Half the world’s forests are gone, but that’s only half the problem

About an acre of forest is lost every second, hobbling one of the main checks on global warming. But it’s not just the impact of fewer trees to worry about.

After trees are cut down, they gradually decay, releasing carbon, degrading the habitat, and threatening species long after the cutting stops. These lagging emissions have an important impact on the battle against global warming, a study released today in the journal Current Biology finds.

Today there are 8.6 billion tons of annual emissions released into the atmosphere as already fallen trees decompose, the equivalent of five to 10 years of global deforestation. That’s roughly the annual amount of total global emissions.

Those lagging emissions are lethal. Researchers found that 144 vertebrate species became extinct due to tropical deforestation from 1950 to 2009, about 20 per cent more than a previous estimate of extinctions in forest-specific vertebrate groups since 1900. As with trees, species losses occur gradually as habitats change, the researchers found.

“No one has ever accounted for this time lag between habitat destruction and the species getting extinct,” said Isabel Rosa, one of the authors of the study and a researcher at the Imperial College of London. “That’s what we aimed for with this study, to understand not only how many species have we lost already as a result of habitat destruction, but also how many more have we committed to extinction due to those fast changes in forest cover.”

The effects could be even greater as the spaces where trees die are converted into farmland in many areas, particularly the Amazon, one of three main tropical rain forest areas, along with the Congo Basin and Southeast Asia. As trees, which keep temperatures cooler, die, tropical areas will get even warmer and come to resemble a different climate, said Jeff Horowitz, the founder of the nonprofit Avoided Deforestation Partners.

The clearing of tropical forests in Indonesia is one of the most pressing global warming issues, as its palm oil and paper plantations are known for disrupting the local ecosystem. Horowitz also cited the global demand for beef, which requires the clearing of trees for pasture.

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