A lack of water in drought years in the Amazon kills some trees that normally absorb CO2, sends additional CO2 into the atmosphere as those trees rot, and temporarily lessens surviving trees' CO2 uptake. Image: K. Didan, University of Arizona via NASA
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.