Face aux émissions de CO2, l'océan ne suit plus

Polar ocean is sucking up less carbon dioxide

Colder waters can hold more dissolved gas. Getty

The ability of the Southern Ocean to remove carbon dioxide from the atmosphere is being eroded by climate change, say environmental researchers. If the trend [tendance] continues, then the ability of this 'carbon sink' to deal with humankind's greenhouse emissions will be impaired.

Roughly half of the carbon dioxide that enters the atmosphere is absorbed by the world's oceans, so as greenhouse emissions increase, the amount taken up by the oceans should increase in proportion. But the new research suggests that the Southern Ocean is not keeping pace with rising emissions. These Antarctic waters are an important sink for carbon dioxide, thanks to ocean currents and cold temperatures - they are thought to account for some 15% of the world's oceanic carbon-storage capacity.

Researchers led by Corinne Le Quéré of the Max Planck Institute of Biochemistry in Martinsried, Germany, took data from 11 coastal monitoring stations in Antarctica and on islands in the Southern Ocean to measure the amount of carbon dioxide being stored and released by the ocean. They then compared this measurements of global atmospheric carbon dioxide levels to work out the change in the performance of the carbon sink.

Since 1981, the percentage of atmospheric carbon dioxide that the Southern Ocean can hold has decreased, the researchers report in a study published online by Science. The trend suggests that, for each decade, the annual capacity of the ocean to store carbon has gone down by 0.08 gigatonnes compared with expectations. On average, the ocean stores between 0.1 and 0.6 gigatonnes a year.

This is a small amount compared with the roughly 8 gigatonnes of carbon dioxide pumped out each year by human activities such as energy generation. But any decline is important, as oceans are an important long-term sink. If humans can bring carbon dioxide emissions under control in the long term, the world's oceans are predicted to sequester between 70% and 80% of the total net anthropogenic emissions of the industrial era.

Winds of change

The main cause of the changes seems to be a relatively rapid increase in average wind strengths over the Southern Ocean, Le Quéré and her team report. These stronger winds, thought to be driven by the depletion of the ozone layer over Antarctic regions, churn up the ocean and bring more dissolved carbon up from the depths.

This was unexpected, says Le Quéré. But when the researchers plugged their data into a computer model and removed these stronger winds, they did indeed find that much of the observed reduction in the carbon sink disappeared.

An increase in global temperature is predicted to worsen the effect, since warmer waters hold less gas.

South to north

"The possibility that in a warmer world the Southern Ocean - the strongest ocean sink - is weakening, is a cause for concern," comments Chris Rapley, director of the British Antarctic Survey in Cambridge, UK.

The Southern Ocean is the only body of water for which this trend has been definitely spotted and quantified, says Le Quéré, although shorter-term studies suggest that a similar process may be occurring in the North Atlantic.

If the phenomenon is happening world-wide, this would undoubtedly affect efforts to stabilize atmospheric greenhouse gases.

A reduction in sink capacities will make it harder for international efforts, such as carbon trading and changes in methods of energy generation, to set achievable targets for stabilizing greenhouse-gas levels. But Le Quéré says that such efforts now need to be redoubled, rather than accepting that greenhouse gas levels will be higher in future. "Targets should depend on the level of danger [from global warming]," she says.