Ocean study sounds another alarm on global warming

The Southern Ocean, a massive storehouse for carbon dioxide from the atmosphere, slowly is losing its capacity to buffer the world from rising concentrations of the greenhouse gas, researchers reported Thursday.

As a result, the report says, carbon dioxide could accumulate in the atmosphere faster than previously expected.

The Southern Ocean, which surrounds Antarctica, accounts for about one-third of all carbon stored in oceans.

The researchers described a vicious cycle in which global warming reduces the ocean's ability to absorb the heat-trapping gas, accelerating the accumulation of carbon dioxide in the atmosphere and 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 at the University of California, San Diego, 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 have been sampling errors.

"I think they make a good case, but I am not entirely convinced," he said.

The degree to which oceans can buffer rising carbon-dioxide emissions is a key uncertainty in predicting temperature increases.

Carbon-dioxide concentrations in the atmosphere are 385 parts per million. Continued burning of fossil fuels has been increasing atmospheric levels of the gas annually by about 2 parts per million.

That rise represents half the carbon dioxide emitted each year. The rest is absorbed, in roughly equal portions, by two "carbon sinks": land vegetation and oceans.

Oceans absorb and expel carbon, coughed up from deep waters where it is stored as carbonic acid.

The new report, published in the journal Science, focused on the Southern Ocean because it is extremely isolated.

With only barren, ice-covered land nearby, researchers could rule out interference from vegetation.

They analyzed data from 11 monitoring stations that measured carbon-dioxide concentration just above the surface of the water. The data covered 1981 to 2004.

Using those readings, researchers estimated how much carbon was being absorbed by the water.

They estimated that the Southern Ocean absorbed 0.6 billion metric tons of carbon in 1981. At the same time, it released 0.3 billion metric tons that had been stored in the ocean, for a net absorption of 0.3 billion metric tons.

By 2004, the ocean was taking in 0.8 billion metric tons of carbon but spitting out 0.5 billion metric tons. The total net amount of carbon absorbed was the same.

But researchers then compared the results with computer predictions of what the ocean should have absorbed given rising atmospheric carbon-dioxide levels. In 2004, the net absorption should have been 0.5 billion metric tons, the report said.

That meant "the ocean sink is weakening," said the lead author, Corinne Le Quéré, an oceanographer at the British Antarctic Survey.