From good to bad

A severe drought in the Amazon last year, coupled with a “once-in-a-century” drought in 2005, has led to the deaths of countless trees and temporarily turned the vast rainforest into a source of CO2 emissions, rather than a sink.

Reporting in the journal Science, scientists from the University of Leeds said that the 2010 drought killed so many trees in Amazonia that emissions last year are likely to exceed the 5.4 billion tons of CO2 annually released by fossil fuel burning in the United States.

The 2005 drought, which caused rainfall shortages over a 734,000-square-mile area, released an estimated 5 billion tons of CO2 into the atmosphere. Last year’s drought, which led to rainfall shortages over a 1.16 million square-mile area, is expected to release even more CO2.

The Amazon usually absorbs 1.5 billion tons of CO2 annually, but the Leeds researchers warned that global warming could reverse that role. “If events like this happen more often, the Amazon rain forest could 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,” said Simon Lewis, an ecologist at Leeds.