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Cutting Down Rainforests Also Cuts Down on Rainfall

As the Amazon rainforest disappears, rainfall falters over a much wider area
Slash and burn agriculture in the Amazon



flickr/Threat to Democracy

When Amazon rainforest disappears, so does Amazon rain.

That's the conclusion of new research that shows deforestation can significantly reduce tropical rainfall far from the area where trees have been cut down.

That's because air passing over forests picks up moisture given off by trees and plants, fueling rains. When those trees disappear, so does some of that rain.

"What we found was this really strong impact -- air that traveled over a lot of forest brought a lot more rain than air that didn't travel over very much forest," said lead author Dominick Spracklen of the University of Leeds.

His research, published yesterday in the journal Nature, helps reconcile a situation that has puzzled scientists.

Climate models project that Amazon deforestation would reduce rainfall regionally. But limited observations show that rainfall in deforested areas is higher than in areas where the rainforest is still intact.

(Scientists believe that when trees are cut down, the bare surfaces left behind absorb more heat than the forest they replaced. And that heat helps draw air upward, pulling moist air in from nearby forested land that increases rainfall in the immediate area.)

But Spracklen's study suggests both the climate model projections and the observations may be correct. Deforestation can reduce rainfall over a wide region, even as it spurs increased rainfall in the immediate area where that deforestation took place.

The analysis has "beautifully reconciled the scale of observations with that of models," Luiz Aragao, an expert in tropical forests at the University of Exeter, said in a commentary published alongside the new study.

Using models and satellites
The study authors' "cutting-edge methodology will allow observations to be used consistently to examine large-scale deforestation impacts on rainfall, and to refine and evaluate current models to support conservation planning in the tropics," Aragao wrote.

The study authors based their analysis on a combination of satellite observations of rainfall and vegetation and an atmospheric circulation model to track the movement of air masses.

"At any point in the tropics, we'd look at rainfall on a particular day and look at how the air traveled to get to that point over the past four to five days," Spracklen said. "By making that comparison, we were able to say that if air has traveled over more forest, do you get less or more rainfall -- or no change?"

The scientists' computer modeling suggests that if deforestation in the Amazon continues at the current pace, it could significantly cut rainfall hundreds of miles away over the next several decades.

By 2050, deforestation could reduce rainfall across the Amazon Basin by 21 percent during dry seasons and 12 percent during wet seasons.

"The Amazon forest is affecting regions hundreds to thousands of kilometers away," Spracklen said -- distances that amount to hundreds of miles.

Deforestation in the Amazon could sharply reduce rainfall in nonforested parts of southern Brazil, a rich agricultural area, as well as Paraguay and Uruguay, his study suggests. And a similar scenario appears to be at play in Africa's Congo Basin.

"If Brazil maintains its rainforest, it's not just helping the [Brazilian] state of Amazonas," Spracklen said.

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

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