The hole in the ozone layer over Antarctica may be impairing the Southern Ocean's ability to mop up carbon dioxide from Earth's atmosphere, according to work presented at a meeting in France today.

Earth's oceans are the largest sink of carbon dioxide, with the Southern Ocean accounting for more than 40% of the annual uptake of the greenhouse gas, says Andrew Lenton, a marine biochemist at the Pierre and Marie Curie University in Paris. In theory, seas should soak up more carbon dioxide as levels of the gas in the atmosphere rise.

But recent measurements have bucked simulations^1–3, showing that the Southern Ocean's surface waters have higher carbon levels than expected, which also makes them more acidic. As a result, the amount of CO₂ that the ocean absorbs each year has also flattened out.

Missing link

What was missing from the models, says Lenton, was stratospheric ozone damage — which, along with the climatic effects of greenhouse-gas emissions, is thought to be behind the observed strengthening of southern winds. These winds, he says, may causing ocean currents that stir up carbon stored in the deep ocean and bring it up to the surface. As part of the five-year CARBOOCEAN project, a research consortium on marine impacts of carbon-dioxide emissions that is meeting in Dourdan from 8–12 December, Lenton and his colleagues built Southern Ocean simulations that coupled the ozone's effects on winds to ocean currents and marine carbon levels.

Until now, these connections have only been studied piecemeal, says Christoph Heinze, a biogeochemical modeller at the University of Bergen in Norway, who was not involved in the study. "It's one of the rare examples where somebody has really looked at several components of the Earth system together," he says.

By running the models both with and without ozone depletion since 1975, the researchers "isolated the signal from ozone depletion", says Lenton's co-worker Francis Codron, an atmospheric scientist at the Dynamic Meteorology Laboratory in Paris.

Including the ozone hole reproduced the carbon sink that has observed by oceanographers. "These sound like very different parts of the system, and yet one affects the other," says Codron.

The signal from ozone, the researchers found, drove a drop in Southern Ocean surface pH of 0.01 units from 1994 to 2004 — half the total pH decline in that period, and one-tenth of the change since the pre-industrial era.

Model muddle

Although the Antarctic ozone hole has stabilized in recent years and is expected to heal in the latter half of this century, climate models that don't include its effects may show an overly optimistic future, says Lenton. Julie Arblaster of the Bureau of Meteorology Research Centre in Melbourne, Australia, agrees: "Understanding recent trends in the Southern Ocean carbon sink is key to understanding future projections of atmospheric CO₂."

Other work, however, casts doubt on that understanding. Scientists from the Leibniz Institute of Marine Sciences in Kiel, Germany, led by Claus Böning, have questioned the altered ocean currents seen in models such as that of Lenton and his colleagues, saying that oceanographic data don't bear them out⁴. Corinne Le Quéré of the University of East Anglia, UK, who studied the Southern Ocean carbon sink¹, says that Lenton's model seems to account for the observed failure of the Southern Ocean to take up more CO₂. But with the controversy over ocean currents, she says, "I don't have very much confidence in the explanation of the process" that the model provides.

Lenton says the contradictions could still be resolved. "What we're arguing over is the mechanism by which the carbon-rich deep water is ventilated," he says. Böning and his colleagues endorse more-detailed ocean models, which show how winds affect small eddies as well as large ocean currents. But these models don't look at carbon-dioxide levels — a crucial omission, says Lenton. A more detailed model, he says, would give much the same results he has reported.

"The great challenge now, for the future, is to run their model with carbon in it," says Lenton — but for now, such a complex simulation is out of reach.

• References

  1. Le Quéré, C. et al. Science 316, 1735–1738 (2008).
  2. Takahashi, T. et al. Deep-Sea Res. II (in the press).
  3. Metzl, N. Deep-Sea Res. II (in the press).
  4. Böning, C. W. , Dispert, A. , Visbeck, M. , Rintoul, S. R. & Schwarzkopf, F. U. Nature Geosci. 1, 864–869 (2008).

Link to article: http://www.nature.com/news/2008/081209/full/news.2008.1292.html