OSLO (Reuters) - A 2005 drought in the Amazon rainforest killed trees and released more greenhouse gas than the annual emissions of Europe and Japan, an international study showed on Thursday.
The report said rainforests from Africa to Latin America may speed up global warming if the climate becomes drier this century. Plants soak up heat-trapping carbon dioxide as they grow and release it when they die and rot.
"The Amazon forest was surprisingly sensitive to drought," said Oliver Phillips, a professor of tropical ecology at Leeds University in England who led the study by 68 scientists.
The experts estimated that the forest had been absorbing 2 billion tons of carbon dioxide a year on average since the 1980s but lost 3 billion in the 2005 drought, which killed trees and slowed growth.
"The total impact was an extra 5 billion tons of carbon dioxide in the atmosphere. That is more than the annual emissions of Europe and Japan combined," Phillips said of the study published in the journal Science.
Paradoxically, the forest's accumulation of carbon before 2005 may have been aided by global warming, which improved plant growth.
But the U.N. Climate Panel projected in a 2007 report that rising temperatures may cause more drought and "lead to gradual replacement of tropical forest by savannah" in the eastern Amazon by mid-century.
The study by a group known as RAINFOR said the 2005 drought especially affected soft-wooded species. "Some species, including some important palm trees, were especially vulnerable," Peruvian botanist and co-author Abel Monteagudo said in a statement. "Drought threatens biodiversity too."
Phillips said the expansion of the Amazon's carbon storage had helped slow global warming since the 1980s. "Just because we've been getting this subsidy doesn't mean we can count on it for ever," he said.
Governments have agreed to work out a new U.N. treaty to fight climate change at a meeting in Copenhagen in December. But many countries are wary of agreeing deeper cuts in industrial emissions because of economic recession.
Many nations want measures to slow deforestation to be part of the deal. Deforestation, often by farmers burning forests to clear land, accounts for about 20 percent of emissions from human activities.
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Editing by Andrew Roche