The tropical forests of Amazonia may be giving up their role as buffers against the continuing buildup of carbon dioxide in the atmosphere, scientists report, a circumstance that could accelerate climate change.

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The warning comes in the new issue of the journal Science, where an international research team reports that the drought in the Amazon during 2010 was even worse than what scientist called the "once-in-a-century" drought of 2005.

Simon L. Lewis of the University of Leeds in the UK and colleagues analyzed a decade of satellite-derived rainfall data which showed that the dry season drought spread over 57 percent of the Amazon Rainforest in 2010, compared with 37 percent in 2005. According to other studies, the 2005 drought was associated with unusually high sea surface temperatures in the Atlantic Ocean — conditions which prevailed again in 2010.

"The two recent Amazon droughts demonstrate a mechanism by which remaining intact tropical forests of South American can shift from buffering the increase in atmospheric carbon dioxide to accelerating it," the scientists write. Growing trees absorb CO2 from the atmosphere. Dying trees give it back.

IMAGES: The top panel are satellite-derived images of the extent of the droughts; the bottom panel is a measure of drought intensity. CREDIT: Science Magazine