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Climate study shows that deforestation of the Amazon could dry out the western United States

Shifts in precipitation patterns would have big consequences for agriculture, forests and municipal water supplies

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Research suggests that deforestation will likely produce a weather cycle over the Amazon consisting of abnormally dry air in the sun-scorched northern Amazon around the equator weighted by wetter air in the cooler south (left). The Princeton-led researchers found that the Amazon pattern would be subject to meandering high-altitude winds known as Rossby waves that move east or west across the planet (center). The Rossby waves would move the dry end of the Amazon pattern directly over the western United States from December to February, while the pattern’s rainy portion would be over the Pacific Ocean south of Mexico (right). Image courtesy Princeton University.

By Summit Voice

FRISCO — Continued deforestation in the Amazon region could have significant impacts on the weather in North America, according to Princeton researchers, who used fine-grained climate models to simulate how precipitation patterns could shift in the future.

Their findings suggest that  total deforestation of the Amazon may significantly reduce rain and snowfall in the western United States — specifically, 20 percent less rain for the coastal Northwest and a 50 percent reduction in the Sierra Nevada snowpack, a crucial source of water for cities and farms in California.

“The big point is that Amazon deforestation will not only affect the Amazon — it will not be contained. It will hit the atmosphere and the atmosphere will carry those responses,” said lead author David Medvigy, an assistant professor of geosciences at Princeton.

Medvigy explained that the rainforest influences various aspects of the surrounding climate, including cloud coverage, heat absorption and rainfall. Previous research has shown that deforestation will likely produce dry air over the Amazon. Using high-resolution climate simulations, the researchers found that the atmosphere’s normal weather-moving mechanics would create a ripple effect that would move that dry air directly over the western United States from December to February.

“It just so happens that one of the locations feeling that response will be one we care about most agriculturally,” he said. “If you change the snowpack in the Sierra Nevada, where most of the irrigation for California’s Central Valley comes from, then by this study deforestation of the Amazon could have serious consequences for the food supply of the United States.”

The Amazon’s fragility and vulnerability — combined with its outsized sway over the climate — add an urgency to better understanding how the forest’s disappearance will affect the larger climate, particularly for agriculturally important areas such as California, Medvigy said.

“We know the Amazon is being deforested, but we don’t know for sure what’s going to happen because of it,” Medvigy said. “Other scientists need to do these simulations and see if they get the same results. If they do, then policymakers will have to take notice.”

Oscillations in the hemispheric circulation known as Rossby Waves that usually put the rainy side of El Niño over southern California would instead subject that region to the dry end of the Amazon pattern. The pattern’s rainy portion would be over the Pacific Ocean south of Mexico.

“We don’t know what the world will be like without the Amazon. We know exactly what happens with El Niño — it’s been studied extensively,” Medvigy said. “Our intention with this paper was to identify an analogy between El Niño and Amazon deforestation. There’s good reason to believe there will be strong climatic similarities between the two. Research like this will give us a handle on what to expect from Amazon deforestation.”

The high resolution of the researchers’ climate model allowed them to see the otherwise subtle pull of the Rossby waves, Medvigy said. The typical model buries finer atmospheric features under a scale of about 200 kilometers — twice the width of the Andes Mountains. Medvigy and his co-authors spotted the intricacies of the Amazon’s future weather pattern using a resolution as fine as 25 kilometers, he said.

The researchers based their simulation on the Amazon’s complete removal, an exaggerated level of destruction needed to produce a noticeable effect, Medvigy said. Nonetheless, clear-cutting of the Amazon marches on, although conservation efforts have significantly slowed deforestation in countries such as Brazil since the mid-2000s. In addition, research has shown that climate change, especially a spike in the global temperature, could wipe out as much as 85 percent of the forest.

The paper, “Simulated changes in Northwest US climate in response to Amazon deforestation,” was published in the Nov. 15 edition of the Journal of Climate. This work was supported by awards from the National Science Foundation.

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