A severe drought last year in the Amazon rainforest outpaced a 2005 dry spell thought to be a once-in-a-century event, a new study finds.

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.

In a normal year, the Amazon rainforest absorbs 1.5 billion metric tons of CO2. But a lack of water in drought years kills some trees that normally absorb CO2, sends additional CO2 into the atmosphere as those trees rot, and temporarily lessens surviving trees' CO2 uptake.

"If drought events continue, the era of intact Amazon forest buffering the increase in atmospheric carbon dioxide may have passed," the scientists wrote in a paper published yesterday in the journal Science.

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.

"Which of those is correct at this stage is unknown, but the droughts being driven by atmospheric greenhouse gas concentrations is in line with some of these global circulation models," Lewis said. About 80 percent of the 23 climate models used by the Intergovernmental Panel on Climate Change predict some degree of drought in the Amazon if greenhouse gas emissions keep climbing, he said.

"The bottom line is that while the future is inherently uncertain, we know enough about rising greenhouse gas concentrations and their potential impact on the drying of the Amazon to warrant big reductions in greenhouse gases, to reduce the possibility that we'll have a very serious impact on the world's biggest rainforest," Lewis said.

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."

"That such a strong drought should follow the 'One in 100-year' drought of 2005 only five years afterwards in 2010 is exceptional, and of concern," said Meir, an expert in tropical forests. "The 2010 drought appears to have been more widespread than in 2005, and this may have important consequences in regions across Amazonia already prone to drought stress. The underlying climatic cause needs to be understood more fully, and its impact quantified, not just on the ecology of the region, but also on society."

Scott Goetz, a senior scientist at the Woods Hole Research Center who has studied the Amazon, said that fleshing out what are "essentially back-of-the-envelope calculations" of the 2010 drought's impact will require additional field measurements and a closer examination of satellite observations of the Amazon's tree canopy.

"The bigger-picture view, however, is that the Amazon has experienced two '100-year' droughts in the past five years, and there is good evidence that the forests are not adapted to drought ... and the bigger trees die first," he said. "There is little doubt that continued droughts of this magnitude and frequency will change the structure of these forests and their ability to sequester carbon from the atmosphere."

Stressed trees no longer store as much CO2
The U.K.-Brazil research team that produced the new study used satellite measurements of dry season rainfall to estimate the magnitude of the two droughts over a study area of 5.3 million square kilometers in the southwest Amazon.

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 scientists were also able to compare the carbon impact of the two droughts. They used field measurements of tree death taken after the 2005 drought at research plots within the Amazon to estimate the tree death and resulting CO2 release from the 2010 drought.

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.

"In 2010, those places that got very, very dry, the driest places, were drier than the very driest places in 2005," he said. The River Negro, the Amazon River's biggest tributary, last year hit its lowest level since record-keeping began in 1910.

"In both years, over part of the Brazilian Amazon there was a state of emergency declared because boats couldn't get food and fuel to some communities," Lewis said. "These droughts have a real impact on people."

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