Amazon rainforest is transitioning to a 'hypertropical' climate — and trees won't survive that for long

The Amazon rainforest is inching toward a "hypertropical" climate regime that has not existed on Earth for at least 10 million years, new research suggests.

Scientists predict this regime will cause more frequent and extreme droughts, which could lead to mass tree dieoffs. By 2100, hot droughts could bake the Amazon for 150 days of the year, extending even into the wet season, according to a study published Wednesday (Dec. 10) in the journal Nature.

"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," study lead author Jeff Chambers, a professor of geography at the University of California, Berkeley, said in a statement.

Scientists think a hypertropical climate last existed between 40 million and 10 million years ago, during the Eocene and Miocene periods. The average global temperature during the middle Eocene was 82 degrees Fahrenheit (28 degrees Celsius) — 25 F (14 C) warmer than the average today — and previous research suggests forests near the equator had fewer mangroves and evergreen trees.

Currently, the Amazon rainforest experiences hot drought conditions a few days or weeks of the year. But due to climate change, the region's dry season — which typically lasts from July to September — is getting longer, and the annual proportion of hotter-than-normal days is increasing.

Chambers and his colleagues analyzed 30 years of temperature, humidity, soil moisture and sunlight intensity data from a patch of forest north of Manaus, a city in the heart of the Brazilian Amazon. The researchers also examined information from sensors that measured water and sap flow inside tree trunks at this site, which helped them understand how the trees coped with drought conditions.

During droughts, trees struggled to access water and stopped absorbing carbon dioxide (CO₂), the researchers found. That's because evaporation rates surged during droughts, reducing soil moisture. Trees responded by closing the pores on their leaves that control water and gas exchange with the atmosphere, so they preserved water. But this simultaneously blocked CO₂ absorption, which in plants is necessary for tissue growth and repair.

As a result, when drought conditions were extreme, a proportion of the trees died from CO₂ starvation. And when soil moisture dropped below a threshold of 33% — meaning only one-third of the soil's pores were filled with water — trees also developed bubbles in their sap that were akin to clots in human blood vessels, preventing normal circulation inside the plants' fluid-filled xylem.

"If there are enough embolisms, the tree just dies," Chambers said. The soil moisture threshold leading to this collapse was remarkably consistent across two El Niño years in 2015 and 2023, and it matched thresholds measured at another study site in the Amazon. "That was really surprising to everyone," he said.

Annual tree mortality in the Amazon rainforest is currently just above 1%, but it could rise to 1.55% by 2100, the researchers found. This may seem insignificant, but it makes a huge difference on the scale of the entire rainforest, Chambers said.

Fast-growing trees were more vulnerable to hot droughts than their slow-growing counterparts, because they needed abundant water and CO₂ to sustain this growth. This suggests slow-growing trees, such as the yellow ipê (Handroanthus chrysanthus) and the Shihuahuaco (Dipteryx micrantha), will eventually dominate the Amazon as temperatures rise — if these trees can cope with increasing water stress and the rate of temperature change, that is.

The results indicate that rainforests in other parts of the world, such as western Africa and Southeast Asia, may also be transitioning to a hypertropical climate regime. This shift has dramatic implications for Earth's carbon cycle, because rainforests absorb huge amounts of CO₂ that would otherwise end up in the atmosphere.

The predictions of what could happen to the Amazon by 2100 assume negligible reductions in CO₂ emissions, so "it's up to us to what extent we're actually going to create this hypertropical climate," Chambers said. "If we're just going to emit greenhouse gases as much as we want, without any control, then we're going to create this hypertropical climate sooner."