The Amazon rainforest is undergoing a transition to a hotter and drier climate, according to a new study led by the University of California, Berkeley. The research, published in Nature on December 10, describes how the region is moving toward what scientists call a “hypertropical” climate—conditions that have not been present on Earth for tens of millions of years.
Researchers warn that if high levels of greenhouse gas emissions continue, these “hot drought” conditions could become common across the Amazon by 2100. Such changes would likely lead to widespread tree die-offs and reduce the forest’s capacity to absorb atmospheric carbon dioxide. This is significant because tropical forests play an important role in absorbing human-generated carbon emissions.
The study explains that global warming is extending the dry season from July to September and increasing temperatures above normal levels. These hot droughts are already causing increased stress among trees, raising mortality rates by 55 percent compared to normal years.
“When these hot droughts occur, that’s the climate that we associate with a hypertropical forest, because it’s beyond the boundary of what we consider to be tropical forest now,” said Jeff Chambers, professor of geography at UC Berkeley and leader of the study. He noted that by 2100, such conditions could last as many as 150 days each year.
Chambers’ team found that once soil moisture drops below about one-third by volume, trees either stop capturing carbon or develop air bubbles in their sap—a process similar to embolisms in humans—which can kill them. Fast-growing species are more affected than slow-growing ones. “We showed that the fast-growing, low wood-density trees were more vulnerable, dying in greater numbers than high wood-density trees,” Chambers said. “That implies that secondary forests might be more vulnerable to drought-induced mortality, because secondary forests have a larger fraction of these types of trees.”
While annual tree mortality is just over one percent under typical conditions, even a small increase has cumulative effects over time. Researchers believe hypertropical climates may also emerge in rainforests outside South America—including western Africa and Southeast Asia—if current trends persist.
Chambers stressed the importance of reducing emissions: “It all depends on what we do,” he said. “It’s up to us to what extent we’re actually going to create this hypertropical climate. If we’re just going to emit greenhouse gasses as much as we want, without any control, then we’re going to create this hypertropical climate sooner.”
Chambers has worked in the Amazon since 1993 and has helped install monitoring equipment at sites north of Manaus with collaborators from Brazil’s Instituto Nacional de Pesquisas da Amazônia (INPA). These instruments track temperature, humidity, sunlight intensity above the canopy, and soil moisture content below ground level.
Data collected over three decades show higher tree death rates following intense droughts—especially among fast-growing species common after logging activities. During severe El Niño-related droughts in 2015 and 2023 at two different sites near Manaus, researchers observed similar thresholds for soil moisture loss leading to rapid declines in transpiration—the movement of water through plants—and subsequent hydraulic stress.
“The really remarkable thing is that the threshold soil moisture content in a different plot with different trees for droughts in different years — 2015 and 2023 — were essentially the same: 0.32 and 0.33,” Chambers said. “That was really surprising to everyone.”
As extreme heat persists during prolonged dry periods, some trees suffer hydraulic collapse due to embolisms forming within their xylem tissue; others starve when they close their pores against water loss but also block intake of carbon dioxide needed for growth.
Analysis using five Earth system models suggests tropical forests are shifting into hotter states previously seen only between ten and forty million years ago—defined here as regions warmer than historical norms with frequent severe droughts.
With further warming predicted this century, such climates will become increasingly common worldwide unless emission trends change significantly soon.
“Present-day hot droughts are harbingers of this emerging climate, providing windows of opportunity to better understand tropical forest responses to increasingly extreme future conditions,” wrote Chambers and co-authors from UC Berkeley; INPA; institutions across Brazil; Europe; Norway; and elsewhere.
The long-term research received support from multiple U.S. and international agencies over thirty years.
