Scientists have been stumped for years over why the southern Amazon rainforest‘s rainy season begins two to three months earlier than they’d expect. But now an international team that includes researchers from NASA and Google has discovered the forest actually triggers its own rainy season, thanks to water vapor off plant leaves. The finding points to one disastrous consequence of deforestation in this part of the world: as trees are cut down, it appears there’s actually less rainfall.


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Monsoon winds and the Intertropical Convergence Zone, which NASA describes as a belt of converging trade winds that shifts depending on the seasons, control when the rainy season begins in many tropical locations, and the southern Amazon experiences both factors. But they don’t kick in until December or January, while the southern Amazon’s rainy season typically begins in the middle of October.

Related: Scientists warn Amazon jungle faces “death spiral”

To try and find out why, the team of scientists led by Jonathon Wright of Tsinghua University scrutinized data on water vapor from NASA’s Tropospheric Emission Spectrometer, aboard the agency’s Aura satellite, together with other satellite measurements, to discover clouds in the southern Amazon at the dry season’s close form via water rising from the rainforest.

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But the southern Amazon’s rainy season already begins nearly a month later than it did back in the 1970’s. Evidence indicates if the region’s dry season stretches longer than five to seven months, there won’t be enough rain for the rainforest to remain a rainforest – it could transition to grassy plains. But the dry season is already a few weeks shorter on average than that benchmark in parts of the southern Amazon.

The new study, published by the journal Proceedings of the National Academy of Sciences of the United States of America, bolsters the idea that deforestation is partly to blame for the delayed start of the rainy season. The rainforest’s capacity to develop clouds dwindles as trees are chopped down. And if deforestation harms the forest to the point where it can’t trigger its own rainy season, the southern Amazon’s rainy season likely wouldn’t commence until December or January.

Such changes could have far-reaching impacts. According to NASA’s Jet Propulsion Laboratory, “The loss of a major Amazonian forest ecosystem could increase Brazilian droughts and potentially disrupt rainfall patterns as far away as Texas.”

Via NASA Jet Propulsion Laboratory

Images via Center for International Forestry Research and Jay on Flickr