They report that a dangerous mix of human-induced devastation and cyclic drought in the Amazon could launch a vicious circle of forest dieback. The drought that killed the trees could intensify because of the intricate relationship between the rainforest and the rainfall, in which trees play a role in maintaining a pattern of precipitation by pumping fallen water back into the atmosphere.

“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. So more droughts can lead to less forest, leading to more droughts, and so on,” says Delphine Clara Zemp, of Germany’s Potsdam Institute for Climate Impact Research, who led the international team of scientists behind the finding.

Amazon forest loss

“Yet the consequences of this feedback between the plants on the ground and the atmosphere above them so far was not clear. Our study provides new insight into this issue, highlighting the risk of self-amplifying forest loss, which comes on top of the forest loss directly caused by the rainfall reduction.”

Dr Zemp and other scientists from Germany, the Netherlands, Sweden and Brazil report in Nature Communications that their results suggest frequent extreme drought events in the region have the potential to destabilise large parts of the Amazon forest.

Forests around the world, and particularly in the tropics, play a powerful role in mitigating climate change. That is because a healthy forest soaks up carbon from the atmosphere and stores it as wood, peat and soil carbon, some of it for decades, some for millennia.

But drought in the Amazon has already taken its toll and threatens to turn what ought to be a “sink” for carbon into a source, as dying trees and parching ground release carbon buried for centuries.

Brazilian scientists have already identified mechanisms for drought in the region, and other researchers have confirmed that humans have been affecting the forest in different ways for many thousands of years.

But the potentially calamitous impact of clearance for mining, logging and ranching, combined with the longer-term impact of human-induced climate change, driven by fossil fuel combustion on a global scale, had to be identified by complex computer simulations.

The scientists found that if rainfall levels fell by half in a dry season, 10% of the forest might be lost through what they call “self-amplification”: that is, dry spells mean that the forests lose even more water, making aridity even more likely, and more harmful.

This seems to have happened in the region 20,000 years ago. If the process went on, this self-amplified forest dieback could afflict 38% of the Amazon Basin. Since by then the drought would be dire, that could put most of the rainforest at risk.

Incalculable consequences

There are huge uncertainties. The latest study is not likely to be the end of the argument. The rainforest is not likely to turn into savannah in this century. But it is home to one of the terrestrial world’s richest habitats, and its loss would have incalculable consequences, for the people in the region and for the rest of the world.

“The Amazon water cycle is, of course, pure physics and biology, but it is also one of nature’s great wonders,” says one of the authors of the report, Henrique Barbosa, from the Universidade de São Paulo, Brazil.

“As powerful as the cycle is, it is also surprisingly susceptible to environmental changes – and humankind is imposing massive perturbations on Amazonia by both cutting down the trees and heating up the air with greenhouse gases, which reduces large-scale moisture transport and precipitation, and ends up affecting even the untouched patches of the forests.”– Climate News Network

• This report was first published in Climate News Network