Amazon rainforest could see less rain as deforestation progresses
Dr. Jess Baker
An analysis of satellite data confirms climate model predictions that rainfall is declining in deforested tropical regions around the world.
This decrease in rainfall could lead to lower crop yields on the farmlands currently seen in deforested areas, leading to a vicious cycle, says Callum Smith of the University of Leeds, UK. “As agricultural yields decline, more forests will be cut down.”
Less rainfall will also increase fire risk, making the remaining forests less likely to survive, he says.
In tropical forests, large amounts of water evaporate from the leaves of trees, and this water can become rain and fall nearby. When forests are cut down, less water is returned to the atmosphere above the forests.
Climate models suggest that this effect reduces precipitation, but studies of observed precipitation changes to date have focused on specific regions rather than the tropics as a whole.
Additionally, some studies have found increased precipitation in deforested areas. This is thought to occur because deforested patches are hotter than the surrounding forest, causing air to rise which causes local rainfall.
To get a better picture, Smith and his colleagues started with a satellite dataset showing how forest cover in the Amazon, Congo, and Southeast Asia changed from 2003 to 2017. We then compared it with various satellite measurements of rainfall in these areas. .
“This is the first time we have done a pantropical assessment using observational data,” says Smith.
Researchers also analyzed the data at various scales ranging from 25 to 40,000 square kilometers. At smaller scales, no change in mean precipitation was observed. Some locations saw an increase, while many saw a decrease.
At larger scales, however, a decrease was seen. At a scale of 40,000 square kilometers, he reduced precipitation by 0.25 millimeters per month for each percentage point of forest lost. This is within the high end of the range suggested by climate models.
If rainfall continues to decrease at the same rate that forests are cut, it will lead to complete deforestation and a 10-20% decrease in rainfall in affected areas. However, once a tipping point is reached, the decline could accelerate.
“There could be catastrophic decreases in precipitation at certain levels,” says Smith.
“The role that rainforests play in producing rainfall has been speculated for some time, but has been difficult to identify and quantify with direct observational evidence,” said Yadvinder Malhi of the University of Oxford.
The study shows that the observed rainfall decline is about the same as model simulations suggest, he says.
But the study only shows correlation, not causation, Malhi said. Other factors, such as changes in sea surface temperature, may also be at play, he says.
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