Le Quéré: Less CO2 Uptake by Southern Ocean Carbon Sink

Southern Ocean Carbon Sink

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If you drove to work or school this morning or used electricity to power the computer on which you're looking at this image, chances are you released carbon dioxide, a greenhouse gas, into the atmosphere. According to the Intergovernmental Panel on Climate Change, people released about 7.8 billion tons (7.8 gigatons) of carbon into the atmosphere in 2005 by burning fossil fuels and making cement, and that number grows every year. What happens to all of the carbon dioxide that people release into the atmosphere? About half stays in the atmosphere, where it warms Earth, and the other half is absorbed by growing plants on land and by the ocean.

As people have put more and more carbon dioxide into the atmosphere, the ocean has responded by soaking up more carbon dioxide—a trend scientists expected to continue for many years. But in 2007, a team of scientists reported in the journal Science that between 1981 and 2004 carbon dioxide concentrations in the Southern Ocean didn’t change at all, even though global atmospheric levels continued to rise. This graph shows the changes scientists expected to see (blue line) compared to their estimate of actual carbon dioxide absorption (red line). The results suggested that the Southern Ocean was no longer keeping pace with human carbon dioxide emissions.

Why has the Southern Ocean started to lag behind human emissions? The answer, believes Corinne Le Quéré, is in the wind. An ocean scientist at the University of East Anglia, Le Quéré led the study that discovered the Southern Ocean’s change of pace. Le Quéré modeled the mechanisms that influence how the ocean takes up carbon and found that winds increased between 1981 and 2004. Winds stirred the ocean and enhanced the upwelling of deep, carbon-rich water. The ocean releases carbon dioxide into the atmosphere in areas where deep water comes to the surface, so increased upwelling allowed the ocean to vent more carbon dioxide. This increased venting made it look like the Southern Ocean was no longer taking up carbon dioxide as quickly as people were pumping it into the atmosphere.

Equally interesting is the cause for the change. The Southern Ocean is already renowned for its high winds and rough seas (see photo). But Le Quéré and her colleagues believe that the ozone hole and global warming combined to bring even stronger winds to the Southern Ocean. “Ozone naturally warms the upper atmosphere because it captures the radiation from the Sun and re-emits it there,” says Le Quéré. “If you deplete ozone, you get a very large cooling in the upper atmosphere.” The huge temperature difference between the ozone hole and the rest of the stratosphere causes strong winds around Antarctica. Uneven warming in different parts of the southern hemisphere from recent global warming may also have created a temperature difference that strengthened the winds. If it turns out that Le Quéré is right about what caused the winds to change, it will be one of the first signs that human activity is affecting the way the ocean takes up and releases carbon dioxide.

To read more, see The Ocean’s Carbon Balance on the Earth Observatory.

• References
• Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland (2007). Changes in Atmospheric Constituents and in Radiative Forcing. 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 UniversityPress, Cambridge, United Kingdom and New York, NY, USA.
• Le Quéré, C., Rödenbeck, C., Buitenhuis E., Conway, T., Langenfelds, R., Gomez, A., Labuschagne, C., Ramonet, M., Nakazawa, T., Metzl, N., Gillet, N., Heimann, M. (2007, June 22). Saturation of the Southern Ocean CO₂ sink due to recent climate change. Science, 316, 1735-1738; published online May 16, 2007.

Photograph courtesy British Antarctic Survey. Graph courtesy Corinne Le Quéré. Caption by Holli Riebeek.

Link to article: http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=18076