Droughts have withered crops in Peru, fires have scorched the Amazon and hydroelectric dams in Ecuador have struggled to keep the lights on as rivers dry up.

Scientists say the cause may lie high above the rainforest, where invisible “flying rivers” carry rain from the Atlantic Ocean across South America.

A new analysis warns that relentless deforestation is disrupting that water flow, and suggests that continuing tree loss will worsen droughts in the southwestern Amazon and could eventually trigger those regions to shift from rainforest to drier savanna — grassland with far fewer trees.

“Flying rivers are one of the forces that actually creates and sustains the Amazon rainforest,” says Matt Finer, a senior researcher with the Amazon Conservation Association, which tracks deforestation and climate threats across the basin and carried out the analysis.

If you break that pump by cutting down too much forest, Finer says, the rains stop reaching where they need to go.

Most of the Amazon’s rainfall starts over the Atlantic Ocean.

Moist air is pushed inland by steady winds that blow west along the equator, known as the trade winds. The forest then acts like a pump, effectively relaying the water thousands of miles westward as the trees absorb water, then release it back into the air.

Brazilian climate scientist Carlos Nobre was among the early researchers who calculated how much of the water vapor from the Atlantic would move through and eventually out of the Amazon basin.

He and colleagues coined the “flying rivers” term at a 2006 scientific meeting, and interest grew as scientists warned that a weakening of the rivers could push the Amazon into a tipping point where rainforest would turn to savanna.

That's important because the Amazon rainforest is a vast storehouse for the carbon dioxide that largely drives the world's warming.

Such a shift would devastate wildlife and Indigenous communities and threaten farming, water supplies and weather stability far beyond the region.

The analysis by Finer's group found that southern Peru and northern Bolivia are especially vulnerable.

During the dry season, flying rivers sweep across southern Brazil before reaching the Andes — precisely where deforestation is most intense.

The loss of trees means less water vapor is carried westward, raising the risk of drought in iconic protected areas such as Peru’s Manu National Park.

Nobre says as much as 50% of rainfall in the western Amazon near the Andes depends on the flying rivers. "All these ecosystems depend on this moisture flow," he says.

Corine Vriesendorp, the director of science at Peru's non-profit group Amazon Conservation, says the changes are already visible.

The last two years have brought the driest conditions the Amazon has ever seen, she notes.

"It's the way Indigenous people think about their year. They have these ecological calendars that are based on when it's drier, when the rain starts, and that's not just about agriculture," Vriesendorp says. "It's about every part of their life."

Protecting intact forests, supporting Indigenous land rights and restoring deforested areas are the clearest paths forward, researchers say.

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Produced by Rosa Ramirez and Steven Grattan.