Tropical Forests Face 2024’s Record Heat and Drought as the New Normal by 2043

The record-shattering heat and drought that gripped the world’s tropical forests during the 2023–24 El Niño event may have been more than an extraordinary anomaly. According to a new study published in Earth’s Future, the journal of the American Geophysical Union, the punishing conditions that affected three quarters of all tropical forests could become the annual norm across most tropical regions within roughly two decades, even under moderate emissions scenarios and even in years when no El Niño is present. The finding transforms what many scientists regarded as a once-in-a-generation climatic shock into a sobering preview of the everyday climate that tropical trees, and the people who depend on them, will have to endure.

El Niño events arise when unusually warm water accumulates in the eastern Pacific Ocean, disrupting weather patterns across the equatorial region. The phenomenon typically brings heavy rainfall to some places, such as Chile and Argentina, while delivering hotter and drier weather to many others, including northern South America and much of southeast Asia. The El Niño that stretched from June 2023 through roughly the same time in 2024, however, unfolded against the backdrop of the warmest global year on record, amplifying its hot and dry signature to unprecedented levels across the tropics.

To quantify the full extent and duration of the event, a team led by Johanna Van Passel, an ecologist at Ghent University, analyzed twelve datasets of heat and drought observations spanning the past 25 years. Their analysis covered nearly 16 million square kilometers, about six million square miles, of tropical forest around the globe, searching for months-long periods roughly half a degree Celsius hotter than a normal month. The scale of the effort allowed the researchers to capture not just peak intensity but the punishing persistence of the anomaly, a factor that is often more decisive for forest survival than any single temperature reading.

The results were stark. Tropical forests were not merely hotter during the 2023–24 El Niño; they remained unusually hot for roughly 18 months on average. Eighty percent of forests were drier than usual, receiving between 22 and 56 fewer millimeters of monthly rainfall than normal for multiple months, and parts of the Amazon endured a full two years of anomalous drought. At the port of Manaus in Brazil, river levels sank to their lowest point since records began more than a century ago. Globally, three quarters of tropical forests experienced extreme heat and drought simultaneously for an average of six months, a combination that stresses trees in particularly damaging ways.

The team then turned to climate projections from 24 Earth system models under a range of emissions scenarios, estimating the likelihood of such conditions occurring in tropical forests in any given year through the end of the century, including years without El Niño influences. Even if humanity achieves carbon neutrality by 2075, the high annual temperatures of 2023 and 2024 will become at least six times more likely in all tropical regions by 2100 compared with the past 25 years. Under any less optimistic scenario, those temperatures become the annual norm in most tropical regions by 2043. Drought projections varied more widely by region: Asia will likely see less frequent drought, but under a high-emissions scenario the low rainfall of 2023–24 could become up to nine times as likely in any given year in the Amazon.

The physiological consequences for trees are severe and, crucially, compounding. When drought strikes, trees close the microscopic pores on their leaves, called stomata, to conserve water. But closing stomata also prevents them from absorbing the carbon dioxide they need for photosynthesis. If droughts become too frequent, Van Passel explained, a tree could effectively be starving, and in the process failing to sequester the planet-warming gas that makes tropical forests such a vital component of the global carbon cycle. Heat presents the opposite dilemma. In intense heat, the internal mechanisms that allow photosynthesis begin to fail, and trees respond by opening their stomata to release water that evaporates and cools the leaf, a process akin to human sweating. When drought coincides with heat, trees have little water to spare for this evaporative cooling.

It is, as Van Passel put it, either dying of lack of water or dying of heat; trees have to choose one. The dilemma extends beyond individual trees to the climate system itself. Water released through evapotranspiration eventually falls back on the forest as rain, accounting for as much as 30 percent of total rainfall in the case of the Amazon. When evapotranspiration wanes in one region, it deprives downwind forests of rainfall. Trees in those areas then reduce their own evapotranspiration to conserve water, and the cycle repeats. Scientists warn that under increasingly hot and dry conditions, this feedback loop could push tropical forests toward widespread tree die-offs, with each suffering forest seeding drought elsewhere. If the forest is suffering, Van Passel noted, it will cause less rain to fall in other parts, and those forests will also start to suffer.

The cascading impacts reach well beyond the canopy. Communities living in tropical forests derive much of their food and livelihood from the plants and animals around them, and intense drought can render the rivers that serve as transportation arteries unnavigable. During 2023 and 2024, some rivers fell so low that residents were stranded and completely disconnected from everyone else, Van Passel said, describing communities that were almost starving because no food was able to reach them. The study’s authors emphasize that the fate of tropical forests and the fate of forest peoples are inseparable: when the forests suffer, so do the people whose lives are interwoven with them.

Despite the grim projections, the researchers stress that the future remains contingent on choices made now. Every step taken to decrease global fossil fuel use improves the probabilities, Van Passel said, acknowledging that the future will be difficult but insisting that it is not hopeless, and that action can still make it less bad. The study, titled The 2023–2024 El Niño as a Preview of Future Tropical Forest Climate, was authored by researchers at Ghent University’s Q-ForestLab, the Laboratoire des sciences du climat et de l’environnement in France, the University of Campinas in Brazil, and Wageningen University in the Netherlands, with the authors declaring no conflicts of interest. Its central message is difficult to ignore: the extremes of 2024 were not a warning of what might come, but a rehearsal for what, on current trajectories, is scheduled to arrive within the lifetime of today’s forest communities.

Subject of Research: Projected frequency of 2023–24 El Niño-level heat and drought extremes in tropical forests under climate change

Article Title: For tropical forests, record 2024 heat and drought could become annual norm within 20 years

Article References: For tropical forests, record 2024 heat and drought could become annual norm within 20 years. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: tropical forests, El Niño, drought, heat extremes, Amazon, climate projections, evapotranspiration, carbon sequestration, Earth's Future, Ghent University, tree mortality, forest communities