Study finds evidence that tropical deforestation stops the rain

Even as miners, loggers and ranchers fell them at record rates, scientists are still learning about all the things that forests do to keep the local and global climate comfortable and stable.

Most recently, they have provided “compelling evidence” for the first time for a link between deforestation and rainfall decline across the tropics as a whole.

“Local people living near deforested regions often report a hotter and drier climate after the forests are cleared. But until now this effect had not been seen in rainfall observations,” study co-author and project supervisor professor Dominick Spracklen of the School of Earth and Environment at the University of Leeds said in a press release. “The study shows the critical importance of tropical forests in sustaining rainfall.”

The study, published in Nature Wednesday, made this crucial link by looking at satellite data of deforestation and rainfall in three key tropical forest regions: the Amazon, the Congo and Southeast Asia. All of these regions had seen significant deforestation between 2003 and 2017, the period observed by the scientists. The researchers looked at rainfall records both in deforested areas and in areas where the forest had been spared. What they found was that the deforested locations were indeed drier — even during the dry season when every drop of rain counts. In the wet season, rainfall fell by as much as 0.6 millimeters a month for every percentage point of clearing.

The study authors also looked at how far the impacts of deforestation would reach, on a scale of 25 to 40,000 square kilometers (approximately 10 to 15,444 square miles), New Scientist explained. They found that the effects increased with the more land included, with no effects discernible within 10 square miles of deforestation, but a monthly rainfall reduction of 0.25 millimeters per percentage point of cleared forest within 15,444 square miles.

Though the study doesn’t prove that deforestation is causing the rainfall decline, it does provide evidence for a longstanding hypothesis that forest loss reduces rainfall because it means there is less evapotranspiration — the word for what happens when the water from leaves wafts up into the atmosphere. If this is true, it could have serious consequences both for tropical forests and the people and animals who depend on them.

“Tropical forests play a critical role in the hydrological cycle through helping to maintain local and regional rainfall patterns,” study lead author and Leeds doctoral researcher Callum Smith said in the press release. “The reduction in rainfall caused by tropical deforestation will impact people living nearby through increased water scarcity and depressed crop yields.”

It’s also bad news for the forests themselves. It adds to concerns that the Amazon rainforest could reach a tipping point after which it can no longer sustain itself by creating its own rain, as The Guardian pointed out. A loss of rainfall can trigger other deforestation feedback loops such as increased wildfire risk as well as hinder forests’ ability to store carbon and harm their biodiversity, according to the press release. The team also looked at the potential future of the Congo rainforest and found that, if current deforestation rates persisted through 2100, rainfall in the region could fall by 8 to 10 percent.

However, there is a silver lining to the disappearing raincloud of the findings — the evidence that deforestation leads to local rainfall loss is also an important short-term argument for forest preservation. For every percentage point of reduced rain, crop yields can fall by 0.5 percent. Since agriculture is the motivator for nearly 90 percent of global deforestation, these findings may give farmers second thoughts about cutting down trees.

“Demonstrating the local benefit of keeping tropical forests standing for the people living nearby has important policy implications,” Spracklen told The Guardian. “I hope our work will provide a strong incentive for policy and decision makers within tropical nations to conserve tropical forests to help maintain a cooler and wetter local climate, with benefits for nearby agriculture and people.”