View source for The Southern Ocean carbon cycle response to historical climate change

:''This page is part of the topic [[Antarctic climate and environment change in the instrumental period]]''

==Introduction==

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>).

==CO<sub>2</sub> fluxes in the Southern Ocean==

[[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>).]]
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.

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).

[[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).]]
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.

==Historical Change - Observed Response==

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.

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.

[[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>).]]
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.

[[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>).]]
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>).

==Historical Changes &ndash; Simulated View==

[[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).]]
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]].

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>).

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.

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.

==Changes in CO<sub>2</sub> inventories==

[[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>).]]
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).

==Concluding Remarks==

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).
==References==
<references />
[[Category:The instrumental period]]
[[Category:The Southern Ocean]]
[[Category:The carbon cycle]]