A Drying Amazon Could Speed Climate Change
As they die and decompose, trees and soil in the Amazon rainforest have traditionally absorbed more carbon dioxide from the atmosphere than they've emitted. But it turns out that during a major drought in 2005, the Amazon put more carbon into the atmosphere than it soaked up.
That worries scientists, who say that the Amazon's role as a "carbon sink" that sucks global-warming carbon dioxide out of the atmosphere may be compromised.
The Amazon is the largest rainforest in the world and consumes about one-fifth of the carbon dioxide produced from burning fossil fuels.
Ecologist Oliver Phillips of England's University of Leeds was part of an international team that studied 30 years of data on the Amazon and compared that with what happened to the rainforest during the 2005 drought there. The team found that the forest emitted far more carbon during the drought "primarily because the trees died faster than before," says Phillips.
At the same time, it absorbed less carbon dioxide because plant growth had slowed down. "So instead of being a sink of about 400 to 500 million tons, the system became a source of somewhere close to a billion tons in that year," he says.
Co-author Dan Nepstad, a forest ecologist from the Moore Foundation in San Francisco, suggests that it may be climate warming that's causing the Amazon to dry up. He says some climatologists think "the warming of the northern tropical Atlantic of 2005 may have been more intense because of global warming." And that warming is believed to have shifted hotter, drier air over the Amazon.
Nepstad says as the climate warms, the Amazon could be caught in a feedback loop, whereby warmer Atlantic waters continue to dry out the Amazon, which in turn contributes more carbon to the atmosphere and accelerates warming.
Phillips is cautious about predicting whether the slip from carbon sink to source will become more frequent. But he notes that computer models of how climate may change in the future do predict a drying of the Amazon and says this latest research confirms those models, at least for one year.
The findings are published in this week's issue of the journal Science.
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MELISSA BLOCK, host:
The world's largest tropical rainforest, the Amazon, may be experiencing some fundamental changes in its weather. That's what a team of scientists concluded this week after studying what's been happening to Amazonian trees over the past 30 years. NPR's Christopher Joyce has more.
CHRISTOPHER JOYCE: The Amazon is well known as home to more different kinds of animals and plants than anywhere else. But climate scientists see the forest for its trees because plants soak up carbon dioxide from the atmosphere as they grow, and the Amazon has a big appetite for this global-warming gas.
In fact, the Amazon rainforest consumes about one-fifth of what people all over the planet put up into the atmosphere from burning coal and oil and natural gas.
But there's a flip side. When plants die and decompose, the rainforest returns a lot of that carbon to the atmosphere. On average, though, the forest helps reduce CO2 in the atmosphere. It takes up more than it emits. Overall, it's been a carbon sink, that is until 2005, when a major drought hit parts of the Amazon.
Ecologist Oliver Phillips was there to see the first signs of change. Phillips, from the University of Leeds in England, was part of a large group of scientists who trekked through the Amazon during and after the drought.
Dr. OLIVER PHILLIPS (Ecologist, University of Leeds): There's little slowing in growth but a bigger kick in terms of killing trees.
JOYCE: They found that trees grew more slowly from lack of water, and they took up less carbon, and because they were dying at a faster-than-normal rate, they were putting tons more carbon back into the air than normal.
Dr. PHILLIPS: So instead of being a sink of about, let's say, 4 to 500 million tons, the system became a source of somewhere close to a billion tons in that year.
JOYCE: They'd never seen that happen before in the Amazon. The scientists wrote up their results in the current issue of the Journal of Science.
Now, droughts are not very common in the Amazon's tropical rainforests. Some scientists, such as forest ecologist Dan Nepstad, with the Moore Foundation, suspect that changes in the Atlantic Ocean may be what's affecting the weather in the Amazon.
Dr. DAN NEPSTAD (Ecologist, Moore Foundation): Global warming may already be having an influence in the Amazon. By changing the sea-surface temperature, the warming of the northern tropical Atlantic of 2005 may have been more intense because of global warming. Some climatologists think so.
JOYCE: 2005 was also the year that unusually warm sea-surface temperatures in the Atlantic spawned Hurricane Katrina.
Nepstad says the Amazon could become part of a giant feedback loop, getting drier from climate change and then emitting more carbon into the atmosphere and making the world's climate even warmer.
Some scientists have even predicted that the Amazon could become a savannah by the end of this century. This scientific team doesn't go that far. Phillips says they aren't sure whether the 2005 drought is, in fact, a single incident or something that could happen more often. But computer models of climate change do predict that a warmer climate could make the southern Amazon drier. How much and when is still unknown.
Phillips says at the very least, their study of the 2005 drought gives them a better idea of what might happen.
Dr. PHILLIPS: I think the interesting thing about what we've done is not so much documenting just what happened in 2005. But saying here's the sensitivity of the forest to make it like a climate shock. So if a drought like 2005 was repeated in the future, we now have a better handle on what's likely to happen.
JOYCE: What could happen is that the forest becomes part of the problem of global warming rather than a solution. Christopher Joyce, NPR News. Transcript provided by NPR, Copyright National Public Radio.
