The Southern Ocean, a massive storehouse for carbon dioxide from the atmosphere, is slowly losing its capacity to buffer the world from rising concentrations of the greenhouse gas, researchers reported Thursday.
As a result, the study said, carbon dioxide could accumulate in the atmosphere faster than expected over the coming decades.
The ocean, which surrounds Antarctica, accounts for about a third of all carbon stored in the world's five oceans.
The researchers described a vicious cycle in which global warming reduced the ocean's ability to absorb the heat-trapping gas. That would then accelerate the accumulation of carbon dioxide in the atmosphere, triggering more warming.
"The buffer doesn't seem to be kicking in as one might expect," said Ralph Keeling, a geochemist at the Scripps Institution of Oceanography in La Jolla, who was not involved in the study.
The findings are controversial. Pieter Tans, a senior scientist at the National Oceanic and Atmospheric Administration in Boulder, Colo., said the measurements of carbon dioxide changes were so subtle that they could easily be sampling errors.
"I think they make a good case, but I am not entirely convinced," he said, adding that there is little evidence that the planet's ability to absorb carbon is fading.
The extent to which the oceans will be able to buffer against rising carbon dioxide emissions is a key uncertainty in predicting temperature increases.
Carbon dioxide concentrations in the atmosphere are now 385 parts per million. The continued burning of fossil fuels has been increasing levels of the gas annually by 2 parts per million.
That rise represents only half the carbon dioxide emitted each year. The rest is absorbed, in roughly equal portions, by two carbon "sinks" -- land vegetation and the oceans.
The oceans also expel carbon, coughed up from deep waters where it is stored as carbonic acid.
The new study, published in the journal Science, focused on the Southern Ocean because it is extremely isolated. With only barren, ice-covered land nearby, the researchers could rule out interference from vegetation.
They analyzed data from 11 monitoring stations in the Southern Ocean that measured carbon dioxide concentration just above the surface. The data covered 1981 to 2004.
Using those readings, they estimated how much carbon the water absorbed. They estimated that in 1981, the Southern Ocean absorbed 0.6 billion metric tons of carbon from the air and released 0.3 billion metric tons, for a net absorption of 0.3 billion metric tons. In 2004, the ocean took in 0.8 billion metric tons of carbon and spat out 0.5 billion metric tons -- resulting in the same net absorption as in 1981.
The researchers compared the results to computer predictions of what the ocean should have absorbed given the rising atmospheric carbon dioxide levels. In 2004, the net absorption should have been 0.5 billion metric tons, the study said.
"The ocean sink is weakening," said lead author Corinne Le Quere, an oceanographer at the British Antarctic Survey.
The changes, she said, are probably the result of temperature increases that have intensified the westerly winds circling Antarctica. The winds stir up the ocean, bringing deep carbon-rich water to the surface. As a result, the surface waters cannot absorb as much carbon dioxide as they would have otherwise.
Le Quere said she believed the phenomenon could apply to other oceans.
Other scientists disagree, saying the Southern Ocean is so cold, deep and isolated that it may be a unique case.