Luciana Gatti, an atmospheric chemist at Brazil’s National Institute for Space Research (INPE), has spent two decades flying on bush planes to collect greenhouse gases from above the Amazon. Climate models have long predicted that, past a certain threshold of deforestation, this vital ecosystem will lose the ability to sustain itself and start turning into a savanna. Still, when Gatti discovered that parts of the forest itself were releasing more carbon than they absorbed, she initially refused to believe her own data. She checked and rechecked her calculations. Ultimately, though, she could not escape the conclusion that the models were already proving accurate. And that was before Jair Bolsonaro became president and started openly encouraging slash-and-burn development, legal or not.

The world’s largest rainforest appears to be reaching a tipping point, losing its natural resilience to droughts and fires. To see what this transformation looks like on the ground, Alex Cuadros accompanied Gatti to the Amazon’s frontier of deforestation, where she and other scientists continue to make important discoveries—even as Bolsonaro strangles their funding. The story also dives deep into the broader science of climate-change tipping points, which has radically reshaped how researchers understand both the history and the future of the global climate. Many now fear that one tipping point may set off another—which means that large-scale dieback in the Amazon could bring devastating consequences for the world as a whole.