents carry water masses into the cavities beneath the ice shelves. There the water interacts with the undersides of the ice shelves either melting the ice or, at times, adding to it by refreezing. The cooled ocean water becomes fresher with the addition of meltwater and emerges from beneath the ice shelves in this modified condition (Nicholls et al, 2009<ref name="Nicholls et al, 2009">Nicholls, K.W., Osterhus, S., Makinson, K., Gammelsrod, T. and Fahrbach, E. 2009. Ice-ocean processes over the continental shelf of the southern Weddell Sea, Antarctica: A review, Reviews of Geophysics, doi:10.1029/2007RG000250, in press.</ref>).<br />
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[[File:Figure 1.7 - Antarctic summer and winter sea ice extent.png|thumb|'''1.7''' Sea ice extent in summer (February 2008) at left, and in winter (September 2008) at right. From NSIDC (http://nsidc.org/cgi-bin/bist/bist.pl?config=seaice_index.)]]<br />
The continent is surrounded for most of the year by a zone of frozen seawater 1 or 2 m thick ([[:File:Figure 1.7 - Antarctic summer and winter sea ice extent.png|Figure 1.7]]). By late austral winter, this sea ice covers an area of 20 &times; 10<sup>6</sup> km<sup>2</sup>, more than the area of the continent itself. At this time of year the northern edge of the sea ice is close to 60&ordm;S around most of the continent, and near 55&ordm;S north of the Weddell Sea. Unlike the Arctic, most of the Antarctic sea ice melts during the austral summer, so that by autumn it only covers an area of about 3 &times; 10<sup>6</sup> km<sup>2</sup>. Most Antarctic sea ice is therefore thin first year ice, with the largest area of multi-year ice being over the western Weddell Sea.<br />
<br />
From time to time substantial areas of ice-free water can form like lakes within the sea ice. These are polynyas, which may occur where relatively warm water rises to the surface or where winds drive sea ice away from the coast (Renfrew et al, 2002<ref name="Renfrew et al, 2002">Renfrew, I.A., King, J.C. and Markus, T. 2002. Coastal polynyas in the southern Weddell Sea: Variability of the surface energy budget, J. Geophys. Res., 107, doi:10.1029/2000JC000720.</ref>). The latter kind of polynyas are &lsquo;factories&rsquo; for the continuous formation of sea ice, which deposits salt in the water making it dense enough to sink and contribute to the formation of new deep water masses (Markus et al, 1998<ref name="Markus et al, 1998">Markus, T., Kottmeier C. and Fahrbach, E. 1998. Ice formation in coastal polynyas in the Weddell Sea and their impact on oceanic salinity, in Antarctic Sea Ice: Physical Processes, Interactions and Variability, edited by M. O. Jeffries, 273-292, AGU, Washington D.C.</ref>).<br />
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An important feature of any polar environment is permafrost, defined as any earth material that remains below 0&ordm;C for two or more consecutive years. The uppermost layer, which seasonally can experience temperatures above 0&deg;C is called the &lsquo;active layer&rsquo;. Permafrost is much less extensive in the Antarctic compared to the Arctic, because of the very large area covered by the major ice sheets. Permafrost underlies most of the ice-free surfaces, some parts of the ice sheet and the glaciated areas, and even some spots of the adjacent sea floor. It is widespread in continental Antarctica, across parts of the Antarctic Peninsula and also in the islands of maritime Antarctica. Here, permafrost is a significant feature of the environment and it is important in the support of terrestrial ecosystems. Permafrost is also found in the McMurdo Dry Valleys and along the narrow coastal zone of East Antarctica. The McMurdo Dry Valleys have lakes with open water in the summer, and the Wright Valley has the Onyx River, which flows for about 2 months each year. Melting snow patches on rock form small runnels at elevations up to around 2,000 m there. The active layer is typically 30-50 cm thick. Permafrost ranges from 240 to 900 m thick in continental Antarctica, and 15 to 180 m thick in maritime Antarctica, and occurs only sporadically in the South Shetland and South Orkney Islands. The temperature of permafrost is &ndash;12 to &ndash;24&deg;C in continental Antarctica but is likely warmer in maritime Antarctica.<br />
==References==<br />
<references /><br />
[[Category:The Antarctic environment in the global system]]<br />
[[Category:The Antarctic ice sheet]]<br />
[[Category:Antarctic sea ice]]</div>Maintenance scripthttp://acce.scar.org/wiki/The_Amundsen_and_Bellingshausen_seas_in_the_instrumental_periodThe Amundsen and Bellingshausen seas in the instrumental period2014-08-06T14:34:05Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[The Southern Ocean in the instrumental period]]''<br />
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The southeast Pacific Ocean (70&deg;W &ndash; 150&deg;W) deserves enhanced scientific interest because of the significant alterations it faces or is supposed to face in a changing climate. Since 1951 annual mean atmospheric temperatures rose by almost 3&deg;C at the Antarctic Peninsula (King, 1994<ref name="King, 1994">King, J.C. 1994. Recent variability in the Antarctic Peninsula, ''International Journal of Climatology'', '''14'''(4), 357-369.</ref>; Turner et al., 2005a<ref name="Turner et al, 2005a">Turner, J., Colwell, S.R., Marshall, G.J., Lachlan-Cope, T.A., Carleton, A.M., Jones, P.D., Lagun, V., Reid, P.A. and Iagovkina, S. 2005a. Antarctic climate change during the last 50 years, ''International Journal of Climatology'', '''25''', 279-294.</ref>) which can be linked to changes in the Bellingshausen Sea, like sea ice retreat (Jacobs and Comiso, 1993<ref name="Jacobs and Comiso, 1993">Jacobs, S.S. and Comiso, J.C. 1993. A recent sea-ice retreat west of the Antarctic Peninsula, ''Geophys. Res. Lett.'', '''20''', 1171-1174.</ref>), increased ocean surface summer temperatures of more than 1&deg;C, enhanced upper-layer salinification (Meredith and King, 2005<ref name="Meredith and King, 2005">Meredith, M.P. and King, J.C. 2005. Rapid climate change in the ocean west of the Antarctic Peninsula during the second half of the 20<sup>th</sup> century, Geophys. Res. Lett., 32, L19604. (doi: 10.1029/2005GL024042)</ref>), the disintegration of smaller ice shelves (Doake and Vaughan 1991<ref name="Doake and Vaughan, 1991">Doake, C.S.M. and Vaughan, D.G. 1991. Rapid disintegration of the Wordie Ice shelf in response to atmospheric warming, ''Nature'', '''350''', 328-330.</ref>), and accelerated retreat of glaciers (Cook et al., 2005<ref name="Cook et al, 2005">Cook, A., Fox, A., Vaughan, D. and Ferrigno, J. 2005, Retreating glacier fronts on the Antarctic Peninsula over the past half-century, ''Science'', '''308''', 541-544.</ref>). The changes can be related to atmospheric variability including the Antarctic Circumpolar Wave (White and Peterson, 1996<ref name="White and Peterson, 1996">White, W.B. and Peterson, R. 1996. An Antarctic Circumpolar Wave in surface pressure, wind, temperature, and sea ice extent, ''Nature'', '''380''', 699-702.</ref>) or the SAM (Hall and Visbeck, 2002<ref name="Hall and Visbeck, 2002">Hall, A. and Visbeck, M. 2002. Synchronous variability in the southern hemisphere atmosphere, sea ice, and ocean resulting from the annular mode, ''Journal of Climate'', '''15'''(21), 3043-3057.</ref>; Lefebvre et al., 2004<ref name="Lefebvre et al, 2004">Lefebvre, W., Goosse, H., Timmermann, R. and Fichefet, T. 2004. Influence of the Southern Annular Mode on the sea ice-ocean system, ''J. Geophys. Res.'', '''109''', C09005, doi:10.1029/2004JC002403.</ref>), which both exhibit extreme values in the southeast Pacific Ocean. Different hydrographic conditions have a severe impact on marine species (e.g., the Antarctic krill) which use the Bellingshausen Sea for breeding and nursery before the larvae mainly drift eastward to the southern Scotia Sea/northwestern Weddell Sea (Siegel, 2005<ref name="Siegel, 2005">Siegel, V. 2005. Distribution and population dynamics of ''Euphausia superba'': summary of recent findings, ''Pol. Biol.'', '''29''', 1-22, doi:10.1007/s00300-005-0058-5.</ref>). A comprehensive field study on Antarctic krill in the Amundsen Sea has yet to be conducted.<br />
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[[File:Figure 4.25 - Amundsen Sea bathymetric chart and hydrographic stations of different cruises.png|thumb|'''4.25''' Bathymetric chart of the Amundsen Sea continental shelf and adjacent deep ocean spotted with the distribution of hydrographic stations of different cruises (colour coded). Fringing ice shelves and glaciers draining the West Antarctic Ice Sheet in light blue (Nitsche et al., 2007<ref name="Nitsche et al, 2007">Nitsche, F.O., Jacobs, S.S., Larter, R.D. and Gohl, K. 2007. Bathymetry of the Amundsen Sea continental shelf: Implications for geology, oceanography, and glaciology, ''Geochem. Geophys. Geosyst.'', '''8''', Q10009, doi:10.1029/2007GC001694.</ref>). The CTD station identifiers refer to the Nathaniel B. Palmer (NBP), the James Clark Ross (JCR) and the Polarstern (ANT).]]<br />
[[File:Figure 4.26 - Cross-section showing warm water penetrating the Pine Island Bay sub-ice shelf cavity.png|thumb|'''4.26''' A cross-section showing penetration of warm water to the sub-ice shelf cavity in Pine Island Bay. Potential temperature of the upper 1,200 m along a band between 100&deg;W and 105&ordm;W projected on a strait transect from the open ocean (left) to Pine Island Bay (right) measured during NBP9402 (see Figure 4.25 for station locations). Due to the ice conditions the stations could not be done along a straight line. The sea floor depth was extracted from the ship&rsquo;s 3.5 kHz echosounder data. Right figure depicts the temperature field in front of Pine Island Glacier with its draft shaded in gray. This short line is along the line of pink dots (sample stations) shown in Figure 4.25. The 1&deg;C isotherm on the continental shelf and slope is marked in red (modified from Hellmer et al., 1998<ref name="Hellmer et al, 1998">Hellmer, H.H., Jacobs, S.S. and Jenkins, A. 1998. Oceanic erosion of a floating Antarctic glacier in the Amundsen Sea, in: Ocean, Ice, and Atmosphere: Interactions at the Antarctic continental margin, S S Jacobs and R F Weiss (eds), ''Antarc. Res. Ser.'', '''75''', 83-99.</ref>). The solid black area indicates the sea bed.]]<br />
Connected via the westward flowing, and in this part, weak coastal current, changes in the Bellingshausen Sea also influence the Amundsen Sea (100&deg;W &ndash; 150&deg;W) which is fringed to the south by the outlets of major ice streams draining the West Antarctic Ice Sheet. A possible collapse of the latter would result in a 5-6 m global sea level rise threatening many low-lying coastal areas around the globe including millions of their residents (Rowley et al., 2007<ref name="Rowley et al, 2007">Rowley, R.J, Kostelnick, J.C., Braaten, D., Li, X. and Meisel, J. 2007. Risk of rising sea level to population and land area, EOS Transactions, 88, 105, 107.</ref>), but is not likely in the next 100 years. The southernmost position of the ACC southern front (Orsi et al., 1995<ref name="Orsi et al, 1995">Orsi, A.H., Whitworth III, T.W. and Nowlin Jr.,W.D. 1995. On the meridional extent and fronts of the Antarctic Circumpolar Current, ''Deep-Sea Res.'', '''42''', 641-673.</ref>) together with a relative narrow continental shelf crisscrossed by numerous channels ([[:File:Figure 4.25 - Amundsen Sea bathymetric chart and hydrographic stations of different cruises.png|Figure 4.25]]) allows Upper Circumpolar Deep Water (UCDW) with temperatures near 1&deg;C to reach the ice shelf edges in the Amundsen and Bellingshausen Seas ([[:File:Figure 4.26 - Cross-section showing warm water penetrating the Pine Island Bay sub-ice shelf cavity.png|Figure 4.26]]). This ocean heat could fuel melting of up to tens of metres per year at deep ice shelf bases. A linear relationship between melt rates beneath Antarctic ice shelves and ocean temperature is roughly linear at 1 m/yr per 0.1&deg;C ocean warming derived from observations of 23 glaciers (Rignot and Jacobs, 2002<ref name="Rignot and Jacobs, 2002">Rignot, E.J., and Jacobs, S.S. 2002. Rapid Bottom Melting Widespread near Antarctic Ice Sheet Grounding Lines, ''Science'', '''296''', 2020-2023.</ref>). The change of ice melt rate to a change in ocean temperature may not follow this same relationship. In a simple box model Olbers and Hellmer (2009<ref name="Olbers and Hellmer, 2009">Olbers, D. and Hellmer, H. 2009. A box model of circulation and melting in ice shelf caverns, Ocean Dynamics, submitted.</ref>) confirm this rate to be consistent with the involved physical processes and investigate the sensitivity. Numerical modeling of ice-ocean interaction beneath Pine Island Glacier (Payne et al., 2007<ref name="Payne et al, 2007">Payne, A.J., Holland, P.R., Shepherd, A.P., Rutt, I.C., Jenkins, A. and Joughin, I. 2007. Numerical modeling of ocean-ice interactions under Pine Island Bay's ice shelf, J. Geophys. Res., 112, doi:10.1029/2006JC003733.</ref>) concludes that the observed thinning at a rate of 3.9 &plusmn; 0.5 m/yr between 1992 and 2001 (Shepherd et al., 2002<ref name="Shepherd et al, 2002">Shepherd, A.P., Wingham, D.J. and Mansley, J.A.D. 2002. Inland thinning of the Amundsen Sea sector, West Antarctica, ''Geophys. Res. Lett.'', '''29''', 1364, doi:10.1029/2001GL014183.</ref>) would correspond to a different rate of ~ 0.25&deg;C warming of the UCDW underneath Pine Island Glacier. Such warming has not been observed on the Amundsen Sea continental shelf, but a 40-year long temperature time series from the nearby Ross Sea exhibits a warming of the off-shore temperature maximum (190-440 m depth) of ~ 0.3&deg;C (Jacobs et al., 2002<ref name="Jacobs et al, 2002">Jacobs, S.S., Giulivi, C.F. and Mele, P.A. 2002. Freshening of the Ross Sea during the late 20<sup>th</sup> century. Science, 297(5580), 386-389, doi:10.1126/science.1069574.</ref>). Temperature variability correlated with changing freshwater fluxes due to basal melting is likely at the fringe of the West Antarctic Ice Sheet. Upper Southern Ocean temperatures increased since the 1950's (Gille, 2002<ref name="Gille, 2002">Gille, S.T. 2002. Warming of the Southern Ocean since the 1950s, ''Science'', '''295'''(5558), 1275-1277, doi:10.1126/science.1065863.</ref>; B&ouml;ning et al., 2008<ref name="B&ouml;ning et al, 2008">B&ouml;ning, C. W., A. Dispert, M. Visbeck, S. R. Rintoul, and F. Schwarzkopf, 2008. Observed multi-decadal ocean warming and density trends across the Antarctic Circumpolar Current, Submitted.</ref>), but the few oceanographic snapshots (e.g., Hofmann and Klinck, 1998<ref name="Hofmann and Klinck, 1998">Hofmann, E.E. and Klinck, J.M. 1998. Thermohaline variability of the waters overlying the West Antarctic Peninsula continental shelf, in: Ocean, Ice, and Atmosphere: Interactions at the Antarctic continental margin, S S Jacobs and R F Weiss (eds), ''Antarc. Res. Ser.'', '''75''', 67-81.</ref>; Walker et al., 2007<ref name="Walker et al, 2007">Walker, D.P., Brandon, M.A., Jenkins, A., Allen, J.T., Dowdeswell, J.A. and Evans, J. 2007. Oceanic heat transport onto the Amundsen Sea shelf through a submarine glacial trough, Geophys. Res. Lett., 34, doi:10.1029/2006GL028154.</ref>) from the Amundsen/Bellingshausen Sea continental margin are insufficient to identify the time scales and strengths of variability or any trends.<br />
==References==<br />
<references /><br />
[[Category:The instrumental period]]<br />
[[Category:The Southern Ocean]]</div>Maintenance scripthttp://acce.scar.org/wiki/Terrestrial_biota_of_the_AntarcticTerrestrial biota of the Antarctic2014-08-06T14:34:04Z<p>Acce: Added tables</p>
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<div>:''This page is part of the topic [[Biota of the Antarctic]]''<br />
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Looked at from the perspective of the terrestrial and freshwater biologists and biogeographers, the various bits of land are subdivided for the purposes of this volume into three zones - the &lsquo;continental Antarctic&rsquo; (comprising most of the continent), the &lsquo;maritime Antarctic&rsquo; (comprising the Antarctic Peninsula and associated islands and archipelagos as well as the South Shetland, South Orkney, and South Sandwich Islands, and Bouvet&oslash;ya), and the &lsquo;sub-Antarctic&rsquo; (those islands that lie in or around the Antarctic Polar Frontal Zone (PFZ).<br />
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Levels of terrestrial macro-biodiversity in the Antarctic are strikingly lower than those of the Arctic, although the sub-Antarctic hosts greater diversity than the maritime and continental regions. This is the case even relative to the superficially environmentally extreme and isolated High Arctic Svalbard and Franz Josef archipelagos at around 80&deg;N. In comparison with about 900 species of vascular (higher or flowering) plants in the Arctic, there are only two on the Antarctic continent and up to 40 on any single sub-Antarctic island. Likewise, the Antarctic and sub-Antarctic have no native land mammals, against 48 species in the Arctic. The continuous southwards continental connection of much of the Arctic is an important factor underlying these differences. Despite the apparent ease of access to much of the Arctic, few established alien vascular plants or invertebrates are known from locations such as Svalbard (R&oslash;nning, 1996<ref name="R&oslash;nning, 1996">R&oslash;nning, O.I. 1996. The flora of Svalbard. Norsk Polarinstitut, Oslo: 184 pp.</ref>; Coulson 2007<ref name="Coulson, 2007">Coulson, S.J. 2007. The terrestrial and freshwater invertebrate fauna of the High Arctic archipelago of Svalbard, ''Zootaxa'', '''1448''', 41-68.</ref>), in comparison with the 200 species introduced to the sub-Antarctic by human activity over only the last two centuries or so (Frenot et al., 2005<ref name="Frenot et al, 2005">Frenot, Y., Chown, S.L., Whinam, J., Selkirk, P., Convey, P., Skotnicki, M. and Bergstrom, D. 2005 Biological invasions in the Antarctic: extent, impacts and implications, ''Biological Reviews'', '''80''', 45-72.</ref>, 2008<ref name="Frenot et al, 2008">Frenot, Y., Convey, P., Lebouvier, M., Chown, S.L., Whinam, J., Selkirk, P.M., Skotnicki, M. and Bergstrom, D.M. 2008. Antarctic biological invasions: sources, extents, impacts and implications. Non-native species in the Antarctic Proceedings, ed. M. Rogan-Finnemore, 53-96. Gateway Antarctica, Christchurch, New Zealand.</ref>). It may be the case that species comparable to the many sub-Antarctic &lsquo;aliens&rsquo;, being cosmopolitan northern hemisphere and boreal &lsquo;weeds&rsquo;, have had greater opportunity to reach polar latitudes by natural means in the north than the south.<br />
<br />
Antarctic and sub-Antarctic floras and faunas are strongly disharmonic, with representatives of many major taxonomic and functional groups familiar from lower latitudes being absent. Sub-Antarctic plant communities do not include woody plants, and are dominated by herbs, graminoids and cushion plants; flowering plants (phanerogams) are barely represented (two species) in the maritime and not at all in the continental Antarctic. Sub-Antarctic floras have developed some particularly unusual elements &ndash; &lsquo;megaherbs are a striking element of the flora of many islands, being an important structuring force within habitats, and a major contributor of biomass (Meurk et al., 1994a<ref name="Meurk et al, 1994a">Meurk, C.D., Foggo, M.N. and Wilson, J.B. 1994a. The vegetation of subantarctic Campbell Island, ''New Zealand Journal of Ecology'', '''18''', 123-168.</ref>,b<ref name="Meurk et al, 1994b">Meurk, C.D., Foggo, M.N., Thompson, B.M., Bathurst, E.T.J., and Crompton, M.B. 1994b. Ion&ndash;rich precipitation and vegetation patterns on subantarctic Campbell Island, ''Arctic and Alpine Research'', '''26''', 281-289.</ref>; Mitchell et al., 1999<ref name="Mitchell et al, 1999">Mitchell, A.D., Meurk, C.D. and Wagstaff, S.J. 1999. Evolution of Stilbocarpa, a megaherb from New Zealand&rsquo;s sub&ndash;antarctic islands, ''New Zealand Journal of Botany'', '''37''', 205-211.</ref>; Fell, 2002<ref name="Fell, 2002">Fell, D. 2002, Campbell Island, Land of the Blue Sunflower. Bateman, Auckland: 143 pp.</ref>; Shaw, 2005<ref name="Shaw, 2005">Shaw, J.D. 2005. Reproductive Ecology of Vascular Plants on Subantarctic Macquarie Island. PhD Thesis, University of Tasmania, Hobart.</ref>; Convey et al., 2006a<ref name="Convey et al, 2006a">Convey, P., Chown, S.L., Wasley, J. and Bergstrom, D.M. 2006a. Life history traits. Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 101-127.</ref>). These plants present an unusual combination of morphological and life history characteristics (Convey et al., 2006a<ref name="Convey et al, 2006a">Convey, P., Chown, S.L., Wasley, J. and Bergstrom, D.M. 2006a. Life history traits. Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 101-127.</ref>), and their dominance on sub-Antarctic islands is thought to have been encouraged by a combination of the absence of natural vertebrate herbivores (Meurk et al., 1994a<ref name="Meurk et al, 1994a">Meurk, C.D., Foggo, M.N. and Wilson, J.B. 1994a. The vegetation of subantarctic Campbell Island, ''New Zealand Journal of Ecology'', '''18''', 123-168.</ref>; Mitchell et al., 1999<ref name="Mitchell et al, 1999">Mitchell, A.D., Meurk, C.D. and Wagstaff, S.J. 1999. Evolution of Stilbocarpa, a megaherb from New Zealand&rsquo;s sub&ndash;antarctic islands, ''New Zealand Journal of Botany'', '''37''', 205-211.</ref>), and possessing adaptive benefits relating to the harvesting and focussing of low light levels and aerosol nutrients (Wardle, 1991<ref name="Wardle, 1991">Wardle, P. 1991. Vegetation of New Zealand. Cambridge University Press, Cambridge: 672 pp.</ref>; Meurk et al., 1994b<ref name="Meurk et al, 1994b">Meurk, C.D., Foggo, M.N., Thompson, B.M., Bathurst, E.T.J., and Crompton, M.B. 1994b. Ion&ndash;rich precipitation and vegetation patterns on subantarctic Campbell Island, ''Arctic and Alpine Research'', '''26''', 281-289.</ref>). The recent anthropogenic introduction of vertebrate herbivores to most sub-Antarctic islands has led to considerable and negative impacts on megaherb-based communities (Frenot et al., 2005<ref name="Frenot et al, 2005">Frenot, Y., Chown, S.L., Whinam, J., Selkirk, P., Convey, P., Skotnicki, M. and Bergstrom, D. 2005 Biological invasions in the Antarctic: extent, impacts and implications, ''Biological Reviews'', '''80''', 45-72.</ref>; Shaw et al., 2005<ref name="Shaw et al, 2005">Shaw, J.D., Bergstrom, D.M. and Hovenden, M. 2005. The impact of feral rats (Rattus rattus) on populations of subantarctic megaherb (Pleurophyllum hookeri), ''Austral. Ecology'', '''30''', 118-125.</ref>; Convey et al., 2006b<ref name="Convey et al, 2006b">Convey, P., Frenot, F., Gremmen, N. and Bergstrom, D. 2006b. Biological invasions. Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 193-220.</ref>).<br />
<br />
The tables below provide summary information on terrestrial biodiversity in the Antarctic.<br />
<br />
{| class="wikitable" style="text-align: center;"<br />
!Zone!!Flowering plants!!Ferns and<br />club-mosses!!Mosses!!Liverworts!!Lichens!!Macro-fungi<br />
|-<br />
|sub-Antarctic||60||16||250||85||250||70<br />
|-<br />
|maritime Antarctic||2||0||100||25||250||30<br />
|-<br />
|continental Antarctic||0||0||25||1||150||0<br />
|}<br />
<br />
'''Table 1.1''' Biodiversity of plant taxa in the three Antarctic biogeographical zones. Note that figures presented are approximate, as it is likely that (i) new species records will be obtained through more directed sampling, (ii) a significant number of unrecognized synonymies are likely to exist and (iii) taxonomic knowledge of some Antarctic groups is incomplete.<br />
<br />
{| class="wikitable" style="text-align: center;"<br />
! style="text-align: left"|Group!!Sub-Antarctic!!Maritime Antarctic!!Continental Antarctic and<br />continental shelf<br />
|-<br />
| style="text-align: left"|Protozoa *||colspan=2|83||33<br />
|-<br />
| style="text-align: left"|Rotifera *||> 59||> 50||13<br />
|-<br />
| style="text-align: left"|Tardigrada||> 34||26||19<br />
|-<br />
| style="text-align: left"|Nematoda *||> 22||28||14<br />
|-<br />
| style="text-align: left"|Platyhelminthes||4||2||0<br />
|-<br />
| style="text-align: left"|Gastrotricha||5||2||0<br />
|-<br />
| style="text-align: left"|Annelida (Oligochaeta)||23||3||0<br />
|-<br />
| style="text-align: left"|Mollusca||3/4||0||0<br />
|-<br />
| style="text-align: left"|Crustacea (terrestrial)||4||0||0<br />
|-<br />
| style="text-align: left"|Crustacea (non-marine)||44||10||14<br />
|-<br />
| style="text-align: left"|'''Insecta (total)'''||210||35||49<br />
|-<br />
| style="text-align: left;padding-left: 1.6em"|Mallophaga||61||25||34<br />
|-<br />
| style="text-align: left;padding-left: 1.6em"|Diptera||44||2||0<br />
|-<br />
| style="text-align: left;padding-left: 1.6em"|Coleoptera||40||0||0<br />
|-<br />
| style="text-align: left"|Collembola||> 30||10||10<br />
|-<br />
| style="text-align: left"|'''Arachnida (total)'''||167||36||29<br />
|-<br />
| style="text-align: left;padding-left: 1.6em"|Araneida||20||0||0<br />
|-<br />
| style="text-align: left;padding-left: 1.6em"|Acarina *||140||36||29<br />
|-<br />
| style="text-align: left"|Myriapoda||3||0||0<br />
|}<br />
<br />
'''Table 1.2''' Biodiversity of native terrestrial invertebrates in the three Antarctic biogeographical zones. Data obtained from Block, in Laws (1984), Pugh (1993<ref name="Pugh, 1993">Pugh, P.J.A. 1993. A synonymic catalogue of the Acari from Antarctica, the sub-Antarctic Islands and the Southern Ocean, ''Journal of Natural History'', '''27''', 323-421.</ref>), Pugh and Scott (2002<ref name="Pugh and Scott, 2002">Pugh, P.J.A. and Scott, B. 2002. Biodiversity and biogeography of non-marine Mollusca on the islands of the southern Ocean, ''Journal of Natural History'', '''36''', 927-952.</ref>), Pugh et al. (2002<ref name="Pugh et al, 2002">Pugh, P.J.A., Dartnall, H.J.G. and McInnes, S.J. 2002. The non-marine crustacea of Antarctica and the islands of the Southern Ocean: biodiversity and biogeography. Journal of Natural History 36:1047-1103.</ref>), Convey and McInnes (2005<ref name="Convey and McInnes, 2005">Convey, P. and McInnes, S.J. 2005. Exceptional, tardigrade dominated, ecosystems from Ellsworth Land, Antarctica, ''Ecology'', '''86''', 519-527.</ref>), Dartnall (2005<ref name="Dartnall, 2005">Dartnall, H.J.G. 2005. Freshwater invertebrates of subantarctic South Georgia, ''J. Nat. Hist.'', '''39''', 3321-3342.</ref>), Dartnall et al. (2005<ref name="Dartnall et al, 2005">Dartnall, H.J.G., Hollwedel, W. and De Paggi, J.C. 2005. The freshwater fauna of Macquarie Island, including a redescription of the endemic water-flea Daphnia gelida (Brady) (Anomopoda: Crustacea), ''Polar Biol.'', '''28''', 922-939.</ref>), Greenslade (2006<ref name="Greenslade, 2006">Greenslade, P. 2006: The Invertebrates of Macquarie Island. Australian Antarctic Division, Kingston, Tasmania, xvi, 326 pp.</ref>), Maslen and Convey (2006<ref name="Maslen and Convey, 2006">Maslen, N.R. and Convey, P. 2006. Nematode diversity and distribution in the southern maritime Antarctic &ndash; clues to history?, ''Soil Biology and Biochemistry'', '''38''', 3141-3151.</ref>). ND - number of representatives of group unknown; * - large changes likely with future research due to current lack of sampling coverage, expertise and/or synonymy.<br />
<br />
Representing the animal kingdom, across the Antarctic and sub-Antarctic there are no native land mammals, reptiles or amphibians and very few non-marine birds. Instead, terrestrial faunas are dominated by arthropods, including various insects, arachnids, the microarthropod groups of mites and springtails, enchytraeids, earthworms, tardigrades, nematodes, beetles, flies and moths, with smaller representation of some other insect groups (Gressitt, 1970<ref name="Gressitt, 1970">Gressitt, J.L. (ed.). 1970. Subantarctic entomology, particularly of South Georgia and Heard Island, ''Pacific Insects Monograph'', '''23''', 1-374.</ref>; Convey, 2007a<ref name="Convey, 2007a">Convey, P. 2007a. Antarctic Ecosystems. In Levin, S.A. (ed), Encyclopedia of Biodiversity, 2<sup>nd</sup> Edition, Elsevier, San Diego (in press).</ref>). Although levels of species diversity are low relative to temperate communities, population densities are often comparable, with tens to hundreds of thousands of individuals per square metre. Few of these invertebrates are thought to be true herbivores, and the decomposition cycle is thought to dominate most terrestrial ecosystems, even in the sub-Antarctic, with the exception of some beetles and moths, although detailed autecological studies are typically lacking (Hogg et al., 2006<ref name="Hogg et al, 2006">Hogg, I.D., Cary, S.C., Convey, P., Newsham, K.K., O&rsquo;Donnell, T., Adams, B.J., Aislabie, J., Frati, F.F., Stevens, M.I. and Wall, D.H, 2006. Biotic interactions in Antarctic terrestrial ecosystems: are they a factor? Soil Biology and Biochemistry, 38, 3035-3040.</ref>). However, despite the preponderance of detritivores, decay processes are slow. Carnivores are also present (spiders, beetles on the sub-Antarctic islands, along with predatory microarthropods and other microscopic groups throughout), but predation levels are generally thought to be insignificant (Convey 1996a<ref name="Convey, 1996a">Convey, P. 1996a. The influence of environmental characteristics on life history attributes of Antarctic terrestrial biota, ''Biological Reviews of the Cambridge Philosophical Society'', '''71''', 191-225.</ref>).<br />
<br />
Although microbial biodiversity is dominant in most Antarctic terrestrial and freshwater systems, these communities are generally considered to be relatively simple, with a limited trophic structure (Hogg et al., 2006<ref name="Hogg et al, 2006">Hogg, I.D., Cary, S.C., Convey, P., Newsham, K.K., O&rsquo;Donnell, T., Adams, B.J., Aislabie, J., Frati, F.F., Stevens, M.I. and Wall, D.H, 2006. Biotic interactions in Antarctic terrestrial ecosystems: are they a factor? Soil Biology and Biochemistry, 38, 3035-3040.</ref>). Furthermore, only a relatively small proportion, 0.33% (Fox and Cooper, 1994<ref name="Fox and Cooper, 1994">Fox, A.J. and Cooper, A.P.R. 1994. Measured properties of the Antarctic Ice Sheet derived from the SCAR Antarctic Digital Database, ''Pol. Rec.'', '''30'''(174), 201-206.</ref>) of the continent&rsquo;s surface area is ice-free and available for terrestrial biota.<br />
<br />
Terrestrial microbial diversity has been explored, although not extensively enough. Data are available particularly from the McMurdo Dry Valleys (Priscu et al., 1998<ref name="Priscu et al, 1998">Priscu, J.C., Fritsen, C.H., Adams, E.E., Giovannoni, S.J., Paerl, H.W., McKay, C.P., Doran, P.T., Gordon, D.A., Lanoil, B.D. and Pinckney, J.L. 1998. Perennial Antarctic lake ice: an oasis for life in a polar desert, ''Science'', '''280''', 2095-2098.</ref>; Gordon et al., 2000<ref name="Gordon et al, 2000">Gordon, D.A., Priscu, J. and Giovannoni, S. 2000. Origin and phylogeny of microbes living in permanent Antarctic lake ice, Microb. Ecol. 39, 197-202.</ref>; Shravage et al., 2007<ref name="Shravage et al, 2007">Shravage, B.V., Dayanando, K.M., Patole, M.S. and Shouche, Y.S. 2007. Molecular microbial diversity of a soil sample and detection of ammonia oxidizers from Cape Evan, McMurdo Dry Valley, Anatarctica, ''Microbiol. Res.'', '''162''', 15-25.</ref>; Babalola et al., 2009<ref name="Babalola et al, 2009">Babalola, O.O., Kirby, B.M., Le Roes-Hill, M., Cook, A.E., Cary, S.C., Burton, S.G. and Cowan, D.A. 2009. Phylogenetic analysis of actinobacterial populations associated with Antarctic Dry Valley mineral soils, ''Environm. Microbiol'', '''11''', 566-576.</ref>) and endolithic communities (de la Torre et al., 2003<ref name="Torre et al, 2003">De La Torre, J.R., Goebel, B.M., Friedmann, E.I. and Pace, N.R., 2003. Microbial diversity of cryptoendolithic communities from the McMurdo Dry Valleys, Antarctica, ''Appl. Environ. Microbiol.'', '''69''', 3858-3867.</ref>; de los Rios et al., 2007<ref name="Rios et al, 2007">De Los Rios, A., Grube, M., Sancho, L.G. and Ascaso, C. 2007. Ultrastrucutral and genetic characterization of endolithic cyanobacterial and genetic characteristics of endolithic cyanobacterial biofilms colonizing Antarctic granite rocks. FEMS Microbol. Ecol., 59, 386-395.</ref>), the Pridz Bay area (Smith M. et al., 2000; Taton et al., 2006<ref name="Taton et al, 2006">Taton, A., Grubisic, S., Balhasart, P., Hodgson, D.A., Laybourn-Parry, J. and Wilmotte, A. 2006. Biogeographical distribution and ecological ranges of benthic cyanobacteria in East Antarctic lakes, ''FEMS Microbiol. Ecol.'', '''57''', 272-289.</ref>) and also from the Antarctic Peninsula (Hughes and Lawley, 2003<ref name="Hughes and Lawley, 2003">Hughes, K.A. and Lawley, B. 2003. A novel Antarctic microbial endolithic community within gypsum crusts, ''Environ. Microbiol.'', '''5''', 555-565.</ref>). Terrestrial dark crusts are found throughout Antarctica and are commonly dominated by cyanobacteria (Broady, 1996<ref name="Broady, 1996">Broady P. 1996. Diversity, distribution and dispersal of Antarctic terrestrial algae, ''Biodiv. Conserv'', '''5''', 1307-1335.</ref>; Mataloni and Tell, 2002<ref name="Mataloni and Tell, 2002">Mataloni, G. and Tell G. 2002. Microalgal communities from ornithogenic soils at Cierva Point, Antarctic Peninsula, ''Polar Biol.'', '''25''', 488-491.</ref>; Adams et al., 2006<ref name="Adams et al, 2006">Adams, B., Bardgett, R.D., Ayres, E., Wall, D.H., Aislabie, J., Bamforth, S., Bargagli, R., Cary, C., Cavacini, P., Connell, L., Convey, P., Fell, J., Frati, F., Hogg, I., Newsham, K.K., O&rsquo;Donnell, A., Russell, N., Seppelt, R. and Stevens, M.I. 2006. Diversity and Distribution of Victoria Land Biota, Soil Biology and Biochemistry 38, 3003-3018.</ref>). Freshwater systems have been sampled in studies of benthic habitats in continental Antarctic lakes (Bowman et al., 2000<ref name="Bowman et al, 2000">Bowman, J.P., Rea, S.M., McCammon, S.A. and McMeekin, T.A. 2000. Diversity and community structure within anoxic sediment from marine salinity meromictic lakes and a coastal meromictic marine basin, Vestfold Hilds, Eastern Antarctica, ''Environ. Microbiol.'', '''2''', 227-237.</ref>; Brambilla et al., 2001<ref name="Brambilla et al, 2001">Brambilla, E., Hippe, H., Hagelstein, A., Tindall, B.J. and Stackebrandt, E. 2001. 16S rDNA diversity of cultured and uncultured prokaryotes of a mat sample from Lake Fryxell, McMurdo Dry Valleys, Antarctica, ''Extremophiles'', '''5''', 23-33.</ref>; Sabbe et al., 2004<ref name="Sabbe et al, 2004">Sabbe, K., Hodgson, D.A., Verleyen, E., Taton, A., Wilmotte, A., Vanhoutte, K. and Vyverman, W. 2004. Salinity, depth and the structure of and composition of microbial mats in continental Antarctic lakes, ''Freshwater Biol.'', '''49''', 296-319.</ref>; Taton et al., 2003<ref name="Taton et al, 2003">Taton, A., Grubisic, S., Brambilla, E., De Wit, R. and Wilmotte, A. 2003. Cyanobacterial diversity in natural and artificial microbial mats of Lake Fryxell (McMurdo Dry Valleys, Antarctica): a morphological and molecular approach, ''Appl. Environm. Microbiol.'', '''69''', 5157-5169.</ref>, 2006<ref name="Taton et al, 2006">Taton, A., Grubisic, S., Balhasart, P., Hodgson, D.A., Laybourn-Parry, J. and Wilmotte, A. 2006. Biogeographical distribution and ecological ranges of benthic cyanobacteria in East Antarctic lakes, ''FEMS Microbiol. Ecol.'', '''57''', 272-289.</ref>; Van Trappen et al., 2002<ref name="Trappen et al, 2002">Van Trappen, S., Mergaert, J., Van Eygen, S., Dawyndt, P., Cnockaert, M.C. and Swings, J. 2002. Diversity of 746 heterotrophic bacteria isolated from microbial mats from ten Antarctic lakes, ''Syst. Appl. Microbiol.'', '''25''', 603-610.</ref>, 2004<ref name="Trappen et al, 2004">Van Trappen, S., Mergaert, J. and Swings, J. 2004. Loktanella salsilacus gen. nov., sp nov., Loktanella fryxellensis sp nov and Loktanella vestfoldensis sp nov., new members of the Rhodobacter group, isolated from microbial mats in Antarctic lakes, ''Int. J. Syst. Evol. Microbiol.'', '''54''', 1263-1269.</ref>, 2005<ref name="Trappen et al, 2005">Van Trappen, S., Vandecandelaere, I., Mergaert, J. and Swings, J. 2005. Flavobacterium fryxellicola sp. nov. and Flavobacterium psychrolimnae sp. nov., novel psychrophilic bacteria isolated from microbial mats in Antarctic lakes, ''Int. J. Syst. Evol. Microbiol.'', '''55''', 769-772.</ref>). These studies have revealed a considerable amount of new biodiversity, concerning various eubacterial phyla including cyanobacteria. The diversity and function of the microbial lake communities have been reviewed by Ellis-Evans (1996<ref name="Ellis-Evans, 1996">Ellis-Evans, J.C. 1996. Microbial diversity and function in Antarctic freshwater ecosystems, ''Biodivers. Converv.'', '''5''', 1395-1431.</ref>). Molecular genetic tools allow specialist habitats such as hot mineral soils (Soo et al., 2009<ref name="Soo et al, 2009">Soo, R.M., Wood, S.A., Grzymski, J.J., McDonald, I.R. and Cary, S.C. 2009. Microbial biodiversity of thermophilic communities in hot mineral soils of TramwayRidge, Mount Erebus, Antarctica, ''Environm. Microbiol.'', '''11''', 715-728.</ref>), cryoconites (Christner et al., 2003<ref name="Christner et al, 2003">Christner, B.C., Kvitko, I.I. and Reeve, J.N. 2003. Molecular identification of bacteria and eukarya inhabiting an Antarctic cryoconite hole, ''Extremophiles'', '''7''', 177-183.</ref>) and droppings of the Ad&eacute;lie penguins (Banks et al., 2009<ref name="Banks et al, 2009">Banks, J.C., Cary, S.C. and Hogg, I.D. 2009. The phylogeography of Adelie penguin faecal flora, ''Environm. Microbiol'', '''11''', 577-588.</ref>) to be studied in detail. Application of these tools to a diverse range of samples from across Antarctica should reveal more diversity and provide insights into distribution patterns, the forces that drive them, the presence and extent of endemism, and the impact of global change.<br />
==References==<br />
<references /><br />
[[Category:The Antarctic environment in the global system]]<br />
[[Category:Antarctic biology]]<br />
[[Category:Terrestrial biology]]</div>Maintenance scripthttp://acce.scar.org/wiki/Terrestrial_biology_over_the_next_100_yearsTerrestrial biology over the next 100 years2014-08-06T14:34:04Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Biological responses to 21st climate climate change]]''<br />
<br />
Antarctica is extremely isolated and unusually cold as a result of its polar location and ice sheet. As a consequence, climate change will impose a complex web of threats and interactions on the plants, animals and microbes living in the ice-free areas of Antarctica. Increased temperatures may promote growth and reproduction, but may also contribute to drought and associated effects. Furthermore, high amongst future scenarios is the likelihood of invasion by more competitive alien species, easily carried there by humans seeking a place of unspoilt wilderness or chasing scientific knowledge. Such invasions are already a reality on many of the sub-Antarctic islands, with consequential and sometimes drastic consequences for the structure and functioning of native biota and ecosystems (Table 4.2) (Frenot et al., 2005<ref name="Frenot et al, 2005">Frenot, Y., Chown, S.L., Whinam, J., Selkirk, P., Convey, P., Skotnicki, M. and Bergstrom, D. 2005 Biological invasions in the Antarctic: extent, impacts and implications, ''Biological Reviews'', '''80''', 45-72.</ref>, 2008<ref name="Frenot et al, 2008">Frenot, Y., Convey, P., Lebouvier, M., Chown, S.L., Whinam, J., Selkirk, P.M., Skotnicki, M. and Bergstrom, D.M. 2008. Antarctic biological invasions: sources, extents, impacts and implications. Non-native species in the Antarctic Proceedings, ed. M. Rogan-Finnemore, 53-96. Gateway Antarctica, Christchurch, New Zealand.</ref>; Convey et al., 2006b<ref name="Convey et al, 2006b">Convey, P., Frenot, F., Gremmen, N. and Bergstrom, D. 2006b. Biological invasions. Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 193-220.</ref>). These invasions carry a clear warning for the future of terrestrial ecosystems on the Antarctic continent where, although a small number of alien species has already become established, none have yet become invasive (Convey, 2008<ref name="Convey, 2008">Convey, P. 2008. Non-native species in Antarctic terrestrial and freshwater environments: presence, sources, impacts and predictions. Non-native species in the Antarctic Proceedings, ed. M. Rogan-Finnemore, 97-130. Gateway Antarctica, Christchurch, New Zealand.</ref>). While dispersal and range changes are also natural processes, sub-Antarctic data indicate that human assistance outweighs the natural frequency of such events by two or more orders of magnitude. Furthermore, regional environmental change in the sub- and maritime Antarctic is likely to act in synergy with anthropogenic transfer, lowering the current barriers to both transfer and establishment that have previously protected Antarctica. Antarctica contains some of the only places on Earth where natural biological phenomena can be studied in their pristine state, but human visitation risks breaking Antarctica&rsquo;s isolation, and threatens Antarctica&rsquo;s unique legacy.<br />
<br />
The consequences even of direct environmental changes might not always be easy to ascertain. For example, many sub-Antarctic islands show increases in mean annual temperature (Bergstrom and Chown, 1999<ref name="Bergstrom and Chown, 1999">Bergstrom, D.M. and Chown, S.L. 1999. Life at the front: history, ecology and changes on southern ocean islands, ''Trends in Ecology and Evolution'', '''14''', 427-477.</ref>). To date, there has been no suggestion that, even at the microclimate level, the increases are likely to exceed the upper lethal limits of most arthropods. However, in some areas, such as Marion Island, it is not only mean temperature that is predicted to change in line with current trends. Rather, the frequency of freeze&ndash;thaw events and occurrence of minimum temperatures are also predicted to increase, because of a greater frequency of cloud-free skies and a lower frequency of snow (which is a thermal insulator) (Smith and Steenkamp, 1990<ref name="Smith and Steenkamp, 1990">Smith, V.R. and Steenkamp, M. 1990. Climatic change and its ecological implications at a sub-Antarctic island, ''Oecologia'', '''85''', 14-24.</ref>; Smith, 2002<ref name="Smith, 2002">Smith, V.R. 2002. Climate change in the sub-Antarctic: An illustration from Marion Island, ''Climate Change'', '''52''', 345-357.</ref>). An increase in the frequency and intensity of freeze&ndash;thaw events could readily exceed the tolerance limits of many arthropods, as recent work both on Marion Island and other south temperate locations has shown (Sinclair, 2001<ref name="Sinclair, 2001">Sinclair, B.J. 2001. Field ecology of freeze tolerance: interannual variation in cooling rates, freeze-thaw and thermal stress in the microhabitat of the alpine cockroach Celatoblatta quinquemaculata, ''Oikos'', '''93''', 286-293.</ref>; Sinclair and Chown, 2005<ref name="Sinclair and Chown, 2005">Sinclair, B.J. and Chown, S.L. 2005. Deleterious effects of repeated cold exposure in a freeze-tolerant sub-Antarctic caterpillar, ''Journal of Experimental Biology'', '''208''', 869-879.</ref>; Slabber, 2005<ref name="Slabber, 2005">Slabber, S. 2005. Physiological plasticity in arthropods from Marion Island: indigenous and alien species. PhD Thesis, Stellenbosch University, South Africa.</ref>). Other biota, such as continental bryophytes and lichens, may also be pushed beyond their tolerance limits if freeze&ndash;thaw frequency increases, especially given the physiological effects of this stress, such as the loss of soluble carbohydrates (Tearle, 1987<ref name="Tearle, 1987">Tearle, P.V. 1987. Cryptogamic carbohydrate release and microbial response during spring freeze-thaw cycles in Antarctic fellfield fines, ''Soil Biology and Biochemistry'', '''19''', 381-390.</ref>; Melick and Seppelt, 1992<ref name="Melick and Seppelt, 1992">Melick, D.R. and Seppelt, R.D. 1992. Loss of soluble carbohydrates and changes in freezing point of Antarctic bryophytes after leaching and repeated freeze-thaw cycles, ''Antarctic Science'', '''4''', 399-404.</ref>). Thus, one of the major consequences of climate change might paradoxically not be an increase in upper lethal temperature stress, but rather an increase in stress at the other end of the temperature spectrum. How organisms are likely to respond to this kind of challenge has not been well investigated, though it is clear that lower lethal temperatures show substantial capacity for both phenotypic plasticity and evolutionary change.<br />
<br />
In ice&ndash;dominated continental and maritime Antarctica, changes to temperature are intimately linked to fluctuations in water availability. Changes to this latter variable will arguably have a greater effect on vegetation and faunal dynamics than that of temperature alone (Convey, 2006<ref name="Convey, 2006">Convey, P. 2006. Antarctic climate change and its influences on terrestrial ecosystems. Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 253-272.</ref>). Future regional patterns of water availability are unclear, but increasing aridity is likely on the continent in the long-term (Robinson et al., 2003<ref name="Robinson et al, 2003">Robinson, S.A., Wasley, J. and Tobin, A.K. 2003. Living on the edge - plants and global change in continental and maritime Antarctica, Global Change Biology 9, 1-37.</ref>). Plant species which show high tolerance of desiccation, such as the moss ''Ceratodon purpureus'', or others such as ''Bryum pseudotriquetrum'', which have a high degree of physiological flexibility with respect to tolerance of desiccation, are more likely to persist under increased aridity than the relatively desiccation-sensitive and physiologically inflexible ''Grimmia antarctici'' (Wasley et al., 2006<ref name="Wasley et al, 2006">Wasley, J., Robinson, S.A., Lovelock, C.E. and Popp, M. 2006. Some like it wet &ndash; an endemic Antarctic bryophyte likely to be threatened under climate change induced drying, ''Functional Plant Biology'', '''33''', 443-455.</ref>). Changes to water availability that cause an increased frequency of desiccation events are likely to negatively impact more strongly those species requiring hydrated habitats (hydric species) than those adapted to surviving shorter or longer periods of water stress (mesic or xeric species) (Davey, 1997<ref name="Davey, 1997">Davey, M.C. 1997. Effects of continuous and repeated dehydration on carbon fixation by bryophytes from the maritime Antarctic, ''Oecologia'', '''110''', 25-31.</ref>).<br />
<br />
In many respects, Antarctic terrestrial organisms are often well-adapted to the stresses of a highly variable environment, possessing features that should permit them to handle predicted levels of change that are often small compared with the natural variability already experienced. Indeed, with reference to temperature increase, resident biota will often be able to take advantage of reduced environmental stress, which will allow longer active periods/seasons, faster growth, shorter life cycles and population increase. Impacts of increased water availability are expected to be similar, although in both instances exactly the reverse consequences can be experienced locally, either directly as a result of decreased water input, or of interactions between increased temperature and water leading to greater evaporation and desiccation stress. Impacts of increased UV-B exposure associated with the spring ozone hole, while subtle, are expected to be negative.<br />
<br />
With increases in the temperature component of current climate change in many locations of the Antarctic, many terrestrial species may respond positively by faster metabolic rates, shorter life cycles and local expansion of populations. But subtle negative impacts can also be predicted (and are perhaps being observed) with regard to increased exposure to UV-B, as this requires greater allocation of resources within the organism to defense and mitigation strategies, reducing resources available for other life history components (Convey, 2006<ref name="Convey, 2006">Convey, P. 2006. Antarctic climate change and its influences on terrestrial ecosystems. Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 253-272.</ref>; Hennion et al., 2006<ref name="Hennion et al, 2006">Hennion, F., Huiskes, A., Robinson, S. and Convey, P. (2006) Physiological traits of organisms in a changing environment, Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 129-159.</ref>; Robinson et al., 2005<ref name="Robinson et al, 2005">Robinson, S.A., Turnbull, J.D. and Lovelock, C.E. 2005. Impact of changes in natural UV radiation on pigment composition, surface reflectance and photosynthetic function of the Antarctic moss, ''Grimmia antarctici'', ''Global Change Biology'', '''11''', 476-489.</ref>; Snell et al, 2009<ref name="Snell et al, 2009">Snell, K.R.S., Kokubun, T., Griffiths, H., Convey, P., Hodgson, D.A. and Newsham, K.K. 2009. Quantifying the metabolic cost to an Antarctic liverwort of responding to an abrupt increase in UV-B radiation exposure, Global Change Biology, 15, DOI: 10.1111/j.1365-2486.2009.01929.x.</ref>). Changes in water availability will also impact on both terrestrial and more stable limnetic environments. Local reduction in water availability in terrestrial habitats can lead to desiccation stress (Convey, 2006<ref name="Convey, 2006">Convey, P. 2006. Antarctic climate change and its influences on terrestrial ecosystems. Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 253-272.</ref>) and subsequent changes in ecosystem structure, as has been reported from Marion Island where there have been dramatic changes in mire communities associated with a substantial decrease in rainfall (Smith, 2002<ref name="Smith, 2002">Smith, V.R. 2002. Climate change in the sub-Antarctic: An illustration from Marion Island, ''Climate Change'', '''52''', 345-357.</ref>).<br />
<br />
The selective pressures experienced by Antarctic terrestrial biota over evolutionary time have resulted in adaptations with emphases on stress tolerance, plasticity and variation in life histories (Convey, 1996a<ref name="Convey, 1996a">Convey, P. 1996a. The influence of environmental characteristics on life history attributes of Antarctic terrestrial biota, ''Biological Reviews of the Cambridge Philosophical Society'', '''71''', 191-225.</ref>). These adaptations have been at the expense of reduced competitive ability, leaving Antarctic ecosystems vulnerable to the impact of colonization by competitors that may be at more advantage under changed climatic conditions (Bergstrom and Chown, 1999<ref name="Bergstrom and Chown, 1999">Bergstrom, D.M. and Chown, S.L. 1999. Life at the front: history, ecology and changes on southern ocean islands, ''Trends in Ecology and Evolution'', '''14''', 427-477.</ref>; Convey et al., 2006b<ref name="Convey et al, 2006b">Convey, P., Frenot, F., Gremmen, N. and Bergstrom, D. 2006b. Biological invasions. Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 193-220.</ref>). These competitors may be either naturally dispersed or have &lsquo;hitch-hiked&rsquo; with humans. As evidenced by the rapid increase in numbers and impacts of non-native species on the sub-Antarctic islands, the frequency of transfer by human agency (anthropogenic introduction) appears to far outweigh that by natural dispersal, not least as it overcomes the &lsquo;dispersal barrier&rsquo; presented by the geographical isolation and survival of environmental extremes required in transit (Frenot et al., 2005<ref name="Frenot et al, 2005">Frenot, Y., Chown, S.L., Whinam, J., Selkirk, P., Convey, P., Skotnicki, M. and Bergstrom, D. 2005 Biological invasions in the Antarctic: extent, impacts and implications, ''Biological Reviews'', '''80''', 45-72.</ref>, 2008<ref name="Frenot et al, 2008">Frenot, Y., Convey, P., Lebouvier, M., Chown, S.L., Whinam, J., Selkirk, P.M., Skotnicki, M. and Bergstrom, D.M. 2008. Antarctic biological invasions: sources, extents, impacts and implications. Non-native species in the Antarctic Proceedings, ed. M. Rogan-Finnemore, 53-96. Gateway Antarctica, Christchurch, New Zealand.</ref>; Whinam et al., 2005<ref name="Whinam et al, 2005">Whinam, J., Chilcott, N. and Bergstrom, D.M. 2005. Subantarctic hitchhikers: expeditioners as vectors for the introduction of alien organisms, ''Biological Conservation'', '''121''', 207-219.</ref>; Convey et al., 2006a<ref name="Convey et al, 2006a">Convey, P., Chown, S.L., Wasley, J. and Bergstrom, D.M. 2006a. Life history traits. Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 101-127.</ref>; Convey, 2008<ref name="Convey, 2008">Convey, P. 2008. Non-native species in Antarctic terrestrial and freshwater environments: presence, sources, impacts and predictions. Non-native species in the Antarctic Proceedings, ed. M. Rogan-Finnemore, 97-130. Gateway Antarctica, Christchurch, New Zealand.</ref>). Furthermore, the combination of increased human visitation across the entire Antarctic region, and the lowering of dispersal and establishment barriers implicit through climate warming, are expected to act synergistically and result in a greater frequency of both transfers and successful establishments.<br />
<br />
Changes in temperature, precipitation and wind speed, even those judged as subtle by climate scientists, will probably have profound effects on limnetic ecosystems through the alteration of their surrounding catchment, and of the time, depth and extent of surface ice cover, water body volume and lake chemistry (with increased solute transport from the land in areas of increased melt) (Quesada et al., 2006<ref name="Quesada et al, 2006">Quesada, A., Vincent, W.F., Kaup, E., Hobbie, J.E., Laurion, I., Pienitz, R., L&oacute;pez-Mart&iacute;nez, J. and Dur&aacute;n, J.J. 2006. In: Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, Eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 221-252.</ref>; Lyons et al., 2006<ref name="Lyons et al, 2006">Lyons, W.B., Laybourn-Parry, J., Welch, K.A. and Priscu, J.C. 2006. Antarctic lake systems and climate change. In: Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 273-295.</ref>; Quayle et al., 2002<ref name="Quayle et al, 2002">Quayle, W.C., Peck, L.S., Peat, H., Ellis-Evans, J.C. and Harrigan, P.R. 2002. Extreme responses to climate change in Antarctic lakes, ''Science'', '''295''', 645-645.</ref>, 2003<ref name="Quayle et al, 2003">Quayle, W., Convey, P., Peck, L., Ellis-Evans, J., Butler, H. and Peat, H. 2003. Ecological responses of maritime Antarctic lakes to regional climate change. In: Domack E, Leventer A, Burnett A, Convey P, Kirby M, Bindschadler R (eds) American Geophysical Union: Monograph Antarctic Peninsula Climate Variability: A Historical and Paleoenvironmental Perspective, Antarctic Research Series; 79, 159-170.</ref>). Indeed, Quayle et al. (2002<ref name="Quayle et al, 2002">Quayle, W.C., Peck, L.S., Peat, H., Ellis-Evans, J.C. and Harrigan, P.R. 2002. Extreme responses to climate change in Antarctic lakes, ''Science'', '''295''', 645-645.</ref>, 2003<ref name="Quayle et al, 2003">Quayle, W., Convey, P., Peck, L., Ellis-Evans, J., Butler, H. and Peat, H. 2003. Ecological responses of maritime Antarctic lakes to regional climate change. In: Domack E, Leventer A, Burnett A, Convey P, Kirby M, Bindschadler R (eds) American Geophysical Union: Monograph Antarctic Peninsula Climate Variability: A Historical and Paleoenvironmental Perspective, Antarctic Research Series; 79, 159-170.</ref>) show that some Antarctic lake systems magnify the already strong signal of regional climatic warming centered on the maritime Antarctic. Predicted impacts of these changes will be varied. A common factor is the changing influence of reduced lake ice and snow cover, which exert strong controls on the abundance and diversity of the plankton and periphyton (Hodgson and Smol, 2008<ref name="Hodgson and Smol, 2008">Hodgson, D.A. and Smol, J.P. 2008. High latitude palaeolimnology. In Polar Lakes and Rivers - Arctic and Antarctic Aquatic Ecosystems, (W. F. Vincent, and J. Laybourn-Parry, Eds.). Oxford University Press, Oxford, UK.</ref>). With increased warming, more of the lake is made available and production increases. Once the central raft of ice melts completely, the plankton and benthos can flourish, and diversity at all levels of the ecosystem increases. In shallow lakes, lack of surface ice cover will also lead to increased wind&ndash;induced mixing. In some areas of East Antarctica, longer periods of open water have led to increased evaporation and, together with sublimation of winter ice cover, have resulted in rapid increases in lake salinity in the last few decades (Hodgson et al., 2006c<ref name="Hodgson et al, 2006c">Hodgson, D.A., Roberts, D., McMinn, A., Verleyen, E., Terry, B., Corbett, C. and Vyverman, W. 2006c. Recent rapid salinity rise in three East Antarctic lakes, ''Journal of Paleolimnology'', '''36''', 385-406.</ref>).<br />
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Increased inputs of melt water into the upper stratified layer of deeper lakes may also increase stability, and this, associated with increased primary production, will lead to higher organic carbon flux. Such a change will have flow&ndash;on effects including potential anoxia, shifts in overall biogeochemical cycles and alterations in the biological structure and diversity of ecosystems (Lyons et al., 2006<ref name="Lyons et al, 2006">Lyons, W.B., Laybourn-Parry, J., Welch, K.A. and Priscu, J.C. 2006. Antarctic lake systems and climate change. In: Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 273-295.</ref>). The predictions of Lyons et al. (2006<ref name="Lyons et al, 2006">Lyons, W.B., Laybourn-Parry, J., Welch, K.A. and Priscu, J.C. 2006. Antarctic lake systems and climate change. In: Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 273-295.</ref>) also serve to illustrate a profound paradigm shift in Antarctic biology that has occurred in the last 20 years. Although they and Convey (2006<ref name="Convey, 2006">Convey, P. 2006. Antarctic climate change and its influences on terrestrial ecosystems. Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 253-272.</ref>) state that we are not yet in a situation where we can develop a quantitative predictive model (or even models) that completely qualifies the response of Antarctic ecosystems to climate change, many of the predictions currently made are based on a foundation of long-term studies and monitoring, such as those at the McMurdo Dry Valleys LTER, or British Antarctic Survey sites on Signy Island. The importance of such long-term programmes cannot be overstated, particularly in national and global research funding environments increasingly predicated on &lsquo;short-termism&rsquo;.<br />
==References==<br />
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[[Category:The next 100 years]]<br />
[[Category:Antarctic biology]]<br />
[[Category:Terrestrial biology]]</div>Maintenance scripthttp://acce.scar.org/wiki/Terrestrial_biology_in_the_instrumental_periodTerrestrial biology in the instrumental period2014-08-06T14:34:03Z<p>Acce: Added table</p>
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<div>:''This page is part of the topic [[Antarctic climate and environment change in the instrumental period]]''<br />
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Contemporary terrestrial and freshwater ecosystems within Antarctica occupy only 0.34% of the continental area (British Antarctic Survey, 2004), the remainder being permanent ice and snow. The combined land area of the isolated sub-Antarctic islands is likewise small. Individual areas of terrestrial habitat are typically &lsquo;islands&rsquo;, whether in the true sense of the word, being surrounded by ocean, or in the sense of being surrounded and isolated by terrain inhospitable to terrestrial biota in the form of ice (Bergstrom and Chown, 1999<ref name="Bergstrom and Chown, 1999">Bergstrom, D.M. and Chown, S.L. 1999. Life at the front: history, ecology and changes on southern ocean islands, ''Trends in Ecology and Evolution'', '''14''', 427-477.</ref>). While the most biologically developed and most studies of terrestrial exposures are found in coastal regions of the continent, particularly along the Antarctic Peninsula and in Victoria Land, terrestrial habitats exist in all sectors of the continent, and both in coastal and inland locations.<br />
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Terrestrial biological research within Antarctica has, however, been much more spatially limited, with major areas of activity restricted to the South Orkney and South Shetland Islands, Anvers Island, the Argentine Islands and Marguerite Bay along the Antarctic Peninsula/Scotia Arc, and the Dry Valleys and certain coastal locations in Victoria Land. Terrestrial and freshwater research along the continental Antarctic coastline has largely been limited to areas in the vicinity of the Schirmacher Oasis, Windmill Islands and Davis Station, Casey Station, and mountain ranges in Dronning Maud Land. Sporadic biological records exist from more widely dispersed locations, but in most cases these relate to single short field visits to these locations, often by non-biologists or non-specialists. Indeed, there remain many instances where the only biological records available, or only species descriptions that exist, derive from the original exploring expeditions of the &lsquo;heroic era&rsquo;. Even where terrestrial biological research is undertaken within a region or by a national operator, both taxonomic and process-based research coverage is extremely uneven across different regions or operators.<br />
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All Antarctic terrestrial ecosystems are simple in global terms, lacking or with low diversity in specific taxonomic or biological functional groups (Block, 1984<ref name="Block, 1984">Block, W. 1984. Terrestrial microbiology, invertebrates and ecosystems. &ndash; In: R.M. Laws (ed), Antarctic ecology, Academic Press, London, 163-236.</ref>; Smith, 1984<ref name="Smith, 1984">Smith, R.I.L. 1984. Terrestrial plant biology of the sub-Antarctic and Antarctic. In: Laws, R.M. (ed.), Antarctic Ecology, 1, Academic Press, London, 61-162.</ref>; Convey, 2001<ref name="Convey, 2001">Convey, P. 2001. Antarctic Ecosystems. In: Encyclopedia of Biodiversity, ed. S.A. Levin. Academic Press, San Diego, Vol. 1, 171-184.</ref>). It is therefore likely that they lack the functional redundancy that is typical of more diverse ecosystems, raising the possibility of new colonists (arriving by both natural and, more recently, human-assisted means) occupying vacant ecological niches. Such colonists could include new trophic functions or levels, threatening the structure and function of existing trophic webs (Frenot et al., 2005<ref name="Frenot et al, 2005">Frenot, Y., Chown, S.L., Whinam, J., Selkirk, P., Convey, P., Skotnicki, M. and Bergstrom, D. 2005 Biological invasions in the Antarctic: extent, impacts and implications, ''Biological Reviews'', '''80''', 45-72.</ref>, 2008<ref name="Frenot et al, 2008">Frenot, Y., Convey, P., Lebouvier, M., Chown, S.L., Whinam, J., Selkirk, P.M., Skotnicki, M. and Bergstrom, D.M. 2008. Antarctic biological invasions: sources, extents, impacts and implications. Non-native species in the Antarctic Proceedings, ed. M. Rogan-Finnemore, 53-96. Gateway Antarctica, Christchurch, New Zealand.</ref>; Convey, 2008<ref name="Convey, 2008">Convey, P. 2008. Non-native species in Antarctic terrestrial and freshwater environments: presence, sources, impacts and predictions. Non-native species in the Antarctic Proceedings, ed. M. Rogan-Finnemore, 97-130. Gateway Antarctica, Christchurch, New Zealand.</ref>). Responses of indigenous biota will be constrained by their typically &lsquo;adversity-selected&rsquo; life history strategies, which have evolved in an environment where abiotic environmental stresses and selection pressures (i.e. properties of the physical environment) far outweigh in importance biotic stresses and pressures (i.e. competition, predation, etc.) (Convey, 1996a<ref name="Convey, 1996a">Convey, P. 1996a. The influence of environmental characteristics on life history attributes of Antarctic terrestrial biota, ''Biological Reviews of the Cambridge Philosophical Society'', '''71''', 191-225.</ref>).<br />
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The growth and life cycle patterns of many invertebrates and plants are fundamentally dependent on regional temperature regimes and their linkage with patterns of water availability (Convey et al., 2006a<ref name="Convey et al, 2006a">Convey, P., Chown, S.L., Wasley, J. and Bergstrom, D.M. 2006a. Life history traits. Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 101-127.</ref>). In detail, the interaction between regional macroclimate and smaller scale ecosystem features and topography define the microclimate within which an organism must live and function. There has to date been remarkably little effort to identify connections between macro and microclimatic scales, or to probe the application of large-scale macroclimatic trends and predictions at microclimatic scale. Distinct patterns in sexual reproduction are evident across the Antarctic flora and are most likely a function of temperature variation - indeed recent increase in the frequency of successful seed production in the two maritime Antarctic flowering plants (Convey, 1996b<ref name="Convey, 1996b">Convey, P. 1996b. Reproduction of Antarctic flowering plants, ''Antarct. Sci.'', '''8''', 127-134.</ref>) is proposed to be a function of warming in this region. In addition, phenology of flowering plants is cued to seasonality in the light regime. In regions supporting flowering plants, wind is assumed to play a major role in pollination ecology of grasses and sedges resulting in cross-pollination. The lack of specialist pollinators in the native fauna, combined with high reproductive outputs in non-wind pollinated species implies a high reliance on self-fertilisation.<br />
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The Antarctic biota shows high development of ecophysiological adaptations relating to cold and desiccation tolerance, and displays an array of traits to facilitate survival under environmental stress (Hennion et al., 2006<ref name="Hennion et al, 2006">Hennion, F., Huiskes, A., Robinson, S. and Convey, P. (2006) Physiological traits of organisms in a changing environment, Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 129-159.</ref>). While patterns in absolute low temperatures are clearly important in determining survival, perhaps more influential is the pattern of the freeze-thaw regime, with repeated freeze-thaw events being more damaging than a sustained freeze event (Brown et al., 2004<ref name="Brown et al, 2004">Brown, C.L., Bale, J.S. and Walters, K.F.A. 2004. Freezing induces a loss of freeze tolerance in an overwintering insect. Proceedings of the Royal Society of London series B, 271, 1507-1511.</ref>; Sinclair and Chown, 2005<ref name="Sinclair and Chown, 2005">Sinclair, B.J. and Chown, S.L. 2005. Deleterious effects of repeated cold exposure in a freeze-tolerant sub-Antarctic caterpillar, ''Journal of Experimental Biology'', '''208''', 869-879.</ref>). How these patterns change in the future will be an area of major importance.<br />
<br />
The response of Antarctic plants to increased UV-B radiation (280-315 nm) associated with the ozone hole, provides an illustration of another suite of ecophysiological/biochemical strategies. Reported responses vary widely between studies, ranging from negative effects on chlorophyll concentrations in tissues, on growth, and evidence of DNA damage in some species (e.g. Ruhland and Day, 2000<ref name="Ruhland and Day, 2000">Ruhland, C.T. and Day, T.A. 2000. Effects of ultraviolet-B radiation on leaf elongation, production and phenylpropanoid concentrations of ''Deschampsia antarctica'' and ''Colobanthus quitensis'' in Antarctica, ''Physiologia Plantarum'', '''109''', 244-251.</ref>; Xiong and Day, 2001<ref name="Xiong and Day, 2001">Xiong, F. and Day, T. 2001. Effect of solar ultraviolet-B radiation during springtime ozone depletion on photosynthesis and biomass production of Antarctic vascular plants, ''Plant Physiol.'', '''125''', 738-751.</ref>; Robinson et al., 2005<ref name="Robinson et al, 2005">Robinson, S.A., Turnbull, J.D. and Lovelock, C.E. 2005. Impact of changes in natural UV radiation on pigment composition, surface reflectance and photosynthetic function of the Antarctic moss, ''Grimmia antarctici'', ''Global Change Biology'', '''11''', 476-489.</ref>; Turnbull and Robinson, 2008<ref name="Turnbull and Robinson, 2008">Turnbull, J.D. and Robinson, S.A. 2008. Accumulation of DNA damage in Antarctic mosses: correlations with ultraviolet-B radiation, temperature and turf water content vary among species, Global Change Biology, 14, in press.</ref>), through little change (e.g. Bj&ouml;rn, 1999<ref name="Bj&ouml;rn, 1999">Bj&ouml;rn, L.-O. 1999. Ultraviolet-B Radiation, the Ozone Layer and Ozone Depletion. In Rozema, J. ed. Stratospheric Ozone Depletion: the effects of enhanced UV-B radiation on terrestrial ecosystems. Leiden, the Netherlands: Backhuys Publishers, 21-37.</ref>; Lud et al., 2002<ref name="Lud et al, 2002">Lud, D., Moerdijk, T.C.W., Van De Poll, W.H., Buma, A.G.J., and Huiskes, A.H.L. 2002. DNA damage and photosynthesis in Antarctic and Arctic Sanionia uncinata (Hedw.) Loeske under ambient and enhanced levels of UV-B radiation, ''Plant Cell and Environment'', '''25''', 1579-1589.</ref>; Boelen et al., 2006<ref name="Boelen et al, 2006">Boelen, P., De Boer, M.K., De Bakker, N.V.J., and Rozema, J. 2006. Outdoor studies on the effects of solar UV-B on bryophytes: Overview and methodology, ''Plant Ecology'', '''182'''(1-2), 137-152.</ref>) to consistent positive effects, such as increased concentrations of UV-B screening pigments (Newsham et al., 2002<ref name="Newsham et al, 2002">Newsham, K.K., Hodgson, D.A., Murray, A.W.A., Peat, H.J., and Lewis Smith, R.I. 2002. Response of two Antarctic bryophytes to stratospheric ozone depletion, ''Global Change Biology'', '''8''', 972-983.</ref>). Previously it has been suggested that higher plants and bryophytes could differ in their abilities to synthesize UV-B screening pigments (Gwynn-Jones et al., 1999<ref name="Gwynn-Jones et al, 1999">Gwynn-Jones, D., Lee, J.A., Johanson, U., Phoenix, G.K., Callaghan, T.V., and Sonesson, M. 1999. The response of plant functional types to enhanced UV-B radiation. In Rozema, J. ed. Stratospheric Ozone Depletion: the effects of enhanced UV-B radiation on terrestrial ecosystems. Leiden, Netherlands: Backhuys Publishers, 173-185.</ref>), but most recent data from Antarctic studies do not support this proposition (e.g. Newsham et al., 2002<ref name="Newsham et al, 2002">Newsham, K.K., Hodgson, D.A., Murray, A.W.A., Peat, H.J., and Lewis Smith, R.I. 2002. Response of two Antarctic bryophytes to stratospheric ozone depletion, ''Global Change Biology'', '''8''', 972-983.</ref>; Newsham, 2003<ref name="Newsham, 2003">Newsham, K.K. 2003. UV-B radiation arising from stratospheric ozone depletion influences the pigmentation of the moss Andreaea regularis, ''Oecologia'', '''135''', 327-331.</ref>; Newsham et al., 2005<ref name="Newsham et al, 2005">Newsham, K.K., Geissler, P., Nicolson, M., Peat, H.J., and Lewis Smith, R.I. 2005. Sequential reduction of UV-B radiation in the field alters the pigmentation of an Antarctic leafy liverwort, ''Environmental and Experimental Botany'', '''54'''(1), 22-32.</ref>; Dunn and Robinson, 2006<ref name="Dunn and Robinson, 2006">Dunn, J.L. and Robinson, S.A. 2006. Ultraviolet B screening potential is higher in two cosmopolitan moss species than in a co-occurring Antarctic endemic moss: implications of continuing ozone depletion, ''Global Change Biology'', '''12''', 2282-2296.</ref>; Clarke and Robinson, 2008<ref name="Clarke and Robinson, 2008">Clarke, L.J., and Robinson, S.A. 2008. Cell wall-bound ultraviolet-screening compounds explain the high ultraviolet tolerance of the Antarctic moss, ''Ceratodon purpureus'', ''New Phytologist'', '''179''', 776-783.</ref>). The majority of Antarctic bryophytes studied have potential UV screening compounds inside their cells and/or attached to their cell walls (Clarke and Robinson, 2008<ref name="Clarke and Robinson, 2008">Clarke, L.J., and Robinson, S.A. 2008. Cell wall-bound ultraviolet-screening compounds explain the high ultraviolet tolerance of the Antarctic moss, ''Ceratodon purpureus'', ''New Phytologist'', '''179''', 776-783.</ref>), suggesting widespread UV screening potential in these species. A recent study has estimated that the cost of synthesising new protective pigment molecules on exposure to UV-B represents approximately 2% of the carbon fixated by a common Antarctic liverwort, analogous to estimates of 1-10% of biomass invested in cryoprotectants or desiccation protectants by many Antarctic terrestrial invertebrates and microbes (Snell et al., 2009<ref name="Snell et al, 2009">Snell, K.R.S., Kokubun, T., Griffiths, H., Convey, P., Hodgson, D.A. and Newsham, K.K. 2009. Quantifying the metabolic cost to an Antarctic liverwort of responding to an abrupt increase in UV-B radiation exposure, Global Change Biology, 15, DOI: 10.1111/j.1365-2486.2009.01929.x.</ref>).<br />
<br />
{| class="wikitable" style="text-align: center;"<br />
! !!Entire<br />sub-Antarctic!!Maritime Antarctic!!South Georgia!!Marion!!Prince Edward!!Crozet!!Kerguelen!!Heard!!MacDonald!!Macquarie<br />
|-<br />
| style="text-align: left;"|Dicotyledons||62||0||17||6||2||40||34||0||0||2<br />
|-<br />
| style="text-align: left;"|Mono-cotyledons||45||2||15||7||1||18||34||1||0||1<br />
|-<br />
| style="text-align: left;"|Pteridophytes||1||0||1||0||0||1||1||0||0||0<br />
|-<br />
| style="text-align: left;"|'''Total non-indigenous plants'''||108||2||33||13||3||59||69||1||0||3<br />
|-<br />
| style="text-align: left;"|Invertebrates||72||2-5||12||18||1||14||30||3||0||28<br />
|-<br />
| style="text-align: left;"|Vertebrates||16||0||3||1||0||6||12||0||0||6<br />
|}<br />
<br />
'''Table 4.2''' The occurrence of alien non-indigenous terrestrial species across Antarctic biogeographical zones (extracted from Frenot et al. (2005<ref name="Frenot et al, 2005">Frenot, Y., Chown, S.L., Whinam, J., Selkirk, P., Convey, P., Skotnicki, M. and Bergstrom, D. 2005 Biological invasions in the Antarctic: extent, impacts and implications, ''Biological Reviews'', '''80''', 45-72.</ref>); see also Greenslade (2006<ref name="Greenslade, 2006">Greenslade, P. 2006: The Invertebrates of Macquarie Island. Australian Antarctic Division, Kingston, Tasmania, xvi, 326 pp.</ref>) for a detailed description of established and transient alien species, and species recorded only synanthropically, from sub-Antarctic Macquarie Island).<br />
<br />
A meta-analysis of the response of polar vegetation to UV-B radiation concludes that Antarctic bryophytes and vascular plants respond in a similar fashion to vegetation from other regions, with UV-B exposure leading to decreased above-ground biomass and height and increased DNA damage (Newsham and Robinson, 2009<ref name="Newsham and Robinson, 2009">Newsham, K.K. and Robinson, S.A. 2009. Responses of plants in polar regions to UV-B exposure: a meta-analysis, Global Change Biology, 15, DOI: 10.1111/j.1365-2486.2009.01944.x.</ref>). Plants also appear able to protect themselves from elevated UV-B radiation through the induction of UV screening pigments (Newsham and Robinson, 2009<ref name="Newsham and Robinson, 2009">Newsham, K.K. and Robinson, S.A. 2009. Responses of plants in polar regions to UV-B exposure: a meta-analysis, Global Change Biology, 15, DOI: 10.1111/j.1365-2486.2009.01944.x.</ref>), although this likely comes at a cost to biomass production (Snell et al., 2009<ref name="Snell et al, 2009">Snell, K.R.S., Kokubun, T., Griffiths, H., Convey, P., Hodgson, D.A. and Newsham, K.K. 2009. Quantifying the metabolic cost to an Antarctic liverwort of responding to an abrupt increase in UV-B radiation exposure, Global Change Biology, 15, DOI: 10.1111/j.1365-2486.2009.01929.x.</ref>). However this meta-analysis does suggest that the method by which UV-B radiation is applied to plants plays an important part in determining the strength of plant response to UV-B.<br />
<br />
The final suite of life history traits includes elements relating to competition and predation. Their potential significance is illustrated by reference to ecosystem changes caused through the introduction of new predatory invertebrates to certain sub-Antarctic islands (e.g. Table 4.2). The introduction of carabid beetles to parts of South Georgia and &Icirc;les Kerguelen, where such predators were previously absent, is leading to extensive changes to local community structure, which threatens the continued existence of some indigenous and/or endemic invertebrates (Ernsting et al., 1995<ref name="Ernsting et al, 1995">Ernsting, G., Block, W., Macalister, H. and Todd, C. 1995. The invasion of the carnivorous carabid beetle Trechisibus antarcticus on South Georgia (subantarctic) and its effect on the endemic herbivorous beetle Hydromedion spasutum, ''Oecologia'', '''103''', 34-42.</ref>; Frenot et al., 2005<ref name="Frenot et al, 2005">Frenot, Y., Chown, S.L., Whinam, J., Selkirk, P., Convey, P., Skotnicki, M. and Bergstrom, D. 2005 Biological invasions in the Antarctic: extent, impacts and implications, ''Biological Reviews'', '''80''', 45-72.</ref>, 2008<ref name="Frenot et al, 2008">Frenot, Y., Convey, P., Lebouvier, M., Chown, S.L., Whinam, J., Selkirk, P.M., Skotnicki, M. and Bergstrom, D.M. 2008. Antarctic biological invasions: sources, extents, impacts and implications. Non-native species in the Antarctic Proceedings, ed. M. Rogan-Finnemore, 53-96. Gateway Antarctica, Christchurch, New Zealand.</ref>). Regional warming has also been predicted to rapidly increase the impact of certain indigenous predators (Arnold and Convey, 1998<ref name="Arnold and Convey, 1998">Arnold, R.J. and Convey, P. 1998. The life history of the world&rsquo;s most southerly diving beetle, ''Lancetes angusticollis'' (Curtis) (Coleoptera: Dytiscidae), on sub-Antarctic South Georgia, ''Polar Biol.'', '''20''', 153-160.</ref>). Providing an analogous impact within the decomposition cycle, detailed studies on Marion Island indicate that indigenous terrestrial detritivores are unable to overcome a bottleneck in the decomposition cycle, hence illustrating a further ecosystem service likely to be strongly influenced by recently introduced non-indigenous species (Slabber and Chown, 2002<ref name="Slabber and Chown, 2002">Slabber, S. and Chown, S.L. 2002. The first record of a terrestrial crustacean, Porcellio scaber (Isopoda, Porcellionidae), from sub-Antarctic Marion Island, ''Polar Biology'', '''25''', 855-858.</ref>).<br />
<br />
The lack of attention to these traits to date is unfortunate, particularly with respect to the understanding of alien species&rsquo; impacts. It is already well known that Antarctic terrestrial biota possess very effective stress tolerance strategies, in addition to considerable response flexibility. The exceptionally wide degree of environmental variability experienced in many Antarctic terrestrial habitats, on a range of timescales between hours and years, means that predicted levels of change in environmental variables (particularly temperature and water availability) are often small relative to the range already experienced. However, as illustrated above with biochemical responses to UV-B exposure, any change in the balance of use of specific strategies carries a quantifiable cost, and carries implications for changes in the allocation of resources within the organism.<br />
<br />
Given the absence of more effective competitors, predicted and observed levels of climate change may be expected to generate positive responses from resident biota of the maritime and continental Antarctic, and this is confirmed in general terms both by observational reports of changes in maritime Antarctic terrestrial ecosystems, and the results of manipulation experiments mimicking the predictions of climate change (Convey, 2003<ref name="Convey, 2003">Convey, P. 2003. Maritime Antarctic climate change: signals from terrestrial biology. In: Antarctic Peninsula Climate Variability: Historical and Palaeoenvironmental Perspectives, eds. E. Domack, A. Burnett, A. Leventer, P. Convey, M. Kirby and R. Bindschadler, pp. 145-158. Antarctic Research Series vol. 79, American Geophysical Union.</ref>, 2007). Over most of the remainder of the continent, biological changes are yet to be reported, as might be expected given the weakness or lack of evidence for clear climate trends over the instrumental period. Potentially sensitive indicators of change have been identified amongst the biota of this region (e.g. Wasley et al., 2006<ref name="Wasley et al, 2006">Wasley, J., Robinson, S.A., Lovelock, C.E. and Popp, M. 2006. Some like it wet &ndash; an endemic Antarctic bryophyte likely to be threatened under climate change induced drying, ''Functional Plant Biology'', '''33''', 443-455.</ref>), particularly in the context of sensitivity to changes in desiccation stress (Robinson et al., 2003<ref name="Robinson et al, 2003">Robinson, S.A., Wasley, J. and Tobin, A.K. 2003. Living on the edge - plants and global change in continental and maritime Antarctica, Global Change Biology 9, 1-37.</ref>). More local scale and short-term trends of cooling over recent decades in the Dry Valleys of Victoria Land have been associated with reductions in abundance of the soil fauna (Doran et al., 2002<ref name="Doran et al, 2002">Doran, P.T., Priscu, J.C., Lyons, W.B., Walsh, J.E., Fountain, A.G., McKnight, D.M., Moorhead, D.L., Virginia, R.A., Wall, D.H., Clow, G.D., Fritsen, C.H., McKay, C.P. and Parsons, A.N. 2002. Antarctic climate cooling and terrestrial ecosystem response, ''Nature'', '''415''', 517-520.</ref>). The picture is likely to be far more complex on the different sub-Antarctic islands as, in addition to various different trends being reported in a range of biologically important variables, many also already host (different) alien invasive taxa, some of which already have considerable impacts on native biota (Frenot et al., 2005<ref name="Frenot et al, 2005">Frenot, Y., Chown, S.L., Whinam, J., Selkirk, P., Convey, P., Skotnicki, M. and Bergstrom, D. 2005 Biological invasions in the Antarctic: extent, impacts and implications, ''Biological Reviews'', '''80''', 45-72.</ref>; Convey, 2007, Table 4.2).<br />
<br />
[[File:Figure 4.61 - Antarctic flowering plants - Deschampsia antarctica and Colobanthus quitensis.png|thumb|'''4.61''' The only two flowering plants native to the Antarctic continent are both restricted to the Antarctic Peninsula. The grass ''Deschampsia antarctica'' may develop into swards covering several to tens of square metres (left), while the pearlwort ''Colobanthus quitensis'' more typically is encountered as individual cushions (right) (photos P. Convey).]]<br />
The best-known and frequently reported example of terrestrial organisms interpreted to be responding to climate change in the Antarctic is that of the two native Antarctic flowering plants (''Deschampsia antarctica'' and ''Colobanthus quitensis'') ([[:File:Figure 4.61 - Antarctic flowering plants - Deschampsia antarctica and Colobanthus quitensis.png|Figure 4.61]]) in the maritime Antarctic (Fowbert and Smith, 1994<ref name="Fowbert and Smith, 1994">Fowbert, J.A. and Smith, R.I.L. 1994. Rapid population increases in native vascular plants in the Argentine Islands Antarctic Peninsula, ''Arctic and Alpine Research'', '''26''', 290-296.</ref>; Smith, 1994<ref name="Smith, 1994">Smith, R.I.L. 1994. Vascular plants as bioindicators of regional warming in Antarctica, ''Oecologia'', '''99''', 322-328.</ref>; Grobe et al., 1997<ref name="Grobe et al, 1997">Grobe, C.W., Ruhland C.T. and Day T.A. 1997. A new population of ''Colobanthus quitensis'' near Arthur Harbor, Antarctica: correlating recruitment with warmer summer temperatures, ''Arctic and Alpine Research'', '''29''', 217-221.</ref>; Gerighausen et al., 2003<ref name="Gerighausen et al, 2003">Gerighausen, U., Br&auml;utigam, K., Mustafa, O. and Peter, H-U. 2003. Expansion of vascular plants on Antarctic Islands a consequence of climate change? In: Antarctic Biology in a Global context (eds Huiskes, AHL, Gieskes WWC, Rozema J, Schorno RML, van der Vies SM, Wolff WJ), 79-83, Backhuys, Leiden.</ref>). At the Argentine Islands numbers of plants increased by two orders of magnitude between the mid 1960s and 1990 (Fowbert and Smith, 1994<ref name="Fowbert and Smith, 1994">Fowbert, J.A. and Smith, R.I.L. 1994. Rapid population increases in native vascular plants in the Argentine Islands Antarctic Peninsula, ''Arctic and Alpine Research'', '''26''', 290-296.</ref>), although it is often overlooked that these increases have not involved any change in the species&rsquo; overall geographic ranges, limited in practice by extensive ice cover south of the current distribution. These increases are thought to be due to increased temperature encouraging growth and vegetative spreading of established plants, in addition to increasing the probability of establishment of germinating seedlings. Additionally, warming is proposed to underlie a greater frequency of mature seed production (Convey, 1996b<ref name="Convey, 1996b">Convey, P. 1996b. Reproduction of Antarctic flowering plants, ''Antarct. Sci.'', '''8''', 127-134.</ref>), and stimulate growth of seeds that have remained dormant in soil propagule banks (McGraw and Day, 1997<ref name="McGraw and Day, 1997">McGraw, J.B. and Day, T.A. 1997. Size and characteristics of a natural seed bank in Antarctica, ''Arctic and Alpine Research'', '''29''', 213-216.</ref>). However, since 1990, there has been no further increase in the Argentine Islands populations, while there has also been no significant warming trend in either the annual or seasonal air temperature data record at this location over the period 1990-2008, which might suggest the link between environmental conditions and plant responses is even closer than initially thought (Parnikoza et al., in press).<br />
<br />
Changes in both temperature and precipitation have already had detectable effects on limnetic ecosystems through the alteration of the surrounding landscape and of the time, depth and extent of surface ice cover, water body volume and lake chemistry (with increased solute transport from the land in areas of increased melt) (Quesada et al., 2006<ref name="Quesada et al, 2006">Quesada, A., Vincent, W.F., Kaup, E., Hobbie, J.E., Laurion, I., Pienitz, R., L&oacute;pez-Mart&iacute;nez, J. and Dur&aacute;n, J.J. 2006. In: Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, Eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 221-252.</ref>; Lyons et al., 2006<ref name="Lyons et al, 2006">Lyons, W.B., Laybourn-Parry, J., Welch, K.A. and Priscu, J.C. 2006. Antarctic lake systems and climate change. In: Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 273-295.</ref>; Quayle et al., 2002<ref name="Quayle et al, 2002">Quayle, W.C., Peck, L.S., Peat, H., Ellis-Evans, J.C. and Harrigan, P.R. 2002. Extreme responses to climate change in Antarctic lakes, ''Science'', '''295''', 645-645.</ref>, 2003<ref name="Quayle et al, 2003">Quayle, W., Convey, P., Peck, L., Ellis-Evans, J., Butler, H. and Peat, H. 2003. Ecological responses of maritime Antarctic lakes to regional climate change. In: Domack E, Leventer A, Burnett A, Convey P, Kirby M, Bindschadler R (eds) American Geophysical Union: Monograph Antarctic Peninsula Climate Variability: A Historical and Paleoenvironmental Perspective, Antarctic Research Series; 79, 159-170.</ref>. The latter authors highlight that some maritime Antarctic lake environmental changes actually magnify those seen in the atmospheric climate, highlighting the value of these locations as model systems to give &lsquo;early warning&rsquo; of potential changes to be seen at lower latitudes. Predicted impacts of such changes will be varied. In shallow lakes, lack of surface ice cover will lead to increased wind&ndash;induced mixing and evaporation and increases in the diversity at all levels of the ecosystem. If more melt water is available, input of freshwater into the mixolimna of deeper lakes will increase stability and this, associated with increased primary production, will lead to higher organic carbon flux. Such a change will have flow&ndash;on effects including potential anoxia, shifts in overall biogeochemical cycles and alterations in the biological structure and diversity of ecosystems (Lyons et al., 2006<ref name="Lyons et al, 2006">Lyons, W.B., Laybourn-Parry, J., Welch, K.A. and Priscu, J.C. 2006. Antarctic lake systems and climate change. In: Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 273-295.</ref>).<br />
<br />
Alien microbes, fungi, plants and animals, introduced directly through human activity over approximately the last two centuries, already occur on most of the sub-Antarctic islands and some parts of the Antarctic continent (Frenot et al., 2005<ref name="Frenot et al, 2005">Frenot, Y., Chown, S.L., Whinam, J., Selkirk, P., Convey, P., Skotnicki, M. and Bergstrom, D. 2005 Biological invasions in the Antarctic: extent, impacts and implications, ''Biological Reviews'', '''80''', 45-72.</ref>, 2008<ref name="Frenot et al, 2008">Frenot, Y., Convey, P., Lebouvier, M., Chown, S.L., Whinam, J., Selkirk, P.M., Skotnicki, M. and Bergstrom, D.M. 2008. Antarctic biological invasions: sources, extents, impacts and implications. Non-native species in the Antarctic Proceedings, ed. M. Rogan-Finnemore, 53-96. Gateway Antarctica, Christchurch, New Zealand.</ref>; Greenslade, 2006<ref name="Greenslade, 2006">Greenslade, P. 2006: The Invertebrates of Macquarie Island. Australian Antarctic Division, Kingston, Tasmania, xvi, 326 pp.</ref>; Convey, 2008<ref name="Convey, 2008">Convey, P. 2008. Non-native species in Antarctic terrestrial and freshwater environments: presence, sources, impacts and predictions. Non-native species in the Antarctic Proceedings, ed. M. Rogan-Finnemore, 97-130. Gateway Antarctica, Christchurch, New Zealand.</ref>, Table 4.2). The level of detail varies widely between locations and taxonomic groups (although at the microbial level, knowledge is virtually non-existent across the entire continent). On sub-Antarctic Marion Island and South Atlantic Gough Island it is estimated that rates of establishment through anthropogenic introduction outweigh those from natural colonization processes by two orders of magnitude or more. Introduction routes have varied, but are largely associated with movement of people and cargo in connection with industrial, national scientific programme and tourist operations. Although it is rare to have a record available of a specific introduction event, and there are undoubtedly instances of natural colonization processes resulting in new establishment, the impact of undoubted human-assisted introductions to some sub-Antarctic islands (particularly South Georgia, Kerguelen, Marion, Macquarie) is substantial and probably irreversible. Thus a range of introduced vertebrates and plants have led to large shifts in ecosystem structure and function, while in terms of overall diversity some islands now host a greater number of non-indigenous than indigenous species of plant. The large majority of aliens are European in origin.<br />
==References==<br />
<references /><br />
[[Category:The instrumental period]]<br />
[[Category:Marine biology]]<br />
[[Category:Terrestrial biology]]</div>Maintenance scripthttp://acce.scar.org/wiki/Terrestrial_biological_responses_to_climate_changeTerrestrial biological responses to climate change2014-08-06T14:34:03Z<p>Tonyp: Changed book section reference to page link</p>
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<div>:''This page is part of the topic [[Biological responses to climate change]]''<br />
<br />
With the exception of lake sediment studies (Hodgson et al., 2004a<ref name="Hodgson et al, 2004a">Hodgson, D.A., Doran, P.T., Roberts, D. and McMinn, A. 2004a. Paleolimnological studies from the Antarctic and subantarctic islands. In: Pienitz R, Douglas MSV, Smol JP (eds) Developments in Palaoenvironmental Research. Long-term Environmental Change in Arctic and Antarctic Lakes, 8, Springer, Dordrecht, 419-474.</ref>), little terrestrial research has set out to examine changes in biodiversity, distributions and abundance over Holocene and longer timescales. There has been a widely held but untested general assumption that, owing to the much more extensive and thicker ice sheets present on the continent at LGM, many if not all contemporary Antarctic terrestrial biota must be recent colonists. That this is not the case has come to recent prominence (Convey and Stevens, 2007<ref name="Convey and Stevens, 2007">Convey, P. and Stevens, M.I. 2007. Antarctic Biodiversity, ''Science'', '''317''', 1877-1878.</ref>; Convey et al., 2008<ref name="Convey et al, 2008">Convey, P., Gibson, J.A.E., Hillenbrand, C-D., Hodgson, D.A., Pugh, P.J.A., Smellie, J.L. and Stevens, M.I. 2008. Antarctic terrestrial life - challenging the history of the frozen continent? Biological Reviews, 83, 103-117.</ref>; Pugh and Convey, 2008<ref name="Pugh and Convey, 2008">Pugh, P.J.A. and Convey, P. 2008. Surviving out in the cold: Antarctic endemic invertebrates and their refugia, ''Journal of Biogeography'', '''35''', 2176-2186.</ref>), and it is becoming clear from biogeographical and molecular evidence that across the continent and also the sub-Antarctic islands the contemporary biota is a result of vicariance and colonization processes that have taken place on all timescales between pre-LGM and pre-Gondwana-breakup (e.g. Stevens et al., 2006<ref name="Stevens et al, 2006">Stevens, M.I., Greenslade, P., Hogg, I.D. and Sunnucks, P. 2006. Southern hemisphere springtails: could any have survived glaciation of Antarctica?, ''Mol. Biol. Evol.'', '''23''', 874-882.</ref>). Nevertheless it is also clear that much of the tiny proportion of Antarctica that is ice-free today has been exposed over only the last few thousand years during post LGM glacial retreat. Through the maritime Antarctic and much of the continental coastline, most exposed ground takes the form of &lsquo;islands&rsquo; of terrestrial habitat of varying size at low altitude and close to the coast, surrounded by either hostile sea or ice (Bergstrom and Chown, 1999<ref name="Bergstrom and Chown, 1999">Bergstrom, D.M. and Chown, S.L. 1999. Life at the front: history, ecology and changes on southern ocean islands, ''Trends in Ecology and Evolution'', '''14''', 427-477.</ref>). Exceptions to this generalization are provided first by the continental Antarctic &lsquo;Dry Valleys&rsquo; of Victoria Land, providing several thousand square kilometres of ground at least some of which has been continuously exposed since about 12 Ma in the Miocene, and second by inland higher altitude nunataks and mountain ranges, some of which will not have been covered at Pleistocene glacial maxima (Stevens and Hogg, 2003<ref name="Stevens and Hogg, 2003">Stevens, M.I. and Hogg, I.D. 2003. Long-term isolation and recent range expansion revealed for the endemic springtail Gomphiocephalus hodgsoni from southern Victoria Land, Antarctica, ''Molecular Ecology'', '''12''', 2357-2369.</ref>; Stevens et al., 2006<ref name="Stevens et al, 2006">Stevens, M.I., Greenslade, P., Hogg, I.D. and Sunnucks, P. 2006. Southern hemisphere springtails: could any have survived glaciation of Antarctica?, ''Mol. Biol. Evol.'', '''23''', 874-882.</ref>).<br />
<br />
Taking the maritime Antarctic as an example, it is thus clear (a) that the large majority of areas of currently ice free ground have been exposed post LGM (while longer term refugia are required to explain contemporary biota distributions, their precise locations remain unknown (Hodgson and Convey, 2005<ref name="Hodgson and Convey, 2005">Hodgson, D.A. and Convey, P. 2005. A 7000-year record of oribatid mite communities on a maritime-Antarctic island: responses to climate change, ''Arctic Antarctic and Alpine Research'', '''37''', 239-245.</ref>; Pugh and Convey, 2008<ref name="Pugh and Convey, 2008">Pugh, P.J.A. and Convey, P. 2008. Surviving out in the cold: Antarctic endemic invertebrates and their refugia, ''Journal of Biogeography'', '''35''', 2176-2186.</ref>)), and at the same time (b) it is clear that most elements of the regional biota have successfully colonized those areas that have been exposed, and done so rapidly, as their terrestrial communities are in most cases entirely typical of this regional biota. Hodgson and Convey (2005<ref name="Hodgson and Convey, 2005">Hodgson, D.A. and Convey, P. 2005. A 7000-year record of oribatid mite communities on a maritime-Antarctic island: responses to climate change, ''Arctic Antarctic and Alpine Research'', '''37''', 239-245.</ref>), using terrestrial arthropod abundances obtained from lake sediment cores on maritime Antarctic Signy Island, identified some differences in relative abundances of two common mite species over time (the last 5+ ka) that they proposed to be consistent with climatic changes that would have altered the balance of the different habitats that these species favour. On a longer pre-LGM Pleistocene timescale, Hodgson et al. (2005<ref name="Hodgson et al, 2005">Hodgson, D.A., Verleyen, E., Sabbe, K., Squier, A.H., Keely, B.J., Leng, M.J., Saunders, K.M. and Vyverman, W. 2005. Late Quaternary climate-driven environmental change in the Larsemann Hills, East Antarctica, multi-proxy evidence from a lake sediment core, ''Quaternary Research'', '''64''', 83-99.</ref>) have described changes in lake diatom communities in some continental East Antarctic lakes proposed to have survived intact throughout the LGM period. They provide evidence that sub-Antarctic diatom taxa present during the last interglacial period were lost from the community as the LGM approached, leaving only continental taxa, and that the sub-Antarctic taxa have not yet returned to the lakes post LGM.<br />
<br />
[[File:Figure 3.29 - Biological colonisations and extinctions in Antarctica since the break-up of Gondwana.png|thumb|'''3.29''' Biological colonisations and extinctions in Antarctica since the break-up of Gondwana; based on molecular, phylogenetic, fossil and sub-fossil evidence. The upper panel shows schematic timelines for the survival of different species on the continent. The lower panel shows the major geological and glaciological events in the evolution of Antarctica that will have influenced the flora and fauna. Note that the geological timescale is non-linear and that most micro-organisms are excluded from this schematic diagram]]<br />
Liquid water and ice-free refugia during ice ages have meant long availability of habitat for some of the biota (e.g. mites, springtails, chironomids), even extending back to the Gondwana era (Allegrucci et al., 2006<ref name="Allegrucci et al, 2006">Allegrucci, G., Carchini, G., Todiscso, V., Convey, P. and Sbordoni, V. 2006. A molecular phylogeny of Antarctic Chironomidae and its implications for biogeographical history. Polar Biology, 29, 320-326.</ref>; Hodgson et al., 2006a<ref name="Hodgson et al, 2006a">Hodgson, D.A., Verleyen, E., Squier, A.H., Sabbe, K., Keely, B.J., Saunders, K.M. and Vyverman, W. 2006a. Interglacial environments of coastal east Antarctica: comparison of MIS 1 (Holocene) and MIS 5e (Last Interglacial) lake-sediment records, ''Quaternary Science Reviews'', '''25''', 179-197.</ref>; Convey et al., 2008<ref name="Convey et al, 2008">Convey, P., Gibson, J.A.E., Hillenbrand, C-D., Hodgson, D.A., Pugh, P.J.A., Smellie, J.L. and Stevens, M.I. 2008. Antarctic terrestrial life - challenging the history of the frozen continent? Biological Reviews, 83, 103-117.</ref>) ([[:File:Figure 3.29 - Biological colonisations and extinctions in Antarctica since the break-up of Gondwana.png|Figure 3.29]]). Nevertheless, the expansion and contraction of the Antarctic ice sheets has undoubtedly led to the local extinction of biological communities on the Antarctic continent during glacial periods (Hodgson et al., 2006a<ref name="Hodgson et al, 2006a">Hodgson, D.A., Verleyen, E., Squier, A.H., Sabbe, K., Keely, B.J., Saunders, K.M. and Vyverman, W. 2006a. Interglacial environments of coastal east Antarctica: comparison of MIS 1 (Holocene) and MIS 5e (Last Interglacial) lake-sediment records, ''Quaternary Science Reviews'', '''25''', 179-197.</ref>). Subsequent interglacial re-colonisation and the resulting present-day biodiversity is then a result of whether the species were vicariant (surviving the glacial maxima in refugia, possibly also requiring them to take advantage of diachronous extension and retreat of ice in particular areas, then recolonising deglaciated areas), arrived through post-glacial dispersal from lower latitude islands and continents that remained ice free (Pugh et al., 2002<ref name="Pugh et al, 2002">Pugh, P.J.A., Dartnall, H.J.G. and McInnes, S.J. 2002. The non-marine crustacea of Antarctica and the islands of the Southern Ocean: biodiversity and biogeography. Journal of Natural History 36:1047-1103.</ref>), or are present through a combination of both mechanisms. Evidence can be found to support both vicariance (Marshall and Coetzee, 2000<ref name="Marshall and Coetzee, 2000">Marshall, D.J. and Coetzee, L. 2000. Historical biogeography and ecology of a continental Antarctic mite genus, Maudheimia (Acari, Oribatida): evidence for a Gondwanan origin and Pliocene-Pleistocene speciation, ''Zoological Journal of the Linnean Society'', '''129''', 111-128.</ref>; Stevens and Hogg, 2003<ref name="Stevens and Hogg, 2003">Stevens, M.I. and Hogg, I.D. 2003. Long-term isolation and recent range expansion revealed for the endemic springtail Gomphiocephalus hodgsoni from southern Victoria Land, Antarctica, ''Molecular Ecology'', '''12''', 2357-2369.</ref>; Hodgson et al., 2005<ref name="Hodgson et al, 2005">Hodgson, D.A., Verleyen, E., Sabbe, K., Squier, A.H., Keely, B.J., Leng, M.J., Saunders, K.M. and Vyverman, W. 2005. Late Quaternary climate-driven environmental change in the Larsemann Hills, East Antarctica, multi-proxy evidence from a lake sediment core, ''Quaternary Research'', '''64''', 83-99.</ref>; Allegrucci et al., 2006<ref name="Allegrucci et al, 2006">Allegrucci, G., Carchini, G., Todiscso, V., Convey, P. and Sbordoni, V. 2006. A molecular phylogeny of Antarctic Chironomidae and its implications for biogeographical history. Polar Biology, 29, 320-326.</ref>; Hodgson et al., 2006a<ref name="Hodgson et al, 2006a">Hodgson, D.A., Verleyen, E., Squier, A.H., Sabbe, K., Keely, B.J., Saunders, K.M. and Vyverman, W. 2006a. Interglacial environments of coastal east Antarctica: comparison of MIS 1 (Holocene) and MIS 5e (Last Interglacial) lake-sediment records, ''Quaternary Science Reviews'', '''25''', 179-197.</ref>; Stevens et al., 2006<ref name="Stevens et al, 2006">Stevens, M.I., Greenslade, P., Hogg, I.D. and Sunnucks, P. 2006. Southern hemisphere springtails: could any have survived glaciation of Antarctica?, ''Mol. Biol. Evol.'', '''23''', 874-882.</ref>; Stevens and Hogg, 2006<ref name="Stevens and Hogg, 2006">Stevens, M.I. and Hogg, I.D. 2006. The molecular ecology of Antarctic terrestrial and limnetic invertebrates and microbes. In: Bergstrom DM, Convey P, Huiskes AHL (eds) Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a global indicator. Springer, Dordrecht, 177-192.</ref>) and dispersal (Hodgson et al., 2006a<ref name="Hodgson et al, 2006a">Hodgson, D.A., Verleyen, E., Squier, A.H., Sabbe, K., Keely, B.J., Saunders, K.M. and Vyverman, W. 2006a. Interglacial environments of coastal east Antarctica: comparison of MIS 1 (Holocene) and MIS 5e (Last Interglacial) lake-sediment records, ''Quaternary Science Reviews'', '''25''', 179-197.</ref>) for a variety of different species, and is based on the level of cosmopolitanism (dispersal model) or endemism (vicariance model) (Gibson and Bayly, 2007<ref name="Gibson and Bayly, 2007">Gibson, J.A.E. and Bayly, I.A.E. 2007. New insights into the origins of crustaceans of Antarctic lakes, ''Antarctic Science'', '''19''', 157-164.</ref>), on direct palaeolimnological evidence, or, most recently, on molecular phylogenetic and evolutionary studies (Marshall and Convey, 2004<ref name="Marshall and Convey, 2004">Marshall, D.J. and Convey, P. 2004. Latitudinal variation in habitat specificity of ameronothroid mites, ''Experimental and Applied Acarology'', '''34''', 21-35.</ref>; Allegrucci et al., 2006<ref name="Allegrucci et al, 2006">Allegrucci, G., Carchini, G., Todiscso, V., Convey, P. and Sbordoni, V. 2006. A molecular phylogeny of Antarctic Chironomidae and its implications for biogeographical history. Polar Biology, 29, 320-326.</ref>; Stevens et al., 2006<ref name="Stevens et al, 2006">Stevens, M.I., Greenslade, P., Hogg, I.D. and Sunnucks, P. 2006. Southern hemisphere springtails: could any have survived glaciation of Antarctica?, ''Mol. Biol. Evol.'', '''23''', 874-882.</ref>; Stevens and Hogg, 2006<ref name="Stevens and Hogg, 2006">Stevens, M.I. and Hogg, I.D. 2006. The molecular ecology of Antarctic terrestrial and limnetic invertebrates and microbes. In: Bergstrom DM, Convey P, Huiskes AHL (eds) Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a global indicator. Springer, Dordrecht, 177-192.</ref>).<br />
<br />
On the oceanic islands, the biotas must have originally arrived via long-distance over-ocean dispersal, with vicariance and terrestrial dispersal playing subsequent roles in shaping the biodiversity across glacial cycles (Marshall and Convey, 2004<ref name="Marshall and Convey, 2004">Marshall, D.J. and Convey, P. 2004. Latitudinal variation in habitat specificity of ameronothroid mites, ''Experimental and Applied Acarology'', '''34''', 21-35.</ref>; Stevens et al., 2006<ref name="Stevens et al, 2006">Stevens, M.I., Greenslade, P., Hogg, I.D. and Sunnucks, P. 2006. Southern hemisphere springtails: could any have survived glaciation of Antarctica?, ''Mol. Biol. Evol.'', '''23''', 874-882.</ref>). Species on Southern Ocean islands show conventional island biogeographic relationships, with variance in indigenous species richness explained by factors including area, mean surface air temperature, and age and distance from continental land masses (Marshall and Convey, 2004<ref name="Marshall and Convey, 2004">Marshall, D.J. and Convey, P. 2004. Latitudinal variation in habitat specificity of ameronothroid mites, ''Experimental and Applied Acarology'', '''34''', 21-35.</ref>). For aquatic species, at least for some groups such as the diatoms, diversity is controlled by the &lsquo;connectivity&rsquo; among habitats with the more isolated regions developing greater degrees of endemism (Vyverman et al., 2007<ref name="Vyverman et al, 2007">Vyverman, W., Verleyen, E., Sabbe, K., Vanhoutte, K., Sterken, M., Hodgson, D.A., Mann, D.G., Juggins, S., Van De Vijver, B., Jones, V.J., Flower, R., Roberts, D., Chepurnov, V.A., Kilroy, C., Vanormelingen, P. and De Wever, A. 2007. Historical processes constrain patterns in global diatom diversity, ''Ecology'', '''88''', 1924-1931.</ref>; Verleyen et al., 2009<ref name="Verleyen et al, 2009">Verleyen, E., Vyverman, W., Sterken, M., Hodgson, D.A., De Wever, A., Juggins, S., Van De Vijver, B., Jones, V.J., Vanormelingen, P., Roberts, D., Flower, R., Kilroy, C., Souffreau, C. and Sabbe, K. 2009. The importance of dispersal related and local factors in shaping the taxonomic structure of diatom metacommunities, ''Oikos'', '''118''', 1239-1249.</ref>).<br />
<br />
==Changing species distributions, abundance and biodiversity==<br />
<br />
Through the Holocene the changing environmental conditions described in [[Regional patterns of holocene climate change in Antarctica]] have caused marked changes in species distributions particularly for those species that have well defined ecological ranges. This is best recorded in palaeolimnological studies where preserved morphological and geochemical fossils provide a detailed record of changing species compositions in response to changes in lake water chemistry and other environmental variables at many sites around the continent.<br />
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The historical record of changing terrestrial species distributions is more sparse. Much of our knowledge is based on changes that have been recently been observed in the Antarctic Peninsula region where increasing temperatures in the last 50 years have resulted in the local expansion of population ranges of a number of plant and animal species, and the establishment (albeit with human assistance) of new species that appear not to have survived on the continent before (Frenot et al., 2005<ref name="Frenot et al, 2005">Frenot, Y., Chown, S.L., Whinam, J., Selkirk, P., Convey, P., Skotnicki, M. and Bergstrom, D. 2005 Biological invasions in the Antarctic: extent, impacts and implications, ''Biological Reviews'', '''80''', 45-72.</ref>; Barnes et al., 2006<ref name="Barnes et al, 2006">Barnes, D.K., Hodgson, D.A., Convey, P., Allen, C.S. and Clarke, A.C. 2006. Incursion and excursion of Antarctic biota: past, present and future, ''Global Ecol Biogeogr'', '''15''', 121-142.</ref>).<br />
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The growth and life cycle patterns of many invertebrates and plants are fundamentally dependent on regional temperature regimes and their linkage with patterns of water availability. Distinct patterns in sexual reproduction are evident across the Antarctic flora and are most likely a function of temperature variation. In addition, phenology (the study of periodic plant and animal life cycle events and how these are influenced by seasonal and interannual variations in climate) of flowering plants is cued to seasonality in the light regime. In regions supporting angiosperms, wind is assumed to play a major role in the pollination ecology of grasses and sedges, resulting in cross-pollination. The lack of pollinators in the native fauna, combined with high reproductive outputs in non-wind pollinated species implies a high reliance on self-pollination.<br />
<br />
The Antarctic biota shows high development of ecophysiological adaptations relating to cold and desiccation tolerance, and displays an array of traits to facilitate survival of these conditions. While patterns in absolute low temperatures are clearly influential in determining survival, perhaps more influential are the patterns of sub-lethal environmental stresses experienced and the freeze-thaw regime, with repeated freeze-thaw events being more damaging than a sustained freeze event. How these patterns change in the future will be an issue of major importance to ecosystems.<br />
<br />
In contrast to many Antarctic marine organisms, the terrestrial biota often has a wide environmental tolerance. It includes some of the most robust life forms on Earth, the cyanobacteria, which can survive extremes of low temperature, water availability, light and high UV radiation (Hodgson et al., 2004b<ref name="Hodgson et al, 2004b">Hodgson, D.A., Vyverman, W., Verleyen, E., Sabbe, K., Leavitt, P.R., Taton, A., Squier, A.H. and Keely, B.J. 2004b. Environmental factors influencing the pigment composition of ''in situ'' benthic microbial communities in east Antarctic lakes, ''Aquatic Microbial Ecology'', '''37''', 247-263.</ref>). These are particularly abundant in extreme habitats, such as parts of the Transantarctic Mountains, where they have no or few competitors. Other groups, however, do have well defined ranges within which they can survive.<br />
<br />
It is already well known that Antarctic terrestrial biota possess very effective stress tolerance strategies, in addition to considerable response flexibility. The exceptionally wide degree of environmental variability experienced in many Antarctic terrestrial habitats, on a range of timescales between hours and years, means that predicted levels of change in environmental variables (particularly temperature and water availability) are often small relative to the range already experienced. Given the absence of colonisation by more effective competitors, predicted and observed levels of climate change may be expected to generate positive responses from resident biota of the maritime and continental Antarctic. The picture is likely to be far more complex on the different subantarctic islands, and many already host (different) alien invasive taxa, some of which already have considerable impacts on native biota (Frenot et al., 2005<ref name="Frenot et al, 2005">Frenot, Y., Chown, S.L., Whinam, J., Selkirk, P., Convey, P., Skotnicki, M. and Bergstrom, D. 2005 Biological invasions in the Antarctic: extent, impacts and implications, ''Biological Reviews'', '''80''', 45-72.</ref>).<br />
==References==<br />
<references /><br />
[[Category:The pre-instrumental period]]<br />
[[Category:Antarctic biology]]<br />
[[Category:Terrestrial biology]]</div>Maintenance scripthttp://acce.scar.org/wiki/Temperature_changes_over_the_21st_centuryTemperature changes over the 21st century2014-08-06T14:34:02Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Atmospheric change over the next 100 years]]''<br />
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[[File:Figure 5.6 - Skin temperature trend over the Twenty First Century.png|thumb|'''5.6''' Skin temperature trend over Twenty First Century in &ordm;C/decade.]]<br />
A significant surface warming over Antarctica is projected over the Twenty First Century. The weighted average of the A1B scenario runs of the CMIP3 models shows an increase of the annual average surface temperature of 0.34&deg;C/decade over land and grounded ice sheets (Bracegirdle et al., 2008<ref name="Bracegirdle et al, 2008">Bracegirdle, T.J., Connolley, W.M. and Turner, J. 2008. Antarctic climate change over the Twenty First Century, Journal of Geophysical Research &ndash; Atmospheres, 113, D03103, doi:03110.01029/02007JD008933.</ref>) ([[:File:Figure 5.6 - Skin temperature trend over the Twenty First Century.png|Figure 5.6]]). All the CMIP3 models show a warming, but with a large range from 0.14 to 0.5&deg;C/decade under the A1B scenario. The difference between the ensemble average projections of each scenario is smaller than the inter-model spread for any given scenario.<br />
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Due to the retreat of the sea ice edge induced by global warming, the largest projected surface warming occurs during the winter when the sea ice extent approaches its maximum, e.g. 0.51 &plusmn; 0.26 C/decade off East Antarctica (Bracegirdle et al., 2008<ref name="Bracegirdle et al, 2008">Bracegirdle, T.J., Connolley, W.M. and Turner, J. 2008. Antarctic climate change over the Twenty First Century, Journal of Geophysical Research &ndash; Atmospheres, 113, D03103, doi:03110.01029/02007JD008933.</ref>).<br />
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[[File:Figure 5.7 - Change in skin temperature by season over the Twenty First Century.png|thumb|'''5.7''' Skin temperature change over the Twenty First Century in &ordm;C/decade. (a) DJF, (b) MAM, (c) JJA and (d) SON.]]<br />
Inland, away from coastal regions there is very little seasonal dependence of the warming trend, which in all seasons is largest over the high-altitude interior of East Antarctica according to the model average ([[:File:Figure 5.7 - Change in skin temperature by season over the Twenty First Century.png|Figure 5.7]]). Despite this large increase of temperature, the surface temperature by the year 2100 will remain below freezing over most of Antarctica and therefore will not contribute significantly to melting.<br />
<br />
The pattern of warming for the next 100 years is different between simulations and observations of temperature change for the latter part of the Twentieth Century. The most notable difference is that the observed and simulated maximum of warming over the Antarctic Peninsula for the latter part of the Twentieth Century is not present in projections of change over the Twenty First Century. That is because although the Peninsula does continue to warm, other parts of Antarctica warm with it.<br />
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[[File:Figure 5.8 - Inter-model standard deviation of Twenty First Century skin temperature change by season.png|thumb|'''5.8''' Inter-model standard deviation of Twenty First Century skin temperature change for individual grid points in &ordm;C/decade. (a) DJF, (b) MAM, (c) JJA and (d) SON.]]<br />
The model consensus for warming is strong for Antarctica as a whole, but there is large uncertainty in the regional detail. One way to measure the significance of a projected change is to calculate a signal to noise ratio of that change. Here the signal is the ensemble average change and the noise is the standard deviation of the inter-model spread. A change can be thought of as &lsquo;significant&rsquo; if larger than the inter-model standard deviation, i.e. a signal to noise ratio of greater than one. In [[:File:Figure 5.8 - Inter-model standard deviation of Twenty First Century skin temperature change by season.png|Figure 5.8]] it can be seen that at most grid points the projected increases of temperature are larger than the inter-model standard deviation. In particular over the Antarctic continent the projected warming shown in [[:File:Figure 5.7 - Change in skin temperature by season over the Twenty First Century.png|Figure 5.7]] is much larger than the inter-model spread ([[:File:Figure 5.8 - Inter-model standard deviation of Twenty First Century skin temperature change by season.png|Figure 5.8]]). This demonstrates strong confidence that there will be a warming at the surface in these regions under the A1B scenario. There is less confidence in the large warming trends around the coast than in the smaller changes over the high interior. This is due to the large uncertainty over the sea ice and ocean projections.<br />
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According to the IPCC report the warming over the Antarctic continent is 0.5-1.0&ordm;C less than over most other landmasses around the globe (apart from south-east Asia and southern South America where increases are the same). The reasons for this are not known. Over the Southern Ocean projected surface warming is much smaller than the global average due to the large heat uptake by the ocean.<br />
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In the mid-troposphere above the continent, the annual ensemble mean warming rate at 500 hPa of 0.28&ordm;C/decade is slightly smaller than the surface warming, with no evidence of the mid-tropospheric maximum that has been observed over the last 30 years (Turner et al., 2006<ref name="Turner et al, 2006">Turner, J., Lachlan-Cope, T.A., Colwell, S.R., Marshall, G.J. and Connolley, W.M. 2006. Significant warming of the Antarctic winter troposphere, ''Science'', '''311''', 1914-1917.</ref>). The mid-tropospheric warming at low latitudes is larger than over and around Antarctica, which increases the baroclinicity and seems to contribute to the southward migration of the storm tracks that is simulated by the CMIP3 models (Yin, 2005<ref name="Yin, 2005">Yin, J. H. 2005. A consistent poleward shift of the storm tracks in simulations of 21<sup>st</sup> century climate, ''Geophysical Research Letters'', '''32''', L18701.</ref>).<br />
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==Extremes==<br />
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Very little work has been done on changes to extremes over Antarctica. Temperature-related extreme indices are available from the CMIP3 archive and have been assessed by Tebaldi et al. (2006<ref name="Tebaldi et al, 2006">Tebaldi, C., Hayhoe, K., Arblaster, J.M. and Meehl, G.A. 2006. going to the extremes: An intercomparison of model-simulated historicak and future changes in extreme events, ''Climate Change'', '''79''', 185-211.</ref>). The heat wave duration index (defined as the maximum period greater than five consecutive days with the daily maximum temperature greater than 5&ordm;C above the 1961-1990 mean daily maximum temperature) shows significant increases along coastal Antarctica, where melting can occur, but largest increases over the interior, where heat waves are not warm enough to cause melting. On even shorter timescales, the magnitude, timing and frequency of extreme events (e.g. temperature minima/maxima, freeze-thaw events) can be of considerable relevance to predictions of future trajectories in terrestrial ecosystems.<br />
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The extreme temperature range between the coldest and warmest temperature of a given year is projected to decrease around coastal Antarctica and show little change over most of the interior of the continent. Further assessment is required to determine the reasons for the projected pattern of changes. One possibility is that the models simulate a larger nighttime than daytime warming. This has been observed during the second half of the Twentieth Century over the Antarctic Peninsula (Hughes et al., 2007<ref name="Hughes et al, 2007">Hughes, G.L., Rao, S.S. and Rao, T.S. 2007. Statistical analysis and time-series models for minimum/maximum temperatures in the Antarctic Peninsula, ''Proceedings of the Royal Society A'', '''463''', 241-259.</ref>).<br />
==References==<br />
<references /><br />
[[Category:The next 100 years]]<br />
[[Category:The Antarctic atmosphere]]<br />
[[Category:Temperature]]</div>Maintenance scripthttp://acce.scar.org/wiki/Temperature_changes_in_the_instrumental_periodTemperature changes in the instrumental period2014-08-06T14:34:01Z<p>Tonyp: Changed book section reference to page link</p>
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<div>:''This page is part of the topic [[Antarctic climate and environment change in the instrumental period]]''<br />
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==Surface temperature==<br />
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Surface temperature trends across the Antarctic can be determined using a number of different forms of data, including the ''in-situ'' observations, satellite infra-red imagery and ice core isotope measurements. In order to get a reasonable estimate of trends it is necessary to use all these data.<br />
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The ''in-situ'' observational record of Antarctic surface temperatures is rather sparse and sporadic before the IGY (see Appendix A in King and Turner, (1997<ref name="King and Turner, 1997">King, J.C. and Turner, J. 1997. Antarctic meteorology and climatology, Cambridge University Press, Cambridge, UK, 409 pp.</ref>)), although the Orcadas series from Laurie Island, South Orkney Islands began in 1903 and the Faraday Station/Argentine Islands record began in 1947. However, we are fortunate in having around 16 stations on the Antarctic continent or islands that have reported on a near-continuous basis since the IGY. In addition, a further six stations started reporting during the 1960s, so that we have around two dozen time series that allow the investigation of temperature trends. Unfortunately, the vast majority of the stations are in the Antarctic coastal region or on the islands of the Southern Ocean, with only Vostok and Amundsen-Scott Station being in the interior of the continent.<br />
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The ''in-situ'' record has been used by several workers to investigate temperature changes across the continent and Southern Ocean (Jacka and Budd, 1991<ref name="Jacka and Budd, 1991">Jacka, T.H. and Budd, W.F. 1991. Detection of temperature and sea ice extent changes in the Antarctic and Southern Ocean. Weller, G., Wilson, C. L., and Severin, B. A. Proceedings of the International Conference on the Role of the Polar Regions in Global Change. June 11-15, 1990, University of Alaska Fairbanks, 63-70. Fairbanks, AK, University of Alaska, Geophysical Institute.</ref>; Jacka and Budd, 1998<ref name="Jacka and Budd, 1998">Jacka, T.H. and Budd, W.F. 1998. Detection of temperature and sea-ice-extent changes in the Antarctic and Southern Ocean, 1949-96, ''Annals of Glaciology'', '''27''', 553-559.</ref>; Jones, 1995<ref name="Jones, 1995">Jones, P.D. 1995. Recent variations in mean temperature and the diurnal temperature range in the Antarctic, ''Geophysics Research Letters'', '''22''' [11], 1345-1348.</ref>; Raper et al., 1984<ref name="Raper et al, 1984">Raper, S.C., Wigley, T.M., Jones, P.D. and Salinger, M. J. 1984. Variations in surface air temperatures: Part 3. The Antarctic, 1957-1982, ''Monthly Weather Review'', '''112''', 1341-1353.</ref>). Many of the records were scattered across a number of data centres and it was unclear as to the amount of quality control that had been carried out on the observations. SCAR therefore initiated the READER (Reference Antarctic Data for Environmental Research) project to bring as many of the observations together as possible, quality control the data and produce a new data base of monthly mean temperatures (Turner et al., 2004<ref name="Turner et al, 2004">Turner, J., Colwell, S. R., Marshall, G. J., Lachlan-Cope, T. A., Carleton, A. M., Jones, P. D., Lagun, V., Reid, P. A. and Iagovkina, S. 2004. The SCAR READER project: Towards a high-quality database of mean Antarctic meteorological observations, ''Journal of Climate'', '''17''', 2890-2898.</ref>). The READER data base is available online at http://www.antarctica.ac.uk/met/READER/.<br />
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The READER data base has now been used in a number of studies concerned with the climate of the Antarctic, including that of Turner et al. (2005a<ref name="Turner et al, 2005a">Turner, J., Colwell, S.R., Marshall, G.J., Lachlan-Cope, T.A., Carleton, A.M., Jones, P.D., Lagun, V., Reid, P.A. and Iagovkina, S. 2005a. Antarctic climate change during the last 50 years, ''International Journal of Climatology'', '''25''', 279-294.</ref>), which considered changes since the start of the routine instrumental record. Here we will use the READER data base and the online meteorological data maintained by Dr. Gareth Marshall (http://www.antarctica.ac.uk/met/gjma/) to examine how Antarctic temperatures have changed over the period of the instrumental record.<br />
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[[File:Figure 4.8a - Near-surface temperature trends for 1951-2006 based on station data.png|thumb|'''4.8a''' Near-surface temperature trends for 1951-2006 based on station data.]]<br />
Surface temperature trends from the station data since the early 1950s illustrate a strong dipole of change, with significant warming across the Antarctic Peninsula, but with little change across the rest of the continent ([[:File:Figure 4.8a - Near-surface temperature trends for 1951-2006 based on station data.png|Figure 4.8a]]). The largest warming trends in the annual mean data are found on the western and northern parts of the Antarctic Peninsula. Here Faraday/Vernadsky Station has experienced the largest statistically significant (&lt;5% level) trend of +0.53 o C/dec for the period 1951-2006. Rothera station, some 300 km to the south of Faraday, has experienced a larger annual warming trend, but the shortness of the record and the large inter-annual variability of the temperatures means that the trend is not statistically significant. Although the region of marked warming extends from the southern part of the western Antarctic Peninsula north to the South Shetland Islands, the rate of warming decreases away from Faraday/Vernadsky, with the long record from Orcadas on Laurie Island, South Orkney Islands only having experienced a warming of +0.20&deg;C/decade. This record covers a 100-year period rather than the 50 years for Faraday. For the period 1951-2000 the temperature trend was +0.13&deg;C/decade.<br />
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[[File:Figure 4.8b - Linear trends of annual mean surface air temperature for 1958-2002.png|thumb|'''4.8b''' Linear trends of annual mean surface air temperature (&deg;C /dec) for the period 1958&ndash;2002. Greens and blues denote cooling; yellows and reds denote warming. Significant trends are indicated by hatching (95% = single hatching; 99% = crosshatching). From Chapman and Walsh (2007<ref name="Chapman and Walsh, 2007">Chapman, W.L. and Walsh, J. E. 2007. A Synthesis of Antarctic Temperatures, ''Journal of Climate'', '''20''', 4096-4117.</ref>)]]<br />
Determining temperature trends across the interior of the Antarctic is difficult as there are only two stations with long records. However, attempts have been made to extrapolate the station trends across the rest of the continent. Chapman and Walsh (2007<ref name="Chapman and Walsh, 2007">Chapman, W.L. and Walsh, J. E. 2007. A Synthesis of Antarctic Temperatures, ''Journal of Climate'', '''20''', 4096-4117.</ref>) produced estimates of annual tends ([[:File:Figure 4.8b - Linear trends of annual mean surface air temperature for 1958-2002.png|Figure 4.8b]]) and found the greatest warming over the Antarctic Peninsula, but with a small warming (~0.1&ordm; C/dec) across West Antarctica and much of East Antarctica. However, they also found cooling in a swath from the South Pole to Halley Station.<br />
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[[File:Figure 4.8c - Winter season temperature trends reconstructed using infrared satellite data.png|thumb|'''4.8c''' Winter season temperature trends reconstructed using infrared satellite data. NS indicates the trends are not significant in this area. From Steig et al. (2009<ref name="Steig et al, 2009">Steig, E.J., Schneider, D.P., Rutherford, S.D., Mann, M.E., Comiso, J. C. and Shindell, D.T. 2009. Warming of the Antarctic ice-sheet surface since the 1957 International Geophysical Year, ''Nature'', '''457''', 459-462.</ref>).]]<br />
Steig et al. (2009<ref name="Steig et al, 2009">Steig, E.J., Schneider, D.P., Rutherford, S.D., Mann, M.E., Comiso, J. C. and Shindell, D.T. 2009. Warming of the Antarctic ice-sheet surface since the 1957 International Geophysical Year, ''Nature'', '''457''', 459-462.</ref>) use statistical climate-field-reconstruction techniques to produce similar fields of trends for the seasons and the year as a whole. The annual trends ([[:File:Figure 4.8c - Winter season temperature trends reconstructed using infrared satellite data.png|Figure 4.8c]]) show significant warming over most of West Antarctica with trends greater than 0.1&ordm; C/dec over the last 50 years. The trends are greatest during the winter and spring.<br />
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There has been a great deal of debate about the causes of the recent temperature changes across the continent. The summer warming on the eastern side of the Antarctic Peninsula has been shown to be a result of anthropogenic activity, and particularly the spring time loss of stratospheric ozone (Marshall et al., 2006<ref name="Marshall et al, 2006">Marshall, G. J., Orr, A., Van Lipzig, N.P.M. and King, J.C. 2006. The impact of a changing Southern Hemisphere Annular Mode on Antarctic Peninsula summer temperatures, ''Journal of Climate'', '''19''' [20], 5388-5404.</ref>). For the continent as a whole Gillett et al. (2009<ref name="Gillett et al, 2009">Gillett, N.P., Stone, D.A., Stott, P.A., Nozawa, T., Karpechko, A.Y., Hegerl, G.C., Wehner, M.F. and Jones, P.D. 2009. Attribution of polar warming to human influence, ''Nature Geoscience'', '''1''', 760-764.</ref>) carried out a formal attribution study to determine whether the observed changes were within the range of natural climate variability or whether they were a result of anthropogenic forcing. They found that recent changes were not consistent with internal climate variability or natural climate drivers alone, and were directly attributable to human influence.<br />
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Prior to the establishment of the research stations in the middle of the Twentieth Century we are reliant on ice core data to investigate surface temperature changes. Many studies have used single and multiple cores to investigate changes at selected sites or to investigate regional change. However, &lsquo;stacking&rsquo; multiple cores can provide insight into Antarctic-wide change. Schneider et al. (2006<ref name="Schneider et al, 2006">Schneider, D.P., Steig, E.J., Van Ommen, T.D., Dixon, D.A., Mayewski, P.A., Jones, J.M. and Bitz, C.M. 2006. Antarctic temperatures over the past two centuries from ice cores, ''Geophysics Research Letters'', '''33''', L16707, doi:10.1029/2006GL027057.</ref>) stacked several isotope records from ice cores to obtain a continental pattern of temperature over the past 200 years. The ice core stack was found to be well correlated with annual mean temperature and the data suggested warming of 0.2&ordm; C since the late nineteenth century. The paper suggested that recent Antarctic cooling is superimposed on longer term warming, with the more recent cooling being attributed to the SAM strengthening.<br />
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There is also evidence of climatic changes over the Southern Ocean. In recent decades, instrumental data recorded at the South African Weather Service station on Marion Island (46&ordm; 32&rsquo; S and 37&ordm; 30&rsquo; E) shows that the local climate of this island has undergone significant changes since the 1960s, mostly in the austral summer. These include a decrease in rainfall, an increase in non-rainy days, changes in wind speed and direction, and an increase in maximum and minimum local air temperature and in nearshore SST. Research suggests that the changes are linked to the well-documented shift of the semiannual oscillation and SAM after about 1980 (Rouault et al., 2005<ref name="Rouault et al, 2005">Rouault, M., Melice, J.L., Reason, C.J.C. and Lutjeharms, J.R.E. 2005. Climate variability at Marion Island, Southern Ocean, since 1960, J. Geophy. Res. &ndash; Oceans, 110, C05007, doi:10.1029/2004JC002492.</ref>).<br />
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Satellite-derived surface temperatures for the Antarctic have been used to investigate the extent of the region of extreme variability, since this was not possible with the sparse station data. King and Comiso (2003<ref name="King and Comiso, 2003">King, J.C. and Comiso, J.C. 2003. The spatial coherence of interannual temperature variations in the Antarctic Peninsula, Geophysics Research Letters, 30, [2 10.1029/2002GL015580].</ref>) found that the region in which satellite-derived surface temperatures correlated strongly with west Peninsula station temperatures was largely confined to the seas just west of the Peninsula. It was also found that the correlation of Peninsula surface temperatures with those over the rest of continental Antarctica was poor, confirming that the west Peninsula is in a different climate regime.<br />
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The warming on the western side of the Antarctic Peninsula has been largest during the winter season, with the winter temperatures at Faraday increasing by +1.03&deg;C/decade over 1950-2006. In this area there is a high correlation during the winter between the sea ice extent and the surface temperatures, suggesting more sea ice during the 1950s and 1960s and a progressive reduction since that time. King and Harangozo (1998<ref name="King and Harangozo, 1998">King, J.C. and Harangozo, S.A. 1998. Climate change in the western Antarctic Peninsula since 1945: observations and possible causes, ''Annals of Glaciology'', '''27''', 571-575.</ref>) found a number of ship reports from the Bellingshausen Sea in the 1950s and 1960s when sea ice was well north of the locations found in the period of availability of satellite data, suggesting some periods of greater sea ice extent than found in recent decades. However, there is very limited sea ice extent data before the late 1970s, so we have largely circumstantial evidence of a mid-century sea ice maximum at this time. At the moment it is not known whether the warming on the western side of the Peninsula has occurred because of natural climate variability or as a result of anthropogenic factors.<br />
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Temperatures on the eastern side of the Peninsula have risen most during the summer and autumn months, with Esperanza having experienced a summer increase of +0.41&deg;C/decade between 1946-2006. This temperature rise has been linked to a strengthening of the westerlies that has taken place as the SAM has shifted into its positive phase (Marshall et al., 2006<ref name="Marshall et al, 2006">Marshall, G. J., Orr, A., Van Lipzig, N.P.M. and King, J.C. 2006. The impact of a changing Southern Hemisphere Annular Mode on Antarctic Peninsula summer temperatures, ''Journal of Climate'', '''19''' [20], 5388-5404.</ref>). Stronger winds have resulted in more relatively warm, maritime air masses crossing the peninsula and reaching the low-lying ice shelves on the eastern side.<br />
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Around the rest of the Antarctic coastal region there have been few statistically significant changes in surface temperature over the instrumental period. The largest warming outside the Peninsula region is at Scott Base, where temperatures have risen at a rate of +0.29&deg;C/decade, although this is not statistically significant. The high spatial variability of the changes is apparent from the data for Novolazarevskya and Syowa, which are 1,000 km apart. The former station has warmed at a rate of +0.25&deg;C/decade between 1962&ndash;2000, which is significant at the 10% level, whereas the record from Syowa shows almost no change over this period.<br />
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One area of the Antarctic where marked cooling has been noted, at least over a relatively short time period, is the McMurdo Dry Valleys. Here automatic weather station (AWS) data for 1986-2000 shows that there has been a cooling of 0.7&deg;C/decade, with the most pronounced cooling being in the summer (the December-February tend was 1.2&deg;C/decade, statistically significant at the 2% level) and autumn (March-May trend of 2.0&deg;C/decade, statistically significant at close to the 10% level). Winter (June-August) and spring (September-November) show small temperature increases of (0.6&deg;C and 0.1&deg;C/decade, which are not significant) (Doran et al., 2002<ref name="Doran et al, 2002">Doran, P.T., Priscu, J.C., Lyons, W.B., Walsh, J.E., Fountain, A.G., McKnight, D.M., Moorhead, D.L., Virginia, R.A., Wall, D.H., Clow, G.D., Fritsen, C.H., McKay, C.P. and Parsons, A.N. 2002. Antarctic climate cooling and terrestrial ecosystem response, ''Nature'', '''415''', 517-520.</ref>). Bertler et al. (2004<ref name="Bertler et al, 2004">Bertler, N.A.N., Barrett, P.J., Mayewski, P.A., Fogt, R.L., Kreutz, K.J. and Shulmeister, J. 2004. El Ni&ntilde;o suppresses Antarctic warming, ''Geophys. Res. Lett.'', '''31''', L15207, doi:10.1029/2004GL020749.</ref>) suggest that this short-term cooling is associated with ENSO-driven changes in atmospheric circulation.<br />
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On the interior plateau, Amundsen-Scott Station at the South Pole has shown a small cooling in the annual mean temperature of -0.05&ordm; C/dec over 1958-2008, although this trend is not statistically significant. This small cooling is thought to be a result of fewer maritime air masses penetrating into the interior of the continent. The data show a cooling throughout the year, with the largest change being during the summer, however, only the annual change is statistically significant. The other plateau station, Vostok, has not experienced any statistically significant change in temperatures, either in the annual or seasonal data, since the station was established in 1958.<br />
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A recent analysis of the ''in-situ'' surface meteorological observations (Chapman and Walsh, 2007<ref name="Chapman and Walsh, 2007">Chapman, W.L. and Walsh, J. E. 2007. A Synthesis of Antarctic Temperatures, ''Journal of Climate'', '''20''', 4096-4117.</ref>) gridded the available observations and analysed the trends. For the period 1958-2002 they found a modest warming over much of the 60&deg;&ndash;90&deg;S region, although the largest warming trends were over the Antarctic Peninsula. They also identified a zone of cooling stretching from Halley Station to the South Pole. They found overall warming in all seasons, with winter trends being the largest at +0.172&deg;C/decade while summer warming rates were only +0.045&deg;C/decade. For the 45 year period the temperature trend in the annual means was +0.082&deg;C/decade. Interestingly the trends computed were very sensitive to start and end dates, with trends calculated using start dates prior to 1965 showing overall warming, while those using start dates from 1966 to 1982 show net cooling over the region. Because of the large interannual variability of temperatures over the continental Antarctic, most of the continental trends are not statistically significant significant through 2002.<br />
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The temperature records from the Antarctic stations suggest that the trends at many locations are dependent on the time period examined, with changes in the major modes of variability affecting the temperature data. Perhaps the largest change in climatic conditions across the high southern latitudes has been the shift in the SAM into its positive phase during austral summer and autumn (see [[Atmospheric circulation changes in the instrumental period#The Southern Annular Mode|The SAM in the instrumental period]]). The SAM has changed because of the increase in greenhouse gases and the development of the Antarctic ozone hole, although the loss of stratospheric ozone has been shown to have had the greatest influence during Austral summer (Arblaster and Meehl, 2006<ref name="Arblaster and Meehl, 2006">Arblaster, J.M. and Meehl G.A. 2006. Contributions of external forcings to Southern Annual Mode trends, ''Journal of Climate'', '''19''', 2896-2905.</ref>). During austral autumn, the causality of the upward SAM trends is not well understood, as stratospheric ozone changes do not appear to play a major role (Fogt et al., In Press). As discussed at many points in this document, the changes in the SAM have influenced many aspects of the Antarctic environment over recent decades.<br />
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[[File:Figure 4.9 - Trends in December-May Z500, surface temperature and 925hPa winds, and SAM contribution.png|thumb|'''4.9''' December-May trends (left) and the contribution of the SAM to the trends (right). Top, 22-year (1979-2000) linear trends in 500 hPa geopotential height. Bottom: 32-year (1969-2000) linear trends in surface temperature and 22-year (1979-2000) linear trends in 925 hPa winds. Shading is drawn at 10 m per 30 years for 500 hPa height and at increments of 0.5&ordm; K per 30 years for surface temperature. The longest vector corresponds to about 4 m/s. From Thompson and Solomon (2002<ref name="Thompson and Solomon, 2002">Thompson, D. and Solomon, S. 2002. Interpretation of recent southern hemisphere climate change, ''Science'', '''296'''(5569), 895-899.</ref>).]]<br />
Thompson and Solomon (2002<ref name="Thompson and Solomon, 2002">Thompson, D. and Solomon, S. 2002. Interpretation of recent southern hemisphere climate change, ''Science'', '''296'''(5569), 895-899.</ref>) considered the surface temperature trends over 1969-2000 and showed that the contribution of the SAM was a warming over the Antarctic Peninsula and a cooling along the coast of East Antarctica ([[:File:Figure 4.9 - Trends in December-May Z500, surface temperature and 925hPa winds, and SAM contribution.png|Figure 4.9]]). They only considered the months of December to May, which was when the largest change in the SAM has taken place. They attributed the trends primarily to changes in the polar vortex as a result of the development of the Antarctic ozone hole. While the major loss of stratospheric ozone occurs in the spring, the greatest changes in the tropospheric circulation, such as the strengthening of the westerlies, has been in the summer and autumn. There is therefore a downward propagation of the vortex strengthening, with it starting in the spring in the stratosphere and moving down through the troposphere to the surface through the summer and autumn. As discussed earlier, the warming on the eastern side of the Antarctic Peninsula has been linked to the stronger westerlies associated with the changes in the SAM. However, the large winter-season warming on the western side of the peninsula appears to be largely independent of changes in the SAM.<br />
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It is very important to determine the surface temperature trends across the high Antarctic plateau, but as noted earlier, there are only two stations with long temperature records. Since the mid-1980s many AWSs have been deployed in the interior, filling important gaps in the observational network. These can provide valuable indications of temperature trends at remote locations, although few AWS systems have been maintained at the same locations since the 1980s and there can be gaps in the data when systems fail during the winter.<br />
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Another means of examining temperature trends is via the infra-red imagery from the polar orbiting satellites. Such imagery can only be used under cloud-free conditions, and provides data on the snow surface rather than at the standard meteorological level of 2 m above the surface, but with such high spatial coverage it provides a very valuable supplement to the ''in-situ'' observations. Comiso (2000<ref name="Comiso, 2000">Comiso, J.C. 2000. Variability and trends in Antarctic surface temperatures from ''in situ'' and satellite infrared measurements, ''Journal of Climate'', '''13''', 1674-1696.</ref>) used the NOAA AVHRR imagery to investigate the trends in skin temperature across the Antarctic over the period 1979-1998. The satellite-derived temperatures were compared with the ''in-situ'' observations from 21 stations and found to be in good agreement with a correlation coefficient of 0.98. The trends showed a cooling across much of the high plateau of East Antarctica and across Marie Byrd Land (-0.1 to -0.2&ordm; C/yr), with the former being consistent with the trends derived by Thompson and Solomon (2002<ref name="Thompson and Solomon, 2002">Thompson, D. and Solomon, S. 2002. Interpretation of recent southern hemisphere climate change, ''Science'', '''296'''(5569), 895-899.</ref>), although the Comiso trends are for annual data and the Thompson and Solomon study is concerned only with the summer and autumn.<br />
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In recent decades many relatively short ice cores have been drilled across Antarctica by initiatives such as the International Trans Antarctic Science Expedition (ITASE) (Mayewski et al, 2006<ref name="Mayewski et al, 2006">Mayewski, P.A. Et AL. 2006. The International Trans-Antarctic Scientific Expedition (ITASE): An overview, ''Annals of Glaciology'', '''41''', 180-185.</ref>). These provide data over roughly the last 200 years and therefore provide a good overlap with the instrumental data. Large-scale calibrations have been carried out between satellite-derived surface temperature and ITASE ice core proxies (Schneider et al., 2006<ref name="Schneider et al, 2006">Schneider, D.P., Steig, E.J., Van Ommen, T.D., Dixon, D.A., Mayewski, P.A., Jones, J.M. and Bitz, C.M. 2006. Antarctic temperatures over the past two centuries from ice cores, ''Geophysics Research Letters'', '''33''', L16707, doi:10.1029/2006GL027057.</ref>). Their reconstruction of Antarctic mean surface temperatures over the past two centuries was based on water stable isotope records from high-resolution, precisely dated ice cores. The reconstructed temperatures indicated large interannual to decadal scale variability, with the dominant pattern being anti-phase anomalies between the main Antarctic continent and the Antarctic Peninsula region, which is the classic signature of the SAM. The reconstruction suggested that Antarctic temperatures had increased by about 0.2&ordm; C since the late nineteenth century. They found that the SAM was a major factor in modulating the variability and the long-term trends in the atmospheric circulation of the Antarctic.<br />
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==Upper air temperature changes==<br />
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[[File:Figure 4.10 - Annual and seasonal 500hPa temperature trends for 1971-2003.png|thumb|'''4.10''' Annual and seasonal 500 hPa (approximately at 5 km above mean sea level) temperature trends for 1971-2003. From Turner et al. (2006<ref name="Turner et al, 2006">Turner, J., Lachlan-Cope, T.A., Colwell, S.R., Marshall, G.J. and Connolley, W.M. 2006. Significant warming of the Antarctic winter troposphere, ''Science'', '''311''', 1914-1917.</ref>).]]<br />
Analysis of Antarctic radiosonde temperature profiles indicates that there has been a warming of the troposphere and cooling of the stratosphere over the last 30 years. This is the pattern of change that would be expected from increasing greenhouse gases, however, the mid-tropospheric warming in winter is the largest on Earth at this level. The data show that regional mid-tropospheric temperatures have increased most around the 500 hPa level with statistically significant changes of 0.5 &ndash; 0.7&ordm;C/decade ([[:File:Figure 4.10 - Annual and seasonal 500hPa temperature trends for 1971-2003.png|Figure 4.10]]) (Turner et al., 2006<ref name="Turner et al, 2006">Turner, J., Lachlan-Cope, T.A., Colwell, S.R., Marshall, G.J. and Connolley, W.M. 2006. Significant warming of the Antarctic winter troposphere, ''Science'', '''311''', 1914-1917.</ref>). [[:File:Figure 4.10 - Annual and seasonal 500hPa temperature trends for 1971-2003.png|Figure 4.10]] indicates warming at many of the radiosonde stations around the continent, but a clear pattern of winter warming is apparent around the coast of East Antarctica and at the pole.<br />
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The warming is represented in the ECMWF 40 year reanalysis, which is not surprising since the radiosonde ascents were assimilated into the system. In fact the warming trends are slightly larger than when computed from the radiosonde data, since there is a known slight cold bias in the early part of the reanalysis data set.<br />
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The exact reason for such a large mid-tropospheric warming is not known at present. However, it has recently been suggested that it may, at least in part, be a result of greater amounts of polar stratospheric cloud (PSC) during the winter (Lachlan-Cope et al., In press). PSCs are a feature of the cold Antarctic winter, forming at temperatures below about -78&ordm; C. However, the Antarctic stratosphere has cooled in recent decades because the greenhouse gas ozone is now missing from the lower stratosphere in spring, and the greenhouse gas carbon dioxide is concentrated in the troposphere and leads to further cooling of the stratosphere. Analysis of stratospheric temperatures in the reanalysis data sets suggest that over the last 30 years the area where PSCs might form in winter has increased in size, so promoting the formation of more PSCs. Once present, PSCs act like any other cloud, giving a warming below their level and cooling above. We have little data on the optical properties of PSCs, but modelling suggests that if the optical depth in the infrared is around 0.5 then a greater amount of PSCs could give a mid-tropospheric warming.<br />
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PSCs are not currently represented explicitly in climate models, but if further research shows that they are responsible for the large winter season mid-tropospheric warming they need to be represented more realistically in the models.<br />
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==Attribution of change==<br />
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Great advances have been made in our understanding of recent temperature changes across the Antarctic in the last few years. We now know that anthropogenic activity, and particularly the presence of the Antarctic ozone hole, has played a large part in the near-surface warming on the eastern side of the Antarctic Peninsula, and a formal attribution study (Gillett et al., 2009<ref name="Gillett et al, 2009">Gillett, N.P., Stone, D.A., Stott, P.A., Nozawa, T., Karpechko, A.Y., Hegerl, G.C., Wehner, M.F. and Jones, P.D. 2009. Attribution of polar warming to human influence, ''Nature Geoscience'', '''1''', 760-764.</ref>) found that that recent changes were not consistent with internal climate variability. However, we still do not know the reasons for the large winter season warming on the western side, although it is thought to be linked to a reduction in sea ice extent since the 1950s.<br />
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The recently discovered large mid-tropospheric warming above the continent in winter is not fully understood at present. If increasing amounts of PSCs are shown to be responsible this will be an interesting Antarctic amplification of the effect of greenhouse gas increases, along with being a side effect of the ozone hole. However, more research is needed to confirm this.<br />
==References==<br />
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[[Category:The instrumental period]]<br />
[[Category:Temperature]]<br />
[[Category:The Antarctic atmosphere]]</div>Maintenance scripthttp://acce.scar.org/wiki/Small-scale_processes_in_the_Southern_OceanSmall-scale processes in the Southern Ocean2014-08-06T14:33:59Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[The Southern Ocean in the instrumental period]]''<br />
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The importance of small-scale processes in surface, bottom and lateral boundary layers is well known. In ice-covered oceans the surface boundary layer is of particular interest (McPhee, 2009) since it controls the heat and momentum exchange between ice and ocean. These exchanges affect ice growth, melt and motion as well as the heat budget of the ocean and the currents (McPhee et al., 2008<ref name="McPhee et al, 2008">McPhee, M.G., Morison, J.H. and Nilsen, F. 2008. Revisiting heat and salt exchange at the ice-ocean interface: Ocean flux and modelling considerations, ''Journal of Geophysical Research'', '''113''', C06014, doi: 10.1029/2007JC004383.</ref>). Appropriate quantification of such processes is a prerequisite for successful modelling the large scale conditions.<br />
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For a long time small-scale processes related to ocean turbulence in the interior were almost neglected in the consideration of large-scale ocean conditions. This resulted from lacking appropriate observation techniques and theoretical concepts for relating small-scale to large-scale processes. Normally a simple energy cascade providing turbulent energy from larger scale ocean current shear to small-scale mixing was assumed, which could be applied in large-scale models by properly selected mixing parameters. However, improved measurements of turbulence suggest that the amount of turbulent energy is much larger in the interior than previously assumed. There, internal waves, not only generated at the continental slope, but as well over rough bottom topography, play a major role in generating intensive mixing in the interior away from the well-known strongly mixed boundary layers. The intensity of internal mixing is so high, that it has to be taken into account to quantitatively understand the meridional overturning circulation. The understanding of spatially and timely important variations in the intensity of mixing and its significant role for large-scale processes prompted research with the aim of detecting by observations whether changes in small-scale processes could affect large-scale conditions i.e. the oceans role in climate.<br />
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It has been estimated (e.g., Wunsch, 1998<ref name="Wunsch, 1998">Wunsch, C. 1998. The work done by the wind on the oceanic general circulation, ''J. Phys. Oceanogr'', '''28''', 2332-2340.</ref>) that up to one-third of the energy required to drive the global ocean&rsquo;s overturning circulation (2-3 TW, see Wunsch and Ferrari (2004<ref name="Wunsch and Ferrari, 2004">Wunsch, C. and Ferrari, R. 2004. Vertical mixing, energy, and the general circulation of the oceans, ''Ann. Rev. Fluid Mech.'', '''36''', 281-314.</ref>) for a review) stems from the work done by the wind on the ACC, and that the bulk of that energy is transferred to the mesoscale eddy field. Current conceptual and numerical models of the Southern Ocean overturning circulation (see Rintoul et al. (2001<ref name="Rintoul et al, 2001">Rintoul, S.R., Hughes, C.W. and Olbers, D. 2001. The Antarctic Circumpolar Current system. In: Eds. G. Siedler, J. Church and J. Gould, Ocean circulation and climate; observing and modelling the global ocean. International Geophysics Series, 77, 271-302, Academic Press.</ref>) and Olbers et al. (2004<ref name="Olbers et al, 2004">Olbers, D., Borowski, D., V&ouml;lker, C. and Wolff, J.-O. 2004. The dynamical balance, transport and circulation of the Antarctic Circumpolar Current, ''Antarct. Sci.'', '''16''', 439-470.</ref>) for two reviews) unanimously highlight the eddies&rsquo; crucial role in transporting water masses and tracers along the sloping isopycnals (surfaces of constant density) of the ACC &mdash; particularly in the upper overturning cell &mdash; as well as in transporting the momentum input by the wind to the level of topographic obstacles, such as ridges, where it may be transferred to the solid Earth.<br />
<br />
Physical processes occurring on length scales on which earth rotation is of similar importance to ocean stratification (the baroclinic Rossby radius of deformation), which are smaller than 5-20 km south of 40&ordm;S, (see Chelton et al., 1998<ref name="Chelton et al, 1998">Chelton, D.B., Deszoeke, R.A., Schlax, M.G., EL Naggar, K. and Siwertz, N. 1998. Geographical variability of the first-baroclinic Rossby radius of deformation, ''J. Phys. Oceanogr.'', '''28''', 433-460.</ref>) play an important role in shaping and driving the circulation of the Southern Ocean. To represent ocean properties realistically measurements and models would need to resolve this scale. However this is normally not possible and the lack of resolution has to be compensated for by assumptions about processes at this scale and below.<br />
<br />
For example, formation of the AABW filling the deepest layers of much of the global ocean abyss is crucially dependent on the convective (i.e. small scale) production of dense, high-salinity shelf waters over the Antarctic continental shelves during periods of sea ice growth (Morales Maqueda et al., 2004<ref name="Maqueda et al, 2004">Morales Maqueda, M.A., Willmott, A.J. and Biggs, N.R.T. 2004. Polynya dynamics: A review of observations and modelling, Rev. Geophys., 42, doi:10.1029/2002RG000116.</ref>), as well as on the heat and freshwater exchanges between those waters and the adjacent ice shelves (e.g. Hellmer, 2004<ref name="Hellmer, 2004">Hellmer, H.H. 2004. Impact of Antarctic shelf basal melting on sea ice and deep ocean properties, Geophys. Res. Lett., 31, doi:10.1029/2004GL019506.</ref>). The properties of Antarctic shelf waters are modified further by turbulent mixing associated with the dissipation of tidal energy taking place over the continental shelves fringing the continent (Egbert and Ray, 2003<ref name="Egbert and Ray, 2003">Egbert, G.D. and Ray, R.D. 2003. Semi-diurnal and diurnal tidal dissipation from TOPEX/Poseidon altimetry, Geophys. Res. Lett., 30, doi:10.1029/2003GL017676.</ref>), including the vicinity of the ice shelf front (Makinson et al., 2006<ref name="Makinson et al, 2006">Makinson, K., Schr&ouml;der, M. and Osterhus, S. 2006. Effect of critical latitude and seasonal stratification on tidal current profiles along Ronne Ice Front, Antarctica, J. Geophys. Res., 111, doi:10.1029/2005JC003062.</ref>) and sub-ice-shelf cavities (Makinson, 2002<ref name="Makinson, 2002">Makinson, K. 2002. Modeling tidal current profiles and vertical mixing beneath Filchner-Ronne Ice Shelf, Antarctica, ''J. Phys. Oceanogr.'', '''32''', 202-215.</ref>). A variety of small-scale processes underlies the conversion of tidal energy into turbulence, most prominent are the breaking of internal gravity waves generated by tidal flows impinging on rough ocean-floor topography (e.g., Robertson et al., 2003<ref name="Robertson et al, 2003">Robertson, R., Beckmann, A. and Hellmer, H. 2003. M<sub>2</sub> tidal dynamics in the Ross Sea, ''Antarct. Sci.'', '''15''', 41-46.</ref>), and the formation of thick frictional boundary layers (e.g., Makinson, 2002<ref name="Makinson, 2002">Makinson, K. 2002. Modeling tidal current profiles and vertical mixing beneath Filchner-Ronne Ice Shelf, Antarctica, ''J. Phys. Oceanogr.'', '''32''', 202-215.</ref>). The importance of these processes is accentuated by the proximity of the Antarctic continental shelves to the critical latitude (where the tidal frequency is equal to the one of inertial oscillations imposed on ocean flow by the Earth&rsquo;s rotation) of the dominant tidal constituent (M<sub>2</sub>), near which the generation of internal tides and frictional boundary layers is most efficient (Robertson, 2001<ref name="Robertson, 2001">Robertson, R. 2001. Internal tides and baroclinicity in the southern Weddell Sea: Part II: Effects of the critical latitude and stratification, ''J. Geophys. Res.'', '''106''', 27017-27034.</ref>; Pereira et al., 2002<ref name="Pereira et al, 2002">Pereira, A.F., Beckmann, A. and Hellmer, H.H. 2002. Tidal mixing in the southern Weddell Sea: Results from a three-dimensional model, ''J. Phys. Oceanogr.'', '''32''', 2151-2170.</ref>). In addition to their role in moulding the properties of shelf waters, tidal fluctuations regulate the flow of those waters across the ice shelf front (Nicholls et al., 2004<ref name="Nicholls et al, 2004">Nicholls, K.W., Makinson, K. and Osterhus, S. 2004. Circulation and water masses beneath the northern Ronne Ice Shelf, Antarctica, J. Geophys. Res., 109, doi:10.1029/2004JC002302.</ref>) and the continental shelf break (e.g. Gordon et al., 2004<ref name="Gordon et al, 2004">Gordon, A. L., Zambianchi, E., Orsi, A., Visbeck, M., Giulivi, C. F., Whitworth III, T. and Spezie, G. 2004. Energetic plumes over the western Ross Sea continental slope, Geophys. Res. Lett., 31, doi: 10.1029/2004GL020785.</ref>), often steered by local topographic features such as canyons. On descending the continental slope in broad sheets or narrow plumes, shelf waters tend to focus in largely geostrophic boundary currents that entrain ambient surface and intermediate waters and detrain ventilated fluid in the offshore direction (Baines and Condie, 1998<ref name="Baines and Condie, 1998">Baines, P.G. and Condie, S. 1998. Observations and modelling of Antarctic downslope flows: a review. In Ocean, Ice and Atmosphere: Interactions at the Antarctic Continental Margin, AGU Antarctic Research Series Vol. 75, S.S. Jacobs and R. Weiss editors, 29-49.</ref>; Hughes and Griffiths, 2006<ref name="Hughes and Griffiths, 2006">Hughes, G.O. and Griffiths, R.W. 2006. A simple convective model of the global overturning circulation, including effects of entrainment into sinking regions, ''Ocean Model.'', '''12''', 46-79.</ref>). The shelf waters&rsquo; descent is promoted further by other smaller scale processes, such as double-diffusion (instabilities due to different diffusivities of heat and salt) and interleaving (layer formation) across the shelf break zone (Foster and Carmack, 1976<ref name="Foster and Carmack, 1976">Foster, T.D. and Carmack, E.C. 1976. Temperature and salinity structure in the Weddell Sea, ''J. Phys. Oceanogr.'', '''6''', 36-44.</ref>; Foster, 1987<ref name="Foster, 1987">Foster, T.D. 1987. Mixing and bottom water formation in the shelf break region of the southern Weddell Sea, Deep-Sea Res. 34, 1771-1794.</ref>), as well as nonlinearities in the equation of state (thermobaricity and cabbeling) and instabilities of the Antarctic Slope Front, which have been reported to generate cyclonic eddies effecting a net downward transport of shelf water (see Baines and Condie, 1998<ref name="Baines and Condie, 1998">Baines, P.G. and Condie, S. 1998. Observations and modelling of Antarctic downslope flows: a review. In Ocean, Ice and Atmosphere: Interactions at the Antarctic Continental Margin, AGU Antarctic Research Series Vol. 75, S.S. Jacobs and R. Weiss editors, 29-49.</ref> for a review). In subsequent stages of its northward journey, the newly formed AABW navigates numerous topographic obstacles and, in so doing, undergoes profound modification due to vigorous turbulent mixing with overlying water masses (e.g. Heywood et al., 2002<ref name="Heywood et al, 2002">Heywood, K.J., Naveira Garabato, A.C. and Stevens, D.P. 2002. High mixing rates in the abyssal Southern Ocean, ''Nature'', '''415''', 1011-1014.</ref>). A large fraction of this modification is likely driven by flows over small sills in confined passages (Bryden and Nurser, 2003<ref name="Bryden and Nurser, 2003">Bryden, H. L. and Nurser, A.J.G. 2003. Effects of strait mixing on ocean stratification, ''J. Phys. Oceanogr.'', '''33''', 1870-1872.</ref>) and mid-ocean ridge-flank canyons (Thurnherr and Speer, 2003<ref name="Thurnherr and Speer, 2003">Thurnherr, A.M. and Speer, K.G. 2003. Boundary mixing and topographic blocking on the Mid-Atlantic Ridge in the South Atlantic, ''J. Phys. Oceanogr.'', '''33''', 848-862.</ref>), although the breaking of internal tides (Simmons et al., 2004<ref name="Simmons et al, 2004">Simmons, H., Hallberg, R. and Arbic, B. 2004. Internal wave generation in a global baroclinic tide model, ''Deep-Sea Res. II'', '''51''', 3043-3068.</ref>) and internal lee waves (Naveira Garabato et al., 2004<ref name="Garabato et al, 2004">Naveira Garabato, A.C., Polzin, K.L., King, B.A., Heywood, K.J. and Visbeck, M. 2004. Widespread intense turbulent mixing in the Southern Ocean, ''Science'', '''303''', 210-213.</ref>) must contribute significantly too.<br />
<br />
[[File:Figure 4.28 - Vertical distribution of turbulent diapycnal diffusivity along the rim of the Scotia Sea.png|thumb|'''4.28''' The level of turbulent energy is high over rough bottom topography up to a distance of several hundred kilometres and down into the deep ocean. As an indicator of turbulent mixing the vertical distribution of log<sub>10</sub> of the turbulent diapycnal diffusivity (in m<sup>2</sup>/sec) which can be measured (top), is displayed along a section following the rim of the Scotia Sea anticlockwise (green dots on the bottom graph). Density surfaces separating water masses (AAIW, Antarctic Intermediate Water; UCDW and LCDW, Upper and Lower Circumpolar Deep Water; AABW, Antarctic Bottom Water) are shown by the thick dashed lines. Crossings of the two main frontal jets of the ACC (SAF, Subantarctic Front; PF, Polar Front) and its southern boundary (SB) are marked in the upper axis. Station positions are indicated by tickmarks at the base of the topography. Reproduced from Naveira Garabato et al. (2004<ref name="Garabato et al, 2004">Naveira Garabato, A.C., Polzin, K.L., King, B.A., Heywood, K.J. and Visbeck, M. 2004. Widespread intense turbulent mixing in the Southern Ocean, ''Science'', '''303''', 210-213.</ref>).]]<br />
While theoretical considerations (Marshall and Naveira Garabato, 2007) and altimetric observations of an energy cascade from smaller to larger motions in the Southern Ocean (Scott and Wang, 2005<ref name="Scott and Wang, 2005">Scott, R.B. and Wang, F. 2005. Direct evidence of an oceanic inverse kinetic energy cascade from satellite altimetry, ''J. Phys. Oceanogr.'', '''35''', 1650-1666.</ref>) endorse the view that much of the eddy field&rsquo;s energy is ultimately transferred toward the ocean floor and dissipated by viscous bottom drag, measurements of oceanic fine structure (Naveira Garabato et al., 2004<ref name="Garabato et al, 2004">Naveira Garabato, A.C., Polzin, K.L., King, B.A., Heywood, K.J. and Visbeck, M. 2004. Widespread intense turbulent mixing in the Southern Ocean, ''Science'', '''303''', 210-213.</ref>; Sloyan, 2005<ref name="Sloyan, 2005">Sloyan, B.M. 2005. Spatial variability of mixing in the Southern Ocean, Geophys. Res. Lett., 32, doi:10.1029/2005GL023568.</ref>; Kunze et al., 2006<ref name="Kunze et al, 2006">Kunze, E., Firing, E., Hummon, J.M., Chereskin, T.K. and Thurnherr, A.M. 2006. Global abyssal mixing inferred from lowered ADCP shear and CTD strain profiles, ''J. Phys. Oceanogr.'', '''36''', 1553-1576.</ref>) suggest that a substantial fraction of the energy is dissipated via the generation and breaking of internal lee waves. These waves induce intense turbulent mixing ([[:File:Figure 4.28 - Vertical distribution of turbulent diapycnal diffusivity along the rim of the Scotia Sea.png|Figure 4.28]]) and, in so doing, contribute to driving the lower cell of the Southern Ocean overturning. It thus appears that the upper and lower overturning cells, often treated as largely independent entities in descriptions of the ocean circulation, may be strongly coupled by smaller scale processes. This proposition is consistent with the concurrent intensification of eddy-driven upwelling along inclined surfaces of constant water density, and turbulent mixing of superimposed water layers of different density (diapycnal mixing) in ACC regions of complex topography. In this view, eddy dampening - which is required to sustain a vigorous meridional circulation in the upper ocean - may be connected to the topographic generation of internal lee waves in the abyss (Naveira Garabato et al., 2007<ref name="Garabato et al, 2007">Naveira Garabato, A.C., Stevens, D.P., Watson, A.J. and Roether, W. 2007. Short-circuiting of the overturning circulation in the Antarctic Circumpolar Current, ''Nature'', '''447''', 194-197.</ref>). Although the patchy indirect evidence available to date points to topographic generation as the key agent in the transfer of eddy energy to the internal wave field in the ACC ([[:File:Figure 4.28 - Vertical distribution of turbulent diapycnal diffusivity along the rim of the Scotia Sea.png|Figure 4.28]]), other mechanisms are likely to enhance this transfer e.g. interaction between internal waves and mesoscale eddies (Polzin, 2007<ref name="Polzin, 2007">Polzin, K.L. 2007. How Rossby Waves Break. Results from POLYMODE and the end of the enstrophy cascade, J. Phys. Oceanogr., in press.</ref>) and the generation of internal waves by unstable mesoscale processes (Molemaker et al., 2005<ref name="Molemaker et al, 2005">Molemaker, J., McWilliams, J.C. and Yavneh, I. 2005. Baroclinic instability and loss of balance, J. Phys. Oceanogr. 35, 1505-1517.</ref>). The occurrence of these processes is indicated by altimetric evidence of an energy cascade at rather small scales in the Pacific sector of the ACC (Scott and Wang, 2005<ref name="Scott and Wang, 2005">Scott, R.B. and Wang, F. 2005. Direct evidence of an oceanic inverse kinetic energy cascade from satellite altimetry, ''J. Phys. Oceanogr.'', '''35''', 1650-1666.</ref>). Nonetheless, it appears that diapycnal mixing in these upper layers of the ACC is primarily driven by the breaking of downward-travelling near-inertial internal waves generated by the wind in the upper-ocean mixed layer (Alford, 2003<ref name="Alford, 2003">Alford, M.H. 2003. Improved global maps and 54-year history of wind-work on ocean inertial motions. Geophys. Res. Lett., 30, doi:10.1029/2002GL016614.</ref>).<br />
<br />
The key point emerging is that relatively small scale processes in the Southern Ocean exert an important influence on the large-scale behaviour of global ocean circulation over a wide range of time scales of climatic significance. This is a highly significant conclusion when one considers that these processes and interactions are often absent, or parameterized with coefficients tuned to the present ocean state, in the models used to simulate the ocean&rsquo;s evolution (see e.g. Wunsch and Ferrari (2004<ref name="Wunsch and Ferrari, 2004">Wunsch, C. and Ferrari, R. 2004. Vertical mixing, energy, and the general circulation of the oceans, ''Ann. Rev. Fluid Mech.'', '''36''', 281-314.</ref>) for a discussion).<br />
==References==<br />
<references /><br />
[[Category:The instrumental period]]<br />
[[Category:The Southern Ocean]]</div>Maintenance scripthttp://acce.scar.org/wiki/Sea_level_observationsSea level observations2014-08-06T14:33:59Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Observations, data accuracy and tools]]''<br />
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==Monitoring sea level==<br />
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As Antarctica could play a potentially large role in 21<sup>st</sup> century sea level change, it is disappointing that we have such poor knowledge of 20<sup>th</sup> century and present-day rates of change of sea level around the continent itself. Of course, this situation is due primarily to the great difficulty of acquiring extended time series of sea level measurements in environmentally hostile areas to the same standard as is possible elsewhere. Most sea level measurements during the 19<sup>th</sup> and 20<sup>th</sup> centuries were made with float and stilling well gauges, technology which presented operational problems in Antarctica. In some locations, there were also major issues to do with datum control (e.g. the establishment of adequate local benchmark networks and maintenance of tide gauge calibration with respect to those marks in local surveys conducted during brief annual visits).<br />
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[[File:Figure 2.28 - Map of the main Antarctic tide gauges.png|thumb|'''2.28''' Main Antarctic tide gauges described in the text.]]<br />
While many short tide gauge measurements have been made around Antarctica, primarily for the determination of tidal parameters (e.g. IHB, 2002<ref name="IHB, 2002">IHB, 2002. Status of natutical charting (part A) and hydrographic surveying (part B) in Antarctica, International Hydrographic Bureau publication S-59. (see also previous editions of this publication with the same number.)</ref>), there are few records that satisfy the quality criteria required by the Permanent Service for Mean Sea Level (Woodworth and Player, 2003<ref name="Woodworth and Player, 2003">Woodworth, P.L. and Player, R. 2003. The Permanent Service for Mean Sea Level: an update to the 21<sup>st</sup> century, ''Journal of Coastal Research'', '''19''', 287-295.</ref>) and that are long enough to be of interest for long-term change studies. The outstanding record is that from Vernadsky station (formerly Faraday station) in the Argentine Islands on the western side of the Antarctic Peninsula ([[:File:Figure 2.28 - Map of the main Antarctic tide gauges.png|Figure 2.28]]). It is now operated by the National Antarctic Scientific Center of Ukraine and contains a conventional float and stilling well gauge maintained in collaboration with the Proudman Oceanographic Laboratory. Hourly sea levels are measured by means of a paper chart recorder, with datum control provided by daily comparisons of tide gauge and tide pole observations. The sea level record from this venerable gauge commenced in 1958, the equipment having been installed at the then British Antarctic Survey Faraday base during the International Geophysical Year, thereby providing the longest sea level time series in Antarctica. The gauge received a major upgrade in the early 1990s when a pressure sensor gauge was added, and a new pressure sensor gauge with satellite transmission capability was installed in 2007.<br />
<br />
The PSMSL data catalogue (www.pol.ac.uk/psmsl) provides one list of sea level data available from Antarctica. Notable records can be found from the Japanese Syowa base from the mid-1970s and from the three Australian bases of Mawson, Davis and Casey from the early 1990s. Other long term records are known to exist that are as yet not included in international data banks. A particularly interesting one, as it is far from the other long records mentioned above, is from the New Zealand Scott base and has been acquired since 2001 with the use of a bubbler pressure gauge attached to the reverse osmosis water pipe for the base, around which there is a permanent gap in the sea ice. France has made major efforts to instrument Dumont D&rsquo;Urville. This station has been operated since 1997 with gaps and various upgrades, and is currently being updated to real time transmission as part of the Indian Ocean Tsunami Warning System. Details of this and other gauges operated by various nations in Antarctica are often included in the national reports of the Global Sea Level Observing System (GLOSS: IOC, 1997<ref name="IOC, 1997">IOC. 1997. Global Sea Level Observing System (GLOSS) implementation plan-1997. Intergovernmental Oceanographic Commission, Technical Series, No. 50, 91pp. and Annexes.</ref>; Woodworth et al., 2003<ref name="Woodworth et al, 2003">Woodworth, P.L., Aarup, T., Merrifield, M., Mitchum, G.T. and Le Provost, C. 2003. Measuring progress of the Global Sea Level Observing System, EOS, Transactions of the American Geophysical Union, 84(50), 16 December 2003, 565, 10.1029/2003EO500009.</ref>) (see www.gloss-sealevel.org). In addition, a list of Antarctic stations with tide gauges is maintained by the Scientific Committee on Antarctic Research (SCAR) (www.geoscience.scar.org/geodesy/perm_ob/tide/tide.htm). However, an important point to make about Antarctic data is that very little of it is downloadable and as readily analysable as data from elsewhere. In particular, much data have been obtained with pressure sensors, which are subject to drifts and biases. Any analyst must consider carefully the possible data problems and the impacts on the application to which they are put.<br />
<br />
==Sea Level Data for Ocean Circulation Studies==<br />
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One application of sea level data is in ocean circulation studies, for which analysts usually require sub-surface pressure (SSP) rather than sea level itself. SSP can be obtained at a tide gauge site either with the use of a shallow-water pressure sensor, or by adding local air pressures to the data from a gauge (e.g. float or acoustic) that records true sea level. An alternative to coastal equipment in such studies is provided by bottom pressure recorders (BPRs), which have been employed in the Drake Passage and at other Antarctic locations at various times since the International Southern Ocean Studies (ISOS) programme (Whitworth and Peterson, 1985<ref name="Whitworth and Peterson, 1985">Whitworth, T. and Peterson, R.G. 1985. Volume transport of the Antarctic Circumpolar Current from bottom pressure measurements, ''Journal of Physical Oceanography'', '''15''', 810-816.</ref>), and more recently since the World Ocean Circulation Experiment of the 1990s (Spencer and Vassie, 1997<ref name="Spencer and Vassie, 1997">Spencer, R. and Vassie, J.M. 1997. The evolution of deep ocean pressure measurements in the U.K, ''Progress in Oceanography'', '''40''', 423-435.</ref>; Woodworth et al., 2002<ref name="Woodworth et al, 2002">Woodworth, P.L., Le Provost, C., Rickards, L.J., Mitchum, G.T. and Merrifield, M. 2002. A review of sea-level research from tide gauges during the World Ocean Circulation Experiment, Oceanography and Marine Biology: An Annual Review, 40, 1-35.</ref>). Many of these deployments have been by UK groups and most records are readily available for analysis via www.pol.ac.uk/ntslf/acclaimdata/bprs.<br />
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Antarctic sea level data have great importance in understanding the variability in the Antarctic Circumpolar Current (ACC), and thereby the role of the ACC in the global climate and, ultimately, in sea level change itself. A series of papers (Woodworth et al., 1996<ref name="Woodworth et al, 1996">Woodworth, P.L., Vassie, J.M., Hughes, C.W. and Meredith, M.P. 1996. A test of the ability of TOPEX/POSEIDON to monitor flows through the Drake Passage, ''Journal of Geophysical Research'', '''101'''(C5), 11935-11947.</ref>; Hughes et al., 1999<ref name="Hughes et al, 1999">Hughes, C.W., Meredith, M.P. and Heywood, K. 1999. Wind-driven transport fluctuations through Drake Passage: a Southern mode, ''Journal of Physical Oceanography'', '''29''', 1971-1992.</ref>; Aoki, 2002<ref name="Aoki, 2002">Aoki, S. 2002. Coherent sea level response to the Antarctic Oscillation. Geophysical Research Letters, 29(20), 1950, doi:10.1029/2002GL015733.</ref>; Hughes et al., 2003<ref name="Hughes et al, 2003">Hughes, C.W., Woodworth, P.L., Meredith, M.P., Stepanov, V., Whitworth, T. and Pyne A.R. 2003. Coherence of Antarctic sea levels, Southern Hemisphere Annular Mode, and flow through Drake Passage, ''Geophysical Research Letters'', '''30'''(9), 1464, doi:10.1029/2003GL017240.</ref>; Meredith et al., 2004<ref name="Meredith et al, 2004">Meredith, M.P., Woodworth, P.L., Hughes, C.W. and Stepanov, V. 2004. Changes in the ocean transport through Drake Passage during the 1980s and 1990s, forced by changes in the Southern Annular Mode, Geophysical Research Letters, 31(21), L21305, 10.1029/2004GL021169.</ref>) have demonstrated that SSP fluctuates similarly around the entire Antarctic continent and that the SSP fluctuations can be related to changes in the circumpolar ocean transport around Antarctica. SSP data can be obtained either from measurements by BPRs deployed to the south of the main ACC axis or from coastal gauges as described above. The relationship between SSP, ACC transport and the SAM applies at least on intra-seasonal timescales (i.e. periods of more than a month and less than a year but excluding the quasi-regular seasonal cycle). It also applies on inter-annual timescales, despite the presence of baroclinic variability in the ocean at these longer periods (Meredith et al., 2004<ref name="Meredith et al, 2004">Meredith, M.P., Woodworth, P.L., Hughes, C.W. and Stepanov, V. 2004. Changes in the ocean transport through Drake Passage during the 1980s and 1990s, forced by changes in the Southern Annular Mode, Geophysical Research Letters, 31(21), L21305, 10.1029/2004GL021169.</ref>). However, the relationship at longer (decadal) timescales remains to be tested. The importance of sea level data in monitoring the circumpolar transport around Antarctica became more apparent with the realisation that many of the other techniques commonly employed are subject to critical aliasing, resulting in unrealistically high estimates of variability (Meredith and Hughes, 2005<ref name="Meredith and Hughes, 2005">Meredith, M.P. and Hughes, C.W. 2005. On the sampling timescale required to reliably monitor interannual variability in the Antarctic circumpolar transport, Geophysical Research Letters, 32(3), L03609, 10.1029/2004GL022086.</ref>).<br />
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Major efforts have been made recently to provide sea level data from Antarctica in real-time, resulting in more rapid determination of ACC transport than has been possible to date (Woodworth et al., 2006<ref name="Woodworth et al, 2006">Woodworth, P.L., Hughes, C.W., Blackman, D.L., Stepanov, V.N., Holgate, S.J., Foden, P.R., Pugh, J.P., Mack, S., Hargreaves, G.W., Meredith, M.P., Milinevsky, G. and Fierro Contreras, J.J. 2006. Antarctic peninsula sea levels: a real time system for monitoring Drake Passage transport, ''Antarctic Science'', '''18'''(3), 429-436.</ref>). This development is also part of a general effort by GLOSS to have as many gauges as possible in the global network delivering data in real-time data, thereby enabling faults to be identified and corrected faster than would otherwise be the case. Rothera real-time data became available in 2007, while data from Vernadsky and King Edward Point, South Georgia in the South Atlantic will follow. The latter will largely replace an older installation at Signy, South Orkney Islands. All such UK data will be obtainable via www.pol.ac.uk/ntslf/acclaimdata. Data from Syowa are available in real-time from www1.kaiho.mlit.go.jp/KANKYO/KAIYO/jare/tide/tide_index.html, while data from Australian stations are available in &lsquo;fast&rsquo; rather than &lsquo;real time&rsquo; mode (i.e. with a short delay of typically 1-2 months).<br />
==References==<br />
<references /><br />
[[Category:Observations, data accuracy and tools]]<br />
[[Category:Sea level]]</div>Maintenance scripthttp://acce.scar.org/wiki/Sea_ice_over_the_last_million_yearsSea ice over the last million years2014-08-06T14:33:58Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[The last million years]]''<br />
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Sea ice is a crucial part of the Earth&rsquo;s climate system and its extent can be inferred for up to the past 220 ka from marine sediments (e.g. Crosta et al., 2004<ref name="Crosta et al, 2004">Crosta, X., Sturm, A., Armand, L. and Pichon, J.J. 2004. Late Qaternary sea ice history in the Indian sector of the Southern Ocean as record by diatom assemblages, ''Marine Micropaleontology'', '''50''', 209-223.</ref>) and for shorter periods from some ice cores (Curran et al., 2003<ref name="Curran et al, 2003">Curran, M.A.J., Van Ommen, T.D., Morgan, V.I., Phillips, K.L. and Palmer, A.S. 2003. Ice core evidence for Antarctic sea ice decline since the 1950s, ''Science'', '''302''', 1203-1206.</ref>; Abram et al., 2007a<ref name="Abram et al, 2007a">Abram, N.J., Mulvaney, R., Wolff, E. and Mudelsee, M. 2007a. Ice core records as sea ice proxies: An evaluation from the Weddell Sea region of Antarctica. Journal of Geophysical Research, 112:D15101.</ref>). Sea ice isolates the polar ocean from the atmosphere and inhibits the exchange of heat and moisture. The formation and melting of sea ice also changes the salinity of the cool surface waters of the polar oceans, so changing their density. This affects the global thermohaline circulation, which, in turn, influences climate around the globe. The high albedo of sea ice also means that it efficiently reflects incoming solar radiation, a process that acts as a positive climate feedback to amplify climate change. Growing sea ice cools the planet; decreasing sea ice warms the planet by exposing more dark and less reflective sea, so enabling more heat to be absorbed than reflected. At the same time decreasing sea ice may also enhance outgassing of CO<sub>2</sub> from the Southern Ocean &ndash; see Stephens and Keeling (2000<ref name="Stephens and Keeling, 2000">Stephens, B.B. and Keeling, R.F. 2000. The influence of Antarctic sea ice on glacial-interglacial CO<sub>2</sub> variations, ''Nature'', '''404''', 171-174.</ref>) &ndash; which may help to explain the close correlation of sea ice extent with the pattern of CO<sub>2</sub> in ice cores (see Crosta et al., 2004<ref name="Crosta et al, 2004">Crosta, X., Sturm, A., Armand, L. and Pichon, J.J. 2004. Late Qaternary sea ice history in the Indian sector of the Southern Ocean as record by diatom assemblages, ''Marine Micropaleontology'', '''50''', 209-223.</ref>).<br />
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==Sea ice extent==<br />
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[[File:Figure 3.17 - Sea ice distribution at the Southern Ocean EPILOG-LGM time slice.png|thumb|'''3.17''' Sea ice distribution at the Southern Ocean EPILOG-LGM (E-LGM) time slice. E-LGM-winter sea ice (E-LGM-WSI) indicates maximum extent of winter sea ice (September concentration &gt;15%). Modern winter sea ice (M-WSI) shows extent of &gt;15% September sea ice concentration according to Comiso et al. (2003<ref name="Comiso et al, 2003">Comiso, J.C., Cavalieri, D.J. and Markus, T. 2003. Sea ice concentration, ice temperature, and snow depth using AMSR-E data, ''IEEE Trans Geosci Remote Sensing'', '''41''', 243, doi:210.1109/TGRS.2002.808317.</ref>). Values indicate estimated winter (September) sea ice concentration in percent derived with Modern Analog Techniques and Generalized Additive Models. Signature legend: (1) concomitant occurrence of cold-water indicator diatom ''F. obliquecostata'' (&gt;1% of diatom assemblage) and summer sea ice (February concentration &gt;0%) interpreted to represent sporadic occurrence of ELGM summer sea ice; (2) presence of WSI (September concentration &gt;15%, diatom WSI indicators &gt;1%); (3) no WSI (September concentration &lt;15%, diatom WSI indicators &lt;1%) (Gersonde et al., 2005<ref name="Gersonde et al, 2005">Gersonde, R., Crosta, X., Abelmann, A. and Armand, L. 2005. Sea-surface temperature and sea ice distribution of the Southern Ocean at the EPILOG Last Glacial Maximum-a circum-Antarctic view based on siliceous microfossil records, ''Quaternary Science Reviews'', '''24''', 869-896.</ref>)]]<br />
Diatom assemblages from some marine sediment cores can be used to indicate whether or not the sea at the core locations was covered with sea ice in the past (Crosta et al., 2004<ref name="Crosta et al, 2004">Crosta, X., Sturm, A., Armand, L. and Pichon, J.J. 2004. Late Qaternary sea ice history in the Indian sector of the Southern Ocean as record by diatom assemblages, ''Marine Micropaleontology'', '''50''', 209-223.</ref>; Justwan and Ko&ccedil;a, 2008<ref name="Justwan and Ko&ccedil;a, 2008">Justwan, A. and Ko&ccedil;a, N. 2008. A diatom based transfer function for reconstructing sea ice concentrations in the North Atlantic, ''Marine Micropaleontology'', '''66''', 264-278.</ref>). Recently, a novel proxy for sea ice studies was established, the so-called IP<sub>25</sub> (Ice Proxy with 25 carbon atoms) produced by diatoms living in the sea ice (Belt et al., 2007<ref name="Belt et al, 2007">Belt S., Masse, G., Rowland, S.J., Poulin, M., Michel, C. and Leblanc, B. 2007. A novel chemical fossil of palaeo sea ice: IP<sub>25</sub>, ''Organic Geochemistry'', '''38''', 16-27.</ref>; Belt et al., 2008<ref name="Belt et al, 2008">Belt, S.T., Mass&eacute;, G., Vare, L.L., Rowland, S.J., Poulin, M., Sicre, M.-A., Sampei, M. and Fortier, L. 2008. Distinctive <sup>13</sup>C isotopic signature distinguishes a novel sea ice biomarker in Arctic sediments and sediment traps, ''Marine Chemistry'', '''112''', 158-167.</ref>). In the Arctic, sediments containing IP<sub>25</sub> have already been dated using radiocarbon methods to at least 9,000 yr (Belt et al., 2007<ref name="Belt et al, 2007">Belt S., Masse, G., Rowland, S.J., Poulin, M., Michel, C. and Leblanc, B. 2007. A novel chemical fossil of palaeo sea ice: IP<sub>25</sub>, ''Organic Geochemistry'', '''38''', 16-27.</ref>). Given reliable proxies and enough cores it is possible to approximately map the position of the sea ice edge back through time and to determine its summer and winter limits. This has been done for the Southern Ocean, for example at the LGM by Gersonde et al. (2005<ref name="Gersonde et al, 2005">Gersonde, R., Crosta, X., Abelmann, A. and Armand, L. 2005. Sea-surface temperature and sea ice distribution of the Southern Ocean at the EPILOG Last Glacial Maximum-a circum-Antarctic view based on siliceous microfossil records, ''Quaternary Science Reviews'', '''24''', 869-896.</ref>) see [[:File:Figure 3.17 - Sea ice distribution at the Southern Ocean EPILOG-LGM time slice.png|Figure 3.17]]. From this exercise it can be seen that at the LGM sea ice was double its present extent in winter; LGM sea ice cover was similarly double its present extent in summer due to greater extent off the Weddell Sea and possibly the Ross Sea (Gersonde et al., 2005<ref name="Gersonde et al, 2005">Gersonde, R., Crosta, X., Abelmann, A. and Armand, L. 2005. Sea-surface temperature and sea ice distribution of the Southern Ocean at the EPILOG Last Glacial Maximum-a circum-Antarctic view based on siliceous microfossil records, ''Quaternary Science Reviews'', '''24''', 869-896.</ref>). This value is however far less than the 5-fold estimate of CLIMAP (1981<ref name="CLIMAP, 1981">CLIMAP. 1981. Seasonal Reconstructions of the Earth's Surface at the Last Glacial Maximum. Map Chart Series MC-36, Geological Society of America, Boulder, CO.</ref>).<br />
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According to Gersonde et al. (2005<ref name="Gersonde et al, 2005">Gersonde, R., Crosta, X., Abelmann, A. and Armand, L. 2005. Sea-surface temperature and sea ice distribution of the Southern Ocean at the EPILOG Last Glacial Maximum-a circum-Antarctic view based on siliceous microfossil records, ''Quaternary Science Reviews'', '''24''', 869-896.</ref>) the LGM sea ice edge in the Atlantic and Indian sectors reached close to 47&ordm;S ([[:File:Figure 3.17 - Sea ice distribution at the Southern Ocean EPILOG-LGM time slice.png|Figure 3.17]]), which is in the modern Polar Frontal Zone and close to the Subantarctic Front that today defines the northern edge of the Antarctic Circumpolar Current. More data are needed to define the sea ice edge in the Pacific sector (Gersonde et al., 2005<ref name="Gersonde et al, 2005">Gersonde, R., Crosta, X., Abelmann, A. and Armand, L. 2005. Sea-surface temperature and sea ice distribution of the Southern Ocean at the EPILOG Last Glacial Maximum-a circum-Antarctic view based on siliceous microfossil records, ''Quaternary Science Reviews'', '''24''', 869-896.</ref>). Data for the extent of summer sea ice suggest that it was at least as extensive as at present, but with a larger than present summer sea ice extent in the Weddell Sea area. The related sea surface temperature calculations show that the Polar Front in the Atlantic, Indian and Pacific sectors would have shifted to the North during the LGM by around 4&ordm;, 5&ndash;10&ordm;, and 2&ndash;3&ordm; in latitude, respectively, compared with their present location. In the Atlantic and Indian sector, the Subantarctic Front would have shifted by around 4&ndash;5&ordm; and 4&ndash;10&ordm; in latitude, respectively. The Subtropical Front displacement would have been minor, by around 2&ndash;3&ordm; and 5&ordm; in latitude in the Atlantic and Indian sector. The net effect would be to steepen the oceanographic fronts in the Polar Frontal Zone, thereby speeding current flow in the jets along those fronts. A northerly displacement of the wind field is also implied.<br />
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[[File:Figure 3.18 - February SSTs and sea ice duration in core SO136-111 and insolation curves.png|thumb|'''3.18''' Comparison of February SSTs and sea ice duration in core SO136-111 with insolation curves. Parameters estimated by Modern Analog Technique 5201/31 are represented by black lines; insolation curves are represented by grey lines. (a) SSTs versus insolation at 65&deg;N for the 15th of June, (b) SSTs versus insolation at 15&deg;N for the 15th of June, (c) SSTs versus insolation at 15&deg;S for the 15th of January, (d) SSTs versus insolation at 60&deg;S for the 15th of January, (e) sea ice cover versus insolation at 65&deg;N for the 15th of June, (f) sea ice cover versus insolation at 15&deg;N for the 15th of June, (g) sea ice cover versus insolation at 15&deg;S for the 15th of January, (h) sea ice cover versus insolation at 60&deg;S for the 15th of January (Crosta et al., 2004<ref name="Crosta et al, 2004">Crosta, X., Sturm, A., Armand, L. and Pichon, J.J. 2004. Late Qaternary sea ice history in the Indian sector of the Southern Ocean as record by diatom assemblages, ''Marine Micropaleontology'', '''50''', 209-223.</ref>).]]<br />
Fluctuations in the position of the sea ice edge through Late Quaternary and Holocene time can be compared with insolation and sea surface temperature data over about the past 220 ka (Crosta et al., 2004<ref name="Crosta et al, 2004">Crosta, X., Sturm, A., Armand, L. and Pichon, J.J. 2004. Late Qaternary sea ice history in the Indian sector of the Southern Ocean as record by diatom assemblages, ''Marine Micropaleontology'', '''50''', 209-223.</ref>) ([[:File:Figure 3.18 - February SSTs and sea ice duration in core SO136-111 and insolation curves.png|Figure 3.18]]).<br />
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Past sea ice extent can also be inferred from assemblages of planktonic organisms or produced biomarkers (such as IP<sub>25</sub>) that exhibit a relationship to the temperature of the surface waters in which they live. Knowing this relationship at the present day, we can use it to transform assemblage data and IP<sub>25</sub> abundances from the recent past into estimates of seawater temperature (Crosta et al., 2004<ref name="Crosta et al, 2004">Crosta, X., Sturm, A., Armand, L. and Pichon, J.J. 2004. Late Qaternary sea ice history in the Indian sector of the Southern Ocean as record by diatom assemblages, ''Marine Micropaleontology'', '''50''', 209-223.</ref>; Mass&eacute; et al., 2008<ref name="Mass&eacute; et al, 2008">Mass&eacute;, G., Rowland, S., Sicre, M-A., Jacob, J., Jansen, E. and Belt, S. 2008. Abrupt climate changes for Iceland during the last millennium: Evidence from high resolution sea ice reconstructions, ''Earth and Planetary Science Letters'', '''269''', 564-568.</ref>) and compare these with oxygen isotope records where these are available. In the Southern Ocean, the data indicate that for core sites well offshore, sea ice formation lags temperature decline at the onset of glaciations by about 1,000 years (probably reflecting the time for Southern Ocean temperatures to become cold enough for sea ice to form at these more northerly sites), but that warming and sea ice retreat are simultaneous during glacial terminations (the transitions from glacial to interglacial) (Crosta et al., 2004<ref name="Crosta et al, 2004">Crosta, X., Sturm, A., Armand, L. and Pichon, J.J. 2004. Late Qaternary sea ice history in the Indian sector of the Southern Ocean as record by diatom assemblages, ''Marine Micropaleontology'', '''50''', 209-223.</ref>).<br />
==References==<br />
<references /><br />
[[Category:The pre-instrumental period]]<br />
[[Category:The last million years]]<br />
[[Category:Antarctic sea ice]]</div>Maintenance scripthttp://acce.scar.org/wiki/Sea_ice_observationsSea ice observations2014-08-06T14:33:57Z<p>Tonyp: Changed book section reference to page link</p>
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<div>:''This page is part of the topic [[Observations, data accuracy and tools]]''<br />
<br />
==The pre-satellite era==<br />
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Since the days of the earliest explorers, ships&rsquo; logs have recorded encounters with sea ice. Captain James Cook frequently reported the presence of sea ice as he tried to push south toward the continent, as did Captain Fabian von Bellingshausen during his exploration in 1831. Mackintosh and Herdman (1940<ref name="Mackintosh and Herdman, 1940">Mackintosh, N.A. and Herdman, H.F.P. 1940. Distribution of the pack-ice in the Southern Ocean, ''Discovery Reports'', '''19''', 285-296.</ref>) compiled a circumpolar map of the monthly variation of the average sea ice edge, based on data from ships&rsquo; logs during the 1920s and 1930s. These were later updated and republished by Mackintosh (1972<ref name="Mackintosh, 1972">Mackintosh, N.A. 1972. Life cycle of Antarctic krill in relation to ice and water conditions, ''Discovery Reports'', '''36''', 1-94.</ref>).<br />
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Whaling vessels also made many valuable observations of the postion of the sea ice edge. The region close to the ice edge is rich in food and many whales congregate there, attracting the whaling fleets. Most of these observations are for the summer season. De la Mare (1997<ref name="Mare, 1997">De La Mare, W.K. 1997. Abrupt mid-twentieth century decline in Antarctic sea-ice extent from whaling records, ''Nature'', '''389''' (6646), 57-61.</ref>) examined the whaling records, which provide the location of every whale caught since 1931. He suggested that there had been a big change in the location of the whaling vessels between the 1940s and the 1970s, with the summer sea ice edge having moved southward by 2.8&deg; of latitude between the mid 1950s and the early 1970s. He inferred this as meaning that there had been a decrease of 25% in the area covered by sea ice. However, there is a great deal of debate over whether or not the locations of whale catches can be translated into ice edge estimates that are comparable to those made from satellite observations. It is particularly unfortunate that there is no overlap between the period covered by the whale catch data and the modern satellite observations of the ice edge. At the moment the de la Mare results are questioned in many quarters and cannot be regarded as proof of a major decrease in sea ice extent between the 1940s and 1970s.<br />
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Coastal stations have also made valuable sea ice observations, albeit from a very limited number of locations. Nevertheless, direct observation of the ice edge from land is difficult because the ice edge extends well into the Southern Ocean for much of the year, out of sight from most coastal observatories. Some island stations, such as Signy in the South Orkney Islands, have provided information on sea ice variability over many years, and revealed details of some important modes of climate variability, such as the Antarctic Circumpolar Wave (Nowlin and Klinck, 1986<ref name="Nowlin and Klinck, 1986">Nowlin, W.D. and Klinck, J.M. 1986. The physics of the Antarctic Circumpolar Current, ''Reviews of Geophysics'', '''24''', 469-491.</ref>).<br />
<br />
==Satellite observations of sea ice extent and concentration==<br />
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From the 1960s it was possible to obtain a broad-scale view of the distribution of sea ice from visible and infra-red satellite imagery. However, the imagery was only of value in cloud-free or partly cloudy conditions, which was a major handicap as the Antarctic sea ice zone is characterised by extensive low cloud cover. With the introduction of reliable satellite passive microwave observations in the early 1970s (Gloersen et al., 1992<ref name="Gloersen et al, 1992">Gloersen, P., Campbell, W.J., Cavalieri, D.J., Comiso, J.C., Parkinson, C.L. and Zwally, H.J. 1992. Arctic and Antarctic Sea Ice, 1978-1987, 290 pp, Washington, DC, NASA.</ref>), the extent (the area bounded by the ice edge, which is often taken as 15% ice concentration) and area (the integrated area of ice within the ice edge) of Antarctic sea ice became confidently measureable.<br />
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The US Nimbus-5 Electrically Scanning Microwave Radiometer (ESMR) was launched in December 1972 and allowed the first all-weather mapping of Antarctic sea ice. This instrument only had one channel at 19 GHz, but the large contrast in the emissivity of sea ice and ice-free ocean enabled the development of an ice concentration algorithm, allowing the production of sea ice concentration maps from 1973 to 1976. The ESMR data provided the first observational data on the growth and decay patterns of sea ice for the entire Antarctic region.<br />
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Despite hopes for further developments in sea ice monitoring with the launch in 1975 of Nimbus-6/ESMR-2, with its dual polarized 37 GHz radiometer the instrument failed to perform well and no useful data were obtained. Further useful passive microwave data were obtained with the launch of the Scanning Multichannel Microwave Radiometer (SMMR), first on board the SeaSat satellite in July 1978, and then on Nimbus-7 in October 1978. The SMMR was a multifrequency system covering five frequencies from 6 to 37 GHz, all of them dual polarized (horizontal and vertical). The sensor was also conically scanning (i.e., incidence angle constant), and with its multifrequency capability ice concentrations were derived at a much better accuracy than with ESMR data. SMMR lasted for about 9 years, before it failed in August 1987, the DMSP/Special Scanning Microwave Imager (SSM/I) was already in operation and provided overlap data from mid-July to mid-August 1987. The SSM/I sensor has only 7 channels from 19 to 89 GHz, among which is the same set used for generating ice concentration maps from SMMR. The sensor is also conically scanning with similar resolutions, but has a wider swath; it has provided continuous data up to the present. The overlap allowed for comparison of the performance of the two radiometers and a confirmation that data from both sensors provided approximately the same results. In May 2002, the EOS/Advanced Microwave Scanning Radiometer (AMSR-E) was launched, and with 14 channels from 6 to 89 GHz, and much higher resolution, it has provided the baseline for sea ice studies.<br />
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The ESMR data set was very valuable and was unique when it first came out, but there were problems using it together with the other sets of data for time series studies. First, since it is a one-channel instrument, the ice concentration data are not as accurate because variations in temperature and emissivity of the ice cover could not be taken into account. Second, it is a horizontally scanning radiometer going from nadir to around 50o with varying resolution and with different incident angles. Third, there were many missing bits in the data stream, causing the elimination of a large fraction of the data and big data gaps in the time series. And fourth, there was no overlap of ESMR and SMMR data to enable assessment of differences of ice edge locations and concentrations derived from the two sensors. For uniformity, and accuracy in the trend analysis in [[Antarctic sea ice cover in the instrumental period#Variability and trends in sea ice using satellite data|Variability and trends in Antarctic sea ice using satellite data]], we use data from the two sets of similar sensors (i.e., SMMR and SSM/I) to evaluate the variability and trends in the ice cover over the last 28 years. We also discuss, how we can utilize ESMR data as well as some ship observations during the pre-satellite era to improve our understanding of the long-term trend.<br />
<br />
SARs flown on spacecraft can provide high resolution data on sea ice and reveal mesoscale ice motion and deformation, the development of leads and polynyas, ice type discrimination, sea ice roughness data and iceberg detection. In addition to the scientific applications of these images, ship operators in polar seas have benefited from recent advances in the near real time processing of SAR data, allowing them to be delivered quickly enough to assist ship navigation in sea ice.<br />
<br />
==Observations of sea ice thickness==<br />
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While the early explorers made many observations of sea ice location and type, their logs do not contain information on sea ice thickness. Only in recent decades have vessels become more ice capable and spent more time south of the ice edge in support of logistic and scientific activities. Consequently the sea ice logs from these ships have become more comprehensive and often include an estimate of sea ice thickness, or ice type from which thickness can be inferred.<br />
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In 1997, SCAR established the Antarctic Sea Ice Processes and Climate (ASPeCt) programme. One of the programe&rsquo;s first objectives was to collate the many disparate sea ice logs kept from icebreakers operating in the Antarctic sea ice zone. This effort focused primarily on the Australian, German, US and Russian national Antarctic programmes, which were known to have dozens of data sets containing information on the concentration, thickness and snow cover characteristics of the Antarctic sea ice zone. The data constituted a compilation of 23,391 individual ship-based observations collected from 81 voyages to Antarctica over the period 1981 &ndash; 2005, plus 1,663 aircraft-based observations. The ship-based observations are typically recorded hourly and include the ship&rsquo;s position, total ice concentration and an estimate of the areal coverage, thickness, floe size, topography, and snow cover characteristics of the three dominant ice thickness categories within a radius of approximately 1 km around the ship (Worby and Allison, 1999<ref name="Worby and Allison, 1999">Worby, A. P. and Allison, I. 1999. A ship-based technique for observing Antarctic sea ice. Part I: Observational Techniques and Results, ''Antarctic CRC Research Report'', '''14''', 1-23.</ref>). Not all observations contain this level of information, but at a minimum the partial ice concentrations and thicknesses (or ice types) were necessary for inclusion in the data set. The data are publicly available via the ASPeCt website (http://www.aspect.aq) or from the Australian Antarctic Data Centre (http://data.aad.gov.au/).<br />
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Time series of sea ice thickness can be measured by moored upward looking sonars. They consist of echosounders moored about 150 m below the ocean surface which record by travel time changes the variation of the ice draft. Mooring motions and changes of the sound velocities require extended corrections. Significant efforts occurred in the Weddell Sea to quantify the annual cycle of sea ice thickness and horizontal transports in the gyre circulation.<br />
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A new method for sea ice thickness measurements is by means of non-destructive electromagnetic inductive (EM) sounding which can be performed by moving an EM instrument above the snow surface either airborne or terrestrially.<br />
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It is hoped that in the future, processing of satellite altimeter data will allow the recovery of sea ice freeboard and therefore ice thickness. This may be easier in the Arctic since there sea ice is generally thicker and there is more multi-year ice present. In the Antarctic, the predominance of first year ice may make this a great challenge.<br />
<br />
==Sea ice extent proxies==<br />
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ITASE and other Antarctic ice coring programmes are in the process of developing proxies for sea ice, a critical component in the climate system, through studies of sulfur compounds such as sulfate and MSA (methane sulphonic acid) (Welch et al., 1993<ref name="Welch et al, 1993">Welch, K.A., Mayewski, P.A. and Whitlow, S.I. 1993. Methanesulphonic acid in coastal Antarctic snow related to sea-ice extent, ''Geophysical Research Letters'', '''20''', 443-446.</ref>; Curran et al., 2003<ref name="Curran et al, 2003">Curran, M.A.J., Van Ommen, T.D., Morgan, V.I., Phillips, K.L. and Palmer, A.S. 2003. Ice core evidence for Antarctic sea ice decline since the 1950s, ''Science'', '''302''', 1203-1206.</ref>; Dixon et al., 2005<ref name="Dixon et al, 2005">Dixon, D., Mayewski, P.A., Kaspari, S., Sneed, S. and Handley, M. 2005. Connections between West Antarctic ice core sulfate and climate over the last 200+ years, ''Annals of Glaciology'', '''41''', 155-166.</ref>, Abram et al., 2007a<ref name="Abram et al, 2007a">Abram, N.J., Mulvaney, R., Wolff, E. and Mudelsee, M. 2007a. Ice core records as sea ice proxies: An evaluation from the Weddell Sea region of Antarctica. Journal of Geophysical Research, 112:D15101.</ref>). ENSO-sea ice connections are noted utilizing ice core MSA and sulfate series over the Ross Sea embayment region (Meyerson et al., 2002<ref name="Meyerson et al, 2002">Meyerson, E.A., Mayewski, P.A., Whitlow, S.I., Meeker, L.D., Kreutz, K.J. and Twickler, M.S. 2002. The extratropical expression of ENSO recorded in a South Pole glaciochemical time series, ''Annals of Glaciology'', '''35''', 430-436.</ref>). The sea ice proxies rest on the premise that biogenic sulfur emissions from within the sea ice zone, when integrated regionally and through time, may be related to the total sea ice extent. This approach has been supported by observed correlations between MSA and sea ice (Curran et al., 2003<ref name="Curran et al, 2003">Curran, M.A.J., Van Ommen, T.D., Morgan, V.I., Phillips, K.L. and Palmer, A.S. 2003. Ice core evidence for Antarctic sea ice decline since the 1950s, ''Science'', '''302''', 1203-1206.</ref>; Foster et al., 2006<ref name="Foster et al, 2006">Foster, A.F.M., Curran, M.A.J., Smith, B.T., Van Ommen, T.D. and Morgan, V.I. 2006. Covariation of sea ice and methanesulphonic acid in Wilhelm II Land, East Antarctica, ''Annals Glaciol.'', '''44''', 429-432.</ref>, Abram et al., 2007a<ref name="Abram et al, 2007a">Abram, N.J., Mulvaney, R., Wolff, E. and Mudelsee, M. 2007a. Ice core records as sea ice proxies: An evaluation from the Weddell Sea region of Antarctica. Journal of Geophysical Research, 112:D15101.</ref>), however it is likely to be dependent upon the regional sea ice regime. For example, the positive relationship observed by Curran et al. is in the East Antarctic, where there is little residual summer sea ice and very little multi-year ice. Results from East Antarctica show evidence of a 20% decline in mean sea ice extent since the mid-20<sup>th</sup> century, consistent with results reconstructed from whaling records (de la Mare, 1997<ref name="Mare, 1997">De La Mare, W.K. 1997. Abrupt mid-twentieth century decline in Antarctic sea-ice extent from whaling records, ''Nature'', '''389''' (6646), 57-61.</ref>) although the validity of such observations is questioned (Ackley et al., 2003<ref name="Ackley et al, 2003">Ackley, S., Wadhams, P., Comiso, J.C. and Worby, A.P. 2003. Decadal decrease of Antarctic sea ice extent inferred from whaling records revisited on the basis of historical and modern sea ice records, ''Polar Res.'', '''22'''(1), 19-25.</ref>). However this continuous sea ice proxy illustrates large decadal-scale variations superimposed on the trend.<br />
<br />
[[File:Figure 2.20 - Correlation pattern of JJA MSLP anomalies and MSA variability at GF12.png|thumb|'''2.20''' The cross correlation pattern with R values for June - August MSLP anomalies for the Indian Ocean and Southern Ocean region and MSA variability at GF12, (97&deg; E) in Queen Mary Land. This shows the strong relationship between MSA and the SAM.]]<br />
Curran et al. (2003<ref name="Curran et al, 2003">Curran, M.A.J., Van Ommen, T.D., Morgan, V.I., Phillips, K.L. and Palmer, A.S. 2003. Ice core evidence for Antarctic sea ice decline since the 1950s, ''Science'', '''302''', 1203-1206.</ref>) have demonstrated that MSA variability in the coastal DSS ice core can be used as a proxy for regional sea ice extent. Accordingly they reconstructed the sea ice extent between 80&deg; to 140&deg; E back to 1840 AD, and showed the consistent decline in sea ice extent since the 1950s (decreasing MSA in the DSS ice). However, regional sea ice extent is a function of ocean temperature and circulation and atmospheric windfield divergence and convergence patterns. Hence, changes in the longwave atmospheric pattern may cause contrasting regional temperature and windfield patterns. Preliminary analysis of MSA concentrations in the GF12 core (Queen Mary Land, East Antarctica) indicates that MSA concentrations increased from 1950 to 1986 when the core was retrieved. These analyses may indicate a growth in regional sea ice to the west of 90&deg; E. This is consistent with increased southerly wind outflow from the Lambert Basin and cool SST anomalies in this region. Hence, ice core sites located at different longitudes have the potential to provide historical data on regional sea ice changes around Antarctica. [[:File:Figure 2.20 - Correlation pattern of JJA MSLP anomalies and MSA variability at GF12.png|Figure 2.20]] shows the June &ndash; August MSLP pattern associated with June - August MSA variations recorded in the GF12 ice core in Queen Mary Land, near 100&deg; E.<br />
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Sea-salt levels have been proposed as an additional proxy of sea ice extent (Wolff et al., 2003<ref name="Wolff et al, 2003">Wolff, E.W., Rankin, A.M. and Rothlisberger, R. 2003. An ice core indicator of Antarctic sea ice production?, Geophysical Research Letters, 30, 2158. (10.1029/2003GL018454.)</ref>) and applied to the long EPICA ice core record (Wolff et al., 2006<ref name="Wolff et al, 2006">Wolff, E.W., Fischer, H., Fundel, F., Ruth, U., Twarloh, B., Littot, G.C., Mulvaney, R., R&ouml;thlisberger, R., De Angelis, M., Boutron, C.F., Hansson, M., Jonsell, U., Hutterli, M.A,, Lambert, F., Kaufmann, P., Stauffer, B., Stocker, T.F., Steffensen, J.P., Bigler, M., Siggaard-Andersen, M.L., Udisti, R., Becagli, S., Castellano, E., Severi, M., Wagenbach, D., Barbante, C., Gabrielli, P. and Gaspari, V. 2006. Southern Ocean sea-ice extent, productivity and iron flux over the past eight glacial cycles, Nature, 440, 491-496 (doi:10.1038/nature04614).</ref>). The basis of this proxy is the observation from near-coastal sites that &lsquo;frost flowers&rsquo; are a significant source of sea-salt species. These &lsquo;flowers&rsquo; form on new ice under calm conditions, producing fragile crystalline structures that are vulnerable to destruction and transport by wind. Salt species from frost flowers provide a distinguishing signature resulting from fractionation of sulfur content relative to other species. At coastal sites, where winter snow can be isolated, this signature is evident, however it appears to diminish in relative importance at sites further inland (M. Curran, personal communication) and its utility as a proxy at sites in the interior has not yet been clearly demonstrated.<br />
==References==<br />
<references /><br />
[[Category:Observations, data accuracy and tools]]<br />
[[Category:Antarctic sea ice]]</div>Maintenance scripthttp://acce.scar.org/wiki/Sea_ice_ecosystems_in_the_instrumental_periodSea ice ecosystems in the instrumental period2014-08-06T14:33:57Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Marine biology in the instrumental period]]''<br />
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It is now clear that decreasing sea ice cover in the southwest Atlantic is due to a decline in ice production along the Peninsula. This can significantly reduce the supply of iron to the surface layer. The formation and presence of sea ice has several effects on the underlying water column and benthos. Deep convection due to brine discharge during ice formation can mix the entire water column down to the sea floor. Downward transported organic particles from the productive surface layer thus become available to benthic filter feeders. This mechanism will be a major source of food supply to the sponge-dominated fauna of Antarctic shelves and explains the apparent lack of gearing of reproduction of the shelf benthos to the ice&ndash;free period, when vertical particle flux from the overlying water column is at its maximum. Upward mixing of water that has contacted the sediment surface will bring iron to the surface layer. This mechanism of convective upward iron transport and subsequent fuelling of surface phytoplankton blooms has been reported from the Peninsula and around islands with shallow shelves where convective winter mixing is sufficient to reach the sediment surface without ice formation. It follows that the ongoing retreat of winter sea ice along the western Peninsula will result in declining depths of winter mixing and hence also in the supply of iron to the water column overlying deeper shelves, where surface warming reduces ice formation. A decrease in downstream spring productivity can be expected, which might be a factor contributing to krill decline in this region. However, it cannot be the only factor because the krill decline began before the retreat of the sea ice.<br />
[[Category:The instrumental period]]<br />
[[Category:Antarctic biology]]<br />
[[Category:Marine biology]]<br />
[[Category:Antarctic sea ice]]</div>Maintenance scripthttp://acce.scar.org/wiki/Sea_ice_change_over_the_21st_centurySea ice change over the 21st century2014-08-06T14:33:57Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Antarctic climate and environment change over the next 100 years]]''<br />
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The average of the CMIP3 model&rsquo;s sea ice extent compares well with observations, although there is a large inter-model spread (Arzel et al., 2006<ref name="Arzel et al, 2006">Arzel, O., Fichifet, T. and Goosse, D.H. 2006. Sea ice evolution over the 20<sup>th</sup> and 21<sup>st</sup> centuries as simulated by current AOGCMs. Ocean Modelling, 12, 401-415.</ref>; Parkinson et al., 2006<ref name="Parkinson et al, 2006">Parkinson, C.L., Vinnikov, K.Y. and Cavalieri, D.J. 2006. Evaluation of the simulation of the annual cycle of Arctic and Antarctic sea ice coverages by 11 major global climate models, J. Geophys. Res., 111, doi:10.1029/2005JC003408.</ref>).<br />
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Models following the A1B scenario show that over the Twenty First Century the annual average total sea ice area is projected to decrease by 2.6 &times; 10<sup>6</sup> km<sup>2</sup>, or 33% (Bracegirdle et al., 2008<ref name="Bracegirdle et al, 2008">Bracegirdle, T.J., Connolley, W.M. and Turner, J. 2008. Antarctic climate change over the Twenty First Century, Journal of Geophysical Research &ndash; Atmospheres, 113, D03103, doi:03110.01029/02007JD008933.</ref>). There is strong consensus among the models for an Antarctica-wide decrease in sea ice; the inter-model standard deviation is low at 0.73 &times; 10<sup>6</sup> km<sup>2</sup> (9%). Arzel et al. (2006<ref name="Arzel et al, 2006">Arzel, O., Fichifet, T. and Goosse, D.H. 2006. Sea ice evolution over the 20<sup>th</sup> and 21<sup>st</sup> centuries as simulated by current AOGCMs. Ocean Modelling, 12, 401-415.</ref>) assessed different measures of sea ice amount using a different subset of 15 of the CMIP3 A1B projections and found Twenty First Century deceases of 34% for sea-ice volume and 24% for sea-ice extent.<br />
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[[File:Figure 5.16 - Twenty First Century sea ice seasonal concentration change.png|thumb|'''5.16''' Twenty First Century sea ice concentration change for (a) DJF, (b) MAM, (c) JJA and (d) SON, showing the difference between the 2080-2099 mean and 2004-2023 mean. Changes are shown in terms of the fraction of the surface covered by sea ice, rather than sea ice percentage, since a spatial plot of sea ice percentage change would show infinite (or very large) increases where concentrations were initially zero (or very small).]]<br />
Most of the simulated ice retreat occurs in winter and spring when the sea ice extent is largest ([[:File:Figure 5.16 - Twenty First Century sea ice seasonal concentration change.png|Figure 5.16]]). The amplitude of the seasonal cycle of sea ice area will therefore decrease. The smaller amount of seasonal ice melting/freezing will affect the ocean due to changes in processes such as brine rejection.<br />
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There is strong confidence in the projected Antarctic-wide decreases of sea ice extent. At a more regional level, decreases of sea ice are less significant (e.g. Lefebvre and Goosse, 2008<ref name="Lefebvre and Goosse, 2008">Lefebvre, W. and Goosse, H. 2008. Analysis of the projected regional sea-ice changes in the Southern Ocean during the twenty-first century, ''Climate Dynamics'', '''30''', 59-76, DOI 10.1007/s00382-007-0273-6.</ref>). One way to measure the significance of a projected change is to calculate the signal to noise ratio of that change. Here the signal is the ensemble average change, and the noise is the standard deviation of the inter-model spread. A change can be thought of as &lsquo;significant&rsquo; if larger than the inter-model standard deviation, i.e. a signal to noise ratio of greater than one. In the regions where sea ice currently remains present throughout the summer, in particular the Weddell Sea, large reductions of sea ice extent are projected. On this there is quite a strong consensus between the models, with the model average reductions larger than the inter-model standard deviation ([[:File:Figure 5.16 - Twenty First Century sea ice seasonal concentration change.png|Figure 5.16]]). However, at the scale of one grid point the confidence of a reduction of sea ice over the Twenty First Century is not significant in many regions (i.e. the magnitude of the change is smaller than one standard deviation of the inter-model spread).<br />
==References==<br />
<references /><br />
[[Category:The next 100 years]]<br />
[[Category:Antarctic sea ice]]</div>Maintenance scripthttp://acce.scar.org/wiki/Sea_ice_biological_and_physical_modelingSea ice biological and physical modeling2014-08-06T14:33:56Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Models of the physical and biological environment of the Antarctic]]''<br />
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Most of our knowledge on sea ice microalgal growth and production was derived from observational data and some experimental work. Recently, new experimental data have revealed the complex and heterogeneous character of the sea ice ecosystem (Thomas and Dieckmann, 2002<ref name="Thomas and Dieckmann, 2002">Thomas, D.N. and Dieckmann, G.S. 2002. Antarctic sea ice-a habitat for extremophiles, ''Science'', '''295''', 641-644.</ref>), and the new information is improving the parameterization of biological sea ice models . The dependence of microalgal growth on light, nutrient and temperature is mechanistic, which means all processes involved are in accordance with purely physical and chemical relationships. However it has often been modeled using empirical relationships, where microalgal growth is parameterized as a function of the most limiting resource (Liebig&rsquo;s law). The difference between empirical and mechanistic models resides in the use of self-adapting variables by the latter to predict future states of microalgal biomass, allowing a dynamic adaptation of the biological processes to transient environmental changes. Such a dynamical approach depends on the physiological status of the cells. For Antarctic sea ice, where microalgal cells are subject to strong seasonal and daily variations in light, temperature and nutrients, only a mechanistic model can well represent the physiological responses of microalgal cells to environmental changes. The sea ice ecosystem differs considerably from water column conditions, and is characterized by extremely low temperatures, high brine salinities and strong downward attenuation of solar radiation. The sea ice seasonal cycle has two main implications for the spatial distribution of its microalgal communities: first, the ice growth season provides a large space surface area available for colonization in e.g. the brine channels of newly formed sea ice, followed by a drastic ice retreat during the transition to summer, dispersing the ice communities into the water column; and second, the role of ice drift during the growth season in the lateral transport of biomass through the seasonal ice zone, which significantly contributes to the horizontal distribution of food resources for other organisms associated with sea ice (e.g. the Antarctic krill and upward organisms further up in the trophic chain). The link between the establishment of microalgal communities and sea ice formation itself begins during direct interactions of ice crystals formed in seawater with individual organisms in the water column. This process is not selective and the mechanism by which microalgal cells, organic matter and other organisms are incorporated into developing sea ice is described in detail by Garrison et al. (1989<ref name="Garrison et al, 1989">Garrison, D.L., Close, A.R. and Reimntz, A. 1989. Algae concentrated by frazil ice: evidence from laboratory and field measurements, ''Antarctic Sci.'', '''1''', 313-316.</ref>) and Weissenberger and Grossmann (1998<ref name="Weissenberger and Grossmann, 1998">Weissenberger, J. and Grossmann, S. 1998. Experimental formation of sea ice: importance of water circulation and wave action for incorporation of phytoplankton and bacteria., ''Polar Biol.'', '''20''', 178-188.</ref>). Garrison (1991<ref name="Garrison, 1991">Garrison, D.L. 1991. Antarctic sea ice biota, ''Am. Zool.'', '''31''', 17-33.</ref>) demonstrated that although the large number of species that inhabit the underlying water column and sea ice brine channels (bacteria, microalgae, protists, small metazoans and some crustaceans), the extreme environmental conditions which the organisms are exposed to are very selective, and most of the sea ice biomass is dominated by small diatoms which are responsible for almost all sea ice primary production. Gleitz et al. (1998) showed that for more than 100 different diatoms species already found in sea ice habitats, less than 20 contribute significantly to the total biomass commonly found in the ice-pack while Lizotte (2001<ref name="Lizotte, 2001">Lizotte, M.P. 2001. The contributions of sea ice algae to. Antarctic marine primary production, ''Am. Zool.'', '''41''', 57-73.</ref>) inferred that Fragilartiopsis cylindrus and ''F. curta'' are the most dominant microalgae in sea ice. This low diversity may be related to the physiological capacity of these diatoms to maintain relatively high growth rates under extreme conditions of low light and temperature when compared to water column species. This apparent dominance of few species is also found between heterotrophic protists that inhabit sea ice. Garrison and Buck (1989<ref name="Garrison and Buck, 1989">Garrison, D.L. and Buck, K.R. 1989. The biota of Antarctic pack ice in the Weddell sea and Antarctic peninsula regions, ''Polar Biol.'', '''10''', 211-219.</ref>), working with pack-ice microbial communities in the Weddell Sea found that ciliates contributed 70% of the total protozoa biomass, followed by heterotrophic flagellates and other small protists, which play a key role in the cycling of material (e.g. excretion of nitrogen) and in controlling microalgal growth (Garrison, 1991<ref name="Garrison, 1991">Garrison, D.L. 1991. Antarctic sea ice biota, ''Am. Zool.'', '''31''', 17-33.</ref>).<br />
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The first attempt to model the Antarctic sea ice primary production was made by Arrigo et al. (1993<ref name="Arrigo et al, 1993">Arrigo, K.R., Kremer, J.N. and Sullivan, C.W. 1993. A simulated Antarctic fast ice ecosystem, ''Journal of Geophysical Research'', '''98'''(C4), 6929-6946.</ref>), where the prognostic variables (C, N and Si) were modeled using static physiological parameterizations as a function of light, temperature and salinity, using a nutrient limitation scheme based on Monod growth kinetics and the Liebig&rsquo;s law. The thermterm static comes from the absence of variables in the model that describe the state of the cells, as well as the use of Redfield ratios to couple the microalgal growth function with the uptake of N and Si to carbon. The Carbon-equivalent (Ceq) for the nutrients N and Si used by Arrigo and co-workers were C:N:Si of 107:17:40 molar-ratio, which was consistent with some sea ice observations (see details in Arrigo et al., 1995<ref name="Arrigo et al, 1995">Arrigo, K.R., Dieckmann, G., Gosselin, M., Robinson, D.H., Fritsen, C.H. and Sullivan, C.W. 1995. High resolution study of the platelet ice ecosystem in McMurdo Sound, Antarctica: Biomass, nutrient, and production profiles within a dense microalgal bloom, ''Marine Ecology Progress Series'', '''127'''(1-3), 255-268.</ref>, 1997<ref name="Arrigo et al, 1997">Arrigo, K.R., Worthen, D.L., Lizotte, M.P., Dixon, P. and Dieckmann, G. 1997. Primary production in Antarctic sea ice, ''Science'', '''276'''(5311), 394-397.</ref>). The shortcoming arising from such an approach is that microalgal growth limitation by light, temperature or salinity, affects only the rate of carbon incorporation in biomass, without any co-limitation and the system is basically controlled by the balance of nutrients and biomass in a fixed ratio (given by the molar ratios). When the environment is nutrient depleted, than growth rate relies only on the Monod growth kinetics. Thus, their model results show that the maximum biomass produced by microalgae depends only on the nutrient availability, and the rate of remineralization. The exact concentration is only related to the molar ratios between carbon, nitrogen and silicon. Although Redfield Ratios are commonly in use by many modelers (Arrigo et al., 1995<ref name="Arrigo et al, 1995">Arrigo, K.R., Dieckmann, G., Gosselin, M., Robinson, D.H., Fritsen, C.H. and Sullivan, C.W. 1995. High resolution study of the platelet ice ecosystem in McMurdo Sound, Antarctica: Biomass, nutrient, and production profiles within a dense microalgal bloom, ''Marine Ecology Progress Series'', '''127'''(1-3), 255-268.</ref>; Fritsen et al., 1998<ref name="Fritsen et al, 1998">Fritsen, C.H., Ackley, S.F., Kremer, J.N. and Sullivan, C.W. 1998. Flood-freeze cycles and microalgal dynamics in Antarctic pack ice. In: Lizotte MP, Arrigo K (eds) Antarctic sea ice: biological processes, interactions and variability, vol 73. American Geophysical Union, Washington, DC, 1-21.</ref>), there are many cases where elemental composition does not match with these ratios and the use of Redfield is questionable.<br />
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There are many approaches to creating a biological model for sea ice microalgal growth and production and modern studies shown that a quasi-realistic model must be based on self-adapting mechanisms to relate the physiological status of sea ice microalgae to their primary production, including grazing by sea ice protozoa. Sea ice microalgae in models must grow over two essential co-limiting nutrients, dissolved inorganic nitrogen [N] and silicate [Si], and biomass is represented in terms of carbon content (P C), chlorophyll-a (PChl), particulate organic nitrogen (PN) and biogenic silica (PSi), allowing variable cellular N:C and Si:C quotas to decouple primary production and inorganic nutrient availability. Light- and temperature dependence of microalgal growth must be included in the model, treated by a set of ordinary differential equations, which describes the balance between light and the chlorophyll-a:carbon ratio, to simulate the photo acclimation mechanism. There is a lack of data about heterotrophic protists feeding rates on microalgae accumulating carbon and nitrogen biomass (ZC and ZN, respectively) and excreting the N-excess to the medium. However, remineralization of silicon is neglected in most experiments, as well as the uptake of other elements, like phosphate, iron, aluminum and vitamins.<br />
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Developing a sea ice coupled biological-physical model, to investigate the influence of transient changes in environmental conditions on sea ice biological communities, must include physiological self-adaptive schemes to simulate microalgal photoacclimation and uptake of dissolved nutrients in response to transient changes of light, temperature and nutrient supply. Grazing may be simulated by the incorporation of heterotrophic protists in the model with specific organic carbon and nitrogen pools, although there is almost no field data about these points. The excretion of nitrogen excess by the sea ice protozoa in some laboratory experiments proved to be of importance in sea ice nutrient dynamics.<br />
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Also, to simulate sea ice biology, the model must be coupled with a one-dimensional thermodynamic sea ice model including a numerical scheme to resolve the heat conduction in sea ice, as well as the incorporation and vertical redistribution of biological material due to brine flux. Belem (2002<ref name="Belem, 2002">Belem, A.L. 2002. Modeling physical and biological processes in Antarctic sea ice. Ph.D. thesis, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven.</ref>) showed that these physical processes play a key role in the vertical structuring of sea ice biological communities and their associated primary production. The inclusion of brine fluxes in the physical model shows that vertical differentiation in the salinity profiles, commonly observed in ice cores collected in the Antarctic pack ice, results from variable ice growth rates and consequently salt partitioning, which cause distinct bulk salinity profiles. These differences associated with the vertical position of the impermeable layer play a key role in the formation of banded layers where high accumulation of biological material is commonly observed.<br />
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However, there are a few key questions to be assessed in such coupled biological-physical model. One of these is a Lagrangian approach, where the time-dependent position of simulated floes extracted from ice velocity fields can be used to compute model forcing parameters (e.g. air temperature, oceanic heat flux, solar radiation). These processes are essential to determine the thermodynamic history of floes and to obtain a good agreement between model results and field observations. During the drift, synoptical changes in environmental conditions lead to a differentiation of vertical sea ice characteristics that most models do not take into account. With exception of the model cited, all other sea ice models have an Eulerian approach.<br />
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One of the most important environmental parameters governing the sea ice primary production is the incoming solar radiation. The model includes the simulation of the spectral solar radiation attenuated by the atmosphere and clouds and a bio-optical sea ice model, determining the vertical distribution of light within sea ice. The solar radiation is also of significant importance for the sea ice thermodynamics, governing the temperature profile within sea ice during the summer months. Simulations with the spectral bio-optical sea ice model in many works showed that part of the marginal pack ice during the winter receives enough light to support biological production. This fact is of extreme importance for overwintering organisms associated with the sea ice.<br />
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[[File:Figure 2.38 - Schematic of incorporation of biological material into Antarctic pack ice.png|thumb|'''2.38''' Schematic representation of the seasonal cycle of the Antarctic pack ice, with emphasis on important processes of incorporation of biological material and the physical evolution of sea ice until the melting season. Adapted from Belem (2002<ref name="Belem, 2002">Belem, A.L. 2002. Modeling physical and biological processes in Antarctic sea ice. Ph.D. thesis, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven.</ref>).]]<br />
Thermodynamic processes controlling ice formation are fundamental in governing the vertical distribution of biological communities and the impermeable layer formed due to vertical temperature gradients and brine segregation acts as a barrier to the flux of biogenic material entrapped in the sea ice. Observed vertical variability in chlorophyll profiles from collected ice cores in the Weddell Sea are caused mainly by differential incorporation rates of biological material during the ice growth. [[:File:Figure 2.38 - Schematic of incorporation of biological material into Antarctic pack ice.png|Figure 2.38]] represents the seasonal cycle of the Antarctic pack ice with emphasis on the physical processes controlling the incorporation and redistribution of biological material within sea ice.<br />
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During autumn, dynamic accumulation of microorganisms in the water column due to frazil ice scavenging results in high incorporation rates of biological material within newly accreted ice layers. During winter, the thermodynamic growth of the pack ice controls the vertical distribution of ice assemblages, mainly due to the effect of the brine flux. With the onset of the melting season in spring and summer and increasing solar radiation over sea ice, the biological activity reaches its maximum followed by a rapid decline in the primary production rates between February and March. The sea ice retreat constrains the primary productivity to persistent ice fields in the summer, characterized by nutrient depleted conditions and heavy snow cover, indicating a significant degree of limitation for microalgal production.<br />
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Belem (2002<ref name="Belem, 2002">Belem, A.L. 2002. Modeling physical and biological processes in Antarctic sea ice. Ph.D. thesis, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven.</ref>) demonstrated in a coupled biological-physical sea ice model that the total sea ice productivity in the Weddell Sea shows an annual carbon production of approximately 11 Tg C. During the winter, the sea ice productivity ranges from 0.16 to 0.6 Tg C per month, contributing to 17% of the total annual production. These results are lower than previous estimates for the Weddell Sea pack ice computed only for infiltration communities associated with flooded snow and snow-ice formation. However, the coupled biological-physical sea ice models described before do not consider the formation of snow-ice and, therefore, infiltration communities are not taken into account. Snow flooding is restricted to the later spring/early summer in some areas of the Weddell Sea and cannot be generalized to the whole seasonal ice zone. Alternatively, most of the ice cores collected in the Weddell Sea indicate that infiltration communities occur in only a small fraction of the pack ice and model results revealed a dominance of interior and bottom ice assemblages.<br />
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The ecological distinction between these two assemblages is relevant to understanding the time course of sea ice colonization. Garrison and Buck (1991<ref name="Garrison and Buck, 1991">Garrison, D.L. and Buck, K.R. 1991. Surface-layer sea ice assemblages in Antarctic pack ice during the austral spring: Environmental conditions, primary production and community structure, ''Marine Ecology Progress Series'', '''75''', 161-172.</ref>) found that most of the sea ice infiltration assemblages are contained within a porous layer of hard ice near the freeboard level (i.e. the space between the ice surface and the seawater level) and apparently developed from internal assemblages. They also showed that interior assemblages are apparently established at the time of ice formation and thus overwinter survival in the ice is an important consideration. In contrast, an infiltration assemblage, which is inoculated when flooding occurs, would be heavily influenced by the composition of the plankton at the time of flooding. The results of coupled sea ice models suggest that the surface assemblages are related to the impermeable characteristic of topmost ice layers. However, the low temperatures associated with this vertical level must limit primary production.<br />
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Therefore, any estimates of the sea ice primary production obtained by models should be considered conservative, since the simulated pack ice consisted only of first-year ice. Many other ice habitats like multi-year ice, rafted ice floes, refrozen leads, infiltration layers, which are potential sites for the settlement of biological sea ice assemblages were not considered. The contribution of these communities to the total sea ice primary production needs to be further investigated. However, sea ice coupled physical and biological models are good tools to understand biological processes and its physical driven processes.<br />
==References==<br />
<references /><br />
[[Category:Observations, data accuracy and tools]]<br />
[[Category:Models]]<br />
[[Category:Antarctic biology]]</div>Maintenance scripthttp://acce.scar.org/wiki/Satellite_observations_of_the_Southern_OceanSatellite observations of the Southern Ocean2014-08-06T14:33:56Z<p>Acce: Changed the topic link at the top of the page to show the correct place in the topic hierarchy</p>
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<div>:''This page is part of the topic [[Observations, data accuracy and tools]]''<br />
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Satellites underpin a vast amount of modern oceanography, and their utility is nowhere greater than in data-poor regions of the Southern Ocean. They include, for example: - altimeters, scatterometers, infra-red and microwave sensors for sea-surface temperature and ice extent and visible-wavelength radiometers for ocean colour. Ensuring their continuity is of paramount importance.<br />
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==Sea Surface Temperature==<br />
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Sea surface temperature is an important geophysical parameter, providing the boundary condition used in the estimation of heat flux at the air-sea interface. On the global scale this is important for climate modelling, study of the Earth's heat balance, and obtaining insight into atmospheric and oceanic circulation patterns and anomalies (such as El Ni&ntilde;o). On the mesoscale, SST can be used to study ocean structure, such as eddies, fronts and upwellings and to assess biological productivity. In the remote Southern Ocean where ''in-situ'' measurements are particularly sparse, satellite observations of SST are key inputs to studies of physical and biological aspects of oceanography.<br />
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Satellite-derived SST is one of the longest duration continuous remote sensing datasets. Observations began in November 1981 using data collected by the AVHRR instruments on the NOAA polar orbiting operational meteorologicall satellites. This series is still operational and plans are in place to continue availability of AVHRR data from the European MetOp and U.S. National Polar-orbiting Operational Environmental Satellite System (NPOESS) series.<br />
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The direct broadcast capability of instruments such as the AVHRR adds value for real-time support to scientific cruises in the Southern Ocean.<br />
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Improved and consistent processing of AVHRR data has resulted in the Pathfinder dataset (http://podaac.jpl.nasa.gov/PRODUCTS/p216.html), which provides global datasets from 1985 to current at 4 km pixel spacing with improved ice and land masks. This has provided improved SST data sets of the Southern Ocean, which is a very cloudy region where detection of cloud and ice has been a problem in the past.<br />
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Other processing efforts include the Global High-Resolution Sea Surface Temperature (GHRSST) project (http://www.ghrsst-pp.org/), which aims to provide a new generation of global multi-sensor high-resolution (&lt;10 km) SST products to the operational oceanographic, meteorological, climate and general scientific community.<br />
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[[File:Figure 2.14 - Southern Ocean averaged SST for January from Aqua MODIS sensor.png|thumb|'''2.14''' Southern Ocean averaged SST for January from Aqua MODIS sensor]]<br />
Satellite SST data are also available from a range of other satellites, including the European Space Agency Along Track Scanning Radiometer (ATSR) instruments and more recently the MODIS instruments on the NASA Terra and Aqua satellites ([[:File:Figure 2.14 - Southern Ocean averaged SST for January from Aqua MODIS sensor.png|Figure 2.14]]).<br />
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Of particular application to the study of climate change are the ATSR instruments onboard the European Space Agency ERS and Envisat satellites. Operating since July 1991, ATSR is now available as consistently processed dataset with a target SST accuracy of 0.3&ordm;C (Corlett et al., 2006<ref name="Corlett et al, 2006">Corlett, G.K., Barton, I.J., Donlan, C.J., Edwards, M.C., Good, S.A., Horrocks, L.A., Llewellyn-Jones, D.T., Merchant, C.J., Minnett, P.J., Nightingale, T.J., Noyes, E.J., O'carroll, A.G., Remedios, J.J., Robinson, I.S., Saunders, R.W. and Watts, J.G. 2006. The accuracy of SST retrievals from AATSR: An initial assessment through geophysical validation against ''in situ'' radiometers, buoys and other SST data sets, ''Advances in Space Research'', '''37''' (4), 764-769.</ref>). This series of accurate space-based observations of SST is to be extended as part of the European Space Agency Sentinel series, onboard Sentinel-3 and currently planned for launch in 2012.<br />
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In addition to SST data derived from thermal infrared measurements, observations are also made using passive microwave measurements. Accuracy and resolution is poorer for SST derived from passive microwave measurements. The advantage gained with passive microwave is that radiation at these longer wavelengths is largely unaffected by clouds, and generally easier to correct for atmospheric effects. Instruments that have been used include the Scanning Multichannel Microwave Radiometer (SMMR) carried on Nimbus-7 and Seasat satellites, the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and data from the AMSR instrument on the NASA EOS Aqua satellite.<br />
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==Altimetry==<br />
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[[File:Figure 2.15 - Satellite altimetry showing high eddy kinetic energy in the core of the ACC.png|thumb|'''2.15''' Satellite altimetry showing high Southern Ocean eddy kinetic energy in the core of the ACC (Gille and Sandwell, 2001<ref name="Gille and Sandwell, 2001">Gille, S.T. and Sandwell, D.T. 2001. Gravity, Bathymetry, and Mesoscale Ocean Circulation from Altimetry, AVISO newsletter, 8, September.</ref>)]]<br />
Satellite altimeters provide data that are helpful for understanding both the ocean circulation (Gille et al., 2000<ref name="Gille et al, 2000">Gille, S.T., Yale, M.M. and Sandwell, D.T. 2000. Correlation of mesoscale ocean variability with seafloor roughness from satellite altimetry, ''Geophys. Res. Lett.'', '''27''', 1251-1254.</ref>; Hughes et al., 2001) and the geophysical characteristics of the sea floor (Sandwell and Smith, 1997<ref name="Sandwell and Smith, 1997">Sandwell, D.T. and Smith, W.H.F. 1997. Marine gravity anomaly from Geosat and ERS-1 satellite altimetry, J. Geophys. Res., 102, 10,039-10,054.</ref>) ([[:File:Figure 2.15 - Satellite altimetry showing high eddy kinetic energy in the core of the ACC.png|Figure 2.15]]). Satellite altimeters provide long-term observations of mesoscale circulation patterns in the Southern Ocean and the variability of features such as the ACC. Satellite altimetry also provides information that is of use in mapping sea surface wind speeds and significant wave heights.<br />
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In the polar oceans, sea ice and its associated snow cover is a major regulator of the heat, mass and momentum between the atmosphere and the ocean. Although ice extent and concentration are routinely measured from space, sea ice and snow thickness, particularly in the Antarctic, are not well measured and are highly uncertain. Methods for estimating sea ice thickness in the Arctic using satellite altimetry may have application for measuring Antarctic sea ice thickness, despite the difficulty of determining the smaller freeboard measurements of dominant first year ice in the Antarctic (Giles et al., 2006<ref name="Giles et al, 2006">Giles, K.A., Laxon, S.W. and Worby, T. 2006. Validating Satellite Radar Altimetry Estimates of Antarctic sea ice Thickness Using the ASPeCt Data set, Proceedings of 2006 American Geophysical Union Fall Meeting.</ref>).<br />
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Radar altimeters are non-imaging radar sensors, which use the ranging capability of radar to measure the surface topographic profile parallel to the satellite track. They provide precise measurements of a satellite&rsquo;s height above the ocean and, if appropriately designed, over land/ice surfaces, by measuring the time interval between the transmission and reception of very short electromagnetic pulses.<br />
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To date, most spaceborne radar altimeters have been wide-beam (pulse-limited) systems operating from low Earth orbits. Radar altimetry data have been collected from a variety of instruments including Seasat (1978), Geosat (1985-1989), ERS-1 (1991-1996), Topex-Poseidon (since 1992), ERS-2 (since 1995), Jason-1 (since 2001) and Envisat (since 2002). The future availability of satellite altimetry observations is assured with new missions such as Jason-2 and the ESA Sentinel satellites. ESA&rsquo;s CryoSat-2 mission will also provide new data from the SIRAL instrument specifically designed to deliver higher resolution measurements for ice sheet and sea ice observations.<br />
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[[File:Figure 2.16 - Schematic of a single ICESat repeat track.png|thumb|'''2.16''' Schematic of a single ICESat repeat track indicating surface altimetry measurements (red) and cloud observations (green/white/pink). Source: NASA/GSFC.]]<br />
In addition to radar altimetry, the first satellite laser altimeter was launched in 2003 onboard the Ice, Cloud, and land Elevation Satellite (ICESat) satellite. The Geoscience Laser Altimeter System (GLAS) is the sole instrument on the ICESat ([[:File:Figure 2.16 - Schematic of a single ICESat repeat track.png|Figure 2.16]]). The main objective of the ICESat mission is to measure ice sheet elevations and changes in elevation through time. Secondary objectives include sea ice thickness, measurement of cloud and aerosol height profiles, land elevation and vegetation cover (Schutz et al., 2005<ref name="Schutz et al, 2005">Schutz, B.E., Zwally, H.J., Shuman, C.A., Hancock, D. and Dimarzio, J.P. 2005. Overview of the ICESat Mission, Geophys. Res. Lett., 32, L21S01, doi:10.1029/2005GL024009.</ref>). NASA&rsquo;s ICESat-2 mission will continue the legacy of ICESat from around 2014.<br />
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==Ocean colour==<br />
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[[File:Figure 2.17 - Phytoplankton bloom off South Georgia.png|thumb|'''2.17''' Phytoplankton bloom off South Georgia in the Southern Ocean acquired by the Aqua MODIS instrument]]<br />
Satellite ocean colour sensors provide data on the concentration of chlorophyll in the ocean surface waters. Variations in ocean chlorophyll concentration contribute information on the biology of the Southern Ocean and are a key input for modeling of the South Ocean ecosystem. In a region characterised by high-nutrient, low-chlorophyll status, ocean colour satellite imagery has greatly enhanced our understanding of the true spatial and temporal extent of phytoplankton blooms ([[:File:Figure 2.17 - Phytoplankton bloom off South Georgia.png|Figure 2.17]]) and has revealed that chlorophyll-a (chl-a) biomass can be particularly high in regions such as the Scotia and Weddell Seas (E.J. Murphy et al., 2006<ref name="Murphy et al, 2006">Murphy, E.J., Watkins, J.L., Trathan, P.N., Reid, K., Meredith, M.P., Thorpe, S.E., Johnston, N.M., Clarke, A., Tarling, G.A., Collins, M.A., Forcada, J., Shreeve, R.S., Atkinson, A., Korb, R., Whitehouse, M.J., Ward, P., Rodhouse, P.G., Enderlein, P., Hirst, A.G., Martin, A.R., Hill, S.L., Staniland, I.J., Pond, D.W., Briggs, D.R., Cunningham, N.J. and Fleming, A.H. 2006. Spatial and temporal operation of the Scotia Sea ecosystem: a review of large-scale links in a krill centred food web, Philosophical Transactions of the Royal Society B: Biological Sciences, 362, (1477), DOI: 10.1098/rstb.2006.1957.</ref>). Combined with other observations, chlorophyll measurements help to provide an understanding of how ocean productivity changes with weather, oceanographic variations such as the El Ni&ntilde;o/South Oscillation, and other fluctuations in ocean temperature &ndash; work that could provide hints as to how future climatic change could affect ocean productivity (Behrenfeld et al., 2006<ref name="Behrenfeld et al, 2006">Behrenfeld, M., O&rsquo;Malley, R., Siegel, D., McClain, C., Sarmiento, J., Feldman, G., Milligan, A., Falkowski, P., Letelier, R. and Boss, E. 2006. Climate-driven trends in contemporary ocean productivity, ''Nature'', '''444''', 752-755.</ref>).<br />
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Parts of the Southern Ocean ecosystem have also been highly perturbed as a result of harvesting over the last two centuries, and significant ecological changes have also occurred in response to rapid regional warming during the second half of the twentieth century. This combination of historical perturbation and rapid regional change suggests that the Scotia Sea ecosystem is likely to show significant change over the next two to three decades, which may result in major ecological shifts. Satellite measurements of chlorophyll will help obtain a comprehensive understanding of the evolution of the Antarctic marine ecosystem.<br />
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The accuracy of chlorophyll measurements varies from region to region globally. In the Southern Ocean it has been observed (Holm-Hansen et al., 2004<ref name="Holm-Hansen et al, 2004">Holm-Hansen, O., Kahru, M., Hewes, C.D., Kawaguchi, S., Kameda, T., Sushin, V.A., Krasovski, I., Priddle, J., Korb, R., Hewitt, R.P. and Mitchell, B.G. 2004. Temporal and spatial distribution of chlorophyll-a in surface waters of the Scotia Sea as determined by both shipboard measurements and satellite data, Deep-Sea Research Part II&mdash;Topical Studies in Oceanography, 51 (2004) (12-13), 1323-1332.</ref>) that satellite data underestimate chlorophyll values recorded ''in situ'' at high chlorophyll concentrations, and slightly over-estimate the shipboard data at lower chlorophyll concentrations.<br />
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Data suitable for measurement of chlorophyll concentration began in 1979 with the launch of the Coastal Zone Colour Scanner (CZCS) onboard the Nimbus-7 satellite. Intermittent collection of data continued until 1986. However, it was not until 1997 that global ocean colour observations resumed with the launch of the SeaWiFS (Sea-Viewing Wide Field-of-View Sensor) instrument onboard the Orbview-2 satellite. SeaWiFS has now collected data for more than a decade, but continuity of ocean colour data continues concurrently with the MODIS instruments on the Terra and Aqua satellites and the MERIS instrument on the Envisat satellite. Future plans for the continuity of satellite chlorophyll measurements include instruments on the ESA Sentinel-2 satellite and the NOAA Polar-orbiting Operational Environmental Satellite System (NPOESS) series.<br />
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==Scatterometer data==<br />
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Radar scatterometer instruments have been designed principally to capture the near-surface wind field over the oceans. By assuming that the energy transmitted back to the radar from the ocean is dependent only upon that component of sea surface roughness that is a product of the frictional interaction of wind on the surface, a model may be used to relate this roughness, through the radar backscatter coefficient, to wind speed and direction. This is possible because the roughness is anisotropic, with crests and troughs generally orthogonal to the wind direction.<br />
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The primary use of scatterometer data in southern high latitudes is as data to be assimilated into numerical weather prediction (NWP) models. For example, Andrews and Bell (1998<ref name="Andrews and Bell, 1998">Andrews, P.L. and Bell, R.S. 1998. Optimizing the United Kingdom Meteorological Office data assimilation for ERS-1 scatterometer winds. Monthly Weather Review, 126, 736-746.</ref>) showed a marked reduction in rms errors of UK Met Office forecasts over the Southern Ocean that assimilated scatterometer winds. However, the spatial resolution of scatterometer data &mdash; 25 km for the instrument on the ERS-1 satellite &mdash; means that it can also be used for case studies. It has been utilised to study both mesoscale and synoptic-scale weather systems around Antarctica (Marshall and Turner, 1997a<ref name="Marshall and Turner, 1997a">Marshall, G.J. and Turner, J. 1997a. Surface wind fields of Antarctic mesocyclones derived from ERS-1 scatterometer data, ''Journal of Geophysical Research'', '''102''', 13907-13921.</ref>, 1999<ref name="Marshall and Turner, 1999">Marshall, G.J. and Turner, J. 1999. Synoptic-scale weather systems observed during the FROST project via scatterometer winds, Weather and Forecasting, 14, 867 877.</ref>) and coastal katabatic winds off the Ross Ice Shelf (Marshall and Turner, 1997b<ref name="Marshall and Turner, 1997b">Marshall, G.J. and Turner, J. 1997b. Katabatic wind propagation over the western Ross Sea observed using ERS-1 scatterometer data, ''Antarctic Science'', '''9''', 221-226.</ref>).<br />
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[[File:Figure 2.18 - Southern Ocean scatterometer and thermal infrared data acquired on 15 Jan 1995.png|thumb|'''2.18''' Near-coincident data over the Southern Ocean on 15 Jan 1995. (a) ERS-1 scatterometer swath acquired between 13:09 and 13:15 GMT; (b) Thermal infrared AVHRR imagery obtained at 13:11 UTC. Wind feathers point in the direction the wind is blowing; half a barb represents 2.5 m/sec, and a full barb 5.0 m/sec. Labelled features are described in the text. (Marshall and Turner, 1999<ref name="Marshall and Turner, 1999">Marshall, G.J. and Turner, J. 1999. Synoptic-scale weather systems observed during the FROST project via scatterometer winds, Weather and Forecasting, 14, 867 877.</ref>; courtesy of the American Meteorological Society).]]<br />
[[:File:Figure 2.18 - Southern Ocean scatterometer and thermal infrared data acquired on 15 Jan 1995.png|Figure 2.18]], taken from Marshall and Turner (1999<ref name="Marshall and Turner, 1999">Marshall, G.J. and Turner, J. 1999. Synoptic-scale weather systems observed during the FROST project via scatterometer winds, Weather and Forecasting, 14, 867 877.</ref>) demonstrates the type of information that scatterometer data can provide about weather systems over the Southern Ocean. Feature A is a mesocyclone and it can be observed in the scatterometer data: wind speeds are greater on the equatorward side and weaker on the poleward side of the centre than the background flow, and the wind field shows troughing in an equatorward direction. Feature B is a trough that is not apparent in the cloud imagery: the scatterometer data indicate the marked wind shear associated with this trough, with the wind direction changing by 135&deg; within the resolution of the data. Finally, feature C is a dissipating synoptic-scale system located close to the Antarctic Peninsula. The scatterometer winds reveal a weak but closed surface circulation.<br />
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Scatterometer data can also provide useful information from ice-covered surfaces (Long et al., 2001<ref name="Long et al, 2001">Long, D.G., Drinkwater, M.R., Holt, B., Saatchi, S. and Bertoia, C. 2001. Global ice and land climate studies using scatterometer image data, EOS, Transactions of the American Geophysical Union, 82, 503 (see also http://www.agu.org/eos_elec/010126e.html).</ref>): the return signal is dependent on the roughness and the dielectric properties (a measure of the ability of a medium to resist the formation of an electric field within it) of the surface and near-surface. Despite the relatively poor spatial resolution, useful information on the thermodynamic state, distribution and dynamics of sea ice at a regional scale can be tracked easily because of the frequent repeat coverage at polar latitudes (1-2 days). Similarly, over terrestrial ice sheets large-scale patterns of seasonal melt can be observed.<br />
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==Synthetic Aperture Radar==<br />
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[[File:Figure 2.19 - Envisat ASAR image showing sea ice around Adelaide Island, West Antarctic Peninsula.png|thumb|'''2.19''' Envisat ASAR image showing sea ice around Adelaide Island, West Antarctic Peninsula]]<br />
Imaging Synthetic Aperture Radar (SAR) systems provide imagery of the ocean and land surfaces by recording the reflected signals of emitted microwave radiation ([[:File:Figure 2.19 - Envisat ASAR image showing sea ice around Adelaide Island, West Antarctic Peninsula.png|Figure 2.19]]). The active nature of these imaging instruments means data can be obtained throughout the year, day or night and regardless of cloud and weather conditions. Whilst initially difficult to interpret, they are useful in resolving features of sea ice, land ice and snow, and the oceans.<br />
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SAR imagery also has wide application to oceanographic studies. Imagery provides information about the interactions at the sea ice edge and the resolution of currents, frontal boundaries and internal waves.<br />
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Available SAR imaging satellites began in 1991 with the launch of the European ERS-1 satellite. Since then, ERS-2, Envisat and Radarsat satellites have provided continuity of C-band SAR imagery. In 1997, Radarsat even made an unprecedented in-orbit manoeuvre to allow the entire Antarctic continent to be imaged including areas south of 80&ordm; S, which are not normally viewable. This resulted in the RAMP dataset (http://bprc.osu.edu/rsl/radarsat/radarsat.html, which has subsequently been updated in 2000 to provide the MAMM (Modified Antarctic Mapping Mission) dataset, which includes repeat images of glaciers in order to determine velocity. Recently, other SAR systems including different wavelength radars have been launched, including Radarsat-2, the Japanese ALOS, TerraSAR-X and COSMO-SkyMed systems. In the future there are plans for continuity of the European SAR systems from the ESA Sentinel series, a Radarsat constellation, a planned addition to the TerraSAR system called Tandem-X that will deliver detailed elevation data and even discussion of a P-band radar system that may allow imaging of internal ice sheet structure.<br />
==References==<br />
<references /><br />
[[Category:Observations, data accuracy and tools]]<br />
[[Category:The Southern Ocean]]</div>Maintenance scripthttp://acce.scar.org/wiki/Ross_Sea_shelf_waters_in_the_instrumental_periodRoss Sea shelf waters in the instrumental period2014-08-06T14:33:54Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[The Southern Ocean in the instrumental period]]''<br />
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The circulation in the Ross Sea is dominated by a wind-driven cyclonic gyre (Treshnikov, 1964<ref name="Treshnikov, 1964">Treshnikov, A.F. 1964. Surface water circulation in the Antarctic Ocean, Information Bulletin of the Soviet Antarctic Expedition, 5, 81-83 (English translation)</ref>) visible as a depression in the steric height transporting Circumpolar Deep Water to the south where by interaction with shelf and slope water Antarctic Bottom Water is produced. It is located south of the mid-ocean ridge between 170&deg; E and 140&deg;W (e.g. Gouretski, 1999<ref name="Gouretski, 1999">Gouretski, V. 1999. The large-scale thermohaline structure of the Ross Gyre, In: Eds G. Spezie and G.M.R. Manzella, Oceanography of the Ross Sea Antarctica, Springer-Verlag Italia, Milano, 77-100.</ref>) with its centre at about 68&deg;S, 164&deg;W shifting to the southeast with depth. The baroclinic transport of 8.5 Sv is significantly smaller than the one of the Weddell gyre. The eastern boundary is given by a southward deflection of the ACC due to the bottom topography. At the southern limb, westward flow transports water as warm as 1.6&deg;C. From property maps Reid (1986<ref name="Reid, 1986">Reid, J.L. 1986. On the total geostrophic circulation of the South Pacific Ocean: flow patterns, tracers and transports, ''Progress in Oceanography'', '''16''', 1-61.</ref>) included the extension up to the Antarctic Peninsula. Antipov et al. (1987<ref name="Antipov et al, 1987">Antipov, N.N., Maslennikov, V.V. and Pryamikov, S.M. 1987. Location and structure of the Polar Frontal zone in the western part of the Pacific sector of the Southern Ocean (in Russian). Biological oceanographic investigations in the Pacific sector of Antarctica, VNIRO, Moscow, 19-32.</ref>), Maslennikov (1987<ref name="Maslennikov, 1987">Maslennikov, V.V. 1987. Secondary frontal discontinuities in the western part of the Antarctic Pacific sector (in Russian), Biological oceanographic investigations in the Pacific sector of Antarctica, VNIRO, Moscow, 32-41.</ref>) and Locarnini (1994<ref name="Locarnini, 1994">Locarnini, R.A. 1994. Water masses and circulation in the Ross Gyre and environments. PhD thesis, Texas A and M University, College Station.</ref>) locate the eastern boundary at 140&deg;W. The continental shelf area is relatively well sampled due to the normally weak ice cover in summer and the presence of several Antarctic stations (Jacobs and Giulivi, 1999<ref name="Jacobs and Giulivi, 1999">Jacobs, S.S. and Giulivi, C.F. 1999. Thermohaline Data and Ocean Circulation on the Ross Sea Continental Shelf. In: Eds G. Spezie and G.M.R. Manzella, Oceanography of the Ross Sea Antarctica, Springer-Verlag Italia, Milano, 3-16.</ref>).<br />
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&lsquo;Shelf waters&rsquo; include a variety of low-temperature, ice-modified, high- and low-salinity water masses found below the ocean surface layers on the Antarctic continental shelf (Whitworth et al., 1998<ref name="Whitworth et al, 1998">Whitworth, T., Orsi, A.H., Kim, S.J., Nowlin, W.D. and Locarnini, R.A. 1998. Water masses and mixing near the Antarctic Slope Front, ''Antarct. Res. Ser.'', '''75''', 1-27.</ref>). Summer salinity profiles spanning about 20 years in High Salinity Shelf Water (HSSW) near Ross Island displayed gradual salinity increases below 200 m and interannual water column shifts that were several times the measurement accuracy and half the annual cycle in McMurdo Sound (Jacobs, 1985<ref name="Jacobs, 1985">Jacobs, S.S. 1985. Oceanographic evidence for land ice/ocean interactions in the Southern ocean. In: Glaciers, Ice Sheets and Sea Level: Effect of a CO<sub>2</sub>-Induced Climatic Change, Report of a Workshop, Seattle, 13-15 Sep 1984, DOE/ER/60235-1, 116-128.</ref>). Atmospheric forcing, sea ice production, HSSW residence time, ice shelf melting and intrusion of Modified Circumpolar Deep Water onto the continental shelf were considered as possible agents of change. Hellmer and Jacobs (1994<ref name="Hellmer and Jacobs, 1994">Hellmer, H.H. and S.S. Jacobs (1994) Temporal changes in shelf water of the southern Ross Sea, ''Antarct. J. of the U.S.'', '''29'''(5), 123-124.</ref>) noted that salinity decreases could also result from fresher upstream source waters.<br />
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Attempts to link observed HSSW changes to multiyear variability in regional sea ice extent, winds and air temperatures revealed the need for longer time series. The data base largely consists of sporadic summer measurements, and both modelers and observers have noted the possibility of aliasing in this shelf water record due to undersampling of a variable inflow. In addition, most measurements were in or near an HSSW eddy, another potential source of variability. Nonetheless, the deep HSSW trend in that area has closely tracked changes at depth along the Ross Ice Shelf and near 500 m throughout the western Ross Sea (Jacobs and Giulivi, 1998<ref name="Jacobs and Giulivi, 1998">Jacobs, S.S. and Giulivi, C.F. 1998. Interannual ocean and sea ice variability in the Ross Sea, ''Antarct. Res. Ser.'', '''75''', 135-150.</ref>; Smethie and Jacobs, 2005<ref name="Smethie and Jacobs, 2005">Smethie, W.M. and Jacobs, S.S. 2005. Circulation and melting under the Ross Ice Shelf: Estimates from evolving CFC, salinity and temperature fields in the Ross Sea, ''Deep-Sea Res. I'', '''52''', 959-978.</ref>).<br />
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Record low salinities at the site in February 2000 led to analyses that more strongly implicated changes in ice-ocean interactions upstream in the Amundsen and Bellingshausen Seas (Jacobs et al., 2002<ref name="Jacobs et al, 2002">Jacobs, S.S., Giulivi, C.F. and Mele, P.A. 2002. Freshening of the Ross Sea during the late 20<sup>th</sup> century. Science, 297(5580), 386-389, doi:10.1126/science.1069574.</ref>). Assmann and Timmermann (2005<ref name="Assmann and Timmermann, 2005">Assmann, K.M. and Timmermann, D.R. 2005. Variability of dense water formation in the Ross Sea, ''Ocean Dynamics'', '''55''', 68-87.</ref>) successfully modeled averaged HSSW salinity profiles, and inferred that the freshening resulted from a Bellingshausen Sea thermal anomaly. Their periodic signal upwelled in the Amundsen Sea, reduced brine drainage near the sea ice edge and induced a subsurface salinity decrease that was advected into the Ross Sea. Interannual salinity variability is high, but the overall trend has been statistically significant and qualitatively consistent with freshening over a much wider area (Jacobs, 2006<ref name="Jacobs, 2006">Jacobs, S.S. 2006. Observations of change in the Southern Ocean, ''Phil. Trans. Roy. Soc. A'', '''364''', 1657-1681.</ref>).<br />
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[[File:Figure 4.27 - Summer temperature and salinity profiles over a 50 year period near Ross Island.png|thumb|'''4.27''' Summer temperature and salinity profiles over a 50 year period, some averaged from adjacent profiles, and dashed if more than 15 km from 168&ordm; 20&rsquo;E, 77&ordm; 10&rsquo;S, near Ross Island. Plotted values are 100 m averages/interpolations of CTD or bottle data, edited or extrapolated where shown by open circles. Horizontal lines depict the salinity ranges at six bottle casts in southern McMurdo Sound over 23 December 1960 &ndash; 9 February 1961 (Tressler and Ommundsen, 1963<ref name="Tressler and Ommundsen, 1963">Tressler, W.L. and Ommundsen, A.M. 1963. Oceanographic studies in McMurdo Sound, Antarctica, IG Bull. 67, Trans. Am. Geophys. Un. 44(1), 217-225.</ref>). Temperatures are referenced to the surface freezing point, ~-1.91&deg;C at a salinity of 34.8. Tfrs is the surface freezing reference temperature.]]<br />
The record of summer shelf water thermohaline properties has recently been extended to 50 years ([[:File:Figure 4.27 - Summer temperature and salinity profiles over a 50 year period near Ross Island.png|Figure 4.27]]), and the study area widened to include profiles in McMurdo Sound. The 50 year salinity trend continues to be near -0.03/decade, while slightly warming temperatures have remained consistent with HSSW formation by surface freezing in winter. HSSW near Ross Island thus serves as an index site to monitor change occurring in the Ross Sea and upstream (eastward) in the Antarctic Coastal Current. The salinity decline appears to derive mainly from increasing continental ice meltwater, and will subsequently change the properties if not the production rates of deep and bottom waters. Over regional areas the lower salinity has raised sea level via the halosteric component of seawater density.<br />
==References==<br />
<references /><br />
[[Category:The instrumental period]]<br />
[[Category:The Southern Ocean]]</div>Maintenance scripthttp://acce.scar.org/wiki/Regional_patterns_of_holocene_climate_change_in_AntarcticaRegional patterns of holocene climate change in Antarctica2014-08-06T14:33:54Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[The Holocene]]''<br />
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In this section we provide a spatial synthesis of records of climate and environmental changes in East Antarctica (EA), the Antarctic Peninsula (AP), and the Ross Sea region (RS). West Antarctica, which is currently understudied, is also briefly described. We build on previous reviews by Ing&oacute;lfsson et al. (1998<ref name="Ing&oacute;lfsson et al, 1998">Ing&oacute;lfsson, &Oacute;., Hjort, C., Berkman, P.A., Bj&ouml;rck, S., Colhoun, E.A., Goodwin, I.D., Hall, B.L., Hirakawa, K., Melles, M., M&ouml;ller, P. and Prentice, M.L. 1998. Antarctic glacial history since the last glacial maximum: an overview of the record on land, ''Antarctic Science'', '''10''', 326-344.</ref>, 2003<ref name="Ing&oacute;lfsson et al, 2003">Ing&oacute;lfsson, &Oacute;., Hjort, C. and Humlum, O. 2003. Glacial and Climate History of the Antarctic Peninsula since the Last Glacial Maximum, Arctic, ''Antarctic and Alpine Research'', '''35''', 175-186.</ref>), Ing&oacute;lfsson and Hjort (2002<ref name="Ing&oacute;lfsson and Hjort, 2002">Ing&oacute;lfsson, &Oacute;. and Hjort, C. 2002. Glacial history of the Antarctic Peninsula since the Last Glacial Maximum-a synthesis, ''Polar Biology'', '''21''', 227-234.</ref>), Ing&oacute;lfsson (2004<ref name="Ing&oacute;lfsson, 2004">Ing&oacute;lfsson, &Oacute;. 2004. Quaternary glacial and climate history of Antarctica. In: Ehlers J, Gibbard PL (eds), Quaternary Glaciations - Extent and Chronology, Part III, Elsevier, 3-43.</ref>), Jones et al. (2000<ref name="Jones et al, 2000">Jones, V.J., Hodgson, D.A. and Chepstow-Lusty, A. 2000. Palaeolimnological evidence for marked Holocene environmental changes on Signy Island, Antarctica, ''The Holocene'', '''10''', 43-60.</ref>) and Hodgson et al. (2004a<ref name="Hodgson et al, 2004a">Hodgson, D.A., Doran, P.T., Roberts, D. and McMinn, A. 2004a. Paleolimnological studies from the Antarctic and subantarctic islands. In: Pienitz R, Douglas MSV, Smol JP (eds) Developments in Palaoenvironmental Research. Long-term Environmental Change in Arctic and Antarctic Lakes, 8, Springer, Dordrecht, 419-474.</ref>) and focus on four main periods, namely: (1) the deglaciation history of currently ice-free regions and the Pleistocene-Holocene transition; (2) the period after the early Holocene, (3) the Mid Holocene warm period or Hypsithermal (MHH), and (4) the past 2,000 years with a focus on Neoglacial cooling, the presence of warm periods, the possibility of a Little Ice Age (LIA) like event, and the recent rapid climate changes documented in instrumental and observational records. It should be noted that these climate periods are not always synchronous in different regions of Antarctica which might in part be due to different degrees of chronological control, or to forcing mechanisms operating at different intensities between regions. At present, with the exception of the Antarctic Peninsula (Bentley et al., 2009<ref name="Bentley et al, 2009">Bentley, M.J., Hodgson, D.A., Smith, J.A., &Oacute; Cofaigh, C., Domack, E.W., Larter, R.D., Roberts, S.J., Brachfeld, S., Leventer, A., Hjort, C., Hillenbrand, C-D. and Evans, J. 2009. Mechanisms of Holocene palaeoenvironmental change in the Antarctic Peninsula region, ''The Holocene'', '''19''', 51-69.</ref>), the role of various forcing mechanisms in regional climate change is poorly described.<br />
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In order to allow comparison between the different records in studies where <sup>14</sup>C dates were not calibrated we list the original <sup>14</sup>C dates (<sup>14</sup>C ka BP), together with the upper and lower limits (at 2-std deviations) of the data (cal. ka BP) generated by the radiocarbon calibration method CALIB 5.0.2 (http://calib.qub.ac.uk/calib/). Radiocarbon dates of marine samples were corrected for the reservoir effect by subtracting 1,300 yrs following the Antarctic standard prior to calibration (i.e. the offset from the global marine reservoir was set at 900 years when using the marine calibration curve; Hughen et al. (2004<ref name="Hughen et al, 2004">Hughen, K.A., Baillie, M.G.L., Bard, E., Beck, J.W., Bertrand, C.J.H., Blackwell, P.G., Buck, C.E., Burr, G.S., Cutler, K.B., Damon, P.E., Edwards, R.L., Fairbanks, R.G., Friedrich, M., Guilderson, T.P., Kromer, B., McCormac, G., Manning, S., Ramsey, C.B., Reimer, P.J., Reimer, R.W., Remmele, S., Southon, J.R., Stuiver, M., Talamo, S., Taylor, F.W., Van Der Plicht, J. and Ce, W. 2004. Marine Radiocarbon Age Calibration 0-26 Cal Kyr Bp, ''Radiocarbon'', '''46''',1059-1108.</ref>)). For lacustrine <sup>14</sup>C ages younger than 11 ka cal yr BP the Southern Hemisphere atmospheric calibration curve was used (McCormac et al., 2004<ref name="McCormac et al, 2004">McCormac, F., Hogg, A., Blackwell, P., Buck, C., Higham, T. and Reimer, P. 2004. Shcal04 Southern Hemisphere Calibration 0-11.0 Cal Kyr Bp, ''Radiocarbon'', '''46''', 1087-1092.</ref>); in all other cases the Northern Hemisphere atmospheric calibration curve (Reimer et al., 2004<ref name="Reimer et al, 2004">Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Bertrand, C.J.H., Blackwell, P.G., Buck, C.E., Burr, G.S., Cutler, K.B., Damon, P.E., Edwards, R.L., Fairbanks, R.G., Friedrich, M., Guilderson, T.P., Hogg, A.G., Hughen, K.A., Kromer, B., McCormac G., Manning S., Ramsey C.B., Reimer R.W., Remmele, S., Southon, J.R., Stuiver, M., Talamo, S., Taylor, F.W., Van Der Plicht, J. and Weyhenmeyer, C.E. 2004. Intcal04 Terrestrial Radiocarbon Age Calibration, 0-26 Cal Kyr Bp, ''Radiocarbon'', '''46''', 1029-1058.</ref>) was applied. The dates of deglaciation of the current ice-free regions are largely derived from <sup>14</sup>C dating of fossils in raised beaches, organic material and fossils in lake sediments, peat deposits and bird colonies; they are thus minimum ages since there is an unknown lag time between deglaciation and colonization of the land by biota (e.g. Gore, 1997<ref name="Gore, 1997">Gore, D.B. 1997. Blanketing snow and ice; constraints on radiocarbon dating deglaciation in East Antarctic oases, ''Antarctic Science'', '''9''', 336-346.</ref>; Ing&oacute;lfsson et al., 2003<ref name="Ing&oacute;lfsson et al, 2003">Ing&oacute;lfsson, &Oacute;., Hjort, C. and Humlum, O. 2003. Glacial and Climate History of the Antarctic Peninsula since the Last Glacial Maximum, Arctic, ''Antarctic and Alpine Research'', '''35''', 175-186.</ref>).<br />
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Summary: In general, geological evidence shows that deglaciation of the currently ice-free regions was completed earlier in EA compared with the AP, but all periods experienced a near-synchronous early Holocene climate optimum (11.5-9 ka BP). Marine and terrestrial climate anomalies are apparently out of phase after the early Holocene warm period, and show complex regional patterns but an overall trend of cooling. A warm mid Holocene Hypsithermal is present in many ice, lake and coastal marine records from all three geographic regions, although there are some differences in the exact timing. In EA and the AP (excluding the northernmost islands) the Hypsithermal occurs somewhere between c. 4 and 2 ka BP, whereas at Signy Island it spanned 3.6-3.4 &ndash; 0.9 ka BP. Despite this there are a number of marine records that show a marine-inferred climate optimum between about 7-3 ka BP and ice cores in the RS sector that show an optimum around 7-5 ka BP, and the EPICA Dome C ice core, and some others, show a weak optimum between 7.5 and 3 ka BP. The occurrence of a later Holocene climate optimum in the RS is in phase with a marked cooling observed in ice cores from coastal and inland locations (Masson et al., 2000<ref name="Masson et al, 2000">Masson, V., Vimeux, F., Jouzel, J., Morgan, V., Delmotte, M., Ciais, P., Hammer, C., Johnsen, S., Lipenkov, V.Y., Mosley-Thompson, E., Petit, J.R., Steig, E.J., Stievenard, M. and Vaikmae, R. 2000. Holocene climate variability in Antarctica based on 11 ice-core isotope records, ''Quaternary Research'', '''54''', 348-358.</ref>; Masson-Delmotte et al., 2004<ref name="Masson-Delmotte et al, 2004">Masson-Delmotte, V., Stenni, B. and Jouzel, J. 2004. Common millennial-scale variability of Antarctic and Southern Ocean temperatures during the past 5000 years reconstructed from the EPICA Dome C ice core, ''The Holocene'', '''14''', 145-151.</ref>). These differences in the timing of warm events in different records and regions points to the interplay of a number of mechanisms that we have yet to identify. Thus there is an urgent need for well-dated, high resolution climate records in coastal Antarctica and particularly in the Dronning Maud Land region and specific regions of the AP to fully understand these regional climate anomalies and to determine the significance of the heterogeneous temperature trends being measured in Antarctica today. There is no geological evidence in Antarctica for an equivalent to the Northern Hemisphere Medieval Warm Period, there is only weak circumstantial evidence in a few places for a cool event crudely equivalent in time to the Northern Hemisphere&rsquo;s Little Ice Age but not in phase (Goosse et al., 2004<ref name="Goosse et al, 2004">Goosse, H., Masson-Delmotte, V., Renssen, H., Delmotte, M., Fichefet, T., Morgan, V., Van Ommen, T., Khim, B.K. and Stenni, B. 2004. A late medieval warm period in the Southern Ocean as a delayed response to external forcing? Geophysical Research Letters, 31, L06203, doi:06210.01029/02003GL019140.</ref>), and it is likely beyond the current signal to noise ratio of ice cores to detect the c. 0.5&deg;C change believed to have occurred at that time.<br />
==Pages in this topic==<br />
#[[Holocene climate change in East Antarctica (EA)]]<br />
#[[Holocene climate change in the Antarctic Peninsula (AP)]]<br />
#[[Holocene climate change in the Ross Sea region (RS)]]<br />
==References==<br />
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[[Category:The pre-instrumental period]]<br />
[[Category:The Holocene]]</div>Maintenance scripthttp://acce.scar.org/wiki/Regional_climate_modelsRegional climate models2014-08-06T14:33:53Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Models of the physical and biological environment of the Antarctic]]''<br />
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GCMs have increased in resolution and complexity since the first models were developed in the middle of the 20<sup>th</sup> Century. However, until recently, little attention has been devoted to modeling of the polar regions. This is partly due to the large amount of computer time needed by the complex atmosphere and ocean models, but also due to a lack of observations and knowledge of the cryospheric components, including sea ice and ice shelves as well as snow, glaciers and permafrost. Compared to the effort devoted to development of parameterizations for mid-latitude processes, the cryosphere is under-represented. Nevertheless, there has been and continues to be improvement in the cryospheric components of GCMs. Even so, because GCMs need by definition to be global, they lack the grid resolution to sufficiently represent or parameterize processes that occur on a subgrid scale &ndash; like the scale of the Antarctic Peninsula. While the spatial resolution of GCMs will increase, their capabilities will continue to be limited by computational constraints such as processor speed and disk storage space.<br />
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Because of this, there is a niche for Regional Climate Models (RCMs), which can be either atmosphere-only models or coupled atmosphere-ocean models, and which can use the GCMs to supply boundary conditions.<br />
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==Atmosphere-only regional climate models and stretched-grid global models==<br />
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There are many aspects of the atmospheric physics and thermodynamics that are very specific to the Antarctic region, including for instance strong and persistent surface inversions, katabatic winds, &lsquo;clear-sky&rsquo; precipitation, etc, which pose special challenges to the models. Some, such as clear sky precipitation, still remain a challenge to represent in models. But other aspects, such as the katabatic wind system, are being handled better now that high resolution regional models are being run.<br />
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Such models have been applied to regions of the Arctic and Antarctic with some success. For example the model Polar MM5 (mesoscale model 5), based on the Penn State model MM5 has been used to examine a number of problems in Antarctic meteorology, including the katabatic wind system (Bromwich et al., 2001<ref name="Bromwich et al, 2001">Bromwich, D. H., Cassano, J.J., Klein, T., Heinemann, G., Hines, K.M., Steffen K. and Box, J.E. 2001. Mesoscale modeling of katabatic winds over Greenland with the Polar MM5, ''Mon. Wea. Rev.'', '''129''', 2290-2309.</ref>).<br />
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The group at the Institute for Marine and Atmospheric Research, University of Utrecht, has also used a regional model to examine many aspects of the Antarctic climate. Their RACMO model is based on the German ECHAM4 model and has been used to look at the impact of the SAM on the atmospheric circulation of the Antarctic (van Lipzig et al., 2006<ref name="Lipzig et al, 2006">Van Lipzig, N.P.M., Marshall, G.J., Orr, A. and King, J.C. 2006. The relation between the Southern Hemisphere Annular Mode and Antarctic Peninsula summer temperatures: Analysis of a high-resolution model climatology, J. Clim., 21(8): 1649.</ref>).<br />
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Such models obviously have a lateral boundary, and boundary conditions are usually obtained from a global run of a coarse resolution model. In addition, it is also necessary to specify ocean forcing, such as sea ice extent/concentration and sea surface temperatures.<br />
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Stretched-grid general circulation models avoid the lateral boundary conditions issue in limited-area models. They are global but the grid may be horizontally stretched to refine resolution over a region of particular interest. For Antarctica, this approach was pioneered at the Glaciology Laboratory in Grenoble, France (Krinner et al., 1997<ref name="Krinner et al, 1997">Krinner, G., Genthon, C. Li, Z.-X. and Le Van, P. 1997. Studies of the Antarctic climate with a stretched grid GCM, ''J. Geophys. Res.'', '''102''', 13731-13745.</ref>). The finest (~60 km over Antarctica) climate change predictions to date have been obtained with this technique (Krinner et al., 2008<ref name="Krinner et al, 2008">Krinner, G., Guicherd, G., Ox, K., Genthon, C. and Magand, O. 2008. Simulations of Antarctic climate and surface mass balance change from 1981-2000 to 2081-2100, ''Journal of Climate'', '''21''', 936-962, DOI: 10.1175/2007/JCLI960.1.</ref>; Genthon et al., 2008<ref name="Genthon et al, 2008">Genthon C., Krinner, G. and Castebrunet, H. 2008. Antarctic precipitation and climate change in the IPCC models: Horizontal resolution and margin vs plateau issues, Annals of Glaciology, in press.</ref>).<br />
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==Coupled regional climate models==<br />
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Implementing a limited area, coupled atmosphere-ocean climate model is much more difficult because of the need to obtain both atmospheric and oceanic data at the lateral boundary and to maintain stability in the atmosphere/ocean fluxes. However, the value of such a system is that important features of the Antarctic climate, such as the under ice shelf cavity, which is not present in the coarse resolution global models, can be included.<br />
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Such coupled models are starting to be developed, but there are many challenges in obtaining good coupling between the elements. They will become increasingly important in the future.<br />
==References==<br />
<references /><br />
[[Category:Observations, data accuracy and tools]]<br />
[[Category:Models]]</div>Maintenance scripthttp://acce.scar.org/wiki/RecommendationsRecommendations2014-08-06T14:33:53Z<p>Tonyp: </p>
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<div>'''Chapter Editor:''' John Turner<br />
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'''Authors:''' Robert Bindschadler, Pete Convey, Guido di Prisco, Eberhard Fahrbach, Dominic Hodgson, Paul Mayewski, Colin Summerhayes and John Turner<br />
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Great advances have been made in recent years into our understanding of Antarctic climate and environmental change. We now know that the climate system of the high southern latitudes is very complex and that there is variability on a range of time scales, with consequent effects on the terrestrial and marine biota. We also know that changes in the atmospheric and oceanic circulation around Antarctica, and the volume of the ice sheets, interact and influence climate at a global scale. Although a great deal of data are now available with which to investigate change &ndash; both in the past and over the next century, there are still major gaps in our knowledge and many areas where we require additional instrumental data gathering and model development.<br />
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The recommendations drawn together here summarise the conclusions we have reached in [[Antarctic climate and environment change in the instrumental period|The instrumental period]] and [[Antarctic climate and environment change over the next 100 years|The next 100 years]], which if treated separately would have led to some duplication. Other recommendations can be found in [[Future developments in Antarctic observation]], on specific observing system requirements, and in [[Concluding remarks on the pre-instrumental period]], on research needed to improve the record of past climate change on geological time scales. In addition, scattered throughout the text there are many statements about additional research requirements.<br />
* Collection of more ''in-situ'' data over the interior of the continent. AWSs are providing extremely valuable observations, but greater continuity in the observing programmes is needed and the maintenance of systems at selected sites for long periods. In addition to programmes describing Antarctic climatic variables at the macro-scale, there is an urgent need for the establishment of longer-term monitoring of biologically relevant microclimatic variables (as currently happens only during short-term biological studies), and the subsequent modelling effort to integrate patterns at the macro- and micro-scales.<br />
* More traverses are required and more records extending back to at least 2000 years are needed at sites selected using information gained from ITASE efforts. Coastal ice core records not easily accessible by traverse need to be sampled, and there needs to be further international collaboration to ensure that coverage over the continent is as complete as possible.<br />
* Continuity of space-based measurements is absolutely essential, since these are the sole major source of data for the whole of the continent and its surrounding ocean, where measurements on the ground or on the sea are difficult, dangerous, and not normally made year-round. We must not go blind to ice sheets at the very moment when their behaviour has started to become highly significant in relation to changes in sea level. Recommendations for satellite observations of the cryosphere are given in the Cryosphere Theme document produced for the Integrated Global Observing Strategy (IGOS) Partnership (http://www.eohandbook.com/igosp/cryosphere.htm).<br />
* Improved satellite systems are required to help estimate the mass balance of the Antarctic as we are at the limit of the present technology.<br />
* More observations of temperature, salinity, biochemical properties, such as oxygen and flow in the Southern Ocean are required. It is essential for observing instruments to be developed and deployed in greater numbers throughout the year and for long periods as part of a Southern Ocean Observing System (SOOS) so that long time series of key oceanographic parameters can be obtained with sufficient spatial coverage. Ship borne observations have to be complemented by those from autonomous systems.<br />
* Intensified use of highly sophisticated marine equipment such as ROVs, AUVs, crawlers, gliders, landers, and remote underwater laboratories would contribute to a significant enhancement in understanding Antarctic ecosystem functioning, and, consequently, provide the basis for improved predictions of the marine ecosystem response to climate and other environmental changes. AUVs and sustained underwater measurements are also key to understanding ocean-ice interaction, which has emerged as a primary driver of recent (and therefore likely future) large ice mass losses.<br />
* The markedly different behaviour of Antarctic sea ice in comparison to that found in the Arctic requires special efforts to obtain ''in-situ'' observations of Southern Ocean sea ice properties.<br />
* The Southern Ocean continues to be under sampled with respect to carbon cycle related properties. The international CO<sub>2</sub> community recommends for this region the construction of a CO<sub>2</sub> ocean data observing system and delivery of CO<sub>2</sub> ocean data products. Such data sets will provide valuable observational evidence for understanding historical climate change, and providing valuable insights into how the Southern Ocean may respond to further change. Such initiatives should be an integral component of the Southern Ocean Observing System required for monitoring and forecasting the ocean&rsquo;s role in Antarctic climate change.<br />
* In terms of simulating the Southern Ocean response to historical climate change, some of the largest uncertainties in our results lie in the hydrological cycle (especially precipitation), the most poorly observed of all the forcing fields. To better understand and quantify the changes due to historical and future climate change, better estimates of the freshwater budget will be required.<br />
* Although there are many observational programmes concerned with change in the Antarctic ice sheet, we still have little data on permafrost and the active layer. The lack of long-term monitoring data precludes drawing any definitive conclusions on the impact of climate change on permafrost in Antarctica and this situation needs to be remedied through new observational initiatives.<br />
* The last few years have also seen great advances in our understanding of terrestrial and marine ecosystems, and studies are now starting to address their resistance, resilience and adaptation to recent climate change. However, fundamentally important baseline biodiversity and biogeographic survey data are still lacking across most of the continent and parts of the surrounding Southern Ocean &ndash; those data and systematic and robust monitoring programmes across a network of representative locations are required to allow anything other than the current ''ad hoc'' and serendipitous approach to identifying biological responses to any aspect of environmental change in Antarctica. We also still require much more information on the links between the high latitude biota and broad-scale climatic factors, such as changes in the tropical atmosphere/ocean system (e.g. ENSO) and the modes of mid- and high latitude climate variability (e.g. the SAM).<br />
* We recommend that the international community implement and monitor progress in the establishment of internationally recommended observing systems such as (a) CryOS (the Cryosphere observing system recommended for the IGOS Partners and adopted by the Group on Earth Observations), and (b) the Global Climate Observing System (GCOS).<br />
* The Protocol on Environmental Protection to the Antarctic Treaty provides strict guidelines for the protection of the Antarctic environment and underscores its value to scientific research. Although rigorous application of the Protocol will help minimize the local impacts of both the tourism industry and national operators, constant vigilance is essential. Conservation measures should focus on achieving a better knowledge of the structure and functioning of Antarctic ecosystems and of the long-term effects of persistent contaminants in Antarctic organisms and food chains, and in developing continental-scale monitoring programmes based upon a network of carefully selected flagship sites.<br />
* Higher horizontal and vertical resolution is needed in climate models to realistically represent many high latitude processes and their effects. Models must take into account in far greater detail than at present the complex orography in the coastal region, the behaviour of the atmospheric boundary layer, eddies in the ocean, and the effects of and sea ice. Sub-grid scale processes e.g. sea ice properties affecting atmosphere-ice-ocean interaction require improved parameterisations. Model outputs are also required at smaller physical scales relevant to the Antarctic habitats and communities, including the establishment and expansion of links between macro and microclimatic processes and trends.<br />
* Climate model formulations need to be modified to recognise that parameters based on the behaviour of the atmosphere at low latitudes do not necessarily reflect processes operating in the Polar Regions, where the atmospheric boundary layer is commonly very stable. These models must include more sophisticated representation of the formation and melting of sea ice and its effects. In addition, the models need to be interactively coupled to ice shelf models so that the impact of changes in ocean circulation and water mass delivery below the shelf can be correctly simulated. This will lead to better predictions of sea level changes that might arise from interactions of the waters of the Southern Ocean with the periphery of the Antarctic Ice Sheet.<br />
* Improved atmospheric chemistry needs to be included so that the models can better represent the effects of the &lsquo;ozone hole&rsquo;, including the important polar stratospheric clouds. Greater spatial and temporal resolution are also imperative if biological processes, particularly on land, are to be integrated into future generations of climate models, and to permit objective tests of predictions of biological relevance. Advanced integrative and spatially explicit ecosystem modelling is needed to predict the future of the marine ecosystem. Such an approach demands widespread samples of ecological key species that are representative for ecological sub-systems, such as plankton, benthos or apex predators and long-term measurements of ecological key processes such as the response to acidification, warming and changes in ice cover and food regime.<br />
* Realistic models are urgently required of the mechanical behaviour of the ice sheet and ice shelves in response to forcing by climate change, to underpin forecasts of likely sea-level rise and of the rates of change of ice sheet decay. To achieve this the next generation of ice sheet models must be able to account for rapid dynamical changes to the flow of glaciers and ice streams.<br />
* Modelling efforts are also required to more fully understand the implications of Antarctic and Southern Ocean climate change throughout the Southern Hemisphere and globally, and ''vice versa''.<br />
* More observations are needed of permafrost, along with model predictions of permafrost change. It is import to expand the Circum-Polar Active Layer Monitoring (South) (CALM-S) network. To improve understanding of the development and evolution of permafrost under changing conditions in the Antarctic there needs to be an expanded Global Terrestrial Network for Permafrost sites (GTN-P) in Antarctica.<br />
* A central location should be established for management of Antarctic permafrost, active-layer, and ground ice data.<br />
* The PERMAMODEL should be applied to predict changes in permafrost distribution under different climate change scenarios, particularly along the Antarctic Peninsula and in maritime East Antarctica.<br />
* Continued long-term and large-scale observations of functional and structural changes in ecosystems are essential to assess the sensitivity of ecological key species and to ground-truth predictive models. The establishment of a series of core long-term biological monitoring sites would be extremely beneficial both in documenting biological responses and trends, and allowing explicit tests of predictive hypotheses.<br />
* More data on the marine biota are required for especially poorly studied areas like the Amundsen Sea, as the basis for the simulation of the impact of a warming ocean on marine biodiversity.<br />
* Physiological and genomic studies currently interpreted as indicating vulnerability of certain Antarctic marine biota need placing in more ecologically realistic (longer term) timescales.<br />
* Individual and species level responses (including resilience/resistance) to environmental variability and change require integration across communities, trophic webs and ecosystems.<br />
* Biological colonisation routes and processes require identification and quantification in both terrestrial and marine environments, as does the relative importance of natural and human-mediated contributions to this process.<br />
* Without a baseline biodiversity survey across much of the continent and Southern Ocean, objective documentation of future biological change and assessment of impacts will be impossible.<br />
* Evidence should be sought for the possible effects of ocean acidification in Southern Ocean organisms.<br />
* Comparisons should be made between southern and northern polar processes to shed light on evolutionary pressures and provide insight into gene selection.<br />
* Many of the above recommendations will benefit from continued integration of cross-disciplinary expertise and approaches.<br />
* Considerable improvement is needed in both the quantification of changes in precipitation (requiring an intense field programme), and the parameterization of the processes that drive precipitation. In due course, especially in the Antarctic Peninsula, biologists need to know what proportion of the precipitation is likely to fall as rain, since rain is immediately available to terrestrial biota.<br />
* A better understanding of ecological driving forces within Antarctic ecosystems (terrestrial and marine) must serve as the basis for developing predictive models of the response of the Antarctic biota to climate change.<br />
[[Category:Recommendations]]</div>Maintenance scripthttp://acce.scar.org/wiki/Prospects_for_marine_invasions_by_non-indigenous_speciesProspects for marine invasions by non-indigenous species2014-08-06T14:33:53Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Marine biology over the next 100 years]]''<br />
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Establishment of non-indigenous species (NIS) is widely considered one of the greatest threats to biodiversity and endemic species. The threat these invaders pose once they have arrived in a new location, established themselves, and begun successfully to reproduce, is to outcompete native species for food or space, eat them or even hybridise with some. This has happened all over the planet, on all continents and most islands (even around Antarctica), across land, freshwater and marine habitats. The result is that over long time periods species have increased, decreased or otherwise altered the geography of their distributions. In the last few centuries, humans have radically altered organismal transport vectors, frequencies, journey times and survival prospects. Of the thousands of species travelling, probably only a small fraction survive and, of those, again only few establish themselves and yet fewer become &lsquo;invasive pests&rsquo;. Even in these few aggressive invasive NIS, there is often a long lag phase between arrival and becoming a pest. Once established in terrestrial habitats, NIS have proved very difficult, often practically impossible to remove &ndash; except in the case of large mammals from small islands. In the sea, though, there is not a single case of an invading NIS being successfully removed. As a result, the global fauna is becoming more homogeneous as these few winners spread from port to port and the many losers will be native species with restricted distributions (endemics).<br />
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The potential for NIS arrivals to affect Southern Ocean biodiversity in the coming century is considerable because of: 1) historic Southern Ocean isolation and its domination by endemic species 2) lack of established NIS and the &lsquo;pristine&rsquo; nature of the environment; 3) the slow response time of native organisms because of their extended generation turnover times; 4) the lack of durophagus predators (consuming prey with hard shells and bones); and 5) accelerating transport opportunities for NIS in a region of intense warming. These points are considered in turn below.<br />
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Firstly the Southern Ocean is the largest marine environment to be &lsquo;semi-isolated&rsquo; for long periods of time - by surrounding deep water, the ACC and the Polar Frontal Zone (PFZ) (see Clarke et al., 2005<ref name="Clarke et al, 2005">Clarke, A., Barnes, D.K.A. and Hodgson, D.A. 2005. How isolated is Antarctica? Trends in Ecology and Evolution, 20, 1-3.</ref>). As a result most marine species and many genera are endemic to one or more regions within the Southern Ocean (Arntz et al., 1997<ref name="Arntz et al, 1997">Arntz, W.E., Gutt, J. and Klages, M. 1997. Antarctic marine biodiversity: an overview. In: Battaglia, B. (ed) Antarctic communities: species, structure and survival. Cambridge University Press, 3-14.</ref>). Their loss regionally thus would also mean a global loss. Experience of NIS arrival in regions with high endemism is that invasion tends to lead to drastic reduction and extinction of many endemic species, at least in terrestrial and fresh water environments. No NIS fauna have yet been found to be established in the Southern Ocean, although several species of assumed NIS algae now grow inside the caldera of Deception Island (Clayton et al., 1997<ref name="Clayton et al, 1997">Clayton, M.N., Wiencke, C. and Kl&ouml;ser, H. 1997. New records and sub-Antarctic marine benthic macroalgae from Antarctica, ''Polar Biol.'', '''17''', 141-149.</ref>; C. Wienke pers. com.). Recently the North Atlantic spider crab (''Hyas araneus'') has been recorded from the Antarctic Peninsula (Tavares and Melo, 2004<ref name="Tavares and Melo, 2004">Tavares, M. and Melo, M.E.S. 2004. Discovery of the first known benthic invasive species in the Southern Ocean: the North Atlantic spider crab ''Hyas araneus'' found in the Antarctic Peninsula, ''Antarct. Sci.'', '''16''', 129-131.</ref>), as have larvae of related subantarctic species (Thatje and Fuentes, 2003<ref name="Thatje and Fuentes, 2003">Thatje, S., and Fuentes, V. 2003. First record of anomuran and brachyuran larvae (Crustacea: Decapoda) from Antarctic waters, ''Polar Biology'', '''26''', 279-282.</ref>), but to date none are believed to be established. However, species have been found travelling in or close to the Southern Ocean in ballast water (Lewis et al., 2003<ref name="Lewis et al, 2003">Lewis, P.N., Hewitt, C.L., Riddle, M. and McMinn, A. 2003. Marine introductions in the Southern Ocean: an unrecognised hazard to biodiversity, ''Marine Pollution Bulletin'', '''46''', 213-223.</ref>) or fouling ship hulls (Lewis et al., 2006<ref name="Lewis et al, 2006">Lewis, P.N., Bergstrom, D.M. and Whinam, J. 2006. Barging in: A Temperate Marine Community Travels to the Subantarctic, ''Biological Invasions'', '''8''', 787-795.</ref>) or marine debris (Barnes and Fraser, 2003<ref name="Barnes and Fraser, 2003">Barnes, D.K.A. and Fraser, K.P.P. 2003. Rafting by five phyla on man-made flotsam in the Southern Ocean, ''Mar. Ecol. Prog. Ser.'', '''262''', 289-291.</ref>). Most recently, Lee and Chown (2007<ref name="Lee and Chown, 2007">Lee J.E. and Chown S.L. 2007. Mytilus on the move: transport of an invasive bivalve to the Antarctic, ''Mar. Ecol Progr. Ser.'', '''339''', 307-310.</ref>) found that mussels (''Mytilus galloprovincialis'') had survived a journey to and from the Southern Ocean on the ship Agulhas. This is an extremely aggressive invader that can smother coastal life in the absence of predation and that has been found to successfully breed at just 1&ordm;C (Lewis, unpublished data).<br />
<br />
Macro-organisms native to the Southern Ocean tend to be characterised by slow development, growth and generational turn over (Arntz et al., 1994<ref name="Arntz et al, 1994">Arntz, W.E., Brey, T. and Gallardo, V.A. 1994. Antarctic zoobenthos, ''Oceanogr. Mar. Biol. Ann. Rev.'', '''32''', 241-304.</ref>). The few species for which we have ample age spectra grow very old (over many decades) and many do not reach the age of first breeding for many years. This means that their ability to respond ecologically to competition for space or food is poor. Their long generation time also drastically slows rates of adaptation compared with potential invaders from temperate regions. Even the long lag times taken by many NIS take before they spread aggressively are well within the span of a single generation of some common Antarctic species. A closely linked (and fourth) point is the potential influence NIS durophagus (crushing) predators would have on Antarctic benthos. Southern Ocean shelf communities show many resemblances to community structure in Tertiary times, for example in lacking durophagus predators and having many shallow echinoderm suspension feeders (see Aronson et al., 2007<ref name="Aronson et al, 2007">Aronson, R., Thatje S., Clarke A., Peck L.S., Blake D.B., Wilga C.D., Seibel B.A. 2007. Climate change and invisibility of the Antarctic benthos, ''Annual Review of Ecological and Evolutionary Systems'', '''38''', 129-154.</ref>). Invasion of even a few crushing predators could cause major changes to communities not adapted to such predation. Both the survival of such NIS and the ability of Antarctica&rsquo;s indigenous species to respond depend on the extent of regional warming.<br />
<br />
The fifth point about the vulnerability of Antarctic marine biodiversity to NIS concerns increasing transport opportunities offered by rapid warming, both in the Scotia Arc and West Antarctic Peninsula region. This is the area most visited by tourist and scientific ships &ndash; which are amongst the most likely vectors for marine NIS (Lewis et al., 2003<ref name="Lewis et al, 2003">Lewis, P.N., Hewitt, C.L., Riddle, M. and McMinn, A. 2003. Marine introductions in the Southern Ocean: an unrecognised hazard to biodiversity, ''Marine Pollution Bulletin'', '''46''', 213-223.</ref>, 2006<ref name="Lewis et al, 2006">Lewis, P.N., Bergstrom, D.M. and Whinam, J. 2006. Barging in: A Temperate Marine Community Travels to the Subantarctic, ''Biological Invasions'', '''8''', 787-795.</ref>). It is also the area of most intense warming, and the only region within the Antarctic to show physiologically meaningful sea temperature increases to date (Meredith and King, 2005<ref name="Meredith and King, 2005">Meredith, M.P. and King, J.C. 2005. Rapid climate change in the ocean west of the Antarctic Peninsula during the second half of the 20<sup>th</sup> century, Geophys. Res. Lett., 32, L19604. (doi: 10.1029/2005GL024042)</ref>). Even the slight surface warming projected to occur in the next century, at least regionally, may decrease the ability of some native Southern Ocean fauna to function &ndash; e.g. to avoid predators (Peck et al., 2004<ref name="Peck et al, 2004">Peck, L.S., Webb, K. and Bailey, D. 2004. Extreme sensitivity of biological function to temperature in Antarctic marine species, ''Functional Ecology'', '''18''', 625-630.</ref>). Conversely, rises in temperature might significantly raise the survival chances and competitiveness of temperate NIS. Crucially some crushing predators, like brachyuran crabs require slightly higher temperatures than those currently prevalent. Rising CO<sub>2</sub> levels in the ocean may also result in secretion of aragonite skeletons (shells) becoming more difficult, which would be more of a problem if NIS durophagus predators were to become established.<br />
<br />
Potential invasions of NIS must be considered in the context of past species transport on various time scales. Species have moved into and out of the Southern Ocean both on evolutionary and ecological time scales (see Barnes et al., 2006<ref name="Barnes et al, 2006">Barnes, D.K., Hodgson, D.A., Convey, P., Allen, C.S. and Clarke, A.C. 2006. Incursion and excursion of Antarctic biota: past, present and future, ''Global Ecol Biogeogr'', '''15''', 121-142.</ref>). This will have occurred to some species even without them physically moving, simply because the Polar Front will have moved back and forth across their habitats between glacial and interglacial cycles. For example benthos on the shelf around the Kerguelen Islands will have been inside and outside the Polar Front multiple times (see Moore et al., 1999<ref name="Moore et al, 1999">Moore, J.K., Abbott, M.R. and Richman, J.G. 1999. Location and dynamics of the Antarctic Polar Front from satellite sea surface temperature data, ''Journal of Geophysical Research'', '''104''', 3059-3073.</ref>). It is likely that considerable movements of species occurred in response to glacial-interglacial cycles. It is therefore possible that there may be many species currently outside the Polar Front that will return, not as NIS, but as natives expelled during glacial maxima. The poor fossilisation conditions and destruction of potential fossils by advancing ice sheets during the onsets of glaciations makes it hard for us to know definitively which species are native. Another potential problem with recognising invading NIS is the patchy level of knowledge of Southern Ocean biodiversity. Arguably the most likely arrival areas of NIS do, however, coincide with the best-known regions (the shallow shelf, the Scotia arc, and the western Antarctic Peninsula). Despite the deep water and oceanographic barriers between Antarctic and temperate environments, there are many mechanisms for transport (Clarke et al., 2005<ref name="Clarke et al, 2005">Clarke, A., Barnes, D.K.A. and Hodgson, D.A. 2005. How isolated is Antarctica? Trends in Ecology and Evolution, 20, 1-3.</ref>; Barnes et al., 2006<ref name="Barnes et al, 2006">Barnes, D.K., Hodgson, D.A., Convey, P., Allen, C.S. and Clarke, A.C. 2006. Incursion and excursion of Antarctic biota: past, present and future, ''Global Ecol Biogeogr'', '''15''', 121-142.</ref>). Organisms may travel on floating debris, such as floating volcanic rock - pumice and driftwood. Some organisms may hitch-hike on megafauna (such as fur seals) and diseases can be introduced especially by highly mobile animals - such as avian influenza by albatrosses. Shipping has undoubtedly drastically increased opportunities for NIS, because of rapid travel from temperate ports (hotspots of invasive NIS) and across oceanographic barriers. Both Lewis et al. (2006<ref name="Lewis et al, 2006">Lewis, P.N., Bergstrom, D.M. and Whinam, J. 2006. Barging in: A Temperate Marine Community Travels to the Subantarctic, ''Biological Invasions'', '''8''', 787-795.</ref>) and Lee and Chown (2007<ref name="Lee and Chown, 2007">Lee J.E. and Chown S.L. 2007. Mytilus on the move: transport of an invasive bivalve to the Antarctic, ''Mar. Ecol Progr. Ser.'', '''339''', 307-310.</ref>) have found known invasive NIS associated with ships in the region &ndash; it seems that it is only a matter of time before the first established invading animal is found. How the native fauna will respond to such an invasion will depend on the chance nature of the invader identity, the area it arrives at (isolated island or Western Antarctic Peninsula) and the pace of climate change.<br />
==References==<br />
<references /><br />
[[Category:The next 100 years]]<br />
[[Category:Antarctic biology]]<br />
[[Category:Marine biology]]</div>Maintenance scripthttp://acce.scar.org/wiki/PrefacePreface2014-08-06T14:33:52Z<p>Tonyp: Replaced book Preface with updated content from John Turner</p>
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<div>The Antarctic continent and Southern Ocean are extremely important parts of the global physical/biological system, linked to the rest of the Earth in often highly non-linear ways. Research has shown that to understand how planet Earth works we need to study it increasingly as a system, and understand the lithosphere, the hydrosphere, the cryosphere, the biosphere and the atmosphere. One of the remotest parts of the Earth system is Antarctica, a continent larger than either Australia or Europe. We will not be able to fully understand how the Earth system works without comprehensive knowledge of the physical, biological, chemical and geological processes taking place within and above Antarctica and its surrounding Southern Ocean. That is a huge challenge given that these processes take place among some of the remotest and harshest environments anywhere on the Earth&rsquo;s surface.<br />
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Much has been achieved in acquiring knowledge of Antarctica&rsquo;s physical, biological, chemical and geological processes, especially since a network of permanent scientific stations was established for the first time on the continent during the International Geophysical Year of 1957-58. Many more results also emerged from the [http://www.ipy.org/ International Polar Year] of 2007-2008.<br />
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With the increasing emphasis on cross-disciplinary Antarctic studies in the early years of this century, [http://www.scar.org/ SCAR] started a series of regular meetings that focussed of developing better links between those working in the physical and biological sciences. Out of this grew an initiative that became known as Antarctic Climate Change and the Environment ([http://www.scar.org/othergroups/acce ACCE]), which was seem as a southern counterpart to the Arctic Climate Impact Assessment ([http://www.acia.uaf.edu/ ACIA]). The goal was to prepare a volume that reviewed our present understanding of the physical and chemical climate system of the Antarctic region, the way it varies through time, and the profound influence of that variation on life on land and in the ocean around the continent. It would also examine predictions of how the system would evolve over the next century under conditions of increasing concentrations of greenhouse gases and recovery of the ozone hole.<br />
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Inputs were obtained through a process of open consultation with the wider community in which scientists affiliated with SCAR or known to be active in climate and environmental sciences in Antarctica or the Southern Ocean were asked for text on key topics. A first draft was then circulated to the wider community for comment in June 2008. The [[editors|Editorial Board]] then modified the text for a second round of open consultation in March 2009. Parties to the Antarctic Treaty, representatives of the Commission for the Conservation of Antarctic Marine Living Resources ([https://www.ccamlr.org/ CCAMLR]), and representatives of the Council of Managers of National Antarctic Programmes ([https://www.comnap.aq/ COMNAP]) were also asked for input. Eventually over 100 scientists from various fields contributed to the volume, which was published in 2009 as Antarctic Climate Change and the Environment. They are listed alphabetically as authors of the appropriate chapters after the chapter editor.<br />
<br />
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Five hundred copies of the volume were published and distributed widely to the Antarctic community and beyond. In addition, individual downloadable chapters were made available on the SCAR web site, so as to encourage its widespread use as a research and teaching resource. The volume was a contribution to the International Polar Year 2007-2008 and to the goals of the World Climate Research Programme ([http://www.wcrp-climate.org/ WCRP]), and in particular to its Climate and Cryosphere programme ([http://www.climate-cryosphere.org/ CliC]), of which SCAR is a co-sponsor. It was also made available to attendees at the [http://unfccc.int/meetings/copenhagen_dec_2009/meeting/6295.php meeting of the UN Framework Convention on Climate Change in Copenhagen] in December 2009, and subsequently to the [http://www.ipcc.ch/ Intergovernmental Panel on Climate Change]. Brief, annual updates on advances in Antarctic climate-related science were also provided to the [http://www.ats.aq/e/ats_meetings_atcm.htm Antarctic Treaty Consultative Meetings]. However, Antarctic science is advancing very rapidly with important papers regularly being reported in the popular press. It was therefore felt that a revision to the original ACCE volume would be appropriate. But rather than produce another hardcopy book it was decided to convert the original material into a wiki so as to make it easier to revise the text.<br />
<br />
The preparation of the ACCE wiki was carried out by Mr Tony Phillips and Mr Guy Phillips using the open source Mediawiki software. We are grateful to the [http://www.antarctica.ac.uk/ British Antarctic Survey] for hosting the wiki.</div>Maintenance scripthttp://acce.scar.org/wiki/Precipitation_changes_over_the_21st_centuryPrecipitation changes over the 21st century2014-08-06T14:33:52Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Atmospheric change over the next 100 years]]''<br />
<br />
The net precipitation (in climate models this is precipitation less evaporation and ablation) over Antarctica is an important factor in the mass balance of the continental ice sheet. Surface sublimation and blowing-snow processes also help to determine the local mass balance of the ice sheet, but these factors have a limited contribution at scales larger than 100 km (Genthon, 2004<ref name="Genthon, 2004">Genthon, C., 2004: Space-time Antarctic surface mass-balance variability from climate models, ''Ann. Glaciol.'', '''39''', 271-275.</ref>) and are not included in climate models. Giovinetto et al (1997<ref name="Giovinetto et al, 1997">Giovinetto, M.B., Yamazaki, K., Wendler, G. and Bromwich, D.H. 1997. Atmospheric net transport of water vapor and latent heat across 60 degrees S, ''J. Geophys. Res.'', '''102''', 11171-11179.</ref>) estimated the various terms and found blowing snow to contribute about 6% to the surface mass balance. Hence, quantification of precipitation changes in high southern latitudes is paramount for resolving the uncertainty surrounding the future of the Antarctic ice sheet.<br />
<br />
==Skill in simulation of precipitation==<br />
<br />
Much of the validation in model performance with regard to precipitation focuses on net precipitation, since this quantity can be calculated using a range of methodologies, and, in particular, is less dependent upon problematic and sparse station measurements of snow accumulation. In an analysis used in the AR4, Uotila et al. (2007<ref name="Uotila et al, 2007">Uotila P., Lynch, A.H., Cassano, J.J. and Cullather, R.I. 2007. Changes in Antarctic net precipitation in the 21<sup>st</sup> century based on Intergovernmental Panel on Climate Change (IPCC) model scenarios, ''J. Geophys. Res.'', '''112''', D10107, doi:10.1029/2006JD007482.</ref>) found that only 5 of 15 global climate models examined were able to simulate long term average values of net precipitation consistent with the range of observations in the Twentieth Century. This range falls approximately between 150 and 190 mm/year, although recent analyses suggest the likely figure is at the high end of this range (Monaghan et al. (2006a<ref name="Monaghan et al, 2006a">Monaghan, A.J., Bromwich, D.H. and Wang, S-H. 2006a. Recent trends in Antarctic snow accumulation from Polar MM5, ''Philosophical Trans. Royal. Soc. A'', '''364''', 1683-1708.</ref>) quoted 182 mm/year). Monaghan et al. (2006a<ref name="Monaghan et al, 2006a">Monaghan, A.J., Bromwich, D.H. and Wang, S-H. 2006a. Recent trends in Antarctic snow accumulation from Polar MM5, ''Philosophical Trans. Royal. Soc. A'', '''364''', 1683-1708.</ref>) combined new records from the International Transantarctic Scientific Expedition (ITASE) with existing ice cores, snow pit and snow stake data, meteorological observations, and validated model fields to reconstruct Antarctic snowfall accumulation over the past 5 decades, and concluded that there had been no trend over that time, although a significant increase in the net precipitation has been reported in the immediate area surrounding the South Pole, based on ''in situ'' measurements (Mosley-Thompson et al., 1999<ref name="Mosley-Thompson et al, 1999">Mosley-Thompson, E., J. F. Paskievitch, A. J. Gow, L. G. Thompson, 1999: Late 20<sup>th</sup> century increase in South Pole snow accumulation, J. Geophys. Res., 104(D4), 3877-3886, 10.1029/1998JD200092.</ref>) and on satellite altimetry (Davis et al., 2005<ref name="Davis et al, 2005">Davis, C.H., Li Y., McConnell, J.R., Frey, M.M. and Hanna, E. 2005. Snowfall-driven growth in East Antarctic Ice Sheet mitigates recent sea-level rise, ''Science'', '''308''', 1898-1901.</ref>). It is worth noting that this South Pole area is very small in comparison with the area studied by Monaghan et al. (2006a<ref name="Monaghan et al, 2006a">Monaghan, A.J., Bromwich, D.H. and Wang, S-H. 2006a. Recent trends in Antarctic snow accumulation from Polar MM5, ''Philosophical Trans. Royal. Soc. A'', '''364''', 1683-1708.</ref>), and may well have been affected by the existence of the station there (a significant snow hill is also associated with Byrd&rsquo;s former station on the Ross Ice Shelf).<br />
<br />
Observational uncertainty, including uncertainties in trend detection, means that an assessment of simulation skill remains particularly difficult. Nevertheless, it is known that specific deficiencies remain in the parameterizations of key processes that drive precipitation. This is particularly true of the polar regions, since polar cloud microphysics remains poorly understood. The importance of the moisture physics is further demonstrated by studies such as that by Turner et al. (2006<ref name="Turner et al, 2006">Turner, J., Lachlan-Cope, T.A., Colwell, S.R., Marshall, G.J. and Connolley, W.M. 2006. Significant warming of the Antarctic winter troposphere, ''Science'', '''311''', 1914-1917.</ref>), who found that local thermodynamic processes were a significant component of the climate change signal in the Antarctic winter. In the context of the SHEBA (Surface Heat Budget of the Arctic &ndash; an experiment carried out in the Beaufort and Chukchi Sea region of the Arctic in 1997-1998) experiment in the Arctic, much work has been done on the development of polar cloud physics parameterizations, but significant challenges remain even in fine scale models (Sandvik et al., 2007<ref name="Sandvik et al, 2007">Sandvik, A., Biryulina, M., Kvamsto, N.G., Stamnes, J.J. and Stamnes, K. 2007. Observed and simulated microphysical composition of arctic clouds: Data properties and model validation, J. Geophys. Res., 112, Art. No. D05205.</ref>). In the Antarctic, there has been less focused development of the physical parameterizations needed to better represent clouds and precipitation, largely due to the absence of an intense field programme to provide the necessary data support for such an endeavour. The precipitation simulations in high southern latitudes by global models result in significant biases (e.g. Covey et al., 2003<ref name="Covey et al, 2003">Covey, C., Achutarao, K.M., Cubasch, U., Jones, P., Lambert, S.J., Mann, M.E., Phillips, T.J. and Taylor, K.E. 2003. An overview of results from the Coupled Model Intercomparison Project (CMIP), ''Global Planet. Change'', '''37''', 103-133, doi:10.1016/S0921-8181(02)00193-5.</ref>), and this is also true, though not as severe, in high resolution, limited area models (e.g. Bromwich et al., 2004a<ref name="Bromwich et al, 2004a">Bromwich, D.H., Guo, Z., Bai, L. and Chen, Q-S. 2004a. Modeled Antarctic precipitation. Part I: spatial and temporal variability, ''J. Climate'', '''17''', 427-447.</ref>; Van de Berg et al., 2005<ref name="Berg et al, 2005">Van De Berg, W.J., Van Den Broeke, M.R., Reijmer, C.H. and Van Meijgaard, E. 2005. Characteristics of the Antarctic surface mass balance (1958-2002) using a regional atmospheric climate model, ''Ann. Glaciol.'', '''41''', 97-104.</ref>). Hines et al. (2004<ref name="Hines et al, 2004">Hines, K.M., Bromwich, D.H., Rasch, P.J. and Iacono, M.J. 2004. Antarctic Clouds and Radiation within the NCAR Climate Models, ''J. Climate'', '''17''', 1198-1212.</ref>) found that in one global model, the simulation of Antarctic climate was highly sensitive to the mixing ratio threshold for autoconversion from suspended ice cloud to falling precipitation. Such sensitivity can only be resolved by measurements that are currently not available.<br />
<br />
An important driving mechanism for precipitation in this region is the atmospheric circulation (e.g. Massom et al., 2004<ref name="Massom et al, 2004">Massom, R.A., Pook, M.J., Comiso J.C., Adams, N., Turner, J., Lachlan-Cope, T. and Gibson, T.T. 2004. Precipitation over the interior East Antarctic Ice Sheet related to midlatitude blocking-high activity, ''J. Climate'', '''17''', 1914-1928.</ref>). The contributions of the multi-year and the synoptic time scales are roughly proportional over the coastal regions, but the synoptic time scale dominates the inland precipitation (Cullather et al., 1998<ref name="Cullather et al, 1998">Cullather, R.I., Bromwich, D.H. and Van Woert, M.L. 1998. Spatial and temporal variability of Antarctic precipitation from atmospheric methods, ''J. Climate'', '''11''', 334-367.</ref>). This component exhibits a close relationship with elevation and makes a positive contribution to transporting moisture from the ocean toward the pole. Global models exhibit significant biases in this regard also. Placed in the context of the available re-analyses, the multi-model ensemble created for the AR4 overestimates pressures over the ice sheet in summer and overestimates cyclone depths, particularly in the west Antarctic region, in all seasons (Lynch et al., 2006<ref name="Lynch et al, 2006">Lynch, A., Uotila, P. and Cassano, J.J. 2006. Changes in synoptic weather patterns in the polar regions in the twentieth and twenty-first centuries, Part 2: Antarctic, ''Int. J. Climatol.'', '''26''', 1181-1199.</ref>). Two issues of particular note in driving these deficiencies are surface forcing and spatial scale. With regard to the former, Stratton and Pope (2004<ref name="Stratton and Pope, 2004">Stratton, R.A. and Pope, V.D. 2004. Modelling the climatology of storm tracks - Sensitivity to resolution. In: The Second Phase of the Atmospheric Model Intercomparison Project (AMIP2) [Gleckler, P. (ed.)]. Proceedings of the WCRP/WGNE Workshop, Toulouse, 207-210.</ref>) have noted that Arctic Model Intercomparison Project-style experiments (that is, model simulations with specified sea surface temperatures) do produce correctly located storm tracks, but often even these are more zonally oriented than is observed. Krinner et al. (2007<ref name="Krinner et al, 2007">Krinner, G., Magand, O., Simmonds, I., Genthon, C. and Dufresne J.L. 2007. Simulated Antarctic precipitation and surface mass balance at the end of the twentieth and twenty-first centuries, ''Clim. Dyn.'', '''28''', 215-230.</ref>) have found that errors in net precipitation on regional scales are moderated when observed sea surface conditions are prescribed. With regard to the latter, Bromwich et al. (2004b<ref name="Bromwich et al, 2004b">Bromwich, D.H., Monaghan, A.J. and Guo, Z. 2004b. Modeling the ENSO modulation of Antarctic climate in the late 1990s with the Polar MM5, ''J. Clim.'', '''17''', 109-132.</ref>) noted that the reanalysis products, and therefore probably global models in general, underestimate Antarctic precipitation in the Twentieth Century. This is attributed to the smooth coastal escarpment in a coarse resolution model, which causes cyclones to precipitate less than they do in reality. Further, if the Antarctic Peninsula is not well resolved in a model, it produces too little lee cyclogenesis (Turner et al., 1998<ref name="Turner et al, 1998">Turner, J., Leonard, S., Lachlan-Cope, T., and Marshall, G.J. 1998. Understanding Antarctic Peninsula precipitation distribution and variability using a numerical weather prediction model, ''Ann. Glaciol.'', '''27''', 591-596.</ref>).<br />
<br />
==Projected changes in precipitation==<br />
<br />
[[File:Figure 5.9 - 21st century change in the annual cycle of Antarctic precipitation under the A1B scenario.png|thumb|'''5.9''' Annual cycle of percentage precipitation changes (averaged over the Antarctic continent) for 2080-2099 minus 1980-1999, under the A1B scenario. Thick lines represent the ensemble median of the 21 Multi-model Data Set models. The dark grey area represents the 25% and 75% quartile values among the 21 models, while the light grey area shows the total range of the models.]]<br />
The lack of a Twentieth Century trend in net precipitation in recent comprehensive analyses is particularly problematic in the context of Twenty First Century model projections. Almost all climate models simulate a continuing robust precipitation increase over Antarctica in the coming century (see [[:File:Figure 5.9 - 21st century change in the annual cycle of Antarctic precipitation under the A1B scenario.png|Figure 5.9]]) The projected precipitation change has a seasonal dependency, and is larger in winter than in summer. In some models, there is also a phase shift, so that, for example, a narrow early winter peak in precipitation evolves to a broad winter peak (Krinner et al., 2007<ref name="Krinner et al, 2007">Krinner, G., Magand, O., Simmonds, I., Genthon, C. and Dufresne J.L. 2007. Simulated Antarctic precipitation and surface mass balance at the end of the twentieth and twenty-first centuries, ''Clim. Dyn.'', '''28''', 215-230.</ref>). Wild et al. (2003<ref name="Wild et al, 2003">Wild, M., Calanca, P., Scherrer, S.C. and Ohmura, A. 2003. Effects of polar ice sheets on global sea level in high-resolution greenhouse scenarios, ''J. Geophys. Res.'', '''108''', 4165, doi:10.1029/2002JD002451.</ref>) reported that, in a simulation in which the CO<sub>2</sub> concentration in the atmosphere doubles, the annual net accumulation over Antarctica increases by 22 mm/year. Similarly, Huybrechts et al. (2004<ref name="Huybrechts et al, 2004">Huybrechts, P., Gregory, J., Janssens, I. and Wild, M. 2004. Modelling Antarctic and Greenland volume changes during the 20<sup>th</sup> and 21<sup>st</sup> centuries forced by GCM time slice integrations, ''Glob. Planet. Change'', '''42''', 83-105.</ref>) analyzed results from an ice sheet model driven by a climate model simulation in which the CO<sub>2</sub> concentration doubles in 60 years, and found an associated increase of 15% to 20% in mean Antarctic precipitation. The increasing net precipitation in most climate models reflects warmer air temperatures and associated higher atmospheric moisture. Evaporation increases also, but does not keep pace with the precipitation increases. Two models, ECHO-G and GFDL-2.1, project a small decrease of net precipitation after the middle of the Twenty First Century. Interestingly, these models fall at either end of the quality scale in assessments of their ability to reproduce Antarctic synoptic climate, according to Uotila et al. (2007<ref name="Uotila et al, 2007">Uotila P., Lynch, A.H., Cassano, J.J. and Cullather, R.I. 2007. Changes in Antarctic net precipitation in the 21<sup>st</sup> century based on Intergovernmental Panel on Climate Change (IPCC) model scenarios, ''J. Geophys. Res.'', '''112''', D10107, doi:10.1029/2006JD007482.</ref>).<br />
<br />
[[File:Figure 5.10 - 21st century change in days with precipitation exceeding five times the daily mean.png|thumb|'''5.10''' Number of days with precipitation exceeding five times the annual daily mean - relative change from the end of the twentieth to the end of the Twenty First Century (in percent). From Krinner et al. (2007<ref name="Krinner et al, 2007">Krinner, G., Magand, O., Simmonds, I., Genthon, C. and Dufresne J.L. 2007. Simulated Antarctic precipitation and surface mass balance at the end of the twentieth and twenty-first centuries, ''Clim. Dyn.'', '''28''', 215-230.</ref>).]]<br />
Only one study has attempted to address the projected changes in the intensity of precipitation. Krinner et al. (2007<ref name="Krinner et al, 2007">Krinner, G., Magand, O., Simmonds, I., Genthon, C. and Dufresne J.L. 2007. Simulated Antarctic precipitation and surface mass balance at the end of the twentieth and twenty-first centuries, ''Clim. Dyn.'', '''28''', 215-230.</ref>) calculated the difference in the number of days per year with daily precipitation exceeding five times the mean daily precipitation for the simulations using the LMDZ4 stretched grid atmospheric model (see [[:File:Figure 5.10 - 21st century change in days with precipitation exceeding five times the daily mean.png|Figure 5.10]]) This particular model experiment suggests that even using this relatively modest measure, which does not allow for the increase in total precipitation, the number of relatively strong precipitation events near the ice sheet domes and ridges increases, particularly in East Antarctica. This indicates an increased frequency of intrusions of moist marine air, in spite of a projected lower future cyclone frequency.<br />
<br />
A separate, and biologically significant, requirement for inclusion in prediction of future precipitation patterns is the ability to differentiate between precipitation as snow and as rain. While this is not relevant across most of East Antarctica, it is a change that is already apparent along the western Antarctic Peninsula and Scotia arc archipelagos. Precipitation falling as rain is immediately available to terrestrial biota, and hence has some more direct impacts on terrestrial ecosystem changes than that falling as snow.<br />
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The scatter among the individual models reported on in the AR4 is considerable. During the first half of the Twenty First Century, models using the A1B scenario predict anything from a maximum upward trend of 0.71 mm/year to a maximum downward trend of 0.13 mm/year. Some models project stronger net precipitation increases in the first half of the Twenty First Century, while other models project stronger increases after the middle of the century. Hence, the confidence in any conclusions arising from AR4 or this report regarding the future trajectory of Antarctic precipitation is extremely limited.<br />
<br />
The study of Bracegirdle et al. (2008<ref name="Bracegirdle et al, 2008">Bracegirdle, T.J., Connolley, W.M. and Turner, J. 2008. Antarctic climate change over the Twenty First Century, Journal of Geophysical Research &ndash; Atmospheres, 113, D03103, doi:03110.01029/02007JD008933.</ref>) estimated that by the end of the century the snowfall rate over the continent would increase by 20% compared to current values, which, if other effects such as melting and dynamical discharge are ignored, would result in a negative contribution to global sea-level rise of approximately 5 cm.<br />
==References==<br />
<references /><br />
[[Category:The next 100 years]]<br />
[[Category:The Antarctic atmosphere]]<br />
[[Category:Precipitation]]</div>Maintenance scripthttp://acce.scar.org/wiki/Pathways_of_research_in_cold-adapted_organisms_and_climate_changePathways of research in cold-adapted organisms and climate change2014-08-06T14:33:51Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Marine biology over the next 100 years]]''<br />
<br />
The development of research on biological adaptations to polar environmental conditions is relatively recent. Adaptations of the polar ichthyofauna in response to environmental change are commanding attention, and the effects of climate change on biodiversity are increasingly considered. There is ample evidence that recent climate changes already caused physiological problems to a broad range of species, drive evolutionary responses (Thomas C D et al., 2001, 2004; Walther et al., 2002<ref name="Walther et al, 2002">Walther, G-R., Post, E., Convey, P., Menzel, A., Parmesan, C., Beebee, T.J.C, Fromentin, J-M., Hoegh-Guldberg, O. and Bairlein, F. 2002. Ecological responses to recent climate change, ''Nature'', '''416''', 389-395.</ref>), and produce micro-evolutionary changes in some species (Rodriguez-Trelles and Rodriguez, 1998<ref name="Rodriguez-Trelles and Rodriguez, 1998">Rodriguez-Trelles, F. and Rodriguez, M.A. 1998. Rapid micro-evolution and loss of chromosomal diversity in Drosophila in response to climate warming, ''Evol. Ecol.'', '''12''', 829-838.</ref>). But species do not live in isolation and it is necessary to evaluate their responses at community and ecosystem levels. Ecologists and physiologists are thus faced with the difficult challenge of predicting the effect of warming not only on individual species, but also on whole communities. For instance, ice-shelf collapse increases the number of icebergs, enhancing the impact by scouring on benthic biodiversity as well as on the food web by altering regional and local current patterns. Although changes in sea temperature are as yet small, increased warming may cause sub-lethal effects on physiological performance and potential disruption in ecological relationships (Clarke et al., 2007<ref name="Clarke et al, 2007">Clarke, A., Murphy, E.J., Meredith, M.P., King, J.C., Peck, L.S., Barnes, D.K.A. and Smith, R.C. 2007. Climate change and the marine ecosystem of the western Antarctic Peninsula, ''Phil. Trans. R. Soc. B'', '''362''', 149-166.</ref>).<br />
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Most of the work at the molecular and ecological levels in cold-adapted habitats has concentrated on Antarctic fish species. Understanding the impact of past, current and predicted environmental change on biodiversity and the consequences for Antarctic-ecosystem adaptation and function is a primary goal. Examination of Antarctic ecosystems undergoing change provides a major contribution to the understanding of evolutionary processes relevant to life on Earth. Key questions include: &ldquo;How well are Antarctic organisms able to cope with daily, seasonal and longer-term environmental changes?&rdquo; and &rdquo;Will climate change result in either relaxation of selection pressure on genomes, or tighter constraints and ultimately the extinction of species and populations?&rdquo;<br />
<br />
Many of the same questions are being asked for the Arctic, not least through pressure from commercial fishing. Geography, oceanography and biology of species inhabiting Arctic and Antarctic polar regions have often been compared (e.g. Dayton et al., 1994<ref name="Dayton et al, 1994">Dayton, P.K., Mordida, B.J. and Bacon, F. 1994. Polar marine communities, ''Am. Zool.'', '''34''', 90-99.</ref>) to outline the differences between the two ecosystems. The northern polar region is characterised by extensive, shallow shelf seas surrounding a largely land-locked ocean, whereas the southern polar region comprises a dynamic and open ocean and a very deep continental shelf (Smetacek and Nicol, 2005<ref name="Smetacek and Nicol, 2005">Smetacek, V. and Nicol, S. 2005. Polar ocean ecosystems in a changing world, ''Nature'', '''437''', 362-368.</ref>). The sea north of the Arctic Circle is almost completely enclosed and influenced by large human populations in extensively colonised terrestrial areas, as well as by industrial activities. The exchange of seawater through the passage between Greenland and the Svalbard Islands was not possible until 27 Ma (Eastman, 1997<ref name="Eastman, 1997">Eastman, J.T. 1997. Comparison of the Antarctic and Arctic fish faunas. Cybium, 12, 276-287.</ref>). The Arctic region was in a high-latitude position by the early Tertiary, but the climate remained temperate with water temperatures of 10-15&deg;C. During the Miocene, about 10-15 Ma ago, Arctic land masses reached their current positions and it is thought that only at that time temperatures dropped below freezing (&ldquo;unipolar ice-sheet mode)&rdquo;]. Some studies indicate that glaciation events in the Arctic began 10-6 Ma ago, whereas in Antarctica glaciation started much earlier (Eastman, 1977). However, recent evidence revises the timing of the earliest Arctic cooling events, strongly supporting the indication of a &ldquo;bipolar symmetry&rdquo; in climate cooling. This suggests simultaneous evolution of ice at the poles and therefore a bipolar transition from &ldquo;greenhouse&rdquo; to &ldquo;icehouse&rdquo;, pointing out the importance of greenhouse-gas changes in driving global climate patterns. According to this revision, the earliest Arctic cooling events are dated much earlier, to approx. 45 Ma.<br />
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Although high latitudes and cold climates are common to both the Antarctic and the Arctic, in many respects the two regions are more dissimilar than similar. The modern polar faunas differ in age, levels of endemism, taxonomic composition, zoogeographic distinctiveness, and in the range of physiological tolerance to various environmental parameters. Because of the isolating barrier of the Polar Front, the climatic features of the Antarctic waters are more extreme and constant than those of the Arctic, where the range of temperature variation is wider, thus facilitating migration and redistribution of the fauna.<br />
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In summary, the Arctic is the connection between the more extreme Antarctic oceanic system and temperate and tropical systems. Comparison of the ecosystems is likely to provide evolutionary insights into the relationship between environment and evolutionary adaptation. We have a remarkable opportunity to develop comparative studies on evolutionary differences between cold-adapted species and on how organisms from the polar habitats are affected by (and respond to) climate change. Comparing southern and northern polar processes may shed light on evolutionary pressures and provide insight into gene selection.<br />
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A detailed assessment of the impacts of climate change in the Arctic has been published (ACIA, 2005<ref name="ACIA, 2005">ACIA, 2005. Arctic Climate Impact Assessment. Cambridge University Press, 1042p.</ref>). ACCE will hopefully provide a similar contribution for the Antarctic.<br />
<br />
Although the impacts of climate change on polar environments are exceeding those envisaged for other regions, and will produce feedbacks with global consequences, they remain difficult to predict because of the complexity of biological responses (Anisimov et al., 2007<ref name="Anisimov et al, 2007">Anisimov, O.A., Vaughan, D.G., Callaghan, T.V., Furgal, C., Marchant, H., Prowse, T.D., Vilhj&aacute;lmsson, H. and Walsh, J.E. 2007. In: Climate Change 2007: impacts, adaptation and vulnerability. Contribution of working group II to the fourth assessment. Report of the Intergovernmental Panel on Climate Change (IPCC), pp. 653-685 (Parry ML, Canziani OF, Palutikof JP, van der Linden PJ, Hanson CE, Eds). Cambridge University Press, Cambridge, UK.</ref>). Climate change may affect every aspect of an organism's biology, from cellular physiology and biochemistry to food web and habitat. Organisms must alter their physiology and biochemistry to cope with changes in enzyme activity and DNA damage, by means of phenotypic responses (occurring within the lifetime by enzyme activation/inhibition and induction/repression of gene regulation), and genotypic responses (occurring over a much longer timescale through the selection of beneficial mutations). Understanding the adaptation-response mechanisms in species living in both polar habitats may help also to understand change at lower latitudes.<br />
<br />
In addition to adaptation, other key research themes include studies of life cycles (tactics and strategies for responding to environment features), micro-evolutionary processes driven by anthropogenic impacts, interactions between changing abiotic conditions (e.g. temperature, UV-B) and biotic responses, modelling interactions between environmental change and organism responses (to facilitate predictions of change), and development of conservation policies in relation to improved understanding of the response of ecosystems to change.<br />
<br />
The challenge for the next decade will be to incorporate the physiological/biochemical viewpoint into the field of evolutionary biology. Such integration may provide more detailed answers than we can provide here to the question of how Antarctic and Arctic biota may respond to global warming, and the extent to which they will be able to adjust to it. Another challenge will be to determine the ability of polar species to repair the effects of changes induced by a wide variety of natural and anthropogenic processes, in the general framework of species and ecosystem responses to change.<br />
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Intensified communication between scientists should lead to a multidisciplinary approach to studying the ecosystem. Analyses of adaptive evolution across the biological spectrum from molecules to species must integrate physiology, biochemistry/molecular biology, morphology, taxonomy, biogeography, ecology, and ethology. Studying the response of evolutionary processes to changes in selection pressures demands collaboration between biologists, physical scientists and modellers. Investigating changes in the physical environment that have driven evolution over geological time requires collaboration with palaeontologists, paleoclimatologists, geophysicists, glaciologists and oceanographers. Statistical and molecular genetic approaches are needed to monitor changes in biodiversity. The multidisciplinary approach will allow links to be established between tectonics, climate change, glacial processes and evolution. For example, palaeobiological data can be used to assess the age of Antarctic habitats and species; these results can then be combined with molecular estimates of divergence time and disturbances of the mechanisms of adaptation.<br />
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The main aim of such cooperation is to identify key ecological processes and their physiological underpinning by using advanced multivariable community analyses and models, as well as mechanistic studies and mechanism-based models. This approach demands an intensive coordination, not only of ongoing, but also of future research activities. Only sound estimation of large-scale biodiversity as a result of evolutionary processes, as well as a synoptic mapping of the most relevant ecological parameters, such as currents, sedimentation, bottom topography, ice cover, will allow verification of different kinds of direct or indirect anthropogenic or natural impacts on benthic and pelagic communities. Physiological studies specify the sensitivities of marine organisms to environmental factors and thereby support a mechanistic understanding of the driving forces behind the patterns observed. Improved communication will help answer questions such as: How will shore systems develop when they no longer experience physical disturbance by sea ice? How will offshore systems change if pelagic algae replace ice-algae - will the zooplankton or the benthos benefit? What are the consequences for apex-predators, e.g. the polar bear in the Arctic?<br />
<br />
This multidisciplinary approach will allow a series of important targets to be tackled including:<br />
<ol start="1"><br />
<li>Links between tectonics, climate evolution, glacial processes and biotic evolution. In particular, we plan to continue to refine our understanding of how the present biota evolved, and why current patterns of biological diversity are what they are. Palaeobiological data can be used to assess the age of Antarctic habitats and species. These results can then be combined with molecular estimates of divergence time to provide a powerful approach to understanding Antarctic biotic evolution (as was used very successfully to determine the history of the notothenioid radiation by the ESF-funded Network on the Biology of Antarctic Fishes). Palaeobiological data can also determine the nature and origin of latitudinal diversity gradients.</li><br />
<li>Links between the physical environment and gene flow. Models of oceanic and atmospheric circulation can be used to predict transport of propagules into (and out of) the Antarctic. Such models can also be used to elucidate advective processes in the Southern Ocean and their impact on gene flow and population dynamics.</li><br />
<li>Links with northern polar studies. Comparison of southern with northern polar processes can elucidate significant evolutionary pressures and provide insight into gene selection.</li><br />
</ol><br />
<br />
Organisms have a limited number of responses that enhance survival in changing environments. They can:<br />
<ol start="1"><br />
<li>Use the margins of internal physiological flexibility and capacity to sustain new biological requirements. Species inhabiting coastal seabed sites around Antarctica are thought to have poorer physiological capacities to deal with change than species elsewhere (Peck, 2005<ref name="Peck, 2005">Peck, L.S. 2005. Prospects for survival in the Southern Ocean: vulnerability of benthic species to temperature change, ''Antarctic Sci.'', '''17''', 497-507.</ref>; Peck et al., 2007<ref name="Peck et al, 2007">Peck, L.S., Morley, S.A., P&ouml;rtner, H.O. and Clark, M.S. 2007. Thermal limits of burrowing capacity are linked to oxygen availability and size in the Antarctic clam ''Laternula elliptica''. Oecologia. Published on line DOI 10.1007/s00442-007-0858-0.</ref>; P&ouml;rtner et al., 2007<ref name="P&ouml;rtner et al, 2007">P&ouml;rtner, H.O., Peck, L.S. and Somero, G.N. 2007. Thermal limits and adaptation: an integrative view (Antarctic Ecology: From Genes to Ecosystems), ''Phil. Trans. R. Soc. B'', '''362''', 2233-2258.</ref>). Experimental data suggest that they die when temperatures are raised by 5&ndash;10&deg;C above the annual average, at which point many species lose the ability to perform essential functions, e.g. swimming in scallops or burying in infaunal bivalve molluscs when temperatures are raised only 2&ndash;3&deg;C (Peck et al., 2004<ref name="Peck et al, 2004">Peck, L.S., Webb, K. and Bailey, D. 2004. Extreme sensitivity of biological function to temperature in Antarctic marine species, ''Functional Ecology'', '''18''', 625-630.</ref>). In short, the margin range appears narrow, thus the efficiency of this ability may be poor. However, the rate at which the temperatures are increased in laboratory experiments is vastly faster than what occurs in nature, and behavioural change may be quite different at much slower rates of environmental change.</li><br />
<li>Adapt to the new conditions and alter the range of biological capacity. This strategy depends on the magnitude and rate of change, and aquatic habitats change temperature at a far slower rate than terrestrial ones, possibly creating fewer adaptation problems for most marine species. The ability to adapt, or evolve new characters to changing conditions depends on many factors including mutation rate, number of gametes produced per reproductive event, number of reproductive events and generation time. Antarctic benthic species grow more slowly than those from lower latitudes (Barnes et al., 2007<ref name="Barnes et al, 2007">Barnes, D.K.A., Webb, K.E. and Linse, K. 2007. Growth rate and its variability in erect Antarctic bryozoans, ''Polar Biol.'', '''30''', 1069-1081.</ref>; Peck, 2002<ref name="Peck, 2002">Peck, L.S. 2002. Ecophysiology of Antarctic marine ectotherms: limits to life, ''Polar Biology'', '''25''', 31-40.</ref>) and develop at rates often 5 &ndash;10 times slower than similar temperate latitude species (Peck, 2002<ref name="Peck, 2002">Peck, L.S. 2002. Ecophysiology of Antarctic marine ectotherms: limits to life, ''Polar Biology'', '''25''', 31-40.</ref>; Peck et al., 2006<ref name="Peck et al, 2006">Peck, L.S., Convey, P. and Barnes, D.K.A. 2006. Environmental constraints on life histories in Antarctic ecosystems: tempos, timings and predictability, ''Biol. Rev.'', '''81''', 75-109.</ref>). They also live to great age, and exhibit deferred maturity (Peck et al., 2006<ref name="Peck et al, 2006">Peck, L.S., Convey, P. and Barnes, D.K.A. 2006. Environmental constraints on life histories in Antarctic ecosystems: tempos, timings and predictability, ''Biol. Rev.'', '''81''', 75-109.</ref>). Data on numbers of embryos produced per reproductive event are scarce. However, there is a cline of increasing size in eggs with latitude (Clarke, 1992<ref name="Clarke, 1992">Clarke, A. 1992. Reproduction in the cold: Thorson revisited, ''Invert. Reprod. and Dev.'', '''22''', 175-184.</ref>). This means fewer eggs are produced per unit effort. Fertilisation kinetic studies also reveal that around two orders of magnitude more sperm are needed for successful fertilisation of eggs of Antarctic marine invertebrates than in temperate species (Powell et al., 2001<ref name="Powell et al, 2001">Powell, D.K., Tyler, P.A. and Peck, L.S. 2001. Effect of sperm concentration and sperm ageing on fertilization success in the Antarctic soft-shelled clam ''Laternula elliptica'' and the Antarctic limpet ''Nacella concinna'', ''Mar. Ecol. Prog. Ser.'', '''215''', 191-200.</ref>). From this and the egg data it is clear that fewer embryos are produced per unit reproductive effort by polar species. Longer generation times and fewer embryos reduce the opportunities to produce novel mutations, and result in poorer capacities to adapt to change than in similar species at lower latitudes.</li><br />
<li>Migrate to sites where conditions are favourable for survival. This depends on ability to disperse and availability of suitable sites. Intrinsic capacities to colonise new sites and migrate away from deteriorating conditions depend on adult abilities to travel over large distances, or for reproductive stages to drift for extended periods. Antarctic benthic species with pelagic (swimming or within the water column) phases have extremely long development times compared to lower latitude species (Peck, 2002<ref name="Peck, 2002">Peck, L.S. 2002. Ecophysiology of Antarctic marine ectotherms: limits to life, ''Polar Biology'', '''25''', 31-40.</ref>). This means their larvae spend extended periods in the water column. However, the balance of species with pelagic phases, compared with purely protected development, appears to be significantly lower in some Antarctic groups, especially molluscs. These groups (without pelagic dispersal phases) clearly have lower dispersal capabilities and capacities to migrate. The geographic context is also important here, and whereas most continents have coastlines extending over a wide range of latitude, Antarctica is almost circular in outline, is isolated from other oceans by the circumpolar current, and its coastline covers few degrees of latitude. In a warming environment this geographical constraint could be construed as supplying few places to migrate to. In contrast, Antarctic species do show unusually wide bathymetric ranges. With a deep shelf and strong connectivity with the continental slope, lack of latitudinal scope for migration may be compensated for by bathymetric migration possibilities. On all three major criteria, Antarctic benthic species can appear less capable than species elsewhere of responding to change in ways that can enhance survival. However, evidence of the vulnerability of Antarctic fauna to climate change is not yet clear cut (for arguments see Barnes and Peck, 2008<ref name="Barnes and Peck, 2008">Barnes, D.K.A. and Peck, L.S. 2008. Vulnerability of Antarctic shelf biodiversity to predicted climate change, ''Climate Research'', '''37''', 149-163.</ref>).</li><br />
</ol><br />
<br />
Although the absence of wide latitudinal gradients in the Antarctic coastal region minimises the advantage of along-coast migration for survival, it highlights the importance of the sub-Antarctic as a critical research area, largely populated by (eurythermal) fish having a broad temperature tolerance. These fish live in a more variable environment, where changes might be faster and larger than in the High Antarctic. Historically, the sub-Antarctic may have been a site of long-term acclimation, because some cold-adapted notothenioids also inhabit sub-Antarctic waters (e.g. South Georgia, Bouvet), where in shallow waters the temperature may reach +4&deg;C. The same concept can also be applied to invertebrates and warm-blooded animals. Because knowledge of their physiological performance is limited it is not known whether microbes behave in a similar way.<br />
<br />
This discussion of latitudinal gradients ignores the fact that both the Ross and Weddell Seas penetrate to high latitudes (close to 85&ordm;S). However, that only becomes significant for most organisms in the event that the ice shelves occupying those shelf seas melt, which is unlikely in the 100 year time scale considered here.<br />
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In 2004, to address many of the current questions about the evolution of the biota in the face of climate change, the Scientific Committee on Antarctic Research (SCAR) launched an 8 year international scientific research programme &ldquo;Evolution and Biodiversity in the Antarctic: the Response of Life to Change&rdquo; (EBA). EBA integrates research across a wide variety of fields, from functional genomics and molecular systematics to ecosystem science and modelling, and draws on and contributes information to a wide rage of related fields, such as climate modelling and tectonics. Its main goal is to provide a platform for interactions between disciplines and researchers to improve understanding of the role of biodiversity in the Earth System and its responses to change, by offering the Antarctic context, and establishing cross-links with the Arctic, thereby enhancing the knowledge needed to support attempts to achieve a sustainable future for all life. EBA will provide SCAR and the international scientific community with the best possible estimate of the consequences for the Antarctic of continued environmental change.<br />
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New information, including the choice of suitable target species, long-term data sets and the concerted efforts from international multidisciplinary programmes, will help EBA to identify the responses of vulnerable species and habitats to climate change. This preliminary step is required to establish efficient strategies aimed at neutralising threats to biodiversity: in particular, before they become hopelessly irreversible, those that are essentially driven by anthropogenic contributions. EBA was selected by ICSU/WMO (the International Council for Science and the World Meteorological Organisation) as a &ldquo;Lead Project&rdquo; for the International Polar Year (IPY 2007-2008). This timely programme enabled the scientific community to address the increasing concerns expressed by the Antarctic Treaty Parties about the responses of Antarctic environments to natural and anthropogenic disturbances, and their request for information regarding ways in which these responses can be distinguished and mitigated to ensure long-term conservation of Antarctic environments and their biodiversity.<br />
<br />
In summary, areas being investigated and internationally coordinated include:<br />
* Cryptic species: to what extent may we have underestimated the diversity of the Antarctic biota and characteristics of Antarctic species?<br />
* Radiations: when did the key radiations of the Antarctic taxa take place?<br />
* Impact of glaciation at sea (evolutionary links between continental shelf and slope or deep-sea species).<br />
* Phylogeography: geographical and bathymetric structure and relationships in the polar biomes.<br />
* Population structure and dynamics in the context of evolutionary biology.<br />
* Dispersal: immigration and emigration of organisms, dispersal, and role of humans as vectors.<br />
* Genetic structure of populations, and extent to which population structures reflect past evolutionary history.<br />
* The extent to which spatially separated populations of polar organisms interact at some level and, consequently, must be considered as metapopulations.<br />
* The role of advective/transport processes in the gene flow and population structure of marine polar organisms.<br />
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In the broader sense such studies will enable biologists:<br />
* To understand the evolution and diversity of life;<br />
* To determine how evolution and diversity have influenced the properties and dynamics of present ecosystems and the global ocean system;<br />
* To make predictions on how organisms and communities are responding and will respond to current and future environmental change.<br />
==References==<br />
<references /><br />
[[Category:The next 100 years]]<br />
[[Category:Antarctic biology]]<br />
[[Category:Marine biology]]</div>Maintenance scripthttp://acce.scar.org/wiki/Ocean_models_of_the_Southern_OceanOcean models of the Southern Ocean2014-08-06T14:33:51Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Models of the physical and biological environment of the Antarctic]]''<br />
<br />
==Introduction==<br />
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All numerical models involve a compromise between cost and physical realism. This is especially true for ocean models because the computational cost is always high, and can be hundreds of times that of a comparable atmospheric model.<br />
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This underlying conflict arises because the scale of ocean features is small compared with the size of the ocean and because the computer effort required is proportional to the cube of the horizontal resolution. However, the time step of integration may be longer for the ocean models because velocities are smaller (stability condition depends on speed). Thus, although ocean models need to resolve smaller scales, they save on time-step compared to atmosphere models.<br />
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Horizontal scales in the ocean are usually determined by the Rossby radius - a measure of how far the lowest internal wave mode can travel before being affected by the Earth's rotation. In the sub-tropics where ocean models were first developed, the Rossby radius is typically 25 km (in the atmosphere the Rossby radius is nearer 250 km). In polar regions, where the ocean is less stratified, this can drop to 8 km or less. The weak stratification also means that the influence of bottom topography is much stronger in polar regions, so when steep topography is involved a fine horizontal resolution is again required.<br />
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The lack of sufficient synoptic data for initialising and validating ocean models is also an issue because it means that any estimate of skill has to be based largely on qualitative judgements. Even so, because we have a good theoretical understanding of the problems involved, long model runs usually show up gross errors. We also have satellites, which provide good data on the surface fields, and the Argo programme is starting to provide a good data set for the subsurface and near surface layers. Detailed studies of density currents, the sea ice field and other key regions provide further local checks on model performance.<br />
<br />
==The major model choices==<br />
<br />
Most ocean models represent the spatial structure of the ocean by storing the model variables on a regular horizontal and vertical grid. A few (Iskandarani et al., 2003<ref name="Iskandarani et al, 2003">Iskandarani, M., Haidvogel, D.B. and Levin, J.C. 2003. A three-dimensional spectral element model for the solution of the hydrostatic primitive equations, ''Journal of Computational Physics'', '''186''', 397-425.</ref>) split the ocean into finite elements within which the spatial variation is represented by linear or higher order functions. The latter approach is widely used by engineers modelling steady state structures, but it has not been widely adopted by oceanographers, possibly because of cost. If the finite elements are not on a regular grid there are also problems with spurious reflection and refraction.<br />
<br />
Arakawa (1966<ref name="Arakawa, 1966">Arakawa, A. 1966. Computational design for long-term numerical integration of the equations of fluid motion: Two-dimensional incompressible flow, Part 1, ''Journal of Computational Physics'', '''1''', 119-143.</ref>) investigates the five standard ways that velocity and other model variables can be arranged on a regular horizontal grid. Of these he shows that two, the Arakawa B and C grids, are most accurate at representing the large-scale circulation of the ocean and atmosphere. The B-grid is slightly better at representing geostrophic flows within the ocean, i.e. flows in which the main balance is between the horizontal pressure gradient and the Coriolis force. For this reason the B-grid is used by many large-scale ocean models such as MOM (Griffies et al., 2004<ref name="Griffies et al, 2004">Griffies, S.M., Harrison, M.J., Pacanowski, R.C. and Rosati, A. 2004. A technical guide to MOM4. GFDL ocean group technical report No. 5. NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, USA.</ref>) and OCCAM (Coward and de Cuevas, 2005<ref name="Coward and Cuevas, 2005">Coward, A. and De Cuevas, B. 2005. The OCCAM 66 level model: model description, physics, initial conditions and external forcing, Tech. rep., Southampton Oceanography Centre, Internal Document No. 99, 83 pp. (http://eprints.soton.ac.uk/18792).</ref>; Webb and de Cuevas, 2007<ref name="Webb and Cuevas, 2007">Webb, D.J. and De Cuevas, B.A. 2007. On the fast response of the Southern Ocean to changes in the zonal wind, ''Ocean Science'', '''3''', 417-427.</ref>).<br />
<br />
The C-grid is slightly better at representing gravity waves and so is usually used for models near the coast where tides are important or where turbulent effects are large, so that flows are not geostrophic. It is also used for some large-scale models (Penduff et al., 2007<ref name="Penduff et al, 2007">Penduff, T., Le Sommer, J., Barnier, B., Treguier, A.-M., Molines, J.-M., and Madec, G., 2007: Influence of numerical schemes on current-topography interactions in 1/4&deg; global ocean simulations, ''Ocean Science'', '''3''', 509-524.</ref>), where the improved representation of gravity waves makes it easier to add sea ice. On the B-grid, gravity waves on the 'black' and 'white' sub-grids (to use a chess analogy) are only weakly coupled. In regions where internal waves are active this can produce an apparently noisy temperature field in the surface layers, which, if left uncorrected, affects the sea ice field.<br />
<br />
In the vertical dimension, three standard schemes are used. Many, following the original Bryan and Cox model, use fixed horizontal layers (Bryan and Cox, 1968<ref name="Bryan and Cox, 1968">Bryan, K. and Cox, M.D. 1968. A non-linear model of an ocean drivem by wind and differential heating: Part 1. Description of the Three-dimensional velocity and density fields, ''Journal of the Atmospheric Sciences'', '''25''' (6), 945-967.</ref>; Semtner, 1974<ref name="Semtner, 1974">Semtner, A.J. 1974. An oceanic general circulation model with bottom topography, Technical Report No. 9, Dept. of Meteorology, UCLA, Los Angeles, CA 90095, USA.</ref>; Cox, 1984<ref name="Cox, 1984">Cox, M.D. 1984. A primitive equation 3-dimensional model of the ocean. Tech. Rep. 1, Geophys. Fluid Dyn. Lab, Natl. Oceanic and Atmos. Admin., Princeton Uni. Press, Princeton, N.J.</ref>). Together with the horizontal grid this generates a 3-D array of grid-boxes, each model variable nominally placed at the centre of each box defining the mean value of the variable within the box. Modern versions usually extend the vertical grid to include a free upper surface, allowing tides and other waves, and a variable thickness bottom box in each column, for better representation of topography.<br />
<br />
The one major problem of the horizontal level scheme arises because most density surfaces within the ocean are gently sloping and there is little mixing between water masses of different density. The level scheme only allows fluxes through the horizontal and vertical faces of each box and this produces spurious numerical mixing between water masses of different density. To overcome this, isopycnal models have been developed (Bleck et al., 1992<ref name="Bleck et al, 1992">Bleck, R., Rooth, C., Hu, D. and Smith, L.T. 1992. Ventilation patterns and mode water formation in a wind- and thermodynamically driven isopycnic coordinate model of the North Atlantic, ''Journal of Physical Oceanography'', '''22''', 1486-1505.</ref>; Hallberg, 1997<ref name="Hallberg, 1997">Hallberg, R.W. 1997. Stable split time stepping schemes for large-scale ocean modelling, ''Journal of Computational Physics'', '''135''', 54-6.</ref>; Chassignet et al., 2007<ref name="Chassignet et al, 2007">Chassignet, E.P., Hurlburt, H.E., Smedstad, O.M., Halliwell, G.R., Hogan, P.J., Wallcraft, A.J., Baraille, R. and Bleck, R. 2007. The HYCOM (HYbrid Coordinate Ocean Model) data assimilative system, ''Journal of Marine Systems'', '''65''', 60-83.</ref>) in which the layers correspond to constant potential density surfaces within the ocean. The scheme works well in removing numerical mixing. It can also handle overflows well. There are problems in handling mixed layers (Gnanadesikan et al., 2007<ref name="Gnanadesikan et al, 2007">Gnanadesikan, A., Griffies, S.M. and Samuels, B.L. 2007. Effects in a climate model of slope tapering in neutral physics schemes, ''Ocean Modelling'', '''16''', 1-16.</ref>), in handling the non-linear effect of temperature on compressibility (so really a unique constant potential density surface does not exist) and in handling regions where weak stratification means that only a few model layers are present. However the gain from reduced numerical mixing is often more important.<br />
<br />
A third scheme, used most often near coastlines, is to split the water column into a fixed number of layers (Haidvogel et al., 1991<ref name="Haidvogel et al, 1991">Haidvogel, D.B., Wilkin, J.L. and Young, R.E. 1991. A semi-spectral primitive equation ocean circulation model using vertical sigma and orthogonal curvilinear horizontal coordinates, ''Journal of Computational Physics'', '''94''', 151-185.</ref>; Song and Haidvogel, 1994<ref name="Song and Haidvogel, 1994">Song, Y.T. and Haidvogel, D. 1994. A semi-implicit ocean circulation model using a generalized topography following coordinate system, ''Journal of Computational Physics'', '''115''', 228-248.</ref>; Blumberg and Mellor, 1987<ref name="Blumberg and Mellor, 1987">Blumberg, A.F. and Mellor, G.L. 1987. A description of a three-dimensional coastal ocean circulation model, In Three-Dimensional Coastal Ocean Models, Heaps, N.S. (Ed.), 1-16, American Geophysical Union, Washington, DC.</ref>; Mellor, 2003<ref name="Mellor, 2003">Mellor, G.L. 2003. Users guide for a three-dimensional, primitive equation, numerical ocean model (June 2003 version), 53 pp., Prog. in Atmos. and Ocean. Sci, Princeton University.</ref>). This then gives extra vertical resolution in the shallow water. In deep water it suffers from the same numerical mixing problem as fixed layer models (Willebrand et al., 2001<ref name="Willebrand et al, 2001">Willebrand, J., Barnier, B., Boening, C., Dieterich, C., Killworth, P., Le Provost, C., Jia, Y., Molines, J. and New, A.L. 2001. Circulation characteristics in three eddy-permitting models of the North Atlantic, ''Progress in Oceanography'', '''48''', 123-161.</ref>). The sloping surfaces also introduce small errors in calculating the horizontal pressure gradient (Shchepetkin and McWilliams, 2003<ref name="Shchepetkin and McWilliams, 2003">Shchepetkin, A.F. and McWilliams, J.C. 2003. A method for computing horizontal pressure-gradient force in an oceanic model with a nonaligned vertical coordinate, Journal of Geophysical Research, 108 (c3), 3090, doi:10.1029/2001JC001047.</ref>). This does not matter in turbulent shallow water but in the deep ocean it can generate spurious currents.<br />
<br />
Once the grid is chosen, grid variables are used to represent the standard momentum and tracer equations describing the ocean (Griffies, 2006<ref name="Griffies, 2006">Griffies, S.M. 2006. Some ocean model fundamentals. in: Ocean Weather Forecasting: an integrated view of Oceanography. E. P. Chassignet and J. Verron, eds., Berlin, Germany: Springer, 19-74.</ref>). They are written so that momentum, heat and salinity are conserved, but as they miss sub-gridscale processes, they can only provide approximate solutions to the full set of differential equations.<br />
<br />
Most of the errors discussed can be reduced by using higher order numerical schemes or by using finer horizontal or vertical resolution. There is even a suspicion that we are starting to see a convergence in the results obtained from the different fine resolution models, so that in the end computational efficiency may be the only criteria left at this level of model choice.<br />
<br />
==Sub-grid scale and process models==<br />
<br />
Within the large scale model framework there are usually a number of process models representing the surface mixed layer, the bottom boundary layer, sea ice, floating ice shelves, small scale mixing and other processes &ndash; especially those that act at a scale smaller than the model resolution. Usually such process models can be adapted for use in all of the different large-scale models and, because a large variety of such process models have been developed, there is usually a selection available for each run of a large scale model (Griffies et al., 2004<ref name="Griffies et al, 2004">Griffies, S.M., Harrison, M.J., Pacanowski, R.C. and Rosati, A. 2004. A technical guide to MOM4. GFDL ocean group technical report No. 5. NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, USA.</ref>). The skill of the final model may depend critically on the choices made at this stage.<br />
<br />
==Sea ice and the ice shelves==<br />
<br />
Sea ice in the Southern Ocean differs from that of the Arctic in two key ways. First, away from the coastline, the ice usually lasts for only a single year, so complex multi-year ice models are not essential. Secondly, the primarily westerly winds mean that the flow is generally divergent, so modelling of ice rheology is not so essential. As a result Southern Ocean models that use simple ice models (i.e. Semtner, 1976<ref name="Semtner, 1976">Semtner, A. 1976. A model for the thermodynamic growth of sea ice in numerical investigations of climate, ''Journal of Physical Oceanography'', '''6''' (3), 379-389.</ref>; Hibler, 1979<ref name="Hibler, 1979">Hibler III, W.D. 1979. A dynamic-thermodynamic sea ice model, ''Journal of Physical Oceanography'', '''9''', 815-846.</ref>), give good results away from the coastlines. Near the coast, especially in parts of the Weddell and Ross Seas, more complicated models are required. In such areas the Hunke and Dukowicz (1997<ref name="Hunke and Dukowicz, 1997">Hunke, E.C. and Dukowicz, J.K. 1997. An Elastic&ndash;Viscous&ndash;Plastic Model for Sea Ice Dynamics, ''Journal of Physical Oceanography'', '''27''' (9), 1849-1867.</ref>) elastic&ndash;viscous&ndash;plastic scheme is often chosen.<br />
<br />
In the past the ocean under floating ice shelves has often been ignored in ocean models. Recently the situation has changed, and successful models of the flows under such ice shelves have been developed. These are discussed later. As confidence in these models develops they are likely to be more widely adopted.<br />
<br />
==Overflows and bottom boundary layer==<br />
<br />
The cooling and ice formation processes that occur on the continental shelf around Antarctica result in an important source of very dense water which eventually sinks down the continental slope to form some of the densest waters of the deep ocean. This 'overflow' occurs in a very thin layer, typically 30 m thick, which has proved almost impossible to represent correctly unless the model itself has a finer vertical resolution (Legg et al., 2006<ref name="Legg et al, 2006">Legg, S., Hallberg, R.R. and Girton, J.B. 2006. Comparison of entrainment in overflows simulated by z-coordinate, isopycnal and non-hydrostatic models, ''Ocean Modelling'', '''11''' (1-2), 69-79.</ref>).<br />
<br />
Initially it was thought that analytic sub-grid scale models could be used (Baringer and Price, 1997<ref name="Baringer and Price, 1997">Baringer, M.O. and Price, J.F. 1997. Momentum and Energy Balance of the Mediterranean Outflow, ''Journal of Physical Oceanography'', '''27''' (8), 1678-1692.</ref>), but with realistic flows and topography these were found to be unstable. The lack of a realistic sub-grid model has its greatest effect on horizontal or z-layer models but schemes such as those of D&ouml;scher and Beckmann (2000<ref name="D&ouml;scher and Beckmann, 2000">D&ouml;scher, R. and Beckmann, A. 2000. Effects of a Bottom Boundary Layer Parameterization in a Coarse-Resolution Model of the North Atlantic Ocean, ''Journal of Atmospheric and Oceanic Technology'', '''17'''(5), 698-707.</ref>) can be used to reduce the error.<br />
<br />
Isopycnal models can be more successful as long as one of the density layers corresponds to the thin descending plume (Willebrand et al., 2001<ref name="Willebrand et al, 2001">Willebrand, J., Barnier, B., Boening, C., Dieterich, C., Killworth, P., Le Provost, C., Jia, Y., Molines, J. and New, A.L. 2001. Circulation characteristics in three eddy-permitting models of the North Atlantic, ''Progress in Oceanography'', '''48''', 123-161.</ref>). Density is not fixed but depends non-linearly on both pressure and water properties, so the assumed existence of constant potential density layers produces small errors. Also the large range of densities found in the ocean, and the necessity of modelling small density differences in regions of weak stratification, means that the number of density layers in the model needs to be large.<br />
<br />
Sigma coordinates have the advantage that extra resolution can be provided near the bottom. The bottom layer also follows the descending plume, its thickness increasing with depth. In principal, constant thickness lower layers can also be added to the z-layer models, the normal preference for global ocean models.<br />
<br />
For long-term climate integrations, the realistic representation of overflows remains a major problem that still needs to be solved. Its importance arises because it affects the replenishment of bottom waters and thus the large-scale vertical structure of the ocean. However for the study of short-term and near surface processes, the effect of any such error is usually small.<br />
<br />
==Upwelling, subduction and the mixed layer==<br />
<br />
Offshore in the Southern Ocean, key processes include the upwelling of dense water in the south, the transport northwards of this water in the surface Ekman layer, and the mixing and sinking of intermediate waters in the north. If the wind stress is correct, then momentum conservation ensures that the total transport in the surface Ekman layer is also correct. The velocity of the Ekman layer, which affects sea ice, depends on near surface mixed layer processes, and these vary from model to model. Thus for any research involving sea ice, a good mixed-layer model is essential.<br />
<br />
Available mixed layer models include those of Pacanowski and Philander (1981<ref name="Pacanowski and Philander, 1981">Pacanowski, R.C. and Philander, S.G.H. 1981. Paramerization of vertical mixing in numerical models of tropical oceans, ''Journal of Physical Oceanography'', '''11''', 1443-1451.</ref>), Mellor and Yamada (1982<ref name="Mellor and Yamada, 1982">Mellor, G.L. and Yamada, T. 1982. Development of a turbulent closure model for geophysical fluid problems, ''Geophysics and Space Physics'', '''20''', 851-875.</ref>), Price et al. (1986<ref name="Price et al, 1986">Price, J.F., Weller, R.A. and Oinkel, R. 1986. Duirnal cycling: Observations and models of the upper ocean response to diurnal heating, cooling and wind mixing, ''Journal of Geophysical Research'', '''91''', 8411-8427.</ref>), Large et al. (1994<ref name="Large et al, 1994">Large, W.G., Williams, J.C. and Doney, S.C. 1994. Oceanic vertical mixing: a review and a model with nonlocal boundary layer parameterization, ''Reviews of Geophysics'', '''32''', 363-403.</ref>) and Gaspar et al. (1990<ref name="Gaspar et al, 1990">Gaspar, P., Gregoris, Y., and Lefevre, J.-M. 1990. A simple eddy kinetic energy model for stimulations of the oceanic vertical mixing: tests at station Papa and Long-Term Upper Ocean Study Site, ''Journal of Geophysical Research'', '''95''', 16179-16193.</ref>), and extend from simple bulk models to detailed turbulent closure models. Their effectiveness has not been seriously tested with the range of conditions (ice, stratification and surface forcing) found in the Southern Ocean, so at present all should be used with caution.<br />
<br />
Both upwelling and subduction involve advection of water along sloping density layers. This is handled best by isopycnal models (Willebrand et al., 2001<ref name="Willebrand et al, 2001">Willebrand, J., Barnier, B., Boening, C., Dieterich, C., Killworth, P., Le Provost, C., Jia, Y., Molines, J. and New, A.L. 2001. Circulation characteristics in three eddy-permitting models of the North Atlantic, ''Progress in Oceanography'', '''48''', 123-161.</ref>). However subduction also involves interaction with the mixed layer and capping at the end of winter and here the isopycnal layer models have problems (Large and Nurser, 1998<ref name="Large and Nurser, 1998">Large, W.G. and Nurser, A.J.G. 1998. Ocean Surface Water Mass Transformations, 317-336 in: Eds: Siedler, G., Church, J. and Gould, J., Ocean Circulation and Climate. Academic Press, San Diego, 715 pp.</ref>).<br />
<br />
==Mixing &ndash; tides, topography, currents==<br />
<br />
The representation of mixing in the ocean is a huge subject, which can only be briefly discussed here. Near the surface the effect of wind and breaking surface waves is included in the mixed layer models. On continental shelves the extra effect of bottom turbulence due to the currents may fully mix the water column, and as tides produce their own currents, their influence also needs to be included.<br />
<br />
Away from the boundaries, vertical mixing occurs primarily due to breaking internal waves. The energy for these waves may come from the wind acting via the surface mixed layer, from the propagation of internal tides and from the interaction of currents with bottom topography. However the processes are still only poorly understood. Most mixing models represent such effects by simple Laplacian diffusion, possibly with larger values near topography. Recent research indicates that in the Southern Ocean mixing is largest in areas of strong bottom currents so there is a case for increasing the values in these regions as well. However while numerical mixing remains a problem it is likely that, except in isopycnal models, the effective vertical mixing will be too large.<br />
<br />
Horizontal mixing is also important in the ocean, especially in frontal regions where gradients are large. In low-resolution models the main sub-gridscale process that needs to included is baroclinic instability. The Gent and McWilliams scheme (Gent and McWilliams, 1990<ref name="Gent and McWilliams, 1990">Gent, P.R. and McWilliams, J.C. 1990. Isopycnal mixing in ocean circulation models, ''Journal of Physical Oceanography'', '''20''', 150-155.</ref>; Gent et al. 1995<ref name="Gent et al, 1995">Gent, P.R, Willebrand, J., McDougall, T.J. and McWilliams, J.C. 1995. Parameterizing eddy-induced tracer transports in ocean circulation models, ''Journal of Physical Oceanography'', '''25''', 463-474.</ref>; Griffies, 1998<ref name="Griffies, 1998">Griffies, S.M. 1998. The Gent&ndash;McWilliams Skew Flux, ''Journal of Physical Oceanography'', '''28''', 831-841.</ref>) has been used to represent such processes, but there are concerns about how realistic it is, especially near the ocean surface or bottom topography. As a result, if frontal regions are important then it is best to use a model with a resolution of less than the Rossby radius.<br />
<br />
Finally, because the Southern Ocean is only weakly stratified, bottom topography effectively steers the currents throughout the whole water column. It is therefore essential that topography is accurately represented. If smoothing is carried out, as it is usually done in sigma coordinate models to reduce errors in the pressure term, then the errors produced by the smoothing need to be addressed.<br />
<br />
==Available models of the Southern Ocean==<br />
<br />
For large-scale studies of the Southern Ocean it is probably best to start with the fine resolution global models for which model data is readily available. These are OCCAM and the Parallel Ocean Model POP (Maltrud and McClean, 2005<ref name="Maltrud and McClean, 2005">Maltrud, M.E. and McClean, J.L. 2005. An eddy resolving global 1/10 degrees ocean simulation, ''Ocean Modelling'', '''8'''(1-2), 31-54.</ref>; Collins et al., 2006<ref name="Collins et al, 2006">Collins, W.D. and coauthors. 2006. The Community Climate System Model Version 3 (CCSM3), ''Journal of Climate'', '''19''', 2122-2143.</ref>) with a resolution of 0.1 degrees or less. POP is available in both the original and the NCAR community versions.<br />
<br />
OCCAM and POP are related to the original Bryan-Cox-Semtner code. If you want to run or develop your own version of this code, the best supported version is MOM (Griffies et al., 2004<ref name="Griffies et al, 2004">Griffies, S.M., Harrison, M.J., Pacanowski, R.C. and Rosati, A. 2004. A technical guide to MOM4. GFDL ocean group technical report No. 5. NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, USA.</ref>, 2005<ref name="Griffies et al, 2005">Griffies, S.M., Gnanadesikan, A., Dixon, K.W., Dunne, J.P., Gerdes, R., Harrison, M.J., Rosati, A., Russell, J.L., Samuels, B. L., Spelman, M.J., Winton, M. and Zhang, R. 2005. Formulation of an ocean model for global climate simulations, ''Ocean Science'', '''1''', 45-79.</ref>) but both POP and OCCAM have made code available, for example, for developing biological models (Popova et al., 2006<ref name="Popova et al, 2006">Popova, E.E., Coward, A.C., Nurser, G.A., De Cuevas, B., Fasham, M.J.R. and Anderson, T.R. 2006. Mechanisms controlling primary and new production in a global ecosystem model &ndash; Part I: Validation of the biological simulation, ''Ocean Science'', '''2''', 249-266.</ref>). The NEMO (Nucleus for European Modelling of the Ocean), ORCA025 and ORCA12 (Madec et al., 1998<ref name="Madec et al, 1998">Madec, G., Delecluse, P., Imbard, M. and L&eacute;vy, C. 1998. OPA8.1 Ocean general circulation model reference manual. Notes de l'IPSL, Universit&eacute; P. et M. Curie, B102 T15-E5, 4 place Jussieu, Paris cedex 5, N&deg;11, 91p.</ref>) models are primitive equation models adapted for regional and global ocean circulation problems. With a resolution of &frac14;&deg; or 1/12&deg; (Madec, 2008<ref name="Madec, 2008">Madec, G. 2008. &quot;NEMO ocean engine&quot;. Note du Pole de mod&eacute;lisation, Institut Pierre-Simon Laplace (IPSL), France, No 27 ISSN No 1288-1619.</ref>) NEMO is intended to be a flexible tool for studying the ocean and its interactions with the others components of the Earth&rsquo;s climate system (atmosphere, sea-ice, biogeochemical tracers etc) over a wide range of space and time scales.<br />
<br />
Other global studies which have been carried out at lower resolution include the HYCOM (Chassignet et al., 2006<ref name="Chassignet et al, 2006">Chassignet, E.P., Hurlburt, H.E., Smedstad, O.M., Halliwell, G.R., Wallcraft, A.J., Metzger, E.J., Blanton, B.O., Lozano, C., Rao, D.B., Hogan, P.J. and Srinivasan, A. 2006. Generalized vertical coordinates for eddy-resolving global and coastal ocean forecasts. Oceanography, 19, 20-31.</ref>, 2007<ref name="Chassignet et al, 2007">Chassignet, E.P., Hurlburt, H.E., Smedstad, O.M., Halliwell, G.R., Hogan, P.J., Wallcraft, A.J., Baraille, R. and Bleck, R. 2007. The HYCOM (HYbrid Coordinate Ocean Model) data assimilative system, ''Journal of Marine Systems'', '''65''', 60-83.</ref>) and POM (Mellor, 2003<ref name="Mellor, 2003">Mellor, G.L. 2003. Users guide for a three-dimensional, primitive equation, numerical ocean model (June 2003 version), 53 pp., Prog. in Atmos. and Ocean. Sci, Princeton University.</ref>) models. HYCOM is an isopycnal model adapted to use level coordinates in the near surface layer. POM is a widely used sigma coordinate model. At low resolution we are also starting to see more operational models. These combine one of the regular models with data from satellites and other sources to provide a more accurate view of the ocean and its circulation (Chassignet and Verron, 2006<ref name="Chassignet and Verron, 2006">Chassignet, E.P. and Verron, J. 2006. Ocean Weather Forecasting: An Integrated View of Oceanography. (Eds.). Springer, 577pp.</ref>).<br />
<br />
==Regional Models==<br />
<br />
A major problem that arises when developing regional models is how best to deal with the open boundary. Flow through the boundary usually dominates the large-scale circulation within the region under study, so any errors seriously affect the results. In such cases the best solution is to specify the boundary conditions using data taken from one of the global models. A less satisfactory solution is to specify the boundary conditions using climatology.<br />
<br />
At the largest scale there have been three major models that cover just the Southern Ocean. The earliest, FRAM, was a rigid-lid level model without sea ice. It relaxed to climatology at the surface and at the open boundary (FRAM Group, 1991<ref name="FRAM Group, 1991">FRAM Group (Webb, D.J. et al). 1991. An Eddy-Resolving Model of the Southern Ocean. EOS, Transactions, ''American Geophysical Union'', '''72'''(15), 169-174.</ref>).<br />
<br />
The later BRIOS model is a sigma co-ordinate model based on Haidvogel's SPEM code (Haidvogel et al., 1991<ref name="Haidvogel et al, 1991">Haidvogel, D.B., Wilkin, J.L. and Young, R.E. 1991. A semi-spectral primitive equation ocean circulation model using vertical sigma and orthogonal curvilinear horizontal coordinates, ''Journal of Computational Physics'', '''94''', 151-185.</ref>), which uses a Hibler type ice model. The Southern Ocean versions use 24 layers in the vertical and have a horizontal resolution of 1.5 degrees or less, with finer resolution in areas such as the Weddell Sea (Beckmann et al., 1999<ref name="Beckmann et al, 1999">Beckmann, A., Hellmer, H.H. and Timmermann, R. 1999. A numerical model of the Weddell Sea: large-scale circulation and water mass distribution, J. Geophys. Res. 104(C10), 23375-23,391.</ref>). It is important because it is the first of the large-scale models to include the ocean under the ice-shelves.<br />
<br />
Another important large-scale model is the isopycnal model which Hallberg and Gnanadesikan (2006<ref name="Hallberg and Gnanadesikan, 2006">Hallberg, R. and Gnanadesikan, A. 2006. The role of eddies in determining the structure and response of the wind-driven Southern Hemisphere overturning: Initial results from the Modelling Eddies in the Southern Ocean project, ''J. Phys. Oceanogr.'', '''36''', 3312-3330.</ref>) used to investigate the effect of horizontal resolution in the Southern Ocean. The model uses 20 density layers but has no sea ice. Of the three model types this is probably the best suited to studies of the transport of mid-depth water masses through the ocean.<br />
<br />
==The BRIOS Model==<br />
<br />
In the past, most large-scale models ignored the regions of ocean under the ice shelves. This was a serious omission but it arose because no suitable computer codes had been developed for handling the revised upper boundary condition.<br />
<br />
Such codes have now been developed, one of the first to be widely used being part of the BRIOS model, discussed above. It was originally used to study flows in the Weddell Sea region (Beckmann et al., 1999<ref name="Beckmann et al, 1999">Beckmann, A., Hellmer, H.H. and Timmermann, R. 1999. A numerical model of the Weddell Sea: large-scale circulation and water mass distribution, J. Geophys. Res. 104(C10), 23375-23,391.</ref>; Timmermann et al., 2002<ref name="Timmermann et al, 2002">Timmermann, R., Hellmer, H.H. and Beckmann, A. 2002. Simulations of ice-ocean dynamics in the Weddell Sea 2. Interannual variability 1985-1993, ''J. Geophys. Res.'', '''107'''(C3), 3025, doi:10.1029/2000JC000742.</ref>) but has also been used elsewhere around Antarctica (Assmann et al., 2003<ref name="Assmann et al, 2003">Assmann, K., Hellmer, H.H. and Beckmann, A. 2003. Seasonal variation in circulation and water mass distribution on the Ross Sea continental shelf, Antarctic Science, 15 (1), 3-11 (doi: 10.1017/S0954102003001007).</ref>). The model extends the sigma coordinate scheme under the ice shelves with the 'ocean surface' coordinate following the bottom contour of the ice shelf. As a result, vertical resolution is good under the ice shelf. The fact that the model can be run in circumpolar mode also means that problems with the open boundary condition have little effect on the shelf circulation. As with other sigma coordinate models the main problems are due to numerical mixing and the necessity to smooth topography to reduce pressure gradient errors.<br />
<br />
==Isopycnal model==<br />
<br />
An alternative approach is that of Holland (Holland et al., 2003<ref name="Holland et al, 2003">Holland, D.M., Jacobs, S.S. and Jenkins, A. 2003. Modelling the ocean circulation beneath the Ross Ice Shelf, Antarctic Science, 15 (1), 13-23 (doi: 10.1017/S0954102003001019).</ref>; Jenkins et al., 2004<ref name="Jenkins et al, 2004">Jenkins, A., Holland, D.M., Nicholls, K.W., Schr&ouml;der, M. and &Oslash;sterhus, S. 2004. Seasonal ventilation of the cavity beneath Filchner-Ronne Ice Shelf simulated with an isopycnic coordinate ocean model, ''J. Geophys. Res.'', '''109''', C01024, doi:10.1029/2001JC001086.</ref>), who has modified the MICOM isopycnal model to include ice shelves. As with other pure isopycnal schemes, vertical resolution is obtained by making a judicious choice of model density levels for the area under study. The model typically uses ten density layers, but the weak stratification of some regions of the ice cavity means that only a few layers are involved, so the effective vertical resolution can be very coarse. However the advantage of the method is that it does not suffer from numerical mixing in the same way as the sigma coordinate model, and there is no pressure gradient error. The model is thus a useful independent check on the circulation.<br />
<br />
==The model of Dinniman et al. (2007<ref name="Dinniman et al, 2007">Dinniman, M.S., Klinck, J.M. and Smith Jr., W.O. 2007. Influence of sea ice cover and icebergs on circulation and water mass formation in a numerical circulation model of the Ross Sea, Antarctica, ''Journal of Geophysical Research'', '''112''', C11013, (doi:10.1029/2006JC004036).</ref>)==<br />
<br />
A second sigma coordinate model for use under the ice shelves has been developed by Dinniman et al. (2007<ref name="Dinniman et al, 2007">Dinniman, M.S., Klinck, J.M. and Smith Jr., W.O. 2007. Influence of sea ice cover and icebergs on circulation and water mass formation in a numerical circulation model of the Ross Sea, Antarctica, ''Journal of Geophysical Research'', '''112''', C11013, (doi:10.1029/2006JC004036).</ref>), based on the ROMS model (Shchepetkin and McWilliams, 2005<ref name="Shchepetkin and McWilliams, 2005">Shchepetkin, A. F. and McWilliams, J. C. 2005. The regional oceanic modeling system (ROMS): a split-explicit, free-surface, ''topography-following-coordinate oceanic model. Ocean Modelling'', '''9'''(4), 347-404.</ref>). Both this and the BRIOS model use 24 layers in the vertical with a concentration of layers towards the top and bottom, so the main differences are at the process model level. Thus where the BRIOS model uses a Hibler ice model, Dinniman et al. preferred to impose an ice climatology based on satellite observations. They did this because during the period of study (2001-2003) the sea ice was affected by large ice islands and behaved in a complex way, which was unlikely to be reproduced by a standard ice model. Such parallel developments need be encouraged because of the insights they give into the strengths and weaknesses of different approaches.<br />
<br />
==Under Ice Shelf Models==<br />
<br />
It is important to have realistic models of the ocean flow under ice shelves because of the crucial role that the shelves play in the climate system of the Antarctic, and their sensitivity to changes in water masses. At present global and regional climate models do not include sub-ice shelf cavities; these must be included in the future and the current generation of ocean/shelf models provides a step in this direction.<br />
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At the moment high resolution ocean models are run across limited areas of the Southern Ocean with atmospheric forcing being provided by the reanalysis data sets or global or regional atmospheric models. These allow the investigation of the changes of water masses under the ice shelves and the interaction with the broader scale ocean environment (e.g. Hellmer, 2004<ref name="Hellmer, 2004">Hellmer, H.H. 2004. Impact of Antarctic shelf basal melting on sea ice and deep ocean properties, Geophys. Res. Lett., 31, doi:10.1029/2004GL019506.</ref>).<br />
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==Concluding comments==<br />
<br />
In future there are likely to be two areas where specialised models of the Southern Ocean need development. The first is in the study of the biology of the Southern Ocean, and especially the communities that develop under the immense areas of sea ice. The second is in the study of land ice and its response to climate change. Here the processes occurring under the ice shelves may have a significant impact.<br />
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For the biological studies, the main weaknesses of the physical models is likely to be in the representation of the surface mixed layer and the detailed properties of the surface ice field. It is easy to suggest possible improvements to the process models, but what are lacking are sets of good year-round data from the Southern Ocean that can be used to test them. Data from the Argo floats is helping to fill the gaps but there is still very little data from the large areas of open ocean covered by sea ice.<br />
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For the flows under the ice shelves data are becoming available from boreholes and by other means. Here a model intercomparison experiment, along the lines of the DYNAMO project (Willebrand et al., 2001<ref name="Willebrand et al, 2001">Willebrand, J., Barnier, B., Boening, C., Dieterich, C., Killworth, P., Le Provost, C., Jia, Y., Molines, J. and New, A.L. 2001. Circulation characteristics in three eddy-permitting models of the North Atlantic, ''Progress in Oceanography'', '''48''', 123-161.</ref>), would be useful. This should compare the results of a sigma coordinate model with isopycnal and z-layer models of comparable vertical and horizontal resolution, and be designed initially to investigate the size and effect of the error terms. Once these are quantified then people would have a lot more confidence in using the models to predict future changes.<br />
==References==<br />
<references /><br />
[[Category:Observations, data accuracy and tools]]<br />
[[Category:Models]]<br />
[[Category:The Southern Ocean]]</div>Maintenance scripthttp://acce.scar.org/wiki/Ocean_change_over_the_next_100_yearsOcean change over the next 100 years2014-08-06T14:33:50Z<p>Tonyp: Changed reference to book chapter to link to relevant figure and page</p>
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<div>:''This page is part of the topic [[Antarctic climate and environment change over the next 100 years]]''<br />
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==Simulation of present-day conditions in the Southern Ocean.==<br />
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Coupled General Circulation Models (CGCMs) used in the framework of the IPCC&rsquo;s AR4 have made significant progress in their representation of high latitude processes compared to earlier model versions (Randall et al., 2007<ref name="Randall et al, 2007">Randall, D.A., Wood, R.A., Bony, S., Colman, R., Fichefet, T., Fyfe, J., Kattsov, V., Pitman, A., Shukla, J., Srinivasan, J., Stouffer, R.J., Sumi, A. and Taylor, K.E. 2007. Climate Models and Their Evaluation. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.</ref>). However, the Southern Ocean remains one of the regions where the largest differences are found between models and observations and between different models. In particular, the transport of the Antarctic Circumpolar Current (ACC) through Drake Passage simulated by those models ranges from &ndash;6 Sverdrup (Sv, where 1 Sverdrup = 1 &times; 10<sup>6</sup>m<sup>3</sup>/sec) (i.e. a westward transport) to more than 300 Sv (eastward transport). Only two models among the nineteen analysed by Russell et al. (2006a<ref name="Russell et al, 2006a">Russell, J.L., Stouffer, R.J. and Dixon, K.W. 2006a. Intercomparison of the Southern Ocean circulation in IPCC coupled model controm simulations, ''J. Clim.'', '''19''', 4560-4575.</ref>) were able to obtain transports that were within 20% of the value estimated from observation (135 Sv), while most of the simulated values were within 50 Sv of this estimate. Those strong biases have been attributed to different factors (Russell et al., 2006b<ref name="Russell et al, 2006b">Russell, J.L., Dixon, K.W., Gnanadesikan, A., Stouffer, R.J. and Toggweiler, J.R. 2006b. The Southern hemisphere westerlies in a warming world: propping open the door to the deep ocean, ''J. Clim.'', '''19''', 6382-6390.</ref>). Some models tend to simulate too low a zonal wind stress in the Southern Ocean or to have a maximum in zonal wind stress that is located too far north compared to observations. As a consequence, the simulated wind stress in the latitude band of the Drake Passage is too low, resulting in a too weak ACC transport in those models (Russell et al., 2006a<ref name="Russell et al, 2006a">Russell, J.L., Stouffer, R.J. and Dixon, K.W. 2006a. Intercomparison of the Southern Ocean circulation in IPCC coupled model controm simulations, ''J. Clim.'', '''19''', 4560-4575.</ref>). The density contrast across Drake Passage appears to be another important driving factor of the ACC transport in models. The errors in the simulation of this density gradient, partly due to problems in estimating the export of North Atlantic Deep Water (NADW) towards the Southern Ocean, could thus play a significant role in explaining the difference in transport between model and observations (Russell et al., 2006a<ref name="Russell et al, 2006a">Russell, J.L., Stouffer, R.J. and Dixon, K.W. 2006a. Intercomparison of the Southern Ocean circulation in IPCC coupled model controm simulations, ''J. Clim.'', '''19''', 4560-4575.</ref>).<br />
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The representation of Southern Hemisphere subpolar gyres in IPCC AR4 climate Models has been investigated by Wang and Meredith (2008<ref name="Wang and Meredith, 2008">Wang, Z. and Meredith, M.P. 2008. Density-driven Southern Hemisphere subpolar gyres in coupled climate models, Geophysical Research Letters, 35(14) 5, pp. 10.1029/2008GL034344.</ref>). The models reproduce three southern subpolar gyres: the Weddell Gyre, Ross Gyre, and Australian-Antarctic Gyre (see [[:File:Figure 1.10 - Major currents south of 20S.png|Figure 1.10]] on [[The role of the Antarctic in the global climate system]]), in agreement with observations. Some models simulate the presence of a subpolar &lsquo;&lsquo;supergyre&rsquo;&rsquo;, with strong connectivity between these three gyres. The gyre strengths and structures show a great range across the various models. The link between the gyre strengths and wind stress curls is weak, indicating that the Sverdrup balance (the theoretical relationship between the wind stress exerted on the surface of the open ocean and the vertically integrated meridional (north-south) transport of ocean water) does not hold for the modelled southern subpolar gyres; instead, the simulated gyre strengths are mainly determined by upper layer meridional density gradients, which are themselves determined predominantly by the salinity gradients. These findings suggest that a correct simulation of salinity is crucial for the simulation of Southern Ocean circulation.<br />
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[[File:Figure 5.14 - Zonal mean difference between observed and simulated temperature and salinity, 1981-2000.png|thumb|'''5.14''' The zonal mean difference between observed (a) temperature (&deg;C) and (b) salinity (psu) and the average of 19 CGCMS simulations for the period 1981-2000 (20C3M simulation), superimposed on the bathymetry. The observations are taken from World Ocean Atlas (2001, http://www.nodc.noaa.gov/OC5/WOA01/pr_woa01.html). The 19 models for which sufficient data for the 20C3M runs was available are CCCMA CGCM3.1, CCCMA CGCM3.1-T63, CNRM CM3, CSIRO MK3.0, CSIRO MK3.5, GFDL CM2.0, GFDL CM2.1, GISS_AOM, GISS MODEL E R, INGV ECHAM4, IPSL CM4, MIROC3 2 HIRES, MIROC3 2 MEDRES, MIUB ECHO G, MPI ECHAM5, MRI CGCM2-3_2A, NCAR CCSM3_0, NCAR_PCM1, AND UKMO HADCM3.]]<br />
When the temperature and salinity averaged over all the models is compared to observations ([[:File:Figure 5.14 - Zonal mean difference between observed and simulated temperature and salinity, 1981-2000.png|Figure 5.14]]), the zonal mean differences are relatively small. There is a tendency to have too warm and too salty water masses around 30-40&deg;S in the depth range 500-1,000 m (Randall et al., 2007<ref name="Randall et al, 2007">Randall, D.A., Wood, R.A., Bony, S., Colman, R., Fichefet, T., Fyfe, J., Kattsov, V., Pitman, A., Shukla, J., Srinivasan, J., Stouffer, R.J., Sumi, A. and Taylor, K.E. 2007. Climate Models and Their Evaluation. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.</ref>). This could be related to a too far northward site of formation of Antarctic intermediate waters (AAIW) in those models. Besides, on average, in the same depth range but in the latitude band 60-70&deg;S, the models tend to simulate too cold and too fresh water masses. This bias could be caused by a too weak inflow of warm and salty waters from the north or by excessive exchanges with the surface. Overall, the representation of the vertical stratification for the ensemble mean seems a reasonable projection, although it conceals significant over and under estimates in several of the models (Russell et al., 2006a<ref name="Russell et al, 2006a">Russell, J.L., Stouffer, R.J. and Dixon, K.W. 2006a. Intercomparison of the Southern Ocean circulation in IPCC coupled model controm simulations, ''J. Clim.'', '''19''', 4560-4575.</ref>).<br />
<br />
==Projections for the Twenty First Century==<br />
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There is no agreement for the projections of the ACC transports among 19 IPCC AR4 models (Wang and Meredith, 2008<ref name="Wang and Meredith, 2008">Wang, Z. and Meredith, M.P. 2008. Density-driven Southern Hemisphere subpolar gyres in coupled climate models, Geophysical Research Letters, 35(14) 5, pp. 10.1029/2008GL034344.</ref>). The ACC transports are significantly increased in eight models, while they are significantly decreased in eight other models. A recent observational study on the evolution of the ACC transport over the past several decades revealed that that there is no increase in the tilt of isopycnals. This indicates that the ACC transport is not sensitive to the significant intensification of the Southern Hemisphere westerlies during the past several decades as suggested by some models, increasing our uncertainties in the future evolution of the ACC transport (B&ouml;ning et al., 2008<ref name="B&ouml;ning et al, 2008">B&ouml;ning, C. W., A. Dispert, M. Visbeck, S. R. Rintoul, and F. Schwarzkopf, 2008. Observed multi-decadal ocean warming and density trends across the Antarctic Circumpolar Current, Submitted.</ref>).<br />
<br />
The Southern Hemisphere subpolar gyres over the Twenty First Century in IPCC AR4 models generally become significantly intensified (Wang and Meredith, 2008<ref name="Wang and Meredith, 2008">Wang, Z. and Meredith, M.P. 2008. Density-driven Southern Hemisphere subpolar gyres in coupled climate models, Geophysical Research Letters, 35(14) 5, pp. 10.1029/2008GL034344.</ref>). This is a consequence of the wind forcing over the subpolar region becoming more cyclonic, associated with the intensification and southward shift of the circumpolar westerlies. Conversely, changes in freshwater forcing and in the transport of the adjacent ACC exert only minor influences. The strengthening of the subpolar gyres will likely have strong impacts on the mass balance of ice shelves and the stability of the Antarctic ice sheets, and could also impact strongly on the transformations of water masses within the subpolar gyres and the exports of dense waters to lower latitudes.<br />
<br />
The changes of ocean temperature and salinity (means over 2091-2100 minus means over 2001-2010) are presented in summer-winter pairs in Figures 5.15(i) through 5.16(vi). For the purposes of averaging, winter is taken as the time of peak sea ice (August, September, October), and summer as the sea ice minimum (February, March, April). Output from 19 models is used in the calculations (see caption to [[:File:Figure 5.14 - Zonal mean difference between observed and simulated temperature and salinity, 1981-2000.png|Figure 5.14]]). This average over the ensemble of models is likely the most reasonable estimate of the future change presently available but, because of the large scatter between the projections performed with different models, the analysis of those future changes has to be taken with caution.<br />
<br />
The SST changes are small compared with those observed in surface air temperature in the climate models, because the heat capacity of the ocean is much larger than that of the atmosphere. Nevertheless both the atmospheric and oceanic temperatures will have an effect on sea ice. [[:File:Figure 5.15(i) - Sea surface temperature change for summer and winter, between 2000 and 2100.png|Figure 5.15(i)]](a) shows that south of 60&ordm;S in summer the SSTs are likely to be warmer than at present in 2100 by between 0.5&ordm;C and 1.0&ordm;C, except in the Amundsen Sea where they are likely to be warmer by 1.0 to 1.25&ordm;C. South of 60&ordm;S in winter ([[:File:Figure 5.15(i) - Sea surface temperature change for summer and winter, between 2000 and 2100.png|Figure 5.15(i)]](b)) the SSTs are likely to be close to what they are now, i.e. between up to 0.5&ordm;C warmer or -0.25&ordm;C cooler than they are at present, except far offshore off Dronning Maud Land, off West Antarctica and off Queen Mary Land, where they may warm to 0.5&ordm;C to 1.0&ordm;C.<br />
<br />
<gallery widths=180px heights=436px><br />
File:Figure 5.15(i) - Sea surface temperature change for summer and winter, between 2000 and 2100.png|'''5.15(i)''' Sea surface temperature (SST) change for (a) summer and (b) winter between 2000 and 2100.<br />
File:Figure 5.15(ii) - Sea surface salinity change for summer and winter, between 2000 and 2100.png|'''5.15(ii)''' Sea surface salinity (SSS) change for (a) summer and (b) winter, between 2000 and 2100.<br />
File:Figure 5.15(iii) - Ocean temperature change at 200m for summer and winter, between 2000 and 2100.png|'''5.15(iii)''' Ocean temperature change at 200 m for (a) summer and (b) winter, between 2000 and 2100.<br />
File:Figure 5.15(iv) - Salinity change at 200m for summer and winter, between 2000 and 2100.png|'''5.15(iv)''' Salinity change at 200 m for (a) summer and (b) winter, between 2000 and 2100.<br />
</gallery><br />
<br />
[[:File:Figure 5.15(ii) - Sea surface salinity change for summer and winter, between 2000 and 2100.png|Figure 5.15(ii)]](a) shows that to the south of 60&ordm;S in summer, the surface waters will be fresher by 0.1 to 0.2 units, with local patches up to 0.3 fresher in the Weddell Sea, in the Ross Sea off Oates Land, and in a few patches elsewhere along the coast. [[:File:Figure 5.15(ii) - Sea surface salinity change for summer and winter, between 2000 and 2100.png|Figure 5.15(ii)]](b) shows that south of 60&ordm;S in winter the pattern is very similar, but the surface waters are fresher over a larger area and there are salinities up to 0.3 units fresher west of the Peninsula.<br />
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[[:File:Figure 5.15(iii) - Ocean temperature change at 200m for summer and winter, between 2000 and 2100.png|Figure 5.15(iii)]] shows that in both summer (a) and winter (b) the bottom water temperatures on the continental shelf at 200 m in 2100 are likely to be warmer by between 0.5&ordm;C and 0.75&ordm;C, except in the Weddell Sea where the warming is less (between 0&ordm;C and 0.5&ordm;C).<br />
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[[:File:Figure 5.15(iv) - Salinity change at 200m for summer and winter, between 2000 and 2100.png|Figure 5.15(iv)]] shows that in both summer (a) and winter (b) the bottom water salinities on the continental shelf in 2100 are likely to be up to 0.1 units fresher than they are now, and up to 0.2 fresher in the Weddell Sea.<br />
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[[File:Figure 5.15(v) - Zonal mean cross section of ocean temperature difference between 2000 and 2100.png|thumb|'''5.15(v)''' Zonal mean cross section of ocean temperature difference for (a) summer and (b) winter, between 2000 and 2100, superimposed on bathymetry.]]<br />
As shown in [[:File:Figure 5.15(v) - Zonal mean cross section of ocean temperature difference between 2000 and 2100.png|Figure 5.15(v)]], regardless of season the bottom waters from the surface down to 4,000 m along the continental margin are expected to warm in winter and summer by around 0.25&ordm;C, with the possibility of warming by up to 0.5&ordm;C or slightly more at depths of 200 to 500m. The warming surface layers in both winter and summer are quite thin &ndash; largely less than 200 m. There is significant warming (0.75 to almost 2&ordm;C in all seasons) at the surface between 40 and 60&ordm;S, in the core regions of the Antarctic Circumpolar Current. The CGCMs reproduce quite well the mid-depth warming observed during the second half of the Twentieth Century, in particular if the effect of volcanic eruptions is taken into account in models (Fyfe, 2006<ref name="Fyfe, 2006">Fyfe, J.C. 2006. Southern Ocean warming due to human influence, ''Geophys. Res. Let.'', '''33''', L19701, doi:10.1029/2006GL027247 .</ref>). During the Twenty First Century, this warming is projected to continue, reaching nearly all depths when averaged over the ensemble of models. Close to the surface, the warming of the Southern Ocean during the Twenty First Century is weaker than in other regions. This is partly related to the large heat storage by the ocean, which removes a large amount of heat from the atmosphere in an area where the ocean covers nearly all the longitudes and where relatively deep mixed layers occur.<br />
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[[File:Figure 5.15(vi) - Zonal mean cross section of salinity difference between 2000 and 2100.png|thumb|'''5.15(vi)''' Zonal mean cross section of salinity difference for (a) summer and (b) winter, between 2000 and 2100, superimposed on bathymetry.]]<br />
As shown in [[:File:Figure 5.15(vi) - Zonal mean cross section of salinity difference between 2000 and 2100.png|Figure 5.15(vi)]], along the continental margin, there is no major change in salinity, except above depths of around 400m, above which surface waters are fresher than at present by up to almost 0.2 at the surface between the coast and about 45&ordm;S. The freshening is clearly very much a surface phenomenon, although the Antarctic Intermediate Water is also slightly fresher.<br />
<br />
The vertical stratification increases in most of the models during the Twenty First Century because of the surface warming and the freshening southward of 45&deg;S ([[:File:Figure 5.15(v) - Zonal mean cross section of ocean temperature difference between 2000 and 2100.png|Figure 5.15(v)]] and [[:File:Figure 5.15(vi) - Zonal mean cross section of salinity difference between 2000 and 2100.png|Figure 5.15(vi)]]). This surface salinity decrease, which has a dominant impact on the density changes at high latitude, is caused by the increase in precipitation minus evaporation at high latitude. Changes in freshwater transport by ocean currents and sea ice could also play a strong role as shown by Bitz et al. (2006<ref name="Bitz et al, 2006">Bitz, C.M., Gent, P.R., Woodgate, R.A., Holland, M.M. and Lindsay, R. 2006. The influence of sea ice on ocean heat uptake in response to increasing CO<sub>2</sub>, ''J. Clim.'', '''19''', 2437-2450.</ref>) in their analysis of the results of the CCSM3 model. The enhanced stratification tends to reduce vertical exchanges and is responsible for a decrease in the vertical heat transfer to the surface from the relatively warm water at depth. This effect contributes to the moderate surface temperature increase simulated in the Southern Ocean during the Twenty First Century. Changes in the stratification alter the isopycnal diffusion in models by modifying the slope of the isopycnals, resulting in a reduction of the heat transport towards the surface (e.g., Gregory, 2000<ref name="Gregory, 2000">Gregory, J.M., 2000. Vertical heat transport in the ocean and their effect on time-dependent climate change, ''Clim. Dyn.'', '''16''', 501-515.</ref>; Bitz et al., 2006<ref name="Bitz et al, 2006">Bitz, C.M., Gent, P.R., Woodgate, R.A., Holland, M.M. and Lindsay, R. 2006. The influence of sea ice on ocean heat uptake in response to increasing CO<sub>2</sub>, ''J. Clim.'', '''19''', 2437-2450.</ref>). By contrast, through the same mechanisms, the stratification increase is responsible for a warming and an increase in salinity of the Southern Ocean at mid-depth in many models.<br />
<br />
As a result of the surface density decrease, the ocean ventilation and in particular the formation of Antarctic Bottom Water decrease in many models although the magnitude of the changes in AABW is strongly variable among the models (e.g., Manabe et al., 1991<ref name="Manabe et al, 1991">Manabe S., Stouffer, R.J., Spelman, M.J. and Bryan, K. 1991. Transient responses of a coupled atmosphere-ocean model to gradual changes of atmospheric CO<sub>2</sub>. I. Annual mean response, ''J. Clim.'', '''4''', 785-818.</ref>; Manabe and Stouffer, 1993<ref name="Manabe and Stouffer, 1993">Manabe, S. and Stouffer, R.J. 1993. Century-scale effects of increased atmospheric CO<sub>2</sub> on the ocean-atmosphere system, ''Nature'', '''364''', 215-218.</ref>; Hirst, 1999<ref name="Hirst, 1999">Hirst, A.C. 1999. The Southern Ocean response to a global warming in the CSIRO coupled ocean-atmosphere model, Environmental Modeling and Software, 227-241.</ref>; Bates et al., 2005<ref name="Bates et al, 2005">Bates, M.L., England, M.H. and Sijp, W.P. 2005. On the multi-century Southern Hemisphere response to changes in atmospheric CO<sub>2</sub>-concentration in a global climate model, ''Meteorol. Atmos. Phys.'', '''89''', 17-36.</ref>; Bitz et al., 2006<ref name="Bitz et al, 2006">Bitz, C.M., Gent, P.R., Woodgate, R.A., Holland, M.M. and Lindsay, R. 2006. The influence of sea ice on ocean heat uptake in response to increasing CO<sub>2</sub>, ''J. Clim.'', '''19''', 2437-2450.</ref>; Bryan et al., 2006<ref name="Bryan et al, 2006">Bryan F.O., Danabasoglu, G., Gent, P.R. and Lindsay, K. 2006. Changes in ocean ventilation during the 21<sup>st</sup> century in the CCSM3, ''Ocean Model.'', '''15''', 141-156.</ref>). This lower ventilation in response to the increase in greenhouse gas concentration in the atmosphere induces a positive feedback mechanism by reducing the heat storage in the Southern Ocean as well as reducing the uptake of carbon dioxide, more carbon remaining in the atmosphere during the Twenty First Century compared to a case with constant ventilation of the Southern Ocean (e.g., Sarmiento et al., 1998<ref name="Sarmiento et al, 1998">Sarmiento, J.L., Hughes, T.M.C., Stouffer, R.J. and Manabe, S. 1998. Simulated response of the ocean carbon cycle to an anthropogenic climate warming, ''Nature'', '''393''', 245-249.</ref>; Matear and Hirst, 1999<ref name="Matear and Hirst, 1999">Matear, R.J. and Hirst, A.C. 1999. Climate change feedback on the future oceanic CO<sub>2</sub> uptake, ''Tellus'', '''51B''', 722-733.</ref>).<br />
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Vertical mixing does not decrease over the Twenty First Century at all locations of the Southern Ocean in all models. Increased wind stresses could result in deeper mixed layers in some areas. The decrease in ice extent could induce stronger cooling in winter that leads to stronger mixing at the new ice edge. Shifts in convection patterns have also been noticed, with decreased mixing in some areas but increases in others (e.g., Bitz et al., 2006<ref name="Bitz et al, 2006">Bitz, C.M., Gent, P.R., Woodgate, R.A., Holland, M.M. and Lindsay, R. 2006. The influence of sea ice on ocean heat uptake in response to increasing CO<sub>2</sub>, ''J. Clim.'', '''19''', 2437-2450.</ref>; Conil and Men&eacute;ndez, 2006<ref name="Conil and Men&eacute;ndez, 2006">Conil, S. and Men&eacute;ndez, C.G. 2006. Climate fluctuations of the Weddell Sea and its surroundings in a transient climate change scenario, ''Clim. Dyn.'', '''27''' (1), 83-99.</ref>).<br />
<br />
Ocean ventilation could be enhanced because of the surface divergence induced by the increase in the wind stress projected during the Twenty First Century. Russell et al. (2006b<ref name="Russell et al, 2006b">Russell, J.L., Dixon, K.W., Gnanadesikan, A., Stouffer, R.J. and Toggweiler, J.R. 2006b. The Southern hemisphere westerlies in a warming world: propping open the door to the deep ocean, ''J. Clim.'', '''19''', 6382-6390.</ref>) argue that this effect could be significantly underestimated in some models because of westerlies located too far north in the control climate. By comparing two model versions of the GFDL model, they show that the model version displaying a too northerly maximum of the zonal wind stress simulates much lower heat (and carbon dioxide) storage in the Southern Ocean during the next centuries than does the version with more realistic winds. In the latter simulation, the effect of the enhanced wind-driven divergence is indeed strong enough to overwhelm the influence of the increased stratification at high latitude.<br />
<br />
==Long-term evolution of the Southern Ocean==<br />
<br />
Only a few studies have addressed the evolution of the Southern Ocean on timescales longer than a century. To do so, numerical experiments have been conducted in which CO<sub>2</sub> concentration is progressively increased during a few decades and then maintained constant over several centuries. All those numerical experiments show a strong warming of the Southern Ocean (Hirst, 1999<ref name="Hirst, 1999">Hirst, A.C. 1999. The Southern Ocean response to a global warming in the CSIRO coupled ocean-atmosphere model, Environmental Modeling and Software, 227-241.</ref>; Bi et al., 2001<ref name="Bi et al, 2001">Bi, D.H., Budd, W.F., Hirst, A.C. and Wu, X.R. 2001. Collapse and reorganisation of the Southern Ocean overturning under global warming in a coupled model, ''Geophys. Res. Let.'', '''28''' (20), 3927-3930.</ref>; Goosse and Renssen, 2001<ref name="Goosse and Renssen, 2001">Goosse, H. and Renssen, H. 2001. A two-phase response of Southern Ocean to an increase in greenhouse gas concentrations, ''Geophysical Research Letters'', '''28''', 3469-3473.</ref>; Bates et al., 2005<ref name="Bates et al, 2005">Bates, M.L., England, M.H. and Sijp, W.P. 2005. On the multi-century Southern Hemisphere response to changes in atmospheric CO<sub>2</sub>-concentration in a global climate model, ''Meteorol. Atmos. Phys.'', '''89''', 17-36.</ref>; Petoukhov et al., 2005<ref name="Petoukhov et al, 2005">Petoukhov, V., Claussen, M., Berger, A., Crucifix, M. Eby, M., Eliseev, A., Fichefet, T., Ganopolski, A., Goosse, H., Kamenkovich, I. Mokhov, I., Montoya, M., Mysak, L.A., Sokolov, A., Stone, P., Wang, Z. and Weaver, A.J. 2005. EMIC Inter-comparison Project (EMIP-CO<sub>2</sub>): Comparative analysis of EMIC simulations of climate, and of equilibrium and transient responses to atmospheric CO<sub>2</sub> doubling, ''Climate Dynamics'', '''25''', 363-385.</ref>) as well as a strengthening of the ACC transport through Drake Passage (Bi et al., 2002<ref name="Bi et al, 2002">Bi, D., Budd, W.F., Hirst, A.C. and Wu, X. 2002. Response of the Antarctic circumpolar current transport to global warming in a coupled model, ''Geophys. Res. Lett.'', '''29''' (24), 2173, doi:10.1029/2002GL015919.</ref>; Bates et al., 2005<ref name="Bates et al, 2005">Bates, M.L., England, M.H. and Sijp, W.P. 2005. On the multi-century Southern Hemisphere response to changes in atmospheric CO<sub>2</sub>-concentration in a global climate model, ''Meteorol. Atmos. Phys.'', '''89''', 17-36.</ref>). The ocean surface temperature changes and the sea ice shrinking obtained after a few centuries are in general as high or even higher in the Southern Ocean than the ones obtained in the Arctic. Indeed, the oceanic heat storage and the reduction of the oceanic heat transport towards the surface that are responsible for a weaker response in the Southern Ocean during the Twenty First Century are less operative during the third millennium. As a consequence, on multi-century timescales, the Southern Ocean is one of the regions of the globe that experiences the largest warming in those simulations.<br />
<br />
The long-term evolution of the Southern Ocean is also associated with changes in ocean currents. As a first step, the general decrease found in AABW formation over the Twenty First Century in many models continues during the following centuries (e.g., Bates et al., 2005<ref name="Bates et al, 2005">Bates, M.L., England, M.H. and Sijp, W.P. 2005. On the multi-century Southern Hemisphere response to changes in atmospheric CO<sub>2</sub>-concentration in a global climate model, ''Meteorol. Atmos. Phys.'', '''89''', 17-36.</ref>). This eventually leads in some simulations to a complete cessation of AABW formation (Hirst, 1999<ref name="Hirst, 1999">Hirst, A.C. 1999. The Southern Ocean response to a global warming in the CSIRO coupled ocean-atmosphere model, Environmental Modeling and Software, 227-241.</ref>; Bi et al., 2001<ref name="Bi et al, 2001">Bi, D.H., Budd, W.F., Hirst, A.C. and Wu, X.R. 2001. Collapse and reorganisation of the Southern Ocean overturning under global warming in a coupled model, ''Geophys. Res. Let.'', '''28''' (20), 3927-3930.</ref>). However, in a second step, the long-term reorganisation of the Southern Ocean leads to an increase of AABW formation (Bi et al., 2001<ref name="Bi et al, 2001">Bi, D.H., Budd, W.F., Hirst, A.C. and Wu, X.R. 2001. Collapse and reorganisation of the Southern Ocean overturning under global warming in a coupled model, ''Geophys. Res. Let.'', '''28''' (20), 3927-3930.</ref>; Bates et al., 2005<ref name="Bates et al, 2005">Bates, M.L., England, M.H. and Sijp, W.P. 2005. On the multi-century Southern Hemisphere response to changes in atmospheric CO<sub>2</sub>-concentration in a global climate model, ''Meteorol. Atmos. Phys.'', '''89''', 17-36.</ref>). This is due to a long term warming of the ocean at depth, which weakens the stratification and finally allows deep-water formation to start again or to be enhanced (Bi et al., 2001<ref name="Bi et al, 2001">Bi, D.H., Budd, W.F., Hirst, A.C. and Wu, X.R. 2001. Collapse and reorganisation of the Southern Ocean overturning under global warming in a coupled model, ''Geophys. Res. Let.'', '''28''' (20), 3927-3930.</ref>). The disappearance of sea ice could also play a role because very strong atmosphere-ocean interactions take place in the zones close to the Antarctic coast that become ice-free even in winter, leading to new sites of deep water formation and thus an enhancement of deep water production (Bates et al., 2005<ref name="Bates et al, 2005">Bates, M.L., England, M.H. and Sijp, W.P. 2005. On the multi-century Southern Hemisphere response to changes in atmospheric CO<sub>2</sub>-concentration in a global climate model, ''Meteorol. Atmos. Phys.'', '''89''', 17-36.</ref>). It has been suggested that those long term changes in ocean circulation are associated with an increase in heat transport and play a significant role in the long-term warming simulated at high southern latitudes (e.g. Goosse and Renssen, 2001<ref name="Goosse and Renssen, 2001">Goosse, H. and Renssen, H. 2001. A two-phase response of Southern Ocean to an increase in greenhouse gas concentrations, ''Geophysical Research Letters'', '''28''', 3469-3473.</ref>). Unfortunately, although this long term shift from a decrease of AABW formation to an increase is found in nearly all the available long-term simulations, the timing of the reversal is strongly depending on the model, ranging from a few centuries at most (Goosse and Renssen, 2001<ref name="Goosse and Renssen, 2001">Goosse, H. and Renssen, H. 2001. A two-phase response of Southern Ocean to an increase in greenhouse gas concentrations, ''Geophysical Research Letters'', '''28''', 3469-3473.</ref>; Bates et al., 2005<ref name="Bates et al, 2005">Bates, M.L., England, M.H. and Sijp, W.P. 2005. On the multi-century Southern Hemisphere response to changes in atmospheric CO<sub>2</sub>-concentration in a global climate model, ''Meteorol. Atmos. Phys.'', '''89''', 17-36.</ref>) to more than a millennium (Hirst, 1999<ref name="Hirst, 1999">Hirst, A.C. 1999. The Southern Ocean response to a global warming in the CSIRO coupled ocean-atmosphere model, Environmental Modeling and Software, 227-241.</ref>; Bi et al., 2001<ref name="Bi et al, 2001">Bi, D.H., Budd, W.F., Hirst, A.C. and Wu, X.R. 2001. Collapse and reorganisation of the Southern Ocean overturning under global warming in a coupled model, ''Geophys. Res. Let.'', '''28''' (20), 3927-3930.</ref>). Furthermore, the freshwater flux from the melting of the Antarctic ice sheet, which could strongly influence stratification and deep water formation in the Southern Ocean on this time-scale, is included in none of those simulations, increasing our uncertainties in the long-term evolution of the system. A recent study (Swingedouw et al., 2008<ref name="Swingedouw et al, 2008">Swingedouw D., Fichefet, T., Huybrechts, P., Goosse, H., Driesschaert, E. and Loutre, M.F. 2008. Antarctic ice-sheet melting provides negative feedbacks on future climate warming, ''Geophysical Research Letters'', '''35''', L17705, doi:10.1029/2008GL034410.</ref>) using a coupled climate-ice sheet model has shown that taking into account the long-term influence of the freshwater flux from the melting of Antarctic ice sheet contributes to the formation of a cold halocline in the Southern Ocean. This limits sea ice cover retreat under global warming and reduces local surface warming compared to simulations where this effect is not taken into account.<br />
<br />
==Conclusions==<br />
<br />
Forecasts of future conditions are made using coupled models. The wide range of results obtained by state of the art models indicates that there are still significant deficiencies in the different models - mainly due to inadequate representations of relevant processes (for example through inappropriate parameterisation), or to insufficiently high resolution. To a large extent model validation occurs through model intercomparison, simply because observations are scarce, which means that arguments may be circular. Surface data from satellites are available for some parameters with sufficient coverage in time and space, but ''in-situ'' data from the ocean interior are still patchy in space and intermittent in time. In the ice-covered areas of the Southern Ocean, ''in-situ'' data are still almost exclusively collected in the context of research expeditions determined in time and space by initiatives to study processes rather than to make repeat measurements, and limited by funding cycles. Operational observations of climate relevant data are still in their infancy, even though the Argo system is a big step forward. Although excellent technologies are available, with a wide range of autonomous devices, such as moored systems, floating systems, gliders, or sensor-carrying animals, the deployment and maintenance of these technologies in a sustained manner is still out of sight.<br />
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To reach that point a Southern Ocean Observing System (SOOS) has to be established. This will require the cooperation of agencies and research institutions in coordinating the use of their resources, but it requires at the same time further development of instrumentation in order to reduce the effort needed to obtain the required measurements with the appropriate time and space resolution. The provision of climate-relevant observations from an established network will release resources from the research community that can be used for process studies in order to improve the representation of those processes in the models.<br />
==References==<br />
<references /><br />
[[Category:The next 100 years]]<br />
[[Category:The Southern Ocean]]</div>Maintenance scripthttp://acce.scar.org/wiki/Observations_of_the_ice_sheetObservations of the ice sheet2014-08-06T14:33:49Z<p>Tonyp: Changed references to book chapters to page links; moved figure 2.22; added links for figures 1.2-1.6; corrected spelling error</p>
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<div>:''This page is part of the topic [[Observations, data accuracy and tools]]''<br />
<br />
==Introduction==<br />
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Since the earliest exploration of Antarctica scientists have visited the ice plateau in search of new discoveries. Despite the logistic and physical difficulties imposed by the environment and weather, field-based observations have been made throughout Antarctica, and have proved invaluable in explaining the dynamic character of this body of ice. An era of international fieldwork, sparked by the IGY of 1957-58, has continued to the recent International Polar Year of 2007-08. In the intervening decades, glaciological fieldwork adopted new technologies and revealed behaviour that could not have been observed by any other means. Some measurements can now be made over wide areas from satellites or aircraft, but direct ''in-situ'' observations are still needed to provide calibration and validation for these surveys. Moreover, fieldwork provides detailed information about the processes that operate to change the Antarctic environment. The combination of fieldwork, aircraft, and satellite measurements has revealed changes in Antarctica on many scales and given a better understanding of their causes.<br />
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==Surface elevation==<br />
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Oversnow traverses provided the first indications of the surface shape of the ice sheet. The existence of broad, flat ice shelves bounded by tall mountain ranges that retained a high, frigid and harsh polar plateau played into the strategies used by explorers to reach the South Pole. However, such sparse sampling could never hope to attain sufficient coverage of the ice sheet to detect its shape in detail. In the mid-1970s, random sampling of the surface from radar altimeters onboard balloons released to circulate above the continent at constant atmospheric pressure greatly increased the mapping of the ice sheet&rsquo;s surface elevation (Levanon et al., 1977<ref name="Levanon et al, 1977">Levanon, N., Julian, P.R. and Suomi, V.E. 1977. Antarctic topography from balloons, ''Nature'', '''268''', 514-516, doi:10.1038/268514a0.</ref>). Radar altimeters use the ranging capability of radar to measure the surface topographic profile beneath the sensor by measuring the time interval between the transmission and reception of very short electromagnetic pulses. These data showed the ice sheet surface topography contained a number of separate domes in East Antarctica and that this ice sheet was much higher than the West Antarctic ice sheet ([[:File:Figure 1.2 - Antarctic topography and bathymetry.png|Figure 1.2]] on [[The Antarctic cryosphere]]). With this information, the drainage basins of individual outlet glaciers could begin to be defined ([[:File:Figure 1.3 - Antarctic surface elevation.png|Figure 1.3]] on [[The Antarctic cryosphere]]).<br />
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Radar altimeters onboard satellites established systematically repeated surface elevation observations of the earth. To date, most spaceborne radar altimeters have been wide-beam (pulse-limited) systems operating from low Earth orbits. Radar altimetry data of Antarctica has been collected since 1978 from a variety of instruments including Seasat (1978), Geosat (1985-1990), ERS-1 (1991-1996) and ERS-2 (since 1995), and Envisat (since 2002). Seasat reached only to 72&ordm; S whereas subsequent missions have increased coverage to 81.5&ordm; S. The measurement accuracy of such systems is sub-metre across the smoother ice sheet interior, but degrades to a few metres as the surface slope increases, especially nearer the coast and in mountainous regions. The future availability of satellite altimetry observations is assured with new missions such as ESA&rsquo;s Sentinel satellite series. ESA&rsquo;s CryoSat mission will also provide new data from the SIRAL instrument specifically designed for ice sheet and sea ice observations.<br />
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[[File:Figure 2.16 - Schematic of a single ICESat repeat track.png|thumb|'''2.16''' Schematic of a single ICESat repeat track indicating surface altimetry measurements (red) and cloud observations (green/white/pink). Source: NASA/GSFC.]]<br />
In addition to radar altimetry, the first satellite laser altimeter was launched in 2003 onboard the ICESat satellite. The Geoscience Laser Altimeter System (GLAS) is the sole instrument on the Ice, Cloud, and land Elevation Satellite (ICESat) ([[:File:Figure 2.16 - Schematic of a single ICESat repeat track.png|Figure 2.16]]). A laser altimeter has the advantage of ranging directly to the surface without penetrating the upper snow layers, but requires cloud-free conditions. Microwave radar is unaffected by clouds, but penetrates several metres into the snow (Legresy and Remy, 1997<ref name="Legresy and Remy, 1997">Legresy, B. and Remy, F. 1997. Altimetric observations of surface characteristics of the Antarctic ice sheet, ''J. Glaciol.'', '''43'''(144), 265-275.</ref>; Arthern et al., 2001<ref name="Arthern et al, 2001">Arthern, R.J., Wingham, D.J. and Ridout, A.L. 2001. Controls on ERS altimeter measurements over ice sheets: Footprint-scale topography, backscatter fluctuations, and the dependence of microwave penetration depth on satellite orientation, J. Geophys. Res., 106(D24), 33,471-33,484.</ref>). Alteration of the snowpack by weather and climate could change the depth of penetration, with a risk that this is mistaken for a real change in the surface elevation. Recent studies have corrected for this effect using an empirical, location-dependent correlation between penetration depth and backscattered power (Wingham et al., 1998<ref name="Wingham et al, 1998">Wingham, D.J., Ridout, A.L., Scharroo, R., Arthern, R.J. and Schum, C.K. 1998, Antarctic elevation change from 1990 to 1996, ''Science'', '''282''', 456-458, (doi:10.1126/science.282.5388.456).</ref>; Zwally et al., 2005<ref name="Zwally et al, 2005">Zwally, H.J., Giovinetto, M., Li, J., Cornejo, H., Beckley, M., Brenner, A., Saba, J. and Yi, D. 2005. Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002, ''Journal of Glaciology'', '''51'''(175), 509-527.</ref>), although some residual error may be unavoidable (Alley et al., 2007<ref name="Alley et al, 2007">Alley, R.B., Anandakrishnan, S., Dupont, T.K., Parizek, B.R. and Pollard, D., 2007, Effect of sedimentation on ice-sheet grounding-line stability. Science, 315, 1838-1841.</ref>). Both radar and laser altimetry can be affected by changes in the density of the upper layers of the snowpack, so these must be estimated separately or corrected for (Arthern and Wingham, 1998<ref name="Arthern and Wingham, 1998">Arthern, R.J. and Wingham, D.J. 1998. The Natural Fluctuations of Firn Densification and Their Effect on the Geodetic Determination of Ice Sheet Mass Balance, ''Climatic Change'', '''40''', 605-624.</ref>; Zwally et al., 2005<ref name="Zwally et al, 2005">Zwally, H.J., Giovinetto, M., Li, J., Cornejo, H., Beckley, M., Brenner, A., Saba, J. and Yi, D. 2005. Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002, ''Journal of Glaciology'', '''51'''(175), 509-527.</ref>).<br />
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The primary objective of the ICESat mission is to measure ice sheet elevations and changes in elevation through time, with a precision of a few centimetres, surpassing that achieved with radar altimetry. Secondary objectives include measurement of cloud and aerosol height profiles, land elevation and vegetation cover, and sea ice thickness (Schutz et al., 2005<ref name="Schutz et al, 2005">Schutz, B.E., Zwally, H.J., Shuman, C.A., Hancock, D. and Dimarzio, J.P. 2005. Overview of the ICESat Mission, Geophys. Res. Lett., 32, L21S01, doi:10.1029/2005GL024009.</ref>). The southern limit of ICESat data is 86&ordm; S. The increase in surface topographic detail in these data is illustrated in [[:File:Figure 1.3 - Antarctic surface elevation.png|Figure 1.3]] on [[The Antarctic cryosphere]] by noting the smoother apparent surface in the region closest to the South Pole; a visual artifact caused by the absence of precise altimetric data in this region.<br />
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Most modern airborne geophysical measurement systems operated in Antarctica include a laser altimeter instrument. These have provided independent validation of ICESat data. Their primary function is to focus on regional studies and refine the spatial pattern of elevation change, as indication of ice-sheet or outlet glacier thickness changes.<br />
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Satellite altimeter systems have the drawback that their data usually do not extend away from the sub-satellite ground track. ICESat&rsquo;s GLAS system can be pointed off-nadir, but these occasions are rare. Photoclinometry, or shape from shading interpolated between precisely measured elevation profiles by using optical imagery (Landsat, ASTER and MODIS). These systems collect images of the surface using reflected sunlight in a variety of spectral bands. Because most of Antarctica is snow covered and the albedo of snow at any given wavelength is relatively constant, the brightness variations of ice-sheet imagery are predominantly related to the surface slope in the direction of the solar illumination (Bindschadler and Vornberger, 1998<ref name="Bindschadler and Vornberger, 1998">Bindschadler, R.A. and Vornberger, P.L. 1998. Changes in West Antarctic ice sheet since 1963 from Declassified Satellite Photography, ''Science'', '''279''', 689-692.</ref>). This relationship allows the image to be used to interpolate surface elevations between satellite altimetry profiles and provide more complete and accurate elevation fields.<br />
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A more traditional use of optical imagery to produce elevations is stereo photogrammetry. This can be problematic over ice sheets where distinct features visible in modest-resolution imagery can be uncommon, but with the recent addition of higher spatial-resolution sensors such as ASTER and SPOT-5, the technique is now more often successful. A dedicated set of SPOT stereo images has been acquired around the Antarctica perimeter and elevation data are being produced as users request them.<br />
<br />
==Bed elevation and ice thickness==<br />
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As little as fifty years ago, relatively few traverses had been made across the interior of Antarctica, and the thickness of the ice was uncertain over much of the continent. Ice thickness can be determined by the seismic method of detonating explosives and recording the delay before arrival of echoes from the base. This can reveal details of the rock or sediment beneath the ice, as well as the ice thickness (Blankenship et al., 1986<ref name="Blankenship et al, 1986">Blankenship, D.D., Bentley, C.R., Rooney, S.T. and Alley, R.B. 1986. Seismic measurements reveal a saturated porous layer beneath an active Antarctic ice stream, ''Nature'', '''322''', 54-57.</ref>). A similar technique using radar, rather than sound waves can be performed from the ground (e.g. Conway et al., 1999<ref name="Conway et al, 1999">Conway, H., Hall, B.L., Denton, G.H., Gades, A.M., Waddington, E.D. 1999. Past and future grounding-line retreat of the West Antarctic Ice Sheet. Science 286:280-283.</ref>; Catania et al., 2006<ref name="Catania et al, 2006">Catania, G.A., Conway, H., Raymond, C.F. and Scambos, T.A. 2006. Evidence for floatation or near floatation in the mouth of Kamb Ice Stream, West Antarctica, prior to stagnation, ''J. Geophys. Res.'', '''111''', F01005, doi:10.1029/2005JF000355.</ref>), or from aircraft (Robin et al., 1970<ref name="Robin et al, 1970">Robin, G. De Q., Swithinbank, C.W.M. and Smith, B.M.E. 1970. Radio echo exploration of the Antarctic ice sheet, ''Int. Assoc. Sci. Hydrol. Publ.'', '''86''', 97-115.</ref>; Mae and Yoshida, 1987<ref name="Mae and Yoshida, 1987">Mae, S. and Yoshida, M. 1987. Airborne radio echo-sounding in Shirase Glacier drainage basin, Antarctica, ''Ann. Glaciol.'', '''9''', 160-165.</ref>; Siegert et al., 2005a<ref name="Siegert et al, 2005a">Siegert, M.J., Pokar, M., Dowdeswell, J.A. and Benham, T. 2005a. Radio-echo layering in West Antarctica: a spreadsheet dataset, ''Earth Surf. Process. Landforms'', '''30''', 1583-1591.</ref>).<br />
<br />
[[File:Figure 2.21 - Twin Otter aircraft fitted with ice penetrating antennas mounted under the wings.png|thumb|'''2.21''' Twin Otter aircraft fitted with ice penetrating antennas mounted under the wings. Aircraft fitted with geophysical instruments have collected a wealth of ice thickness data throughout Antarctica.]]<br />
Airborne radar (see [[Meteorological and ozone observing in the Antarctic#Snow Accumulation|Snow accumulation]]), sometimes flown by airplanes based at remote field camps, is now the principal method of determining ice thickness ([[:File:Figure 2.21 - Twin Otter aircraft fitted with ice penetrating antennas mounted under the wings.png|Figure 2.21]]). Present technology allows the surface of the underlying rock and sediment to be measured to a vertical precision of tens of metres. Interpolating the rugged bed topography between surveyed tracks remains difficult, and even now there are large regions of Antarctica where few measurements have been recorded. Nevertheless, compiling data from many survey flights can give a good estimate of the volume of ice stored in Antarctica. The most recent compilation of these data is BEDMAP (Lythe et al., 2000<ref name="Lythe et al, 2000">Lythe, M.B., Vaughan, D.G. and the BEDMAP Consortium. 2000. BEDMAP - bed topography of the Antarctic. 1:10,000,000 scale map. BAS (Misc) 9. Cambridge, British Antarctic Survey.</ref>) and BEDMAP-2 is underway. [[:File:Figure 1.5 - Antarctic ice thickness.png|Figure 1.5]] on [[The Antarctic cryosphere]] is a visual illustration of the Antarctic bed elevation field that has resulted from this data compilation. From these data, the total volume of the Antarctic ice sheet is calculated as equivalent to 57 m of sea level. Of this, 52 m is locked away in the thick ice of East Antarctica, and around 5 m is stored in the more dynamic ice sheet covering West Antarctica (Lythe et al., 2000<ref name="Lythe et al, 2000">Lythe, M.B., Vaughan, D.G. and the BEDMAP Consortium. 2000. BEDMAP - bed topography of the Antarctic. 1:10,000,000 scale map. BAS (Misc) 9. Cambridge, British Antarctic Survey.</ref>).<br />
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Several areas of Antarctica have recently benefited from focused airborne-geophysical surveys. Coastal Dronning Maud Land (Steinhage et al., 1999<ref name="Steinhage et al, 1999">Steinhage, D., Nixdorf, U., Meyer, U. and Miller, H. 1999. New maps of the ice thickness and subglacial topography in Dronning Maud Land, Antarctica, determined by means of airborne radio echo sounding, ''Ann. Glaciol.'', '''29''', 267-272.</ref>), Thwaites Glacier (Holt et al., 2006<ref name="Holt et al, 2006">Holt, J.W., Blankenship, D.D., Morse, D.L., Young, D.A., Peters, M.E., Kempf, S.D., Richter, T.G., Vaughan, D.G. and Corr, H.F.J. 2006. New boundary conditions for the West Antarctic Ice Sheet: Subglacial topography of the Thwaites and Smith glacier catchments, ''Geophys. Res. Lett.'', '''33''', L09502, doi:10.1029/2005GL025561.</ref>), Pine Island Glacier (Vaughan et al., 2006<ref name="Vaughan et al, 2006">Vaughan, D.G., Corr, H.F.J., Ferraccioli, F., Frearson, N., O'hare, A., Mach, D., Holt, J.W., Blankenship, D.D., Morse, D.L. and Young, D.A. 2006. New boundary conditions for the West Antarctic ice sheet: Subglacial topography beneath Pine Island Glacier, ''Geophys. Res. Lett.'', '''33''', L09501, doi:10.1029/2005GL025588.</ref>) have been surveyed in detail. These and similar surveys reveal the amounts of ice present. They also characterise deep troughs in the underlying bed that make some regions more vulnerable than others to rapid change. Furthermore they provide geometric boundary conditions needed for modeling the evolution of the ice sheet.<br />
<br />
==Velocity==<br />
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[[File:Figure 2.22 - Speed of Bindschadler and MacAyeal Ice Streams, West Antarctica.png|thumb|'''2.22''' Speed of Bindschadler (right) and MacAyeal (left) Ice Streams, West Antarctica. Flow direction is generally from top to bottom of figure with speed indicated by color superimposed on a mosaic of 14 Landsat images enhanced to highlight surface features. Faster ice occurs in the central portion of each ice stream but the spatial complexity indicates a spatial variation in the lateral, longitudinal and basal forces that determine internal stresses, strains and speeds of the ice. Velocity in metres/year.]]<br />
In-situ observations of ice-flow can be made by repeatedly surveying the position of marker poles carried along by the ice (e.g. Naruse, 1979<ref name="Naruse, 1979">Naruse, R. 1979. Thinning of the ice sheet in Mizuho Plateau, East Antarctica, ''J. Glaciol.'', '''24''', 45-52.</ref>). Nowadays this same direct approach makes use of precise locations derived from GPS. Modern dual-frequency GPS receivers and post-processing of data can give positional accuracy of centimetres or better (King, 2004<ref name="King, 2004">King, M.A. 2004. Rigorous GPS data-processing strategies for glaciological. Applications, ''J. Glaciol.'', '''50''', 601-607.</ref>). The movement of AWS over time along the ice sheets, which have GPS on-board, can also provide invaluable information about ice sheet velocity. Ferrell AWS, located on the Ross Ice Shelf, is estimated to have moved approximately 0.65 - 0.75 km/year over a period from 1979 to 2003. Ice motion is almost everywhere greater than a metre per year, so surface velocities are measured to within one percent in a single year. For faster flowing ice, the relative accuracy is even higher. Point velocities measured in this way provide constraints to calibrate remote sensing methods such as feature tracking or satellite radar interferometry (discussed below).<br />
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Features seen from space can serve the same role as surveyed marker poles or GPS antennas. Sequential imagery, once coregistered, offer the opportunity to measure the surface motion of features identified on multiple images. Image coregistration is simplified when there is exposed rock visible. When not, long-wavelength surface forms can be used, because these are surface expressions of fixed basal undulations and, therefore, also fixed in space. Image processing techniques are then employed to first filter out the higher wavelength features and then cross-correlate static long-wavelength features in multiple images. Once coregistered, the same cross-correlation techniques are employed to determine surface feature motion (Scambos et al., 1992<ref name="Scambos et al, 1992">Scambos, T.A., Dutkiewicz, M.J., Wilson, J.C. and Bindschadler, R.A. 1992. Application of image cross-correlation to the measurement of glacier velocity using satellite image data, ''Remote Sensing of Environment'', '''42''', 177-186.</ref>). Most often applied to sequential Landsat imagery with a 30 m pixel resolution, the errors in measured displacements are 2-3 pixels with roughly equal parts random error associated with tracking individual features, and systematic error associated with image coregistration.<br />
<br />
The optical imagery record began in earnest in 1972 with the launch of ERTS-1, which became the first of the Landsat series. Earlier and recently declassified satellite photographs have extended a sparse data set back to 1960, soon after the first year that satellites were used for military and national security purposes. Spatial resolution, temporal coverage and radiometric sensitivity have steadily improved to the present, when sub-metre pixel resolution, near daily coverage and extraordinary surface detail can be obtained. However, not all of these are available from any single sensor, and the use of the data will help determine what sensor is most suitable.<br />
<br />
[[File:Figure 2.23 - Surface velocity of the Siple and Gould Coast regions of West Antarctica.png|thumb|'''2.23''' Surface velocity (in metres/year) of the Siple and Gould Coast regions of West Antarctica. Flow direction, indicated by the arrows, is generally left to right, and speed, indicated by colour, increases from left to right. The underlying layer is a portion of the Radarsat mosaic with the Transantarctic Mountains in the upper right and the Ross Ice Shelf on the right. The network of tributaries begins to emerge from the surrounding ice at speeds of about 100 m per year and is guided by unseen valleys in the underlying bed. From top to bottom, the major ice streams (red) are the Whillans (with the green Crary Ice Rise in its mouth), Bindschadler and MacAyeal Ice Streams.]]<br />
Much information about land ice and snow is obtained from SAR imagery, in particular from SAR Interferometry (InSAR) techniques. This method, which uses phase differences in returned radar signals, is able to measure centimetric changes in surface displacement over very wide areas ([[:File:Figure 2.23 - Surface velocity of the Siple and Gould Coast regions of West Antarctica.png|Figure 2.23]]). These new techniques have provided information to monitor glacier discharge (Pritchard and Vaughan, 2007<ref name="Pritchard and Vaughan, 2007">Pritchard, H.D. and Vaughan, D.G. 2007. Widespread acceleration of tidewater glaciers on the Antarctic Peninsula, J. Geophys. Res., 112, F03S29, doi:10.1029/2006JF000597.</ref>), map grounding lines (Gray et al., 2002<ref name="Gray et al, 2002">Gray, L.; Short, N.; Bindschadler, R.; Joughin, I.; Padman, L.; Vornberger, P.; Khananian, A. 2002. Radarsat interferometry for Antarctic grounding-zone mapping, ''Annals of Glaciology'', '''34''', 269-276.</ref>; Yamanokuchi et al., 2005<ref name="Yamanokuchi et al, 2005">Yamanokuchi, T., Doi, K. and Shibuya, K. 2005. Validation of grounding line of the East Antarctic Ice Sheet derived by ERS-1/2 interferometric SAR data, ''Polar Geoscience'', '''18''', 1-14.</ref>), contribute to mass balance studies (Rignot and Thomas, 2002<ref name="Rignot and Thomas, 2002">Rignot, E. and Thomas, R.H. 2002. Mass balance of polar ice sheets, Science, 297 (5586), 1502-1506 AUG 30 2002.</ref>), determine glacier and ice sheet motion (Fischer et al., 2003<ref name="Fischer et al, 2003">Fischer, A.H., Rott, H. and Bj&ouml;rnsson, H., 2003. Observation of recent surges of Vatnaj&ouml;kull, Iceland, by means of ERS SAR interferometry, ''Annals of Glaciology'', '''37''', 69-76.</ref>) and monitor propagation of ice shelf rifts (Fricker et al., 2002<ref name="Fricker et al, 2002">Fricker, H.A., Young, N.W., Allison, I. and Coleman, R. 2002. Iceberg calving of the Amery Ice Shelf, East Antarctica: New field and remote sensing data, ''Annals of Glaciology'', '''34''', 81-88.</ref>). In contrast, an approach taken in matching the amplitudes of SAR images can be used in correlating scenes over a long time period. Nakamura et al. (2007<ref name="Nakamura et al, 2007">Nakamura K., Doi, K. and Shibuya, K. 2007. Estimation of seasonal changes in the flow of Shirase Glacier using JERS-1/SAR image correlation, ''Polar Sci.'', '''1'''(2-4), 73-84.</ref>) applied the techniques to the Shirase glacier, one of the fastest-moving ice streams in the Antarctica, and obtained the seasonal variation of velocity near the grounding line, that is the highest in summer and lowest in winter.<br />
<br />
Repeated measurements of horizontal velocity using the GPS technique have shown that the speed of ice streams can change on very short timescales. The speed of ice tens or hundreds of kilometres inland can vary from hour to hour, responding to forcing by the tides (Bindschadler et al., 2003<ref name="Bindschadler et al, 2003">Bindschadler, R. A., King, M.A., Alley, R.B., Anandakrishnan, S. and Padman, L. 2003, Tidally controlled stick-slip discharge of a West Antarctic ice stream, ''Science'', '''301''', 1087-1089.</ref>). Other areas respond differently to tides. Flow speed on the Rutford Ice Stream follows a 14 day cycle (Gudmundsson, 2006<ref name="Gudmundsson, 2006">Gudmundsson, G.H. 2006. Fortnightly variations in the flow velocity of Rutford Ice Stream, West Antarctica, Nature, 444, 1063-1064 doi:10.1038/nature05430.</ref>). Analysing the response of ice streams to known tidal forcing has opened up a new way of studying the friction that impedes the motion of ice.<br />
<br />
[[File:Figure 2.24 - Composite of the Antarctic ice sheet surface speed.png|thumb|'''2.24''' Composite of surface speed. North of 80&ordm; S the velocities are derived from the Modified Antarctic Mapping Mission (MAMM) whose data were collected by Radarsat-1 in Autumn 2000. South of 80&ordm; S the velocities are calculated balance velocities. Overlain on these two data sets are flow lineations (in white) from the SAR imagery indicating the location of faster ice motion and shear. Speed is represented by a color log<sub>10</sub> scale (0 in deep blue to 1,000 m/yr in red) to illustrate the wide range and patterns of surface flow. The underlying layer is a gray-scale version of the Radarsat mosaic where bright and dark areas correspond to regions of high and low radar backscatter. From Jezek, 2008<ref name="Jezek, 2008">Jezek, K. C. 2008. The RADARSAT-1 Antarctic Mapping Project. BPRC Report No. 22, Byrd Polar Research Center, The Ohio State University, Columbus, Ohio, 64 p.</ref>.]]<br />
A meaningful diagnostic of the ice sheet state is to calculate the &ldquo;balance velocity&rdquo; and compare it with the measured surface velocity. The balance velocity is calculated from the ice thickness, accumulation pattern and flow direction (defined as the direction of maximum surface gradient). It is neither a measured, nor an actual velocity, but corresponds to the depth-averaged velocity at any point that is required to transport all the mass accumulated upstream of that point. If the actual velocity equals the balance velocity everywhere, then the shape of the ice sheet remains unchanged. Thus, balance velocity becomes a convenient way of comparing the geometric shape and meteorological input to the ice speed. [[:File:Figure 1.6 - Antarctic ice sheet balance velocity.png|Figure 1.6]] on [[The Antarctic cryosphere]] presents the continental pattern of balance velocity. It compares favourably to the surface speed ([[:File:Figure 2.24 - Composite of the Antarctic ice sheet surface speed.png|Fig 2.24]]) indicating that a majority of the ice sheet is near equilibrium. Areas of change, including areas changing very rapidly, do occur and will be discussed in [[The ice sheet in the instrumental period]].<br />
<br />
==Mass balance==<br />
<br />
The overall condition of an ice sheet is usually described as its &ldquo;mass balance&rdquo;. It is a fundamental expression of the ice sheet&rsquo;s health. Mass balance is the sum of all mass inputs to the ice sheet, such as accumulation, minus all mass removals, such as melting. It expresses the rate of growth or shrinkage of the ice sheet and relates directly to the amount of mass transferred to the oceans to affect sea level. Ice leaving the grounded region of the ice sheet affects sea level immediately, even if it remains attached to the ice sheet in the form of a floating ice shelf. Icebergs calved from floating ice shelves and sub-ice shelf melting remove mass from the ice sheet, but do not alter sea level. Sometimes ice shelf mass changes are included in mass balance calculations, because they reflect changes in ice sheet volume, but care must be taken not to equate these mass balance quantities to Antarctica&rsquo;s contribution to sea level without removing the ice shelf component.<br />
<br />
There are currently three independent methods of measuring changes in the mass of the grounded portion of the ice sheet: satellite altimetry, ice-budget, and gravity surveys. All are subject to errors from different sources: principally, snow compaction and electromagnetic penetration for altimetry; thickness errors and accumulation errors for the ice-budget; unmodelled postglacial rebound and atmospheric effects for gravity surveys. The problem is over-determined (three methods for measuring one number) and each of the methods has errors that are (more or less) independent of the others. This means that a better assessment of the state of balance of the ice sheet should be possible by combining all the observations. There are problems doing this, because all the measurements are recorded at different locations, and over different time intervals. A promising approach is to use data assimilation methods similar to those used by numerical weather prediction centres to produce daily weather forecasts (Arthern and Hindmarsh, 2006<ref name="Arthern and Hindmarsh, 2006">Arthern, R.J. and Hindmarsh, R.C.A. 2006. Determining the contribution of Antarctica to sea-level rise using data assimilation methods, Phil. Trans. R. Soc. A, 364, 1841-1865. doi:10:1098/rsta.2006.1801.</ref>). This should allow a better analysis of the present-day changes, and opens up the possibility of forecasting the evolution of the ice sheet over the coming decades. Even before such formal methods are applied, comparison of the three approaches provides a valuable consistency check.<br />
<br />
The concept of mass balance can also be applied to any portion of the ice sheet. To measure the amount of ice transported, the thickness and the profile of velocity through the column are needed. If the base is slippery, or the ice is afloat, an assumption that ice speed is constant at all depths throughout the column is appropriate. Then the ice flux is simply the product of surface velocity and thickness. For ice thickness of the order of kilometres, the depth error from radar surveys is of the order of one percent, and dominates over the velocity error. If shearing is present in the column it must be corrected for, and this can introduce errors larger than one percent in the flux. The accuracy that can be achieved for the flux leaving any particular drainage basin is typically of the order of ten percent of the turnover (e.g. Fricker et al., 2000<ref name="Fricker et al, 2000">Fricker, H., Warner, R. and Allison, I. 2000. Mass balance of the Lambert Glacier&ndash;Amery Ice shelf system, East Antarctica: a comparison of computed balance fluxes and measured fluxes, ''J. Glaciol.'', '''46''' (155), 561-570.</ref>; Rignot and Thomas, 2002<ref name="Rignot and Thomas, 2002">Rignot, E. and Thomas, R.H. 2002. Mass balance of polar ice sheets, Science, 297 (5586), 1502-1506 AUG 30 2002.</ref>).<br />
<br />
Since measurements began, the overall mass balance of the ice sheet (i.e., the rate of change of the total mass of ice held within it) has been slightly negative, but with large uncertainties. This belies the enormous regional variability of mass changes that express different dynamic characteristics and responses to different climate forcings. Some signals, such as thinning of the Amundsen Sea sector in West Antarctica, are common to all three approaches, and this agreement provides a strong confirmation that this part of Antarctica is contributing to sea level rise. Meanwhile East Antarctica seems close to balance, or thickening slightly, according to these independent assessments.<br />
<br />
Point measurements of elevation change, achieved by repeat GPS positioning, can also contribute to mass balance studies (Smith et al., 1998<ref name="Smith et al, 1998">Smith, A.M., Vaughan, D.G., Doake, C.S.M., ET AL. 1998. Surface lowering of the ice ramp at Rothera Point, Antarctic Peninsula, in response to regional climate change, ''Ann. Glaciol.'', '''27''', 113-118.</ref>; Hamilton, 2005<ref name="Hamilton, 2005">Hamilton, G.S. 2005. Spatial patterns in mass balance of the Siple Coast and Amundsen Sea sectors, West Antarctica, ''Ann. Glaciol.'', '''41''', 105-106.</ref>; Wendt et al., 2009<ref name="Wendt et al, 2009">Wendt, A., G. Casassa, A. Rivera, and J. Wendt (2009). Reassessment of ice mass balance at Horseshoe Valley, Antarctica. Antarctic Science. doi:10.1017/S0954102009002053.</ref>). The vertical component of position derived by GPS is less accurate than the horizontal. The vertical motion of the marker poles must be corrected for along-slope advection, gradients of snow accumulation, and snow compaction. Placing the markers at the bottom of boreholes drilled through the upper layers of the ice sheet, where most of the density changes occur, can lessen the impact of variable snow compaction on these measurements (Hamilton, 2005<ref name="Hamilton, 2005">Hamilton, G.S. 2005. Spatial patterns in mass balance of the Siple Coast and Amundsen Sea sectors, West Antarctica, ''Ann. Glaciol.'', '''41''', 105-106.</ref>). The submergence velocity of markers, compared with the long-term rate of snow accumulation (from ice cores), provides a local estimate of the state of balance of the ice sheet.<br />
<br />
==Ice shelves==<br />
<br />
Ice shelves, the floating fringe of the ice sheet, are often overlooked because of the absence of a direct impact on sea level, however they have a very important indirect effect. Ice shelves impart forces on grounded glaciers and ice streams, slowing the delivery of ice to the ocean (Thomas, 1973<ref name="Thomas, 1973">Thomas, R.H. 1973. The creep of ice shelves: Theory, ''Journal of Glaciology'', '''12''' (64), 45-53.</ref>). This means that collapse or melting of a floating ice shelf can trigger a subsequent rise in sea level, as glaciers accelerate when this force is removed. This force is determined not only by the size, lateral extent and temperature of the ice shelf, but also its shape (Walker and Holland, 2007<ref name="Walker and Holland, 2007">Walker, R. and Holland, D.M. 2007. A two-dimensional coupled model for ice shelf - ocean interaction, ''Ocean Modelling'', '''17''', 123-139.</ref>). Drilling allows temperature profiles through the ice to be recorded, and samples of the ice to be recovered for mechanical tests of strength and deformation strength (e.g. Rist et al., 2002<ref name="Rist et al, 2002">Rist, M.A., Sammonds, P.R., Oerter, H. and Doake, C.S.M. 2002. Fracture of Antarctic shelf ice, J. Geophys. Res., 107(B1), 10.1029/2000JB000058.</ref>). Local mass balance assessments are important, as melt rates at the base of ice shelves can reach several metres a year. Drilling provides access to the oceanographic environment beneath the ice shelf, so that the processes that control melting from the base can be investigated (Nicholls and Jenkins, 1993<ref name="Nicholls and Jenkins, 1993">Nicholls, K.W. and Jenkins, A. 1993, Temperature and salinity beneath Ronne Ice Shelf, Antarctica, J. Geophys. Res., 98, 22,553-22,568.</ref>; Craven et al., 2004<ref name="Craven et al, 2004">Craven, M., Allison, I., Brand, R., Elcheikh, A., Hunter, J., Hemer, M. and Donoghue, S. 2004. Initial borehole results from the Amery Ice Shelf hot-water drilling project, ''Ann. Glaciol.'', '''39''', 531-539.</ref>). Phase-sensitive radar can be used to measure thinning rates (Corr et al., 2002<ref name="Corr et al, 2002">Corr, H.F.J., Jenkins, A., Nicholls, K.W. and Doake, C.S.M. 2002. Precise measurement of changes in ice-shelf thickness by phase-sensitive radar to determine basal melt rates, Geophys. Res. Lett., 29(8), 10.1029/2001GL014618.</ref>). This can provide a direct estimate of the basal melt-rate with an accuracy of a few centimetres per year, revealing whether the ice shelf is in a steady state (Jenkins et al., 2006<ref name="Jenkins et al, 2006">Jenkins, A., Corr, H.F.J., Nicholls, K.W., Stewart, C.L., Doake, C.S.M. and Christopher S.M. 2006. Interactions between ice and ocean observed with phase-sensitive radar near an Antarctic ice-shelf grounding line, ''J. Glaciol.'', '''52''' (178), 325-346.</ref>). GPS observations can record the flow velocities of the floating ice, and how it is changing (King et al., 2007<ref name="King et al, 2007">King, M.A., Coleman, R., Morgan, P.J. and Hurd, R.S. 2007. Velocity change of the Amery Ice Shelf, East Antarctica, during the period 1968-1999, ''J. Geophys. Res.'', '''112''', F01013, doi:10.1029/2006JF000609.</ref>). These ''in-situ'' observations provide an important test of melt rates, flow rates and thinning rates observed using satellite.<br />
<br />
The very definition of where the ice shelves begin needs quantification, and its sensitivity to local ice thickness changes lends it to be an important diagnostic observable. The transition from grounded ice to floating occurs at the &ldquo;grounding line&rdquo;. It can be mapped with optical imagery by noting the change in surface roughness: floating ice is smoother due to the absence of basal friction, which supports smaller spatial scale surface undulations on grounded ice. Vertical flexing of the grounding line also can be observed either by interferometric SAR analysis or repeat laser altimetry (Vaughan, 1995<ref name="Vaughan, 1995">Vaughan, D.G. 1995, Tidal Flexure at Ice Sheet Margins, ''Journal of Geophysical Research'', '''100''' (B4), 6213-6224.</ref>; Padman and Fricker, 2005<ref name="Padman and Fricker, 2005">Padman, L. and Fricker, H.A. 2005. Tides on the Ross Ice Shelf observed with ICESat, Geophysical Research Letters, 32, No. 14, Art. No. L14503, July 29, 2005.</ref>).<br />
<br />
The sensitivity of the grounding line to thickness changes results from a shallow basal slopes. Relatively small changes of a few metres in thickness are effectively amplified by many orders of magnitude, depending on the bed slope, to horizontal displacements of hundreds of metres, or even kilometres. In many locations, the grounding line is extremely complex, and patches of ephemerally grounded ice must be included in the observations.<br />
<br />
==Subglacial hydrology==<br />
<br />
Ice flow rates in the faster moving ice streams and glaciers of Antarctica are orders of magnitude greater than can be explained by ice deformed by gravitational forces. The additional speed is the result of basal sliding and directly related to lubrication at the ice-bed interface. The degree of lubrication is controlled by the presence of water and sediment (glacial till) underneath the ice. A supply of water from melting at the base of the ice sheet can pressurise subglacial water to the point that the ice is close to flotation. This lessens the load on the underlying sediment and allows it to deform easily, lubricating the sliding. Drag from the edge can be comparable to drag from beneath, even when the width of an ice stream greatly exceeds its thickness. The energy that is needed for melting is provided partly by geothermal heat, and partly by frictional heating (Raymond et al., 2001<ref name="Raymond et al, 2001">Raymond, C.F., Echelmeyer, K.A., Whillans, I.M. and Doake, C.S.M. 2001. Ice stream shear margins. In: The West Antarctic Ice Sheet: Behavior and environment (R.B. Alley and R.A. Bindschadler, eds.). Washington DC: American Geophysical Union, ''Antarctic Research Series'', '''77''', 137-155.</ref>). Frictional heating is greater for faster sliding, but is also modulated by the degree of lubrication, so a feedback loop links melting to lubrication, speed, friction, and further melting. Depending upon the environment, this loop may reinforce itself, so that ice streams accelerate or decelerate rapidly. In other circumstances variations in velocity are muted, especially if other physical controls, such as trough geometry, limit the margin migration (Raymond et al., 2001<ref name="Raymond et al, 2001">Raymond, C.F., Echelmeyer, K.A., Whillans, I.M. and Doake, C.S.M. 2001. Ice stream shear margins. In: The West Antarctic Ice Sheet: Behavior and environment (R.B. Alley and R.A. Bindschadler, eds.). Washington DC: American Geophysical Union, ''Antarctic Research Series'', '''77''', 137-155.</ref>). Field measurements have shown that ice streams can flow steadily, have episodes of rapid flow, or shut down completely, depending on details of supply, storage and transport of water and sediment (Retzlaff and Bentley, 1993<ref name="Retzlaff and Bentley, 1993">Retzlaff, R. and Bentley, C.R. 1993. Timing of stagnation of Ice Stream C, West Antarctica, from short-pulse radar studies of buried surface crevasses, Journal of Glaciology, 39, No. 133, 553-561.</ref>; Stokes et al., 2007<ref name="Stokes et al, 2007">Stokes, C.R., Clark, C.D., Lian, O.B. and Tulaczyk, S. 2007. Ice stream sticky spots: A review of their identification and influence beneath contemporary and palaeo-ice streams, ''Earth-Science Reviews'', '''81''', 217-249.</ref>).<br />
<br />
In addition to the direct observations of surface velocity (discussed earlier), seismic methods have illuminated some of the processes that control ice stream flow. Although the base of ice streams tend to be well lubricated, the importance of small areas of concentrated friction has been identified by seismicity characteristic of their stick-slip motion (Anandakrishnan and Alley, 1994<ref name="Anandakrishnan and Alley, 1994">Anandakrishnan, S. and Alley, R.B. 1994. Ice stream C, Antarctica, sticky spots detected by microearthquake monitoring, ''Ann. Glaciol.'', '''20''', 183-186.</ref>). These &lsquo;sticky spots&rsquo; provide significant retardation to the flow of ice and have a number of possible causes, including reduction of basal water pressure by freezing, channel formation, or redirection of subglacial water flow (recently reviewed by Stokes et al., 2007<ref name="Stokes et al, 2007">Stokes, C.R., Clark, C.D., Lian, O.B. and Tulaczyk, S. 2007. Ice stream sticky spots: A review of their identification and influence beneath contemporary and palaeo-ice streams, ''Earth-Science Reviews'', '''81''', 217-249.</ref>). Analysis of waveforms reflected from the bed during active seismic sounding can reveal information about whether the sediments are mobile and fluidized, or whether they are lodged and strong enough to support large shear stress retarding the flow of ice (Smith, 2007<ref name="Smith, 2007">Smith, A.M. 2007. Subglacial Bed Properties from Normal-Incidence Seismic Reflection Data, ''J. Env. and Eng. Geophys.'', '''12''', 3-13.</ref>).<br />
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Understandably, there are not many direct observations from beneath the ice streams, but in several places access to the bed has been achieved by hot-water drilling. Video cameras lowered down the borehole (Carsey et al., 2002<ref name="Carsey et al, 2002">Carsey, F., Behar, A., Lane, A.L., Realmuto, V. and Engelhardt, H. 2002. A borehole camera system for imaging the deep interior of ice sheets, ''J. Glaciol.'', '''48''' (163), 622-628.</ref>) have revealed clear ice at the bottom of Kamb ice stream, supporting the interpretation that basal freezing may have contributed to its shutdown around 140 years ago (Vogel et al., 2005<ref name="Vogel et al, 2005">Vogel, S.W., Tulaczyk, S., Kamb, B., Engelhardt, H., Carsey, F., Behar, A., Lane, A. and Joughin, I. 2005. Subglacial Conditions During And After Stoppage of an Antarctic Ice Stream: Is Reactivation Imminent, ''Geophys. Res. Lett.'', '''32''' (14), L14502, doi:10.1029/2005GL022563.</ref>). Underneath the ice stream, the camera revealed a water layer 1.6 metres in depth at one site, but just centimetres or less at another. The water flux and linkages between such water cavities are important in determining the basal water pressure and this, in turn, affects the amount of lubrication (Christoffersen and Tulaczyk, 2003<ref name="Christoffersen and Tulaczyk, 2003">Christoffersen, P. and Tulaczyk, S. 2003. Response of subglacial sediments to basal freeze-on, 1. Theory and comparison to observations from beneath the West Antarctic Ice Sheet, ''J. Geophys. Res.'', '''108'''(B4), 2222, doi:10.1029/2002JB001935.</ref>; Vogel et al., 2005<ref name="Vogel et al, 2005">Vogel, S.W., Tulaczyk, S., Kamb, B., Engelhardt, H., Carsey, F., Behar, A., Lane, A. and Joughin, I. 2005. Subglacial Conditions During And After Stoppage of an Antarctic Ice Stream: Is Reactivation Imminent, ''Geophys. Res. Lett.'', '''32''' (14), L14502, doi:10.1029/2005GL022563.</ref>). The route taken by subglacial water flow is sensitive to surface and bed topography, and changes in surface slope that alter this routing may affect the water pressures, and hence the flow of ice (Alley et al., 1994<ref name="Alley et al, 1994">Alley, R.B., Anandakrishnan, S., Bentley, C.R. and Lord, N. 1994, A water-piracy hypothesis for the stagnation of Ice Stream C, ''Antarctica. Ann. Glaciol.'', '''20''', 187-194.</ref>).<br />
<br />
The discovery of thicker reservoirs of subglacial water has led to more direct inference from repeat satellite observations that large volumes of water move between subglacial lakes (Gray et al., 2005<ref name="Gray et al, 2005">Gray, L., Joughin, I., Tulazcyk, S., Spikes, V.B., Bindschadler, R. and Jezek, K. 2005. Evidence for subglacial water transport in the West Antarctica Ice Sheet through three-dimensional satellite radar interferometry, Geophysical Research Letters, 32, L03501, doi:10.1029/2004GL021387,.</ref>; Wingham et al., 2006b<ref name="Wingham et al, 2006b">Wingham, D.J., Siegert, M.J., Shepherd, A. and Muir, A.S. 2006b. Rapid discharge connects Antarctic subglacial lakes, Nature, 440, pp. 1033-1036 , doi:10.1038/nature04660.</ref>; Fricker et al., 2007<ref name="Fricker et al, 2007">Fricker, H.A., Scambos, T.A., Bindschadler, R. and Padman, L. 2007. An Active Subglacial Water System in West Antarctica Mapped from Space, ''Science'', '''315''', 1544, doi: 10.1126/science.1136897.</ref>) and that these larger volumes of water transfer change ice flow rates. These build on a series of discoveries that flow rates change on a wealth of time scales from minutes to millennia. A directed programme of fieldwork has illuminated many causes of this fast and changeable flow (e.g. Alley and Bindschadler, 2001<ref name="Alley and Bindschadler, 2001">Alley, R.B. and R.A. Bindschadler. 2001, The West Antarctic Ice Sheet: Behavior and environment. Washington DC: American Geophysical Union, Antarctic Research Series vol. 77, 296 pp.</ref>; Bindschadler, 2006<ref name="Bindschadler, 2006">Bindschadler, R. 2006. The environment and evolution of the West Antarctic ice sheet: setting the stage, ''Phil. Trans. R. Soc. A'', '''364''', 1583-1605, doi:10.1098/rsta.2006.1790.</ref>). Much of this dynamism can ultimately be traced to the slipperiness of basal sediment pressurised by meltwater.<br />
<br />
==Mass balance data from ice cores==<br />
<br />
Ice core records offer a valuable tool to assess natural variability and recent trends in snow accumulation. Climate model predictions lead to a general expectation of an increase in Antarctic precipitation with projected warming. Such an increase acts to offset sea level increases arising from mass loss. Recent results, based on records from ice cores, snow pits and stakes suggest an absence of a warming signal in precipitation over the last 50 years (Monaghan et al., 2006a<ref name="Monaghan et al, 2006a">Monaghan, A.J., Bromwich, D.H. and Wang, S-H. 2006a. Recent trends in Antarctic snow accumulation from Polar MM5, ''Philosophical Trans. Royal. Soc. A'', '''364''', 1683-1708.</ref>), with no significant changes in overall precipitation. The data do show, however, a large temporal and spatial variability that underscores the need for mass balance studies to include long-term records from a number of sites.<br />
<br />
ITASE research reveals high variability in surface mass balance, such that single cores, stakes, and snowpits do not always represent the geographical and environmental characteristics of a local region (Richardson and Holmlund, 1999<ref name="Richardson and Holmlund, 1999">Richardson, C. and Holmlund, P. 1999. Regional and local variability in shallow snow layer depth from a 500 km continuous radar traverse on the polar plateau, central Dronning Maud Land, East Antarctica, ''Annals of Glaciology'', '''29''', 10-16.</ref>; Frezzotti et al., 2004<ref name="Frezzotti et al, 2004">Frezzotti, M., Pourchet, M., Flora, O., Gandolfi, S., Gay, M., Urbini, S., Vincent, C., Becagli, S., Gragnani, R., Proposito, M., Severi, M., Traversi, R., Udisti, R. and Fily, M. 2004. New estimations of precipitation and surface sublimation in East Antarctica from snow accumulation measurements, ''Climate Dynamics'', '''23''', 803-813.</ref>; Spikes et al., 2004<ref name="Spikes et al, 2004">Spikes, V.B., Hamilton, G.S., Arcone, S.A., Kaspari, S. and Mayewski, P.A. 2004. Variability in accumulation rates from GPR profiling on the West Antarctic Plateau, ''Ann. Glaciol.'', '''39''', 238-244.</ref>; Nishio et al., 2002<ref name="Nishio et al, 2002">Nishio, F., Furukawa, T., Hashida, G., Igarashi, M., Kameda, T., Kohno, M., Motoyama, H., Naoki, K., Satow, K., Suzuki, K., Takata, M., Toyama, Y., Yamada, T. and Watanabe, O. 2002. Annual-layer determinations and 167 year records of past climate of H72 ice core in east Dronning Maud Land, Antarcitaca, ''Ann. Glacio.'', '''35''', 471-479.</ref>). For example, Frezzotti et al. (2004<ref name="Frezzotti et al, 2004">Frezzotti, M., Pourchet, M., Flora, O., Gandolfi, S., Gay, M., Urbini, S., Vincent, C., Becagli, S., Gragnani, R., Proposito, M., Severi, M., Traversi, R., Udisti, R. and Fily, M. 2004. New estimations of precipitation and surface sublimation in East Antarctica from snow accumulation measurements, ''Climate Dynamics'', '''23''', 803-813.</ref>) show that spatial surface mass balance variability at sub-kilometre scales (as is typically represented in ice cores) overwhelms temporal variability at the century scale for a low-accumulation site in East Antarctica. Emerging data collected by ITASE and associated deep ice core projects reveals systematic biases in long-term estimates of surface mass balance compared to previous compilations; the biases are presumably related to the small-scale spatial variability (Oerter et al., 1999<ref name="Oerter et al, 1999">Oerter, H., Graf, W., Wilhelms, F., Minikin, A. and Miller, H. 1999. Accumulation studies on Amundsenisen, Dronning Maud Land, Antarctica, by means of tritium, dielectric profiling and stable-isotope measurements: first results from the 1995-96 and 1996-97 field seasons, ''Ann. Glaciol.'', '''29''', 1-9.</ref>; Frezzotti et al., 2004<ref name="Frezzotti et al, 2004">Frezzotti, M., Pourchet, M., Flora, O., Gandolfi, S., Gay, M., Urbini, S., Vincent, C., Becagli, S., Gragnani, R., Proposito, M., Severi, M., Traversi, R., Udisti, R. and Fily, M. 2004. New estimations of precipitation and surface sublimation in East Antarctica from snow accumulation measurements, ''Climate Dynamics'', '''23''', 803-813.</ref>, Magand et al., 2004<ref name="Magand et al, 2004">Magand, O., Frezzotti, M., Pourchet, M., Stenni, B., Genoni, L. and Fily, M. 2004. Climate variability along latitudinal and longitudinal transects in East Antarctica, ''Annals of Glaciology'', '''39''', 351-358.</ref>; Rotschky et al., 2004<ref name="Rotschky et al, 2004">Rotschky, G., Eisen, O., Wilhelms, F., Nixdorf, U. and Oerter, H. 2004. Spatial distribution of surface mass balance on Amundsenisen plateau, Antarctica, derived from ice-penetrating radar studies, ''Annals of Glaciology'', '''39''', 265-270.</ref>). The extensive use, along ITASE traverses, of new techniques like geolocated GPR profiling integrated with core data, provides detailed information on surface mass balance (Richardson and Holmlund, 1999<ref name="Richardson and Holmlund, 1999">Richardson, C. and Holmlund, P. 1999. Regional and local variability in shallow snow layer depth from a 500 km continuous radar traverse on the polar plateau, central Dronning Maud Land, East Antarctica, ''Annals of Glaciology'', '''29''', 10-16.</ref>; Urbini et al., 2001<ref name="Urbini et al, 2001">Urbini, S., Gandolfi, S. and Vittuari, L. 2001. GPR and GPS data integration: examples of application in Antarctica, ''Annali di Geofisca'', '''44''' (4), 687-702.</ref>; Arcone et al., 2005<ref name="Arcone et al, 2005">Arcone, S.A., Spikes, V.B., Hamilton, G. and Mayewski, P.A. 2005. Continuity, vertical resolution and origin of stratigraphy in 400-Mhz short-pulse radar profiles of firn in West Antarctica, ''Annals of Glaciology'', '''39''', 195-200.</ref>; Rotschky et al., 2004<ref name="Rotschky et al, 2004">Rotschky, G., Eisen, O., Wilhelms, F., Nixdorf, U. and Oerter, H. 2004. Spatial distribution of surface mass balance on Amundsenisen plateau, Antarctica, derived from ice-penetrating radar studies, ''Annals of Glaciology'', '''39''', 265-270.</ref>). At some sites stake farm and ice core accumulation rates differ significantly, but isochronal layers in firn, detected with GPR, correlate well with ice core chronologies (Frezzotti et al., 2004<ref name="Frezzotti et al, 2004">Frezzotti, M., Pourchet, M., Flora, O., Gandolfi, S., Gay, M., Urbini, S., Vincent, C., Becagli, S., Gragnani, R., Proposito, M., Severi, M., Traversi, R., Udisti, R. and Fily, M. 2004. New estimations of precipitation and surface sublimation in East Antarctica from snow accumulation measurements, ''Climate Dynamics'', '''23''', 803-813.</ref>). Several GPR layers within the upper 100 m of the surface have been surveyed over continuous traverses of 5,000 km and can be used as historical benchmarks to study past accumulation rates (Spikes et al., 2004<ref name="Spikes et al, 2004">Spikes, V.B., Hamilton, G.S., Arcone, S.A., Kaspari, S. and Mayewski, P.A. 2004. Variability in accumulation rates from GPR profiling on the West Antarctic Plateau, ''Ann. Glaciol.'', '''39''', 238-244.</ref>).<br />
==References==<br />
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[[Category:Observations, data accuracy and tools]]<br />
[[Category:The Antarctic ice sheet]]</div>Maintenance scripthttp://acce.scar.org/wiki/Observations_of_terrestrial_biologyObservations of terrestrial biology2014-08-06T14:33:47Z<p>Maintenance script: Importing text file</p>
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<div>:''This page is part of the topic [[Observations, data accuracy and tools]]''<br />
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[[File:Figure 2.30 - Commonly used Antarctic terrestrial biogeographic regions.png|thumb|'''2.30''' Commonly used terrestrial biogeographic regions within the Antarctic.]]<br />
For the purposes of this volume, the Antarctic terrestrial and freshwater biome includes the main continental landmass (the &lsquo;continental Antarctic&rsquo; to biologists), the Antarctic Peninsula and associated islands and archipelagos (South Shetland, South Orkney, South Sandwich Islands, Bouvet&oslash;ya) (the &lsquo;maritime Antarctic&rsquo;), and the sub-Antarctic islands which lie on or about the Antarctic Polar Frontal Zone (PFZ) ([[:File:Figure 2.30 - Commonly used Antarctic terrestrial biogeographic regions.png|Figure 2.30]]). These geographic regions are also meaningful biogeographical regions (see Smith, 1984<ref name="Smith, 1984">Smith, R.I.L. 1984. Terrestrial plant biology of the sub-Antarctic and Antarctic. In: Laws, R.M. (ed.), Antarctic Ecology, 1, Academic Press, London, 61-162.</ref>; Chown and Convey, 2006<ref name="Chown and Convey, 2006">Chown, S.L. and Convey, P. 2006. Biogeography. Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 55-69.</ref>, 2007<ref name="Chown and Convey, 2007">Chown, S.L. and Convey, P. 2007. Spatial and temporal variability across life&rsquo;s hierarchies in the terrestrial Antarctic, Philosophical Transactions of the Royal Society of London, ''series B'', '''362''', 2307-2331.</ref>; Huiskes et al., 2006<ref name="Huiskes et al, 2006">Huiskes, A.H.L., Convey, P. and Bergstrom, D. 2006. Trends in Antarctic terrestrial and limnetic ecosystems: Antarctica as a global indicator, Trends in Antarctic Terrestrial and Limnetic Ecosystems: Antarctica as a Global Indicator, eds. Bergstrom, D.M, Convey, P. and Huiskes, A.H.L. Springer, Dordrecht, 1-13.</ref>; Convey, 2007b<ref name="Convey, 2007b">Convey, P. 2007b. Influences on and origins of terrestrial biodiversity of the sub-Antarctic islands, Papers and Proceedings of the Royal Society of Tasmania, 141, 83-93.</ref>)<br />
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Over both short and long timescales, there are three major potential colonisation mechanisms likely to have played a role in shaping contemporary Antarctic biodiversity and biogeography, these being simple transport in the air column, incidental transport on other biota and debris, and transport on/in the ocean. Although both oceanic and atmospheric circulation patterns have acted to isolate Antarctica from lower latitudes, at least since the initiation of the Antarctic Circumpolar Current, this barrier is certainly not a hermetic seal, and terrestrial environments of Antarctica and the sub-Antarctic have experienced a fairly constant if low level rate of invasion from temperate or closer regions over evolutionary time, as well acting as a source and exporting biota northwards (Barnes et al., 2006<ref name="Barnes et al, 2006">Barnes, D.K., Hodgson, D.A., Convey, P., Allen, C.S. and Clarke, A.C. 2006. Incursion and excursion of Antarctic biota: past, present and future, ''Global Ecol Biogeogr'', '''15''', 121-142.</ref>).<br />
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Terrestrial and freshwater biological knowledge is unevenly distributed both across these regions and across the different biological groups present (Adams et al., 2006<ref name="Adams et al, 2006">Adams, B., Bardgett, R.D., Ayres, E., Wall, D.H., Aislabie, J., Bamforth, S., Bargagli, R., Cary, C., Cavacini, P., Connell, L., Convey, P., Fell, J., Frati, F., Hogg, I., Newsham, K.K., O&rsquo;Donnell, A., Russell, N., Seppelt, R. and Stevens, M.I. 2006. Diversity and Distribution of Victoria Land Biota, Soil Biology and Biochemistry 38, 3003-3018.</ref>; Chown and Convey, 2007<ref name="Chown and Convey, 2007">Chown, S.L. and Convey, P. 2007. Spatial and temporal variability across life&rsquo;s hierarchies in the terrestrial Antarctic, Philosophical Transactions of the Royal Society of London, ''series B'', '''362''', 2307-2331.</ref>; Peat et al., 2007<ref name="Peat et al, 2007">Peat, H.J., Clarke, A. and Convey, P. 2007 Diversity and biogeography of the Antarctic flora, ''Journal of Biogeography'', '''34''', 132-146.</ref>). Historically, biological research effort has focused on areas easily accessible from research stations, with an understandable but unfortunate tendency to select those areas with obvious biological development. In practice, this means that the majority of biological research (relating to both biodiversity survey and the study of biological adaptation and function) has taken place at a limited number of locations around the continent, and focused on a limited number of organisms. Prime amongst these locations are Signy Island (South Orkney Islands), various locations on the South Shetland Islands, the western coast of the Antarctic Peninsula (Anvers Island, the Argentine Islands, Marguerite Bay), locations along the Victoria Land coastline, and the Victoria Land Dry Valleys. Less accessible and more &lsquo;barren&rsquo; areas have historically not received priority for logistic support or science funding, meaning that very little information is available from most inland regions, and many coastal areas and islands remote from research stations. Furthermore, in terms of important but subtle aspects of biodiversity research, in particular the functional role of organisms within an ecosystem, and the provision of ecosystem services, autecological studies of most species in most groups are non-existent (Convey, 1996a<ref name="Convey, 1996a">Convey, P. 1996a. The influence of environmental characteristics on life history attributes of Antarctic terrestrial biota, ''Biological Reviews of the Cambridge Philosophical Society'', '''71''', 191-225.</ref>; Hogg et al., 2006<ref name="Hogg et al, 2006">Hogg, I.D., Cary, S.C., Convey, P., Newsham, K.K., O&rsquo;Donnell, T., Adams, B.J., Aislabie, J., Frati, F.F., Stevens, M.I. and Wall, D.H, 2006. Biotic interactions in Antarctic terrestrial ecosystems: are they a factor? Soil Biology and Biochemistry, 38, 3035-3040.</ref>). This means that functional interpretations of the biology of Antarctic terrestrial biota are often based on (untested) generalisations from the literature on a small number of species that have been targeted, and that on related species and genera from lower latitude ecosystems.<br />
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While biodiversity records are obviously available from a much wider range of locations across the continent than the focus areas mentioned, these are often the result of single field campaigns (sometimes directly involving an appropriate specialist, more often opportunistic collections subsequently passed to specialists). It is rare, even where specialists are engaged in field studies, for organised and replicated surveys to be completed. This is owing to multiple reasons - the practical logistics of supporting remote fieldwork, the very patchy distribution and small physical scale of habitats, the typically aggregated distributions of many of the biota involved, and the potential environmental impact and damage caused by sampling sensitive and fragile habitats. Many of the records that do exist are of limited taxonomic usefulness, while even where identifications to species level are available, they often represent the work of a single taxonomist and, especially for the smaller groups of soil invertebrates (e.g. nematodes, tardigrades), have not been re-assessed for veracity since the original collections, in some cases made by the early exploring expeditions (Adams et al., 2006<ref name="Adams et al, 2006">Adams, B., Bardgett, R.D., Ayres, E., Wall, D.H., Aislabie, J., Bamforth, S., Bargagli, R., Cary, C., Cavacini, P., Connell, L., Convey, P., Fell, J., Frati, F., Hogg, I., Newsham, K.K., O&rsquo;Donnell, A., Russell, N., Seppelt, R. and Stevens, M.I. 2006. Diversity and Distribution of Victoria Land Biota, Soil Biology and Biochemistry 38, 3003-3018.</ref>). In many cases, the type material originally described no longer exists or is too degraded to be useful. The contemporary shortage of specialist taxonomic expertise is a problem recognised globally (references from Sands et al., 2008<ref name="Sands et al, 2008">Sands, C.J., Convey, P., Linse, K. and McInnes, S.J. 2008. Assessing meiofaunal variation among individuals: an example using Tardigrada, ''BMC Ecology'', '''8''', 7.</ref>), but is particularly acute with reference to the faunal, floral and microbial groups that constitute Antarctic terrestrial and freshwater ecosystems.<br />
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A new tool recently available to Antarctic biologists is that of molecular taxonomy (e.g. Sands et al., 2008<ref name="Sands et al, 2008">Sands, C.J., Convey, P., Linse, K. and McInnes, S.J. 2008. Assessing meiofaunal variation among individuals: an example using Tardigrada, ''BMC Ecology'', '''8''', 7.</ref>). This relies on the use of DNA or RNA sequence substitutions that build up over evolutionary time in order to calculate what are in effect likelihood trees (phylogenetic trees) expressing the evolutionary relationships between different organisms. There are many assumptions inherent in this approach, in particular relating to the rate of substitution over time, how to integrate molecular and classical taxonomic studies, and how to independently &lsquo;ground truth&rsquo; the dating of divergence events, but its utility is now generally accepted. Even without attempting to interpret relationships, the use of selected DNA sequences as a molecular &lsquo;barcode&rsquo; identifying specific taxa is also becoming widely accepted, and can be seen as an alternative means of assessing biodiversity in the absence of either appropriate taxonomic expertise, or of distinguishing morphological characters.<br />
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The limitations of contemporary survey data available as a baseline against which to compare and monitor future trends are amply illustrated by the bryophyte flora, one the best-known and researched groups of Antarctic biota. Peat et al. (2007<ref name="Peat et al, 2007">Peat, H.J., Clarke, A. and Convey, P. 2007 Diversity and biogeography of the Antarctic flora, ''Journal of Biogeography'', '''34''', 132-146.</ref>), based on a comprehensive dataset of confirmed herbarium and literature records, provide a visual illustration and quantification of the level of diversity knowledge of bryophytes across the continent by the simple and coarse means of dividing the continental area in one degree latitude/longitude boxes, identifying all boxes that include at least one ice-free area of &gt; 100 m<sup>2</sup>, and then identifying how many of these have at least one herbarium or verified literature record of a plant&rsquo;s occurrence. On this basis, almost exactly 50% of boxes identified have no plant records (although it is then not possible to separate those that have simply not been visited from those that have had any form of visit or survey). Database compilations of diversity for other major groups of Antarctic terrestrial biota are less spatially representative even than that of the bryophytes, but are now becoming available (references in Pugh and Convey, 2008<ref name="Pugh and Convey, 2008">Pugh, P.J.A. and Convey, P. 2008. Surviving out in the cold: Antarctic endemic invertebrates and their refugia, ''Journal of Biogeography'', '''35''', 2176-2186.</ref>; Convey et al., 2008<ref name="Convey et al, 2008">Convey, P., Gibson, J.A.E., Hillenbrand, C-D., Hodgson, D.A., Pugh, P.J.A., Smellie, J.L. and Stevens, M.I. 2008. Antarctic terrestrial life - challenging the history of the frozen continent? Biological Reviews, 83, 103-117.</ref>), starting to generate a baseline against which future changes can be compared.<br />
==References==<br />
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[[Category:Observations, data accuracy and tools]]<br />
[[Category:Terrestrial biology]]</div>Maintenance script