United States:Researchers say global warming is having a drastic effect on the world's massive carbon dioxide storehouse, writes Alan Zaremboin Los Angeles.
The Southern Ocean, a massive storehouse for carbon dioxide from the atmosphere, is slowly losing its capacity to buffer the world against rising concentrations of the greenhouse gas, researchers have reported.
As a result, the study reported on Thursday, 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 the oceans.
The researchers described a vicious cycle in which global warming reduces the ocean's ability to absorb the heat-trapping gas; that then accelerates the accumulation of carbon dioxide in the atmosphere and triggers more warming.
"The buffer doesn't seem to be kicking in as one might expect," said Ralph Keeling, a geochemist at 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, Colorado, said the measurements of carbon dioxide changes were so subtle that they easily could have been sampling errors.
"I think they make a good case, but I am not entirely convinced," he said, adding that there was little evidence so far that the planet's ability to absorb carbon was fading. The degree to which the oceans will be able to create a 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 atmospheric levels of the gas annually by about two 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 both absorb and 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 analysed data from 11 monitoring stations in the Southern Ocean that measured carbon dioxide concentration just above the surface of the water.
The data covered the period from 1981 to 2004. Using those readings, they estimated how much carbon was being absorbed by the water.
They estimated that in 1981 the Southern Ocean absorbed 0.6 billion metric tonnes of carbon from the air. At the same time, it released 0.3 billion metric tonnes that had been stored in the ocean, for a net absorption of 0.3 billion metric tonnes.
By 2004 the ocean was taking in 0.8 billion metric tonnes of carbon but spitting out 0.5 billion metric tonnes. The total net amount of carbon absorbed by the ocean was the same.
But then the researchers compared the results with 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 tonnes, according to the study.
That meant "the ocean sink is weakening", said the lead author, Corinne Le Quere, an oceanographer at the British Antarctic Survey.
The changes, she said, were most likely 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.
Le Quere said she believed the phenomenon could apply to other oceans. Other scientists disputed that, saying the Southern Ocean is so cold, deep and isolated that it may be a unique case.
- (LA Times-Washington Post service)