Amazon Is Shifting Into a 'Hypertropical' State Unseen For Millions of Years

A new study of the Amazon rainforest has found the region is shifting toward a 'hypertropical' state as droughts become longer, hotter, and more frequent.

These conditions have "no current analogue" according to the international team of researchers behind the study. Trees are becoming exposed to whole new levels of stress, and the Amazon's capacity to absorb carbon dioxide is being reduced, too.

So drastic are the contemporary and impending changes, based on data gathered across the Amazon over more than three decades, that the researchers coined a new descriptor: 'hypertropical'. We're talking about conditions that haven't previously existed on Earth for millions of years.

Related: Amazon Lakes Became Hotter Than a Hot Tub, Killing Hundreds of Dolphins

The researchers looked at how trees, and the soil they're rooted in, respond to periods of high temperatures and drought. As these periods intensify, they offer a brief window into what could be the new normal within the next 100 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," says geographer Jeff Chambers, from the University of California, Berkeley.

The study used measurements taken in the field over more than 30 years. In this picture, a scientist is measuring a leaf's photosynthesis rate. (Jeff Chambers/UC Berkeley)

Models created from the data gathered by Chambers and his colleagues show that these hot droughts are likely to be even more common by 2100, and will occur year-round – even during the wet season (around December to May).

Trees are predicted to die at higher rates due to reduced soil moisture that can trigger two related problems: hydraulic failure, where air bubbles block the transport of water within the trees, and carbon starvation, where the closure of leaf pores in an attempt to save water then impacts photosynthesis.

Field measurements show this is already happening now, in the extremes of the current Amazon climate. If it becomes hypertropical, those extremes will happen much more often – potentially increasing the tree mortality rate by 55 percent.

"We showed that the fast-growing, low wood-density trees were more vulnerable, dying in greater numbers than high wood-density trees," says Chambers.

"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."

Part of the study focused on two specific Amazon sites affected by the 2015 and 2023 droughts, which were driven by unusually warm El Niño events. The critical water threshold was the same across both sites and both years – suggesting a potentially widespread shift.

Most hypertropical forests will emerge in the Amazon region, the researchers predict, though they are also likely to show up in Africa and Asia, too. These forests could flip from being carbon sinks to carbon contributors, as trees die off.

These projections are based on extensive data, and it's another sobering reminder of how important forests are to the balance of the atmosphere – and what happens if they're lost.

"It all depends on what we do," says Chambers. "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 gases as much as we want, without any control, then we're going to create this hypertropical climate sooner."

The research has been published in Nature.