Parts of the Amazon Go From Absorbing Carbon Dioxide to Emitting It

“The ability to build back a margin of safety” through reforestation is very real, he said, and could help bring back the role of trees in producing the moisture within the forests. “I don’t think you’ll ever get it back to what it was, but you can certainly improve it,” he said.

The forests are a critical part of the region’s water cycle; moisture put into the air by trees is responsible for as much as 35 percent of the region’s rainfall, according to some estimates.

By managing the forests with carbon sequestration, hydrology and biodiversity in mind, he said, “you get multiple benefits.” Referring to the changes to the Amazon, he said “it’s come a lot sooner than anybody thought 30 years ago because of the extensive use of fire and climate change. But put some water in there, and it will change.”

Any changes may be long in coming and will face political opposition. Brazil’s president, Jair Bolsonaro, has overseen skyrocketing deforestation in the Amazon. The government, under increasing pressure, has recently announced plans to counter the trend, but spikes in deforestation have continued.

In an accompanying article in Nature, Scott Denning, a professor in the department of atmospheric science at Colorado State University wrote that the paper’s “ atmospheric profiles show that the uncertain future is happening now.”

In an emailed response to questions, Dr. Denning praised the new study as the first real large-scale measurement — from various altitudes across thousands of kilometers and remote sectors — of the phenomenon, an advance beyond the traditional measurement at forest sites. The results show “that warming and deforestation in eastern Amazonia have reversed the carbon sink at regional scale and that the change is actually showing up in atmospheric CO2,” he wrote.

The research flights could be grueling. Dr. Gatti said that she took Dramamine before fights, but one colleague refused to get into the plane, saying that his wife was pregnant and “it’s too risky.” She replied, “I have two kids!”