Reduced rainfall across the Amazon basin is causing large areas of forests to die, which could be amplifying drought conditions across the region.

Researchers from the Potsdam Institute for Climate Impact Research believe that this process, known as self-amplifying forest loss, could cause a vicious circle of drought and further forest loss across the Amazon region, according to a study published in Nature Communications.

"We already know that on the one hand, reduced rainfall increases the risk of forest dieback, and on the other hand, forest loss can intensify regional droughts," lead-author of the study, Delphine Clara Zemp, said in a statement, calling the Amazon "one of the tipping elements in the Earth system."

The survival of tropical forests depends on the very water that they hold, according to PIK researchers, who studied water fluxes across the region to determine the level of interaction between vegetation and the atmosphere.

In the Amazon, one of the most important carbon sinks on the planet, trees evaporate the moisture that will eventually rain back onto them.

"As powerful as the cycle is, it is also surprisingly susceptible to environmental changes," co-author Henrique M.J. Barbosa from the Universidade de Sao Paulo said in a statement. "And humankind is imposing massive perturbations on Amazonia by both cutting down the trees and heating up the air with greenhouse gases."

This, Barbosa explained, has the effect of reducing large-scale moisture transportation and rainfall.

(a) Amazon system in equilibrium. (b) Initial forest loss triggered by decreasing oceanic moisture inflow, which reduces evapotranspiration. (c) As a result, the rainfall regime is altered in another location, leading to further forest loss and reduced moisture transport.

While the Amazon spans nine countries, covering an estimated 2.6 million square miles, some 60 percent of the basin lies in Brazil.

After many years of decelerated deforestation across the Amazon basin, in 2015, Brazil reported a rise for the first time in nearly a decade and at the end of 2016, the government released a study showing that deforestation in the Amazon had jumped 26 percent from the previous year.

Researchers at the Postdam Institute worry that another logging spike could push the Amazon into a vicious dieback cycle that would include hotter dry seasons, more forest loss, and continued drought. They estimate that in addition to direct forest loss due to reduced rainfall, 10 percent of the forest could be lost due to the effects of self-amplification.

This forest–atmosphere feedback cycle could cause forest dieback in 38 percent of the Amazon basin. Combined with the direct effects of drought, this could mean that most of the Amazon would eventually be at risk. Even if rainfall were to remain stable, the intensification of the dry season could be strong enough to tip the ecosystem, turning large swaths of the Amazon into savannah.

"Projected rainfall changes for the end of the 21st century will not lead to complete Amazon dieback," Carl Schleussner, from Berlin-based scientific think tank Climate Analytics and PIK, said. "But our findings suggest that large parts of it are certainly at risk."

Researchers have documented a similar effect in the Cerrado, the vast tropical savannah in central Brazil that has higher rates of deforestation than the Amazon due to the intensification of agribusiness across the region. As the Cerrado is deforested and less water is cycled back into the atmosphere, rainfall has dropped.

The diversity of landscapes is a predictor for how resilient they will be to this dieback cycle, researchers emphasized. The more plant species found in any given forest, the less vulnerable it will be to self-amplifying forest loss.

"Since every species has a different way of reacting to stress, having a great variety of them can be a means for ecosystem resilience," co-author Marina Hirota from the Federal University of Santa Catarina, said. Preserving biodiversity, she added, may be an important tool used to stabilize tropical forests and the transport of water into the atmosphere.

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