To mitigate the effects of climate change, we can do a simple thing: plant trees.

This is hardly a new idea, but a team of scientists at ETH Zurich has calculated just how much tree cover we would need to capture enough atmospheric CO2. The answer, they say, is 4.4 billion hectares of new forests.

“We mapped the global potential tree coverage to show that 4.4 billion hectares of canopy cover could exist under the current climate,” they elucidate in a study published in the journal Science. “Excluding existing trees and agricultural and urban areas, we found that there is room for an extra 0.9 billion hectares of canopy cover, which could store 205 gigatonnes of carbon in areas that would naturally support woodlands and forests.”

Current ecosystems could support an extra 0.9 billion hectares of continuous forest, which would increase the current rate of forested areas by 25%. A large-scale reforestation effort would include more than 500 billion trees and would lock up more than 200 gigatons of additional carbon once they trees have reached maturity. As a result, we could cut the atmospheric carbon pool by about 25%.

“We all knew restoring forests could play a part in tackling climate change, but we had no scientific understanding of what impact this could make,” says Prof. Tom Crowther, a British ecologist who was senior author of the study. “Our study shows clearly that forest restoration is the best climate change solution available today and it provides hard evidence to justify investment.”

A massive global forest restoration project would need to be undertaken soon. Troublingly, the opposite is happening, however. Forests, especially in tropical areas such as Borneo and the Amazon region, continue to be felled at a steady pace, which poses a grave threat to local biodiversity.

Between 2001 and 2015 as much as 3 million hectares of forest was lost permanently worldwide to agriculture-driven deforestation. Yet this amounted to just 27% of the total loss. Another 73% of the loss during that period was due to logging, wildfires and other causes.

Worse: ongoing climate change, which is expected to bring prolonged droughts and extended heatwaves, could shrink the area where forests could thrive. According to the researchers’ estimate, global canopy cover could shrink by another 223 million hectares by 2050, especially in the tropics.

That is why there’s urgent need for action. We should start planting trees ASAP on a large scale, the researchers say.

“If we act now, this could cut carbon dioxide in the atmosphere by up to 25%, to levels last seen almost a century ago,” Crowther says. “However, it will take decades for new forests to mature and achieve this potential. It is vitally important that we protect the forests that exist today, pursue other climate solutions, and continue to phase out fossil fuels from our economies in order to avoid dangerous climate change.”

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