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The top-line findings of a massive analysis of the growth rate of more than 20,000 tropical trees in more than 30 countries may at first seem reassuring: that droughts over the past century have had a minimal effect on their growth. Because tropical forests play a crucial role in naturally sequestering carbon dioxide (CO₂) from the atmosphere—combating global warming—the trees' resilience should be a relief.

However, , published on July 31 in the journal Science, also raises concerns that this resilience is highly likely to wane as the planet continues to warm and the frequency of droughts increases. As announced by the United Nations last year, the global effort to reach the "net zero" target for carbon emissions by 2050 will not succeed at the current rate of progress—resulting in more frequent and severe extreme weather events.

More immediately, the large group of co-authors behind the new paper estimates that each dry year may cause enough tree mortality to result in extra CO₂ emissions roughly equivalent to that of Germany in a year, according to their data when combined with separate long-term forest monitoring studies.

One of the paper's co-authors is Erika Zavaleta, professor of ecology and evolutionary biology at the University of California, Santa Cruz. Acclaimed for her groundbreaking research on biodiversity's importance to people and nature—and on effective responses to climate changes—Zavaleta said the bottom line is that we should still expect declines in tropical forest carbon uptake as global warming leads to hotter, drier conditions.

"We found that tropical trees are fairly resilient—one might say surprisingly so—to intermittent droughts," Zavaleta said. "But I want to be careful: This study did not measure how climate change is affecting loss of tropical forests to increasing wildfires, clearing, disease and more; and deforestation itself continues to threaten them."

In their study, the researchers used the largest collection of tropical tree-ring data to date: more than 20,000 samples from nearly 500 locations in 36 countries. They calculated that, on average across the tropics, trees grew 2.5% less during drought years compared to years with normal or above-average precipitation.

"I agree, that's not a huge change, and that is positive because it means that increasingly dry conditions might have less devastating effects on tropical forest carbon uptake than feared," said Zavaleta, who, along with her former Ph.D. student Paulo Quadri, contributed tree rings from high-mountain tropical species endemic to the Mexican volcanic arc.

Quadri personally hiked to numerous sites near the treeline, the alpine upper limit for tree growth, and collected the cores for two chapters of his dissertation research on how tropical treeline and subalpine tree distributions are responding to climate changes. This new study analyzed tree-ring data collected at 483 locations spread across the tropics: from wet and warm Amazonian forests, to dry forests in southern Africa and cooler Asian mountain forests.

First, the researchers determined the driest years since 1930 for all locations. They then calculated how much narrower the tree rings were during those years, compared to normal years. They also measured the width of tree rings in the two years following a drought and found that this growth dip largely disappeared in the year following the drought.

Locally stronger impacts

The effects of droughts differed significantly between the research areas, however. Trunk (stem) growth slowed down much more than the global average—by over 10%—at a quarter of the locations. This happened mainly in hot and dry regions, such as in northeastern Brazil and southern Africa, according to lead author Pieter Zuidema, a professor of forest ecology and forest management at Wageningen University & Research in the Netherlands.

He explained how the effects of droughts can be more extreme in hot, dry regions because trees shed their leaves quickly, and soil retains less moisture. The study found weaker effects in wetter areas such as Amazonian forests.

The researchers also warn that the resilience of tropical trees could weaken as droughts occur more frequently and intensify under aggravating climate change. They reported this happening at some research sites during the past decades. "Recent droughts have already caused stronger reductions in stem growth compared to earlier droughts," Zuidema said. "It gets harder for trees to recover."

When the researchers combined their data with information from other forest-monitoring studies, they estimated that each dry year may cause approximately 0.1% additional tree mortality—resulting in extra CO₂ emissions from rotting dead wood. Across the world's tropical areas, that seemingly small percentage roughly amounts to the equivalent of annual emissions for all of Germany. That's because, across the tropics, there are an estimated 800 billion trees, according to a 2024 study accounting for those more than 10 centimeters in diameter.

Taken together, lower carbon-absorption rates by tropical forests due to more frequent droughts slowing tree growth, along with extra emissions caused by higher tree mortality, the findings underscore the need for actions to curb climate change and prevent consequences that will accelerate it further.

"Many dry years in a row—or permanent drying, or much more severe drought episodes—all likely affect mortality more than we saw in the study," Zavaleta said. "Dying trees have effects on more than carbon: They affect wildfire and landslide risk, wildlife habitat, disease outbreaks, water quality and more. Those are all potential areas for further research and timely action."