For decades, the dry season in the Amazon rainforest has been getting drier. A new study, published on Tuesday, found that about 75 percent of the decrease in rainfall is directly linked to deforestation.
The study, in Nature Communications, also found that tree loss was partly responsible for increased heat across the Amazon. Since 1985, the hottest days in the Amazon have warmed by about 2 degrees Celsius. About 16 percent of that increase, the researchers found, was because of deforestation.
Marco Franco, an assistant professor at the University of São Paulo who led the study, said he was surprised by the findings. “We were expecting to see deforestation as a driver, but not this much,” he said. “It tells us a lot about what’s going on in the biome.”
The Amazon rainforest is often called the lungs of the planet because its trees help to regulate the global climate by absorbing planet-warming carbon dioxide. But decades of large-scale logging and burning in the forest have recently flipped that script, and parts of the region have become net producers of greenhouse gases.
The Amazon also steers regional weather patterns. Its trees pull water from the soil and, through a process known as transpiration, release that moisture through tiny pores on their leaves. There are hundreds of billions of trees in the Amazon Basin, and the water they collectively release into the air is estimated to contribute more than 40 percent of all the region’s rainfall.
“You can imagine a tree as a big water pump,” said Callum Smith, a postdoctoral researcher at the University of Leeds in England who studies tropical deforestation and was not involved in the Nature Communications study. “If you chop down a tree, it reduces the moisture going into the atmosphere.”
In 2023, Dr. Smith led a study that used large satellite data sets to better understand the link between deforestation and decreases in rainfall. He said scientists have long known about this effect, but weather changes don’t always show up in the same place as deforestation, making the relationships tricky to quantify.
The new study, Dr. Smith said, used sophisticated analytical methods and represented a step forward in knowledge.
Luiz Machado, a professor of climate and meteorology at the University of São Paulo and an author of the new study, said “everybody knows” the climate in the Amazon has changed because of climate change and deforestation. But until now, he said, “nobody knew exactly what each of these things contributed.”
The study analyzed 29 sections of the Amazon Basin within the borders of Brazil and used large sets of satellite data to separate out influences like evolving landscapes, changing climate and shifting weather conditions between 1985 and 2020.
While Dr. Franco and Dr. Machado found that deforestation had driven 74.5 percent of the decrease in precipitation across the basin, they emphasized this was just the average. Regions with more deforestation suffered greater rainfall losses, they noted.
In the tropics, the year is split into two seasons: dry and wet. The study found that deforestation was causing rainfall loss in both seasons. But because the effect on the dry season was much more pronounced, the researchers said, they decided to focus on that period. (For the purposes of the study, the definition of dry season varied across areas. Countrywide, it is generally considered to run from June through November.)
Even during the dry season, ecosystems depend on rain. “As a joke, we used to say that the rainy season in the Amazon was when it rained all day, and the dry season in the Amazon rained every day,” Dr. Machado said. “But that’s not true anymore.”
Less rainfall doesn’t just mean less water for plants and animals. As the forest becomes drier, it becomes more prone to wildfires, which, in turn, eliminate more trees. The region has been plagued by slash-and-burn agriculture, in which fires are used to clear big tracts of land for ranching and farming. Sometimes, these fires burn out of control.
In 2024, more than 40 million acres of the Amazon rainforest went up in smoke. And in the first half of 2025, according to observations by Brazil’s space agency, deforestation was already 27 percent higher than in the same period the year before. It’s a feedback loop that threatens to keep rainfall on the decline, the researchers say.
And it’s not just wild ecosystems that suffer. Brazil’s largest farming hubs are directly adjacent to the Amazon. Adequate rainfall for crops in these regions, the researchers said, also requires a healthy forest.
Farmers in states like Mato Grosso, Dr. Franco said, are already losing crops to drought. In 2024, the state went 150 days straight without rain.
“If you don’t have rainfall, then you don’t have rain for farming in Brazil,” he said. “This isn’t something for the future. This is already happening now.”
Sachi Kitajima Mulkey covers climate and the environment for The Times.