Extreme heat and drought may foreshadow conditions that tropical forests will face more often, but the evidence does not show that every tropical forest has already entered a single “new normal.” A 2025 Nature study combines more than 30 years of tree-demography records with field measurements from central Amazon sites during the 2015–16 and 2023 El Niño droughts, then compares those observations with climate-model projections.
What the Amazon study observed
A long record and two droughts
The study brings together more than 30 years of annually resolved forest-demography data and measurements of how trees functioned during the 2015 and 2023 El Niño droughts. That combination lets the authors examine both how trees responded during hot droughts and how mortality patterns varied over a much longer period. The field observations come from central Amazon forest sites; they are not measurements from every tropical forest.
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The drought periods should not be conflated. The IPCC dates the 2015–16 Amazon drought from September 2015 to May 2016. The study also analyzes field measurements from the 2023 El Niño drought.
2023 was not only an El Niño story
The Science Panel for the Amazon describes the 2023 drought as occurring with El Niño and a warmer-than-usual Tropical North Atlantic. Its 2023 statement also places the event in a regional history that includes severe Amazon droughts in 2005, 2010 and 2015–16. The panel says the 2023 drought could represent a new normal if climate change is not stopped; that is regional context, distinct from the Nature study’s site measurements and projections.
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What happens to trees during a hot drought
Water stress can intensify as rainless days continue
At the study site, tree transpiration—the movement of water through a tree and its release from leaves—fell rapidly after soil moisture passed an identified threshold. As rainless days continued beyond that point, drought stress intensified. The paper’s abstract describes rising potential risks of hydraulic failure, when a tree can no longer move water effectively, and carbon starvation, when stress limits the carbon resources it needs to maintain vital functions.
The study’s abstract does not establish one soil-moisture threshold that applies to all tropical forests. Soil, species and local conditions differ, so the site finding should not be turned into a universal cutoff.
Tree responses differ
Mortality rose during intense droughts, with particularly high risk among fast-growing pioneer trees with low wood density. This points to uneven impacts: a forest is not a uniform collection of trees, and the same drought can affect species differently.
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The Nature authors call present-day hot droughts “harbingers” of an emerging climate: observed extremes that offer a window into conditions tropical forests may encounter in the future. Their broader claim rests on climate-model projections, not on a finding that all tropical forests have already shifted into a stable new state.
| Evidence | What it supports | What it does not establish |
|---|---|---|
| Central Amazon forest records and field measurements, Nature study (2025) | How tree demography and tree function responded at the studied sites, including during the 2015 and 2023 droughts. | That the same response or soil-moisture threshold applies everywhere in the tropics. |
| CMIP6 climate projections analyzed by the Nature study (2025) | Under high-emissions scenarios, a large area of tropical forest could move toward hotter “hypertropical” conditions by 2100. | That this projected climate is already the present-day condition of all tropical forests, or that the outcome is certain. |
| Science Panel for the Amazon statement (2023) | Regional context on severe Amazon droughts and the combination of El Niño with a warmer Tropical North Atlantic in 2023. | A substitute for the Nature study’s site observations or its climate-model analysis. |
“New normal” also has a technical use in climate-extremes research: a 2017 paper proposed a framework for identifying when an extreme becomes common in model projections and applied it to global temperatures in 2015. That global-temperature analysis is not evidence that Amazon drought conditions have already become normal.
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How to read the warning
- It is a warning based on converging evidence. Long-term tree records, drought-time field measurements and model projections address different parts of the question; none alone demonstrates a uniform future for every tropical forest.
- Heat and water availability matter together. The field results connect declining soil moisture and continued rainless days with reduced transpiration and greater physiological stress, rather than treating rainfall totals as the only measure of risk.
- The future depends on emissions. The projected expansion of hypertropical conditions is described under high-emissions scenarios, so it is a conditional projection, not a forecast that ignores future choices.
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