Amazon Is Shifting Toward a 'Hypertropical' Climate — Mass Tree Losses Could Follow

New research published in Nature warns that the Amazon rainforest is shifting toward a "hypertropical" climate regime not seen on Earth for at least 10 million years. The change—driven by warming and an increasing number of extremely hot, dry days—could push the forest beyond the limits that sustain its current tree communities and carbon-absorbing capacity.

What the Study Found

Lead author Jeff Chambers, a geography professor at the University of California, Berkeley, and colleagues analyzed 30 years of local climate and physiological data from a forest site north of Manaus, Brazil. They combined temperature, humidity, soil moisture and sunlight records with sensors that tracked water and sap flow inside tree trunks to observe how trees respond to hotter, drier conditions.

The researchers project that, under high-emissions scenarios, the Amazon could experience roughly 150 days per year of "hot drought" by 2100, with drought conditions extending into what is now the wet season. During these events, evaporation rates rise and soil moisture falls; trees close their stomata to conserve water, which also halts carbon dioxide (CO2) uptake and limits growth and repair.

Physiological Collapse: CO2 Starvation and Xylem Embolism

When droughts become extreme, two processes drive tree mortality. First, stomatal closure prevents CO2 uptake and can lead to effective "CO2 starvation." Second, the team identified a consistent soil moisture threshold: when soil moisture falls below about 33% (only one-third of pore space contains water), trees develop embolisms—air bubbles in xylem vessels—that block water transport. The researchers observed this threshold repeatedly, including during El Niño years 2015 and 2023, and found similar values at another Amazon site.

Slow-growing trees like the Shihuahuaco (Dipteryx micrantha) are more resistant to drought conditions than fast-growing trees. | Credit: Angela Ponce for The Washington Post via Getty Images

"If there are enough embolisms, the tree just dies," Chambers said, underscoring how a relatively small change in average conditions can trigger widespread mortality.

Ecological And Climate Implications

Current annual tree mortality in the Amazon is just above 1%; the study estimates it could rise to about 1.55% by 2100 under scenarios with limited reductions in greenhouse-gas emissions. While the percentage increase appears small, the absolute loss across the entire basin would be substantial and would weaken the rainforest's role as a global carbon sink.

Fast-growing species—those that rely on plentiful water and CO2 to sustain rapid growth—were found to be most vulnerable. If those species decline, slower-growing trees such as the yellow ipê (Handroanthus chrysanthus) and shihuahuaco (Dipteryx micrantha) could become relatively more dominant, provided they tolerate intensifying water stress and rapid warming.

The authors note that similar transitions may be underway in other tropical forests, including parts of western Africa and Southeast Asia. Because tropical rainforests absorb large amounts of atmospheric CO2, a regional shift to hypertropical conditions would have major consequences for the global carbon cycle and climate feedbacks.

What Can Change the Outcome?

The study's projections assume limited reductions in CO2 emissions. As Chambers emphasized, the timing and severity of a hypertropical shift depend on future emissions pathways: stronger mitigation could delay or reduce these impacts, while continued high emissions would accelerate them.

Bottom line: Warming-driven increases in the frequency and intensity of hot droughts could push the Amazon into a climate regime that stresses trees physiologically, raising mortality and altering species composition with broad consequences for carbon storage and biodiversity.