Why is rainfall declining in the Amazon? New research says deforestation is the leading driver

For decades the world’s largest rainforest has been getting drier. A new study published in Nature Communications disentangles how much of this shift can be blamed on humanity’s warming of the planet and how much on the felling of trees within the Amazon itself. The answer: More than many had expected.

Researchers led by Marco Franco and Luiz Augusto Toledo Machado at the University of São Paulo analyzed 35 years of land-use and atmospheric data across Brazil’s “Legal Amazon,” a territory of roughly 5 million square kilometers that covers most of the biome. Using statistical models, they separated the effect of global greenhouse-gas increases from local forest loss. Their results suggest deforestation accounts for nearly three-quarters of the rainfall decline and about one-sixth of the warming during the dry season since the mid-1980s.

The numbers are stark. Between 1985 and 2020, dry-season rainfall fell by about 21 millimeters per year, of which 15.8mm—74.5%—was linked to deforestation. Maximum daily temperatures rose by around 2°C, with 0.39°C, or 16.5%, attributed to tree loss. The rest was due to global climate change. The findings provide quantitative backing to what ecologists and meteorologists have long suspected: forest clearance alters not only carbon balances but also local weather.

Amazonian trees are active players in climate. Each day, billions draw water from the soil and release it into the air through transpiration, supplying more than 40% of the region’s rainfall. This moisture rises into the atmosphere and condenses, forming clouds that later return as rain. The process also creates localized low-pressure zones, pulling in moist air from surrounding areas in what some scientists call the “biotic pump.” Strip away forest and the cycle falters. Earlier research suggested that for every percentage point drop in forest cover, rainfall declines by 3mm a year. The new study confirms that relationship while clarifying the respective roles of land use and global warming.

The consequences extend far beyond the basin. The “flying rivers” of vapor generated by trees influence weather in Brazil’s agricultural heartlands and as far as the Andes. Disrupting this pump-like system can weaken rainfall far from the forest, intensifying drought risks.  Studies also suggest that deforestation is altering the South American monsoon, increasing the risk of drought in central and southeastern Brazil. The Amazon endured record dry spells in 2023 and 2024, disrupting river transport and straining hydropower.

The effects are nonlinear. The sharpest changes in rainfall and temperature occur when 10–40% of forest cover is lost. Early incursions therefore have disproportionate climatic effects. In the most heavily cleared areas, maximum temperatures rose by more than 1.2°C due to deforestation alone, while rainfall fell by more than 50mm during the dry season.

Global emissions still matter. The concentration of carbon dioxide in the region rose by roughly 87 parts per million over the study period, driven largely by fossil-fuel burning elsewhere. Methane levels increased by about 173 parts per billion, again almost entirely from global sources. But while greenhouse gases accumulate slowly and evenly, deforestation reshapes weather rapidly and locally.

Looking ahead, the authors estimate that if recent deforestation rates persist, by 2035 the region could see dry-season rainfall fall by another 7mm and temperatures climb by 0.6°C. The Amazon’s climate could begin to resemble that of the drier Cerrado savanna or even the semi-arid Caatinga to its east. Such a shift would test the resilience of the forest’s more than 11,000 known tree species and the communities who rely on them. It could also weaken rainfall over distant agricultural zones that depend on the Amazon’s “flying rivers,” with implications for food production across much of South America.

So far, about 13.2% of the original Amazon forest biome has been lost to deforestation and other causes, according to the Monitoring of the Andean Amazon Project (MAAP), with large areas further degraded by logging and fire. The eastern portion has fared worse, with 31% already gone. Recent developments highlight both progress and setbacks: Deforestation in the Brazilian Amazon dropped 30.6% in 2024 to its lowest level in nine years, yet fires in 2024 destroyed more than 4.6 million hectares of primary forest. That surge, more than double the average annual loss of the previous decade, was fueled by record heat, the worst drought in 70 years, and El Niño. The contrast underscores how governance gains under President Luiz Inácio Lula da Silva can be quickly undermined by intensifying climate pressures and the long legacy of past mismanagement.

For now, the message is clear. Global climate change is making the Amazon hotter, but deforestation is making it both hotter and drier. The rainforest’s fate—and the weather systems it sustains—depends not only on global emissions but also on choices made within its borders.

Banner image: Sunrise over the Amazon. Image by Rhett Ayers Butler

Citation:

Citation: Franco, M.A., Rizzo, L.V., Teixeira, M.J. et al. How climate change and deforestation interact in the transformation of the Amazon rainforest. Nat Commun 16, 7944 (2025). https://doi.org/10.1038/s41467-025-63156-0

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Rhett Butler Editor

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