The increase in CO₂ levels in the atmosphere due to human activities is partly responsible for global warming. By absorbing almost 15% of anthropogenic carbon released every year, the Southern Ocean is one of the main sinks for atmospheric CO₂. But its effectiveness is decreasing, even as the level of atmospheric carbon jas continued to increase over the last few years. Until now, the saturation of the carbon sink in the Southern Ocean has not been correctly simulated by the climate models used.

In order to improve these simulations, a collaborative team of climatologists, modelers and oceanographers was set up. Their objective: to develop a model that more accurately simulates the Southern Ocean's ability to act as a carbon sink. To do this, the researchers based their studies on the IPSL's coupled ocean/atmosphere model, which integrates the carbon cycle (and thus the evolution of greenhouse gases, such as CO₂). The key novel feature of this model is that it takes into account changes in the concentration of stratospheric ozone from 1975 until the present day. As Nicolas Metzl, LOCEAN/IPSL researcher and OISO coordinator, points out: "The simulations obtained with this model more accurately reproduce the oceanic observations obtained in the field over the last few years".

Above all, this study highlights two major phenomena with regard to the Southern Ocean: a significant reduction in CO2 uptake, which is not compensated in the other oceans, as well as an acceleration in the acidification of high southern latitude oceanic water. Between 1987 and 2004, around 2.3 billion tons of carbon was not taken up by the oceans. This corresponds to a relative reduction of nearly 10% of the global oceanic carbon uptake. The simulations thus reveal how perturbations to the upper atmosphere (in this case, the ozone hole) interact with greenhouse gases and the oceanic carbon cycle: they lead to stronger westerly winds in the Southern Ocean, which in turn lead to surface oceanic water being mixed with deeper water, rich in CO₂, thus limiting the absorption of atmospheric carbon by surface water.

This is the first time that the impact of the ozone hole on the oceanic carbon cycle has been simulated in a global climate model. These results suggest that the climate models used until now have overestimated oceanic carbon uptake and underestimated ocean acidification. They underline the importance of taking ozone into account in future modeling, particularly by the IPCC, which will make it possible to improve future climate predictions. The Southern Ocean is a region that is particularly sensitive to global warming. Predicting the consequences of such changes more accurately is fundamental, not just with regard to the global carbon balance (saturation of air-sea fluxes) but also marine resources (impact of acidification).

More information: Stratospheric ozone depletion reduces ocean carbon uptake and enhances ocean acidification. Lenton, A., F. Codron, L. Bopp, N. Metzl, P. Cadule, A. Tagliabue and J. Le Sommer. Geophysical Research Letters. 20 June 2009.

Source: CNRS