A 10-million-year leap backwards: Amazon climate becomes hypertropical

In order to understand what the Amazon could become tomorrow, scientists are studying what's happening there today. Major droughts in recent years, which are becoming longer and hotter, are no longer seen as mere climatic anomalies: they are becoming commonplace for a possible future.

This is the central message of a study published in the journal Nature, which analyzes over thirty years of data collected in the heart of the Amazon rainforest and introduces a concept destined for debate: the "hypertropical" climate.

A climate unseen on Earth for millions of years
The term hypertropical describes conditions that are warmer than 99% of all historically observed tropical climates, accompanied by much more frequent and intense droughts, a climatic regime which, researchers explain, has not occurred on Earth for at least 10 million years, and which could take hold over vast areas of the Amazon by 2100 if greenhouse gas emissions continue at current levels.

30 years of data in the forest
The work, led by Jeffrey Chambers of the University of California at Berkeley, combines field observations, ecophysiological measurements and global climate models. The data comes from dedicated research plots north of Manaus, in central Brazil, where sensors installed in the trunks and in the soil have tracked the trees' response to climatic stress year after year.

During the El Niño-related droughts of 2015 and 2023, a critical threshold became apparent: when soil moisture falls to around a third of normal levels, tree transpiration rates drop rapidly. This is a defensive reaction, but a costly one. By closing their stomata to limit water loss, trees also reduce their absorption of carbon dioxide, essential for growth and tissue repair. If the heat persists, bubbles can form in the tree sap and interrupt water transport, a process that researchers liken to an embolism. At this stage, the risk of mortality increases sharply, either through water failure or lack of carbon.

The most fragile species and the role of secondary forests
Not all species react in the same way. The most vulnerable are those with rapid growth and low wood density, which show higher mortality rates than trees with denser wood. "This implies that secondary forests could be more vulnerable... because they harbour a higher proportion of these types of trees," explains Jeffrey Chambers; a crucial factor in a region where vast areas have regenerated after deforestation and fires.

Simulations based on climate models from phase 6 of the Coupled Model Intercomparison Project indicate that, under high-emissions scenarios, the Amazon could experience up to 150 days a year of 'intense drought' by the end of the century. These episodes could occur even during today's rainiest months. "When these intense droughts occur, it's this climate that we associate with a hypertropical forest," says Chambers, emphasizing that these conditions "go beyond what we consider a tropical forest today".

Annual tree mortality in the Amazon is currently just over 1%, but could rise to around 1.55% by 2100. This is a seemingly modest increase which, on the scale of a forest the size of the Amazon, translates into massive tree loss and a significant reduction in carbon absorption capacity. During some particularly dry years, the forest has already released more CO₂ than it has stored.

A laboratory for tomorrow's climate
One of the most striking aspects of the study is the consistency of the results: the same stress signals appear at different sites and at different times. Forest response to heat and drought seems to follow predictable patterns. This is why, the authors explain, current droughts provide a unique window of opportunity to observe the current extreme conditions that could become commonplace tomorrow.

"It all depends on what we do," concludes Jeffrey Chambers. Without a drastic reduction in greenhouse gas emissions,  hypertropical climate is likely to rapidly move from the status of a scientific hypothesis to that of a reality that the planet's largest rainforest will have to face.

(MP/©GreenMe.it, Source: Nature/Translation and adaptation: The Global Nature/Pic: Jaime Dantas via Unsplash )