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Amazon Droughts Concern Scientists

February 4, 2011

A long-lasting drought that affected the Amazon Rain Forest last year was worse than the once-in-a-lifetime drought that the region suffered in 2005, and a team of British and Brazilian scientists say it may have a bigger impact on global warming than the US does in a year.

The widespread drought has raised concerns about the rainforest’s future as a major absorber of carbon emissions, the scientists said in a study released Thursday.

Frequent severe dry spells like the ones in 2005 and 2010 risk turning the Amazon from a greenhouse gas eater into a source of the gases, which could definitely accelerate global warming, the report said.

Since the droughts killed off many trees, the team predicts that the Amazon will not be able to absorb as much carbon dioxide as usual in the coming years. Even worse, rotting trees may release as much as 5 billion tons of CO2 into the atmosphere in the years to come, almost as much as the entire United States emitted from fossil fuel usage in 2009.

“If events like this happen more often, the Amazon rain forest would reach a point where it shifts from being a valuable carbon sink slowing climate change to a major source of greenhouse gases that could speed it up,” lead author Simon Lewis, an ecologist at the University of Leeds, told Reuters.

Lewis is the scientist who received an apology from the Sunday Times newspaper last year for their report on the so-called “AmazonGate” affair.

He said it is “difficult to detect patterns from just two observed droughts, but to have them close together is concerning.”

Both droughts were associated with abnormally warm seas in the Atlantic Ocean off the Brazilian coast. “If that turns out to be driven by escalating greenhouse gas concentrations in the atmosphere, it could imply that we’ll see more drought years in the near future,” Lewis told BBC News.

“If events like this do happen more often, the Amazon rainforest would reach a point where it shifts from being a valuable carbon sink slowing climate change to a major source of greenhouse gases,” he said.

Some computer models — especially the one developed at UK’s Hadley Center — project more droughts across the region as the Earth continues on a warming trend, in turn diminishing the ability to absorb CO2.

There are several potential ways in which global warming can turn greenhouse gas-absorbing forests into carbon emitters.

In the Amazon, the main instrument is simply that trees die and rot. Those trees are then not available to absorb CO2 from the air.

Scientists used data from a US/Japanese satellite — Tropical Rainfall Measuring Mission (TRMM) — that monitors rainfall along a belt that extends either side of the equator for their research.

Observation of data showed that the 2010 drought covered more than 1.8 million square miles, whereas the 2005 drought covered only 1.2 million square miles.

Scientists were able to study the impact on trees following the 2005 dry spell, and then work out a relationship between the loss of rainfall and the release of carbon. In an average year, the basin absorbs about 1.6 billion tons of CO2 from the atmosphere. But due to the 2005 drought, scientists determined about 5.5 billion tons of CO2 was released over a number of years.

The study found that last year’s drought caused a shortage of rainfall over a 1.86 million square-mile expanse of forest, compared to 734,000 square miles in the 2005 drought. The 2010 drought was also more severe, causing higher tree mortality and having three major epicenters, while the 2005 drought was more concentrated in the southwestern Amazon.

As a result of the 2010 drought, the study forecasts that the Amazon forest would not absorb the usual 1.6 billion tons of CO2 form the atmosphere in both 2010 and 2011. And dead and dying trees would release 5.5 billion tons of greenhouse gases over the next several years, making the total impact pass 8 billion tons.

“We will not know exactly how many trees were killed until we can complete forest measurements on the ground,” said Dr Paulo Brando, from Brazil’s Amazon Environmental Research Institute (IPAM), one of the study’s researchers.

“It could be that many of the drought susceptible trees were killed off in 2005, which would reduce the number killed last year. On the other hand, the first drought may have weakened a large number of trees so increasing the number dying in the 2010 dry season,” said Brando.

“Our results should be seen as an initial estimate. The emissions estimates do not include those from forest fires, which spread over extensive areas of the Amazon during hot and dry years. These fires release large amounts of carbon to the atmosphere,” he said.

“Two unusual and extreme droughts occurring within a decade may largely offset the carbon absorbed by intact Amazon forests during that time. If events like this happen more often, the Amazon rainforest would reach a point where it shifts from being a valuable carbon sink slowing climate change, to a major source of greenhouse gasses that could speed it up,” added Lewis.

“Considerable uncertainty remains surrounding the impacts of climate change on the Amazon. This new research adds to a body of evidence suggesting that severe droughts will become more frequent leading to important consequences for Amazonian forests. If greenhouse gas emissions contribute to Amazon droughts that in turn cause forests to release carbon, this feedback loop would be extremely concerning. Put more starkly, current emissions pathways risk playing Russian roulette with the world’s largest rainforest,” Lewis said.

Leeds University is part of a research group that maintains more than 125 land stations in the Amazon Rain Forest. If funds take hold, the team plan to visit them all in the coming months to gather data on the number of tree deaths that could provide a more accurate estimate of last year’s drought and the affect it put on global emissions.

The potential fate of the Amazon under climate change came into focus early last year when, as one of a series of attacks on the Intergovernmental Panel on Climate Change (IPCC), the Sunday Times accused the panel of having included an unconfirmed claim that as much as 40 percent of the rainforest could be affected by climate change in the future.

Lewis sent the newspaper a stack of scientific papers to back the IPCC’s case for the concern over the region’s vulnerability. He told the newspaper that the IPCC had sourced its statement to a report from the World Wildlife Fund (WWF), when it should have referenced the scientific papers WWF had used in its report.

“In fact, the IPCC’s Amazon statement is supported by peer-reviewed scientific evidence,” the Sunday Times acknowledged in its apology.

Commenting on the so-called “AmazonGate” affair, Dr Lewis noted: “The notion that the Amazon is potentially very vulnerable to droughts linked to climate change was reasonable and defensible at the time, and is consistent with the new findings.”

“If greenhouse gas emissions contribute to Amazon droughts that in turn cause forests to release carbon, this feedback loop would be extremely concerning,” he said. “Put more starkly, current emissions pathways risk playing Russian roulette with the world’s largest rainforest.”

The Universities of Leeds, Sheffield and the Instituto de Pesquisa Ambiental da Amazonia (IPAM) in Brazil collaborated on the research. The work was funded by the Royal Society, Gordon and Betty Moore Foundation and the US National Science Foundation.

The paper entitled ‘The 2010 Amazon Drought’ by Simon L Lewis, Paulo M Brando, Oliver L Phillips, Geertje MF van der Heijden and Daniel Nepstad is published in the journal Science on Friday February 4, 2011.

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