The Amazon is getting drier as deforestation closes atmospheric rivers

Deforestation has reduced rainfall over the Amazon, suggesting the rainforest could reach a catastrophic tipping point sooner than expected.

Satellite observations and rain gauge measurements show that rainfall in the southern Amazon basin has dropped by 8 to 11 percent between 1980 and 2019. Tree cover in this part of the Amazon shrank by 16 percent over roughly the same period, largely because the forest was cleared and burned for cattle ranching.

The northern Amazon basin suffered much less deforestation and experienced only a slight increase in precipitation, which was not statistically significant.

While recently studies linked deforestation to drier weather within a radius of 300 kilometers, the new research found this link across a basin more than 3,000 kilometers wide. This shows that the destruction of the rainforest can also harm nearby ranches and soybean farms, he says Dominick Spracklen at the University of Leeds in Great Britain, who worked on the new study.

“Some people in agribusiness may see a patch of forest as wasted land [they] “That piece of forest works really hard to maintain the regional rainfall that our agriculture benefits from.”

Global warming is also drying out the Amazon rainforest, with extreme drought leading to record wildfires in 2024. But atmospheric modeling by Spracklen and his colleagues showed that deforestation caused a 52 to 75 percent decrease in rainfall.

Prevailing winds transport moisture from the Atlantic Ocean, which falls as rain over the Amazon. Evaporation and transpiration by plants return three quarters of this water into the atmosphere. Further downwind, it falls again as rain and returns to the atmosphere for half a dozen or more cycles, fueling “flying rivers” that carry moisture throughout the rainforest.

If an area of ​​forest is leveled, more than half of the rainwater in that area drains into streams and begins to drain back into the ocean. This intercepts the flying rivers of moisture and reduces the amount of precipitation. Spracklen and his colleagues found that it also reduces the atmospheric instability that leads to the formation of storm clouds.

With fewer trees to slow it down, the wind blows faster and carries more moisture out of the region.

Unlike past research, the study combines both data and modeling to explain exactly how deforestation weakens rainfall, he says Yadvinder Malhi at Oxford University.

“The atmosphere becomes smoother, it glides in some ways. Moisture can travel further from the forest area because there is less friction on the ground,” says Malhi. “So there are some interesting secondary atmospheric processes that are not normally captured.”

Scientists fear that the combined effects of heat, drought and deforestation could push the Amazon to the tipping point of turning into a savannah, but it’s uncertain how close to that it will be. Spracklen and his colleagues found that climate models underestimate the impact of deforestation on precipitation by as much as 50 percent, suggesting that the rainforest could reach this tipping point much sooner than expected.

ON studies last year found a 37 percent chance of some Amazon die-off by 2100 if global warming, currently at 1.4°C, reaches 1.5°C. While that wouldn’t necessarily mean the rainforest would turn into a savannah, it would mean a scrub forest containing fewer species and less carbon, Spracklen says.

“Amazon is more sensitive than we think, which is bad news,” he says. “Maybe we’re closer to the threshold of deforestation than we thought. But I think there’s a lot of uncertainty.”

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