WASHINGTON — The 2023-24 El Niño brought simultaneous extreme heat and drought to 75% of tropical forests for months on end. Now, a new study indicates that those conditions may have been a preview of annual norms for future tropical forests just a couple of decades away, even under moderate-emissions scenarios, and even outside of El Niño years.
Marked by a buildup of unusually warm water in the eastern Pacific, El Niño events disrupt normal weather patterns across the broader equatorial region, bringing heavy rain to some places (like Chile and Argentina) and hotter, drier weather to many others (like northern South America and much of southeast Asia). The El Niño lasting from June of 2023 through roughly the same time in 2024, however, took place against the background of the warmest global year on record, amplifying hot and dry conditions.
As the climate continues to warm, the stress of ever-more-frequent heat and drought could kill trees, limit forests' carbon-storing abilities and trigger self-perpetuating rainfall losses. It could also cut off trade routes and food sources for people in tropical forests.
"Many of the people that live in these forests are very interconnected with the forest," said Johanna Van Passel, an ecologist at Ghent University and lead author of the study. "When the forests suffer, so do they."
The study will appear in Earth's Future, AGU's journal for research on the state of the planet and its inhabitants and their future resilience in the Anthropocene.
Within our lifetimes, extremes could become common
To get a better handle on the full extent and duration of the 2023-24 El Niño, Van Passel and her team analyzed 12 datasets of heat and drought observations over the past 25 years. Their study covered nearly 16 million square kilometers (about 6 million square miles) of tropical forest around the globe, looking for months-long periods roughly half a degree Celsius hotter than in a normal month.
All tropical forests were not just hotter during the 2023-24 El Niño, they found, but stayed unusually hot for roughly 18 months, on average. 80% of forests were drier than usual (getting 22 to 56 fewer millimeters of monthly rain than normal, for multiple months), with parts of the Amazon experiencing a full two years of anomalous drought. At the port of Manaus in Brazil, water levels sank to their lowest level since records began over a century ago. Globally, three quarters of tropical forests endured extreme heat and drought simultaneously for an average of six months.
Considering climate projections from 24 Earth system models under a range of emissions scenarios, the team then estimated the likelihood of these conditions occurring in tropical forests in any given year through the end of the century, even non-El-Niño years.
Even if humanity reaches carbon neutrality by 2075, they found, the high annual temperatures of 2023 and 2024 will become at least six times more likely in all tropical regions by 2100, compared to the past 25 years. In any less optimistic climate scenario, they become the annual norm in most tropical regions by 2043 — El Niño or no El Niño.
"I will probably not live to see the end of the century, so that feels very far away," Van Passel said. "But the fact that…according to the average of all these models, this is already happening in 20 years — that was very striking."
Projections for drought varied more widely. Asia will likely see less frequent drought, but for the Amazon, under a high-emissions scenario, the low rainfall of 2023-24 could become up to nine times as likely to occur in any given year.
That matters because drought-stressed trees close pores on their leaves (called stomata), avoiding water loss but also preventing themselves from absorbing the carbon dioxide they need to survive. If droughts become too frequent, Van Passel said, "your tree could be starving" — and, in the process, failing to sequester the planet-warming gas.
From one forest's suffering, a cascade of impacts
Other problems arise when heat and drought overlap. In intense heat, the internal mechanisms that allow trees to photosynthesize start to fail. To cool off, trees open their stomata to release water which then evaporates, a process akin to human sweating. But if a drought is happening at the same time, trees have little water to spare for this "sweating" function, better known as evapotranspiration.
"It's either dying of lack of water or dying of heat," Van Passel said. "They have to choose one."
Water released to the air through evapotranspiration eventually falls back on the forest as rain — as much as 30% of the total rainfall, in the case of the Amazon. So when evapotranspiration wanes in one region, it deprives others of rainfall. Trees in those areas then reduce their own evapotranspiration to save water, and the cycle continues. Under increasingly hot, dry conditions, scientists say, this cycle could push tropical forests towards widespread tree die-offs.
"You have this feedback loop," Van Passel said. "If the forest is suffering, it will cause less rain to fall in other parts, and those forests will also start suffering."
The impacts cascade to communities living in tropical forests, too, whom Van Passel said derive much of their food and livelihood from plants and animals in the forest. Intense drought can also make it difficult to navigate the rivers that often serve as transportation arteries.
"During 2023-24, some rivers were so low that these people were stuck and completely disconnected from everyone else," Van Passel said. "They were almost starving because no food was able to reach them."
She stressed the importance of taking action to mitigate suffering for tropical forests and the people who depend on them.
"With every step we can…decrease global fossil fuel use, the probabilities become better," Van Passel said. "The future will be difficult…But it's not hopeless, and we can do something to make it less bad."
