Every year, fossil fuel combustion spews tens of billions of tons of planet-warming carbon dioxide into the atmosphere. And for decades, Earth’s forests, along with oceans and soils, have sucked back about a third of that, creating a vacuum known as the land carbon sink. But as deforestation and wildfires ravage the world’s forests, scientists are beginning to worry that this crucial balancing act could be threatened.

A study published Wednesday in the journal Nature found that despite the turmoil, the world’s forests have been absorbing a steady amount of carbon over the past three decades.

“It looks stable, but it may be masking the problem,” said Yude Pan, a senior research scientist with the U.S. Forest Service and lead author of the study, which included 16 other authors from around the world.

With the world’s forests undergoing dramatic changes, with some releasing more carbon than they absorb, Pan warns that better forest management is needed. “I really hope this study makes people realize how much carbon is lost through deforestation,” Pan said. “We need to protect this carbon sink.”

About 10 million hectares of forest—an area the size of Portugal—are burned to the ground each year, and increasingly intense wildfires are nearly doubling that damage. The planet has lost so many trees that experts have warned that forests could soon reach a tipping point, where this crucial carbon sink starts to emit more planet-warming gases than it absorbs. Some studies have suggested that the Amazon rainforest, often called the lungs of the world, is already there.

Using data going back to 1990, the researchers analyzed manual measurements of tree species, size and mass from 95 percent of the world’s forests to calculate the amount of carbon stored over three decades. For each biome studied — temperate, boreal and tropical forests — the researchers considered how long-term changes to the landscape altered the region’s carbon-sucking power.

In the boreal forest, the world’s largest land biome that stretches across the top of the Northern Hemisphere, researchers found a dire situation. Over the study period, these cold-loving tree species lost 36 percent of their carbon storage capacity as logging, wildfires, pests and drought ravaged the land.

Some regions fare worse than others: In Canada, wildfires have turned boreal forests into a source of carbon emissions. In the forests of Asian Russia, similar conditions caused the region to lose 42 percent of its sinking power.

It’s the clear result of decades of worsening fires. A study published in June in Nature looked at 21 years of satellite data and was the first to confirm that the frequency and size of extreme wildfires has more than doubled worldwide. The change is particularly dramatic in boreal forests, where they are occurring more than 600 percent more often each year.

“I was just shocked by the scale of it,” said Calum Cunningham, a postdoctoral researcher at the University of Tasmania and lead author of the bushfire study.

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Below the equator, where tropical forests make up more than half the world’s tree cover, the global carbon sequestration study found a complicated, three-part equation. Agricultural deforestation has led to a 31 percent loss of the carbon-storing power of old-growth forests. But new plant life has reclaimed vast swaths of abandoned farmland, and the carbon-storing power of these younger forests has offset the losses from logging. While continued deforestation still produces more emissions, the study found that when these gains and losses are added together, tropical forests are nearly carbon neutral.

So how has the world managed to keep the overall balance? The answer lies in temperate forests, where the carbon sink has increased by 30 percent. The study found that decades of reforestation efforts, largely through nationwide programs in China, are finally paying off. But the trend may not last. In China, urbanization and logging have begun to erode tree cover. In the United States and Europe, wildfires, droughts and pests have reduced the carbon sink of temperate forests by 10 and 12 percent, respectively.

Forest management efforts, along with emissions levels, will determine how this all plays out. A paper in Nature last year found “striking uncertainty” in the continued potential for carbon storage in U.S. forests, highlighting the need for protection and restoration efforts.

Chao Wu, a postdoctoral researcher at the University of Utah who led the 2023 study, said reducing emissions should be the biggest priority for solving the climate crisis. “But the other big piece is nature-based climate solutions, and the forest is going to be a really important part of that,” Wu said.

Richard Houghton, a senior scientist at the nonprofit Woodwell Climate Research Center who contributed to the latest carbon storage study, says it’s “in some ways fortunate” that the global forest carbon sink has remained stable.

To keep it that way, Houghton and Pan said more restoration and less logging are needed in all biomes, but especially in tropical forests, where 95 percent of deforestation occurs. “We don’t have enough conservation,” Houghton said, adding that protecting forests has improved biodiversity and ecosystem health. “There’s always more reason to do better.”