Around 75% of the world’s tropical forest areas experienced at least six months of unprecedented simultaneous drought and extreme heat during the 2023–2024 El Niño, raising concerns that such conditions could become increasingly common over the next two decades if greenhouse gas emissions are not reduced.
A study scheduled for publication in the journal Earth’s Future on October 7 warns that the extreme conditions experienced during the last El Niño could offer a preview of the future climate facing tropical forests. Researchers found that rising global temperatures could intensify droughts damage ecosystems and weaken the ability of forests to absorb carbon dioxide from the atmosphere.
The Amazon is among the regions most vulnerable to this emerging climate pattern. Under a high emissions scenario the likelihood of droughts as severe as those recorded during the past two years could increase ninefold in the Amazon by the end of the century even in years without an El Niño event.
The study highlights the risk that tropical forests could temporarily shift from absorbing carbon dioxide to releasing carbon into the atmosphere when exposed to severe heat and drought. This could further intensify global warming.
Researchers combined satellite observations, ground based measurements and projections from 24 climate models to examine past conditions from 2000 to 2024 and potential changes through 2100. They assessed four greenhouse gas emissions scenarios and compared extreme weather with the usual climate conditions in different regions.
The analysis found that high average temperatures could become at least six times more likely across tropical regions, even under a lower emissions scenario.
Corresponding author Johanna Van Passel, a scientist at Ghent University in Belgium and visiting researcher at the Institute of Biology at the State University of Campinas (IB-UNICAMP) in Brazil said the area affected by drought and heat during the 2023–2024 El Niño was among the largest and most extensive recorded in the previous 25 years.
Co-author Peter Groenendijk a biologist and professor at IB-UNICAMP said comparing the impacts of a single El Niño event with future climate projections helped researchers identify how extreme conditions could become more frequent.
The researchers also warned that continued global warming could make the effects of future El Niño events even more severe.
Rainfall patterns and forest health
The study found that approximately 80% of months experiencing extreme heat occurred during the rainy season. This could affect forest ecosystems differently from heat occurring during the dry season.
During rainy periods, warmer conditions and greater sunlight can sometimes increase photosynthesis because water remains available for transpiration and leaf cooling. However, when extreme heat coincides with drought, limited soil moisture can prevent trees from cooling themselves. Water transport through their internal tissues may fail, leading to canopy damage and tree mortality.
Prolonged exposure to simultaneous heat and drought can weaken trees and reduce the resilience of entire forest ecosystems. Researchers said further investigation is needed to understand the precise mechanisms driving these changes.
While temperatures are projected to rise across tropical regions, rainfall patterns are expected to vary geographically. The Amazon, Central America, southern Africa and parts of eastern Asia are projected to become drier, while other tropical regions may experience increased rainfall.
Under the two highest emissions scenarios the low rainfall levels recorded in some areas during 2023–2024 could become between twice as likely in northeastern Brazil and nine times as likely in the Amazon by 2100.
Amazon faces increasing drought and fire risks
The findings align with other research from Brazil’s National Institute for Space Research, which indicates that parts of the Amazon are already experiencing conditions previously projected for future decades.
These changes include longer dry seasons, extending from around four months to six months, and rainfall deficits exceeding 150 millimetres. During the 2023–2024 El Niño period, average burned areas increased by 9%, while forest degradation alerts rose by 19%, according to the studies cited in the report.
The researchers warn that the combination of drought, fire and forest degradation is intensifying, reducing the ecosystem’s ability to recover.
The consequences extend beyond forests. During the 2023–2024 El Niño, river levels in Brazil’s Amazonas state fell to record lows, isolating communities. More than 300 river dolphins were also reported dead as lake waters warmed.
A survey by MapBiomas found that 2,172 of the 5,673 human settlements in the Amazon region, equivalent to 38%, were highly exposed to drought between September and November 2024.
These impacts threaten local livelihoods, biodiversity, water availability and the global climate system.
Emissions cuts could limit future damage
El Niño is a natural climate phenomenon caused by abnormal warming of tropical Pacific Ocean waters. It influences atmospheric circulation, raises global temperatures and changes rainfall patterns in different parts of the world.
Although the 2023–2024 event was not classified as a “super El Niño”, researchers said its impacts were intensified by the combination of the natural climate phenomenon and human driven global warming. The event coincided with record breaking global temperatures, with 2024 reaching approximately 1.55°C above the pre-industrial average according to the World Meteorological Organization.
The study warns that future extreme events could become more damaging, but their severity will depend significantly on the trajectory of greenhouse gas emissions and continued reliance on fossil fuels.
Researchers emphasised that the difference between low and high emissions scenarios is substantial, meaning that rapid and sustained emissions reductions could help limit the risks.
The findings underline the importance of protecting tropical forests, reducing greenhouse gas emissions and strengthening climate resilience. Without decisive action, extreme heat and drought could increasingly undermine the ability of tropical forests to regulate the climate preserve biodiversity and absorb carbon dioxide.