View source for Attribution of ice sheet changes in the instrumental period

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The Antarctic ice sheet is known to respond slowly to large and sustained climate changes, but the new awareness that it can also respond rapidly to other changes makes it difficult to attribute a particular change in the ice sheet to a particular causal event or events such as recent/anthropogenic climate change. The inescapable fact is that ice sheet behaviour manifests itself as the superposition of multiple responses on multiple time scales to multiple environmental changes.

Further complicating the situation is that we do not know the changes that were occurring in the ice sheet prior to the period of satellite observations (which in the case of ice sheets began in earnest in 1992), let alone a century ago, and we have very little knowledge as to whether natural changes in the ice sheet over past millennia were smooth or step-wise and abrupt. The context for current changes must be understood by inference. Caution must be used in extrapolating current changes into the future.

The two areas of most rapid glaciological change are the Antarctic Peninsula ice shelves and the Amundsen Sea sector outlet glaciers. A possible trigger for the sudden collapse of ice shelves in the Antarctic Peninsula is pressurisation of crevasses filled with surface meltwater (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>). The supply of meltwater increases with surface warming, placing a limit on the regions where ice shelves are viable (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>). Some of the observed surface warming in the northern Antarctic Peninsula can be attributed to changes in global circulation, specifically to changes in the SAM (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>). The SAM exhibits considerable decadal variability, but 40-year trends similar to those observed are reproduced in global climate models forced by a combination of ozone depletion, and increasing 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>). The stronger circumpolar westerlies bring warmer, northwesterly winds across the northern Antarctic Peninsula. In the eastern Peninsula, this trend is further amplified because northwesterly winds lead to downslope F&ouml;hn winds across the ice shelves, promoting warmer winter temperatures and longer melt seasons in summer (e.g. 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>). The loss of sea ice cover in the Amundsen and Bellingshausen seas (Jacobs and Comiso, 1997<ref name="Jacobs and Comiso, 1997">Jacobs, S.S. and Comiso, J.C. 1997. Climate variability in the Amundsen and Bellingshausen Seas, ''Journal of Climate'', '''10'''(4), 697-709.</ref>) may also affect the climate of the Antarctic Peninsula, and the viability of its ice shelves. Thinning from enhanced melting by ocean heat supply at the base of the ice shelf (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>), or softening at the margins (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>; Khazendar et al., 2007<ref name="Khazendar et al, 2007">Khazendar, A., Rignot, E. and Larour, E. 2007. Larsen B Ice Shelf rheology preceeding its disintegration inferred by a control method, ''Geophysical Research Letters'', '''34''', L19503, doi:10.1029/2007GL030980.</ref>), may have weakened ice shelves and predisposed them to collapse. While they are present, the ice shelves impart forces on the grounded glaciers that drain into them. Once ice shelves have disintegrated, these forces are removed, and the grounded glaciers accelerate (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>; De Angelis and Skvarca, 2003<ref name="Angelis and Skvarca, 2003">De Angelis, H. and Skvarca, P. 2003. Glacier surge after ice shelf collapse, ''Science'', '''299''', 1560-1562.</ref>; 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>) and start to thin (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>). This increases their discharge of ice into the ocean, and contributes to sea level rise.

The cause of the glaciological changes in the Amundsen Sea embayment is still an open topic of research. Ice sheet models have been used to show that changes similar to those observed can be caused by loss of basal friction in a small part of the ice sheet near the grounding line, perhaps caused by floatation as the grounding line retreats (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>; 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>). Thinning of ice shelves by basal melt (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>), softening of their margins (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>; Khazendar et al., 2007<ref name="Khazendar et al, 2007">Khazendar, A., Rignot, E. and Larour, E. 2007. Larsen B Ice Shelf rheology preceeding its disintegration inferred by a control method, ''Geophysical Research Letters'', '''34''', L19503, doi:10.1029/2007GL030980.</ref>), or shortening by iceberg-calving (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>) would also affect the force imposed on upstream glaciers. The ice shelves in the Amundsen Sea Embayment have been thinning (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>; Bindschadler, 2002<ref name="Bindschadler, 2002">Bindschadler, R.A. 2002. History of lower Pine Island Glacier, West Antarctica, from Landsat. imagery, ''J. Glaciol.'', '''48''' (163), 536-544.</ref>), and the grounding line of Pine Island Glacier has retreated as sections have thinned and gone afloat (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>). This thinning, together with the observation that many independent ice streams are behaving similarly, has been taken to imply that changes in the heat supplied from the ocean are responsible for glaciological change in this sector (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>). One mechanism for a variable supply of heat is episodic delivery of relatively warm CDW onto the continental shelf (Dinniman and Klinck, 2004<ref name="Dinniman and Klinck, 2004">Dinniman, M. S., and Klinck, J. M. 2004. A model study of circulation and cross-shelf exchange on the west Antarctic Peninsula continental shelf. Deep Sea Research Part II: Topical Studies in Oceanography, 51(17), 2003-2022.</ref>). CDW water occupies a deep layer, below the continental shelf break (700 to 1,100 m), but can be induced to upwell onto the continental shelf, especially via troughs (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>), where it becomes available for basal ice shelf melting. Water layers in the depth of 700 to 1100 m are observed to be warming significantly further off shore (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>), and there is evidence from salinity and other measurements over the past 40 years that increased melting is resulting from heat supplied by warmer CDW to northern West Antarctic ice shelves (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>). According to coupled ocean atmosphere models, the upwelling of CDW is partly controlled by atmospheric circulation patterns (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>). There is some correspondence between periods of enhanced heat supply predicted by a coupled model, and periods of observed glacier acceleration (Thoma et al., 2008<ref name="Thoma et al, 2008">Thoma, M., Jenkins, A., Holland, D. and Jacobs, S. 2008. Modelling Circumpolar Deep Water intrusions on the Amundsen Sea continental shelf, Antarctica, Geophys. Res. Lett., 35, L18602. doi:10.1029/2008GL034939.</ref>). Direct observation of temporal changes in the delivery of circumpolar deep water to the ice shelf, and of the consequences for inland ice sheet flow are needed to test such models. To attribute the glaciological changes in the Amundsen Sea sector to a particular climate forcing will require a better understanding of the variability in ice sheet flow, how that flow is influenced by melting beneath ice shelves, and how the oceanic heat delivered to the ice shelves can change under the different atmospheric circulation patterns produced by various scenarios of radiative forcing.
==References==
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[[Category:The instrumental period]]
[[Category:The Antarctic ice sheet]]