El Niño is back with a vengeance. A mammoth patch of warm water is growing in the equatorial Pacific Ocean, and the US National Oceanic and Atmospheric Administration is giving four-in-five odds that the disruptive climate phenomenon will turn into a “very strong” event by the end of the year. Some forecasters say this El Niño is on track to become the strongest in at least 150 years.
For the Amazon rainforest, the timing could hardly be worse.
Trouble in the Amazon
El Niño events, which happen roughly once every two to seven years, upset weather patterns around the globe. For the Amazon basin, they typically trigger severe droughts. The most recent El Niño, which formed in June 2023 and had dissipated by June 2024, hit the rainforest especially hard by drying out large sections1, shrinking major rivers to historic lows2 and helping to fuel the worst fire season in more than two decades3 (see ‘Tree troubles’). Some researchers suggest that the developing El Niño, coming on top of warming and drying caused by climate change, might push the rainforest past its ‘tipping point’ — shorthand for the threshold at which a runaway cascade of drying, tree death and wildfire would begin to spread irreversibly across the Amazon basin. If that were to happen, rainforest would give way to savannah and degraded woodland over decades, potentially releasing an amount of carbon equivalent to several years of global emissions and dramatically reducing rainfall across South America and beyond4.
Source: Ref. 3
The impacts of the current El Niño might not be as severe as the gloomiest forecasts suggest; much depends on how strong it becomes, and how much of the forest burns. But the Amazon is entering this test in a weakened state, and some researchers who know it well are uneasy. “If this next El Niño really is as strong as we think it will be, it might result in things like huge wildfires in the Amazon, spreading across areas that were unimaginable before,” says Bernardo Flores, a specialist in Amazon resilience at the Juruá Institute in Manaus, Brazil. “We will see how much of the forest might die from it.”
Past trauma
Part of the concern, researchers say, is that large areas of the Amazon might still be recovering from the severe droughts of 2023–24. Hao Bai, a forest ecologist at Peking University in Beijing, and his colleagues determined this by analysing 33 years of radar satellite data that tracked two indicators of forest health — how much moisture the forest holds and how much living plant matter it contains — across the Amazon basin. During the 2023–24 droughts, the researchers found, radar measurements associated with both indicators fell to their lowest levels since satellite records began in 19925. The authors estimate that less than half of the affected forest will have returned to pre-drought conditions within seven years — the slowest recovery from a major Amazon drought in the satellite record.
To Bai and his colleagues, these findings underscore the Amazon’s increasing vulnerability to intense droughts spurred by El Niño and human-caused climate change. If major droughts continue to strike before the forest has recovered from previous ones, the repeated stress could weaken the forest, leaving it less able to withstand climate extremes.
When will the Amazon hit a tipping point?
The forest might also be nearer to its tipping point than previous estimates implied, according to a study6 led by Nico Wunderling, an Earth-system modeller at Goethe University in Frankfurt, Germany. The Amazon forest itself generates much of the rain that falls in the basin, so Wunderling and his colleagues — including Flores — examined how this might change by modelling the movement of moisture in the atmosphere and testing the impacts of warming and deforestation.
Without deforestation, their model indicates, the Amazon would be relatively stable in global warming up to 3.7–4 °C above pre-industrial levels. If deforestation clears between 22 and 28% of the rainforest, however, warming of just 1.5–1.9 °C could be enough to trigger a collapse that would eventually turn up to 80% of the forest into degraded woodland and savannah-like ecosystems. The world is fast approaching 1.5 °C of warming. The Amazon, depending on how it’s measured, is about 17% deforested, says Wunderling.
“We’re not too far away,” he adds.
Flores says the rainforest might be even closer to a climate threshold than the study suggests, because the authors didn’t fully account for some other harmful forces4 — chiefly wildfires, which add enormous stress to the forest and which El Niño events intensify.
Once a patch of the rainforest burns, Flores says, it remains flammable for decades. “And then if we continue to have these escalating El Niños — these El Niños that get stronger and stronger — then it’s very concerning,” he continues. “The area of flammable forest just increases and increases, and it can become exponential.” Depending on how much of the rainforest burns during the current El Niño, Flores says, “it could basically cross the tipping point”.
“We may have already crossed the tipping point,” he adds. “We don’t know. But the thing is, if we have crossed it, or we might cross it very soon, this is very concerning.”
Record low water levels were recorded in the Solimões River, one of the largest tributaries of the Amazon River, during a drought that followed the 2023–24 El Niño.Credit: Michael Dantas/AFP via Getty
This has long been a point of contention among scientists who study the rainforest: where exactly the Amazon’s tipping point lies, and how far away it is. A further complication, they say, is that it might not be obvious that it has passed until after the fact.
Some researchers, including Dolors Armenteras, a fire ecologist at the National University of Colombia in Bogotá, say that parts of the Amazon might have already passed their local tipping points, but that a basin-wide point of no return has yet to be breached. “I think it’s a reasonable possibility for parts of the Amazon,” she says, “not for the whole basin.”
Carlos Nobre, an Earth-system scientist at the University of São Paulo in Brazil who, in the 1990s, was among the first to propose that the Amazon might face a crucial climate threshold, echoes Armenteras. “The southern Amazon is close to a tipping point,” he says, but “not the northern Amazon or the northwestern Amazon”.
Climate tipping points — too risky to bet against
Luciana Gatti, an atmospheric scientist at Brazil’s National Institute for Space Research (INPE) in São José dos Campos who has been measuring the Amazon’s carbon balance from small aircraft for more than two decades, has watched the warning signs accumulate. The southeastern Amazon, her research shows, has already flipped from a carbon sink to a carbon source, releasing more carbon than it is able to store7. Wildfires ignited during the 2023–24 El Niño “produced the strongest emissions since 2010 in the Amazon”, she says, adding that she is “really afraid about the consequences” of a potentially stronger El Niño arriving so soon.
