By Paul Rincon Science reporter, BBC News
One of Earth's most important absorbers of carbon dioxide (CO2) is failing to soak up as much of the greenhouse gas as it was expected to, scientists say.
The decline of Antarctica's Southern Ocean carbon "sink" - or reservoir - means that atmospheric CO2 levels may be higher in future than predicted.
These carbon sinks are vital as they mop up excess CO2 from the atmosphere, slowing down global warming.
The study, by an international team, is published in the journal Science.
This effect had been predicted by climate scientists, and is taken into account - to some extent - by climate models. But it appears to be happening 40 years ahead of schedule.
The data will help refine models of the Earth's climate, including those upon which the predictions of the Intergovernmental Panel on Climate Change (IPCC) are based.
Of all the CO2 emitted into the atmosphere, only half of it stays there; the rest goes into carbon sinks.
There are two major natural carbon sinks: the oceans and the land "biosphere". They are equivalent in size, each absorbing a quarter of all CO2 emissions.
The Southern Ocean is thought to account for about 15% of all carbon sinks.
It was assumed that, as human activities released more CO2 into the atmosphere, ocean sinks would keep pace, absorbing a comparable percentage of this greenhouse gas.
The breakdown in efficiency of these sinks was an expected outcome, but not until the second half of the 21st Century.
Lead researcher Corinne Le Quere and colleagues collected atmospheric CO2 data from 11 stations in the Southern Ocean and 40 stations across the globe.
Measurements of atmospheric CO2 allowed them to infer how much carbon dioxide was taken up by sinks. The team was then able to see how efficient they were in comparison to one another at absorbing CO2.
"Ever since observations started in 1981, we see that the sinks have not increased [in their absorption of CO2]," Corinne LeQuere told the BBC's Science in Action programme.
"They have remained the same as they were 24 years ago even though the emissions have risen by 40%."
The cause of the decline in the Southern Ocean sink ... full text