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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.

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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.”