Climate change fueled the remarkable 2023 drought that drained major rivers, fueled huge wildfires and threatened the livelihoods of millions of people in the Amazon rainforest, scientists said on Wednesday.

Deforestation of the Amazon, the world’s largest and most biodiverse rainforest, has decreased rainfall and weakened the ability of trees and soil to retain moisture, researchers found. That made drought more acute and caused the forest to be less resilient to environmental destruction and events like wildfires.

The Amazon River — the world’s largest by volume — and several of its tributaries reached their lowest levels in 120 years of record-keeping last year. One fifth of the world’s freshwater flows through the rainforest.

A severe drought would have still occurred if humans hadn’t so profoundly changed the climate. But the burning of fossil fuels gave it the ranking of “exceptional,” the highest category in the U.S. Drought Monitor classification system, according to the study published by the World Weather Attribution initiative, an international collaboration among scientists that focuses on rapid analysis of extreme weather events.