"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," says David Medvigy. (Credit: BBC World Service/Flickr)

Without Amazon forests, weather could mimic El Niño

Total deforestation of the Amazon could mean 20 percent less rain for the coastal Northwest and a 50 percent reduction in the Sierra Nevada snowpack.

Previous research has shown that deforestation will likely produce dry air over the Amazon. Using high-resolution climate simulations, the researchers are the first to find 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.

Specifically, a denuded Amazon would develop a weather cycle consisting of abnormally dry air in the sun-scorched northern Amazon around the equator weighted by wetter air in the cooler south.

Research has speculated that this pattern would be similar to the warm-water climate pattern El Niño, which during the winter months brings heavy precipitation to southern California and the Sierra Nevada region while drying out the Pacific Northwest.

The simulation shows that the water equivalent of the snowpack by April 1 decreased in range and depth from pre-deforestation levels (left) when the Amazon was cleared (right). The depth is measured in centimeters with the redder areas indicating more snow. (Credit: David Medvigy/Princeton)
The simulation shows that the water equivalent of the snowpack by April 1 decreased in range and depth from pre-deforestation levels (left) when the Amazon was cleared (right). The depth is measured in centimeters with the redder areas indicating more snow. (Credit: David Medvigy/Princeton)

The researchers found that the Amazon pattern would be subject to the same meandering high-altitude winds known as Rossby waves that distribute the El Niño system worldwide from its source over the Pacific Ocean.

Rossby waves are instrumental forces in Earth’s weather that move east or west across the planet, often capturing the weather of one region—such as chill Arctic air—and transporting it to another.

Because the Amazon pattern forms several thousand miles to the southeast from El Niño, the researchers report, the Rossby waves that 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.

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). (Credit: David Medvigy/Princeton)
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 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). (Credit: David Medvigy/Princeton)

First author David Medvigy, an assistant professor of geosciences at Princeton University, explains that the findings stand as one possible outcome of Amazon deforestation in regions outside of South America—consequences that scientists are working to understand. The rainforest influences various aspects of the surrounding climate, including cloud coverage, heat absorption, and rainfall.

US food supply

“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,” Medvigy says.

“It just so happens that one of the locations feeling that response will be one we care about most agriculturally,” he says. “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 figure above shows the change (in millimeters of rain per day) in daily average precipitation after total Amazon deforestation compared to before deforestation. The pink to dark-pink range indicates a drop in precipitation of up 1.6 mm less per day once the Amazon is gone. Areas with statistically significant changes are hatched. (Credit: David Medvigy/Princeton)
The figure above shows the change (in millimeters of rain per day) in daily average precipitation after total Amazon deforestation compared to before deforestation. The pink to dark-pink range indicates a drop in precipitation of up 1.6 mm less per day once the Amazon is gone. Areas with statistically significant changes are hatched. (Credit: David Medvigy/Princeton)

Because the exact result of Amazon deforestation is impossible to know currently, the behavior and impact of El Niño provides one of the best ideas of how the loss of the Amazon could play out, Medvigy says.

Studies have suggested since 1993 that an Amazon without trees will develop an El Niño-like pattern, the researchers report. The researchers then focused on the northwestern United States because the region is typically sensitive to El Niño.

“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 says. “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 says. 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 says.

Fragile rainforests

The researchers based their simulation on the Amazon’s complete removal, an exaggerated level of destruction needed to produce a noticeable effect, Medvigy says. 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 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 says.

“We know the Amazon is being deforested, but we don’t know for sure what’s going to happen because of it,” Medvigy says. “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.”

Medvigy worked with second author Robert Walko, a senior scientist in the division of meteorology and physical oceanography at the University of Miami; Martin Otte, a postdoctoral fellow at the US Environmental Protection Agency; and Roni Avissar, a University of Miami professor of meteorology and physical oceanography.

The paper appears in the Journal of Climate. The National Science Foundation supported the work.

Deforestation will likely produce dry air over the Amazon. The researchers' model indicates that the surface temperature in the Amazon region would increase by up to 2 degrees Celsius (darkest green) over a 14-year period following deforestation. The region of Amazon deforestation is boxed. (Credit: David Medvigy/Princeton)
Deforestation will likely produce dry air over the Amazon. The researchers’ model indicates that the surface temperature in the Amazon region would increase by up to 2 degrees Celsius (darkest green) over a 14-year period following deforestation. The region of Amazon deforestation is boxed. (Credit: David Medvigy/Princeton)

Source: Princeton University

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