Will loss of Amazon forests reduce rain?

U. LEEDS (UK) — The destruction of tropical rainforests could reduce rain across the Amazon basin in the dry season by as much as 21 percent by 2050, experts report.

The loss could have potentially devastating consequences for people living in and near the Amazon and Congo forests, researchers say.

A new study published in Nature shows that for the majority of the Earth’s tropical land surface, air passing over extensive forests produces at least twice as much rain as air passing over little vegetation. In some cases these forests increased rainfall thousands of kilometers away.


“We were surprised to find that this effect occurs strongly across more than half of the tropics. We found that the Amazon and Congo forests maintain rainfall over the periphery of the forest basins—regions where large numbers of people live and rely on rainfall for their livelihoods,” says Dominick Spracklen from the School of Earth and Environment at the University of Leeds.

“Our study implies that deforestation of the Amazon and Congo forests could have catastrophic consequences for the people living thousands of kilometers away in surrounding countries.”

Scientists have debated whether vegetation increases rainfall for hundreds of years. It is well established that plants put moisture back in the air through their leaves by a process known as evapotranspiration, but the quantity and geographical reach of rainfall generated by large forests has—until now—been unclear.

While there is plenty of anecdotal evidence that forests significantly increase rainfall, until now there has been a lack of observational evidence.

The team used newly available NASA satellite observations of rainfall and vegetation, along with a model that predicts atmospheric wind flow patterns, to explore the impact of the Earth’s tropical forests.

“We looked at what had been happening to the air over previous days—where it came from and how much forest it had traveled over,” Spracklen says.

To understand the relationship in detail, researchers investigated the journey of air masses arriving over different parts of the forest, to see the cumulative amount of leaf cover the air had moved over during the previous ten days, not just the amount of vegetation it was over when it rained. This showed that the more vegetation the air had traveled over, the more moisture it carried and more rain was produced.

“The observations show that to understand how forests impact rainfall, we need to account for how air has interacted with vegetation during its journey through the atmosphere often over thousands of kilometers,” says co-author Stephen Arnold.

“This has significant implications for how policymakers should consider the environmental impacts of deforestation, since its effects on rainfall patterns may be felt not only locally, but on a continental scale.”

The findings show the importance of initiatives to protect tropical forests.

“Brazil has recently made progress in slowing the historically high rates of deforestation across the Amazon and our study emphasizes that this progress must be maintained if impacts on rainfall are to be avoided,” Spracklen says.

“The Amazon forest maintains rainfall over important agricultural regions of Southern Brazil, while preserving the forests of the Congo Basin increases rainfall in regions of Southern Africa where rain-fed agriculture is important. Increased drought in these regions would have severe implications for their mostly subsistence farmers.”

The study was funded by NERC.

Source: University of Leeds

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