(Bloomberg) -- Humanity has pushed atmospheric carbon dioxide levels almost 50% higher than they were before industrialization. That dramatic number would be even higher without tropical forests, which have been absorbing as much as 17% of CO₂ emissions along the way.

Unfortunately, rainforests can’t capture carbon like they used to.

In a new study using 30 years of data from pristine Amazon and African tropical forests, researchers found the actual rate CO₂-reduction rate peaked a quarter-century ago. These rainforests absorbed about a third less CO₂ over the past decade than they did the 1990s, according to the study published in the journal Nature. That’s a difference of 21 billion metric tons—or roughly similar to a decade of fossil-fuel emissions from the U.K., Canada, Germany, and France combined.

South American forests began their decline more quickly than their African counterparts, which showed slowing only around 2010. The Amazon, which is world’s largest tropical forest, may turn into a source of emissions by 2035 if it continues to lose the ability to store new carbon at its current rate. “We’ve always taken it for granted,” says Wannes Hubau, a researcher at the Royal Museum for Central Africa in Belgium, “but now we see that it may not last.”

Hubau and Simon Lewis, professor in the School of Geography at the University of Leeds, led the study with more than 100 co-authors. The dataset is made up of hundreds of thousands of standardized measurements taken on 565 remote forest plots in as many as 20 African and South American nations. 

“These are the largest remaining tracts of intact tropical forests on the planet,” says Johanne Pelletier, a fellow at Cornell University who wasn’t involved with the study but is familiar with its findings. Without the forests’ help, “the CO₂ that we produce will accumulate in the atmosphere faster. The impacts of climate change will happen faster. It will also make it harder and more costly to stabilize the climate system.”

The decline of tropical forests as long-terms vaults for carbon dioxide is a momentous change in how Earth systems cope with humanity’s penchant for turning carbon-rich minerals into atmospheric gas. The researchers note that their grim findings may also influence global climate policy debates. Scientific models had been projecting that forests would continue to consume our CO₂ emissions for decades to come. If those projections turn out to be wrong, the study warns, nations have much less time “to meet any given commitment to limit the global heating of the Earth.”

Deforestation and wildfires are widely understood to destroy tropical biomes—but those can be limited or at least managed somewhat by government conservation and forest policies. The authors of the Nature paper demonstrate that trees are dying at higher rates regardless of human-caused damage. Rising CO₂, volatile temperatures, and more prevalent drought may be affecting the internal dynamics of the tropical forests themselves, Hubau says, and those are very difficult for scientists to model. 

The fact remains, however, that “the greatest direct threat to intact tropical forests is deforestation and forest degradation, which remains unabated,” says Cornell’s Pelletier.

Forests outside the tropics have stepped up their appetite for CO₂ even as tropical forests have passed their peak. Northern, or boreal, forests surpassed tropical areas in the amount of CO₂ absorbed over the past two decades. All forests and land vegetation together absorb about 30% of human CO₂ emissions. Oceans draw down about another 25% of emissions, leaving just 45% of emissions up in the air.

The study is “not good news in terms of the carbon cycle and CO₂ sequestration by tropical forests in Africa and Amazonia,” says Thomas Lovejoy, senior fellow in biodiversity and environmental science at the UN Foundation, who wasn’t involved in the research. “The biggest take-home is that for a properly functioning planet, atmospheric CO₂ levels need reduction.”

To contact the author of this story: Eric Roston in New York at eroston@bloomberg.net

To contact the editor responsible for this story: Jillian Goodman at jgoodman74@bloomberg.net, Aaron Rutkoff

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