Researchers from the United Kingdom and Brazil also said the pair of droughts have raised concerns that the forest could be approaching a point where it ceases to be a carbon “sink,” absorbing more carbon dioxide from the atmosphere than it produces, and flips to a carbon source.

Both the 2005 and 2010 droughts were the result of a “very, very unusual” weather pattern linked to higher sea surface temperatures in the Atlantic Ocean, said lead author Simon Lewis, a tropical forests expert at the University of Leeds.

But the scientist said it’s not clear whether the droughts are the product of a random shift in weather patterns or whether they are driven, at least in part, by climate change.

An ‘urgent call’ to understand risks of stress Researchers who were not involved with the new study said it was a good initial estimate of the impact of last year’s drought but cautioned that the ultimate effect of the dry spell is not yet clear.

Patrick Meir, a professor of ecosystem science at the University of Edinburgh, said the study “represents an urgent call to understand better the risk drought poses to the region.”

Their calculations show the 2010 drought affected 3 million square kilometers of rainforest, compared with 1.9 million square kilometers during the 2005 dry spell.

The researchers say that, in drought years, water stress keeps the rainforest from absorbing the 1.5 billion metric tons of CO2 it would sop up in a normal year.

The drought in 2010 could release another 5 billion metric tons of CO2 into the atmosphere as trees killed by the drought decompose, roughly equal to the annual U.S. output of the heat-trapping gas. That dwarfs the approximately 2 billion metric tons of CO2 that entered the atmosphere as a consequence of the 2005 drought.

‘A real impact on people’ The scientists say their calculations are preliminary and don’t account for emissions from forest fires, to which the forest is more vulnerable during drought years, and drought-driven changes in soil decomposition.

Field measurements of tree death since last year’s drought are also needed to refine the estimate of the weather event’s impact on the carbon cycle.

But Lewis said there’s no question that the 2010 drought was more extreme than the 2005 event.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500