Reforestation could be a powerful tool against climate change, according to a new study. Pictured, Shenandoah National Park in Virginia, October 2013.
CNN  — 

Restoring the world’s lost forests could remove two thirds of all the planet-warming carbon that is in the atmosphere because of human activity, according to a new study.

Since the industrial revolution, humans have added around 300 billion tons of extra carbon to the atmosphere – mainly through burning fossil fuels – which is heating the planet to dangerous levels. But trees naturally remove carbon from the atmosphere, storing it above and below ground.

A new study, carried out by researchers at Swiss university ETH Zurich and published Thursday in the journal Science, has calculated that restoring degraded forests all over the world could capture about 205 billion tons of carbon in total. Global carbon emissions are currently around 10 billion tons per year.

Degraded forests

The researchers identified ecosystems around the world that would naturally support some level of tree cover, but have become “degraded” – deforested for timber, for example, or turned into farmland that has since been abandoned. They excluded areas that are currently used as urban or agricultural land, or that would naturally be grasslands or wetlands, because these ecosystems can themselves be valuable carbon stores, as well as supporting biodiversity.

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They concluded that there’s enough suitable land to increase the world’s forests by about a third. That would give the planet more than a trillion extra trees and 900 million hectares of additional tree canopy, an area about the size of the United States.

A map fom the study, showing the potential for tree cover, excluding desert, agricultural and urban areas.

‘Most effective solution’

The researchers say their data shows global tree restoration to be the most effective way to tackle climate change.

Tom Crowther, the study’s senior author, told CNN, “This is way bigger than the next best solution, and this is by far the cheapest.

“The best restoration projects out there that we know of are restoring billions of trees at 30 cents a tree. Scaled up to the numbers we’re talking about it’s $300 billion.”

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The study found that most of the land suitable for restoring forests trees is in six countries – Russia (151 million hectares), USA (103 million hectares), Canada (78 million), Australia (58 million), Brazil (50 million), and China (40 million).

Russia has the greatest potential for forest restoration, according to the study.

But it also warned that the amount of land suitable for supporting new forests is shrinking because of climate change, and that the area available for forest restoration could be reduced by a fifth by 2050.

The researchers say that while most existing models predict that climate change will increase global tree cover, their own models found that although a warmer climate is likely to increase tree cover in northern areas, such as Siberia, that will be outweighed by climate change shrinking denser forests in tropical regions.