Rainfall is decreasing in tropical areas around the world where forests are being cut down, an analysis of satellite data has shown, confirming the predictions of climate models.
This decline in rainfall will lead to falling yields in the farmland now found in regions where forest has been cleared and could lead to a vicious cycle, says Callum Smith at the University of Leeds in the UK. “As agricultural yields decline, more forests will be chopped down.”
Lower rainfall will also increase the risk of fires and reduce the odds of the remaining forest surviving, he says.
Vast amounts of water evaporate from the leaves of trees in tropical forests, and this water may fall as rain in nearby areas. When forests are cut down, less water is returned to the atmosphere above forests.
Climate models suggest this effect will reduce rainfall, but until now, studies of observed changes in rainfall have focused on specific regions, rather than across the tropics generally.
What’s more, a few studies have found increases in rainfall in some deforested areas. That is thought to occur because deforested patches are hotter than the surrounding forest, leading to rising air that triggers local rainfall.
To get a better overall picture, Smith and his colleagues started with a satellite dataset showing how forest cover in the Amazon, the Congo and South-East Asia changed from 2003 to 2017. They then compared this with various satellite measurements of rainfall in these regions.
“This is the first time that anyone has done a pan-tropical assessment with observational data,” says Smith.
The researchers also analysed the data at different scales, ranging from 25 to 40,000 square kilometres. At smaller scales, they found no change in average rainfall – there were increases in some places but just as many decreases.
But at larger scales they found a decrease. At the 40,000-square-kilometre scale, there was a 0.25 millimetre decline in rainfall per month for every 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 as forests are felled, total deforestation would lead to a 10 or 20 per cent decline in rainfall in affected regions. But it is possible the decline could accelerate when a tipping point is reached.
“There could be a catastrophic fall-off in precipitation at a certain level,” says Smith.
“The role that rainforests play in generating rainfall has been speculated about for quite a while but has been hard to pin down and quantify with direct observational evidence,” says Yadvinder Malhi at the University of Oxford.
This work shows that the observed rainfall decline is roughly what model simulations suggest, he says.
But the study shows only correlation, not causation, Malhi notes. It is possible that other factors such as sea surface temperature changes are playing a role too, he says.
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