A quarter of atmospheric CO2 from human activity is absorbed and stored by the oceans – and a new study reveals how massive whirlpools push carbon into the depths of the Southern Oceans, with implications for our understanding of global warming
A joint Australia-United Kingdom study has revealed that atmospheric carbon resulting from human activities is absorbed by the ocean through gigantic funnels in the Southern Ocean. For the first time, measurements of salinity, temperature and carbon have allowed scientists to show the presence of several plunging currents a thousand kilometres wide, via which carbon travels from the ocean surface to the depths.
Oceanographers from British Antarctic Survey and Australia's national research agency, the Commonwealth Scientific and Industrial Research Organisation, published their findings in the journal Nature Geoscience
on July 29. By absorbing 25 per cent of atmospheric CO2 from human activity, and storing it for thousands of years, the oceans play a major role in buffering global warming. And the Southern Ocean surrounding Antarctica contributes 40 per cent of this absorption.
The ocean is layered in a similar way as an onion, with very little to connect the different layers. A few regions around the world are vital in overturning deep and shallow layers of the ocean, allowing carbon to be locked away from the atmosphere for centuries. However, despite its importance for climate, it has always been unclear how the carbon dissolved in the surface layer is overturned and injected in the deep seas from the surface layer.
It has long been thought that wind was the major process in pushing water and carbon down in the deep seas in the Southern Ocean. Wind friction at the ocean surface moves large slabs of water, and generates convergence of water in some regions, which then slowly sinks down in the water column. The present study demonstrates for the first time from observations that massive whirlpools known as eddies have also a primary role in connecting the surface and the deep ocean.
Eddies are similar to the weather systems of the ocean. These vortices are like cyclones and anticyclones but in the ocean. They churn the water and, similar to wind, create large horizontal displacement of water with convergence zone leading to the sinking of water and dissolved-carbon. The Southern Ocean being the most energetic of the world, many eddies are formed, and efficient funnels between the layers of the ocean are therefore developed.
Five or six regions, a thousand kilometres wide, were found in the Southern Ocean, where eddies associated with the effect of wind and prevailing ocean currents create funnels injecting carbon into the deep seas. One of the most intense is located directly south west of Australia; another is located in southern Chile. Improving our understanding of the mechanisms of carbon drawdown in the deep-seas, this research feeds into a scientific controversy born a few years ago: could the ocean ever stop absorbing CO2?
A study published in 2007 in Science
suggested that intensification of the winds in the southern hemisphere due to human activity could reduce the effectiveness of carbon sinks. Indeed, by stirring deep layers with the surface water, intensifying winds could cause outgassing of buried carbon dioxide. Ultimately, the ocean would no longer play its role as a carbon sponge. The present study shows, however, that vortices counter-balance the effect of wind.
Is that good news for global warming? In principle yes, but it is yet not known what will be the effect of climate change on vortices. Hence the importance of integrating them into climate models to estimate their evolution and future impact on the climate. Vortices occur at scales too small compared to the grids of climate models currently being used by the Intergovernmental Panel on Climate Change. International modelling teams are already working to refine the resolution of climate models and ultimately better integrate the importance of eddy-processes for the next IPCC report.Dr Jean-Baptiste Sallée is from the British Antarctic Survey and is the lead author of the study