Researchers Find That Stratospheric Ozone Depletion Reduces Southern Ocean Carbon Uptake

9 July 2009

Researchers in France have found that ozone depletion above Antarctica has significantly reduced the Southern Ocean’s ability to absorb atmospheric CO₂ and has also accelerated acidification of southern polar waters, despite lesser CO₂ uptake. A paper on their work was published online 20 June in the AGU journal Geophysical Research Letters.

The Southern Ocean is a major carbon sink, absorbing almost 15% of anthropogenic carbon released every year. However, its effectiveness is decreasing, even as the level of atmospheric carbon has continued to increase over the last few years. (Earlier post.) Until now, the saturation of the carbon sink in the Southern Ocean has not been correctly simulated by the climate models used.

Andrew Lenton and colleagues at the National Center for Scientific Research used a model to calculate the changes in ocean–atmosphere carbon balances for the period 1975–2004 that included the impacts of changing ozone concentrations. The researchers compared two scenarios: one that accounted for ozone depletion and another in which no ozone alterations occurred.

The results revealed that, 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.

In addition, about 50% of the observed 0.02 unit decline in seawater pH during the study period—i.e., ocean acidification—could be attributed to the addition of CO₂ in the upper ocean as a result of the ozone-hole-induced upwelling, the researchers found.

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).

Resources

• Lenton, A., F. Codron, L. Bopp, N. Metzl, P. Cadule, A. Tagliabue, and J. Le Sommer (2009), Stratospheric ozone depletion reduces ocean carbon uptake and enhances ocean acidification, Geophys. Res. Lett., 36, L12606, doi: 10.1029/2009GL038227\