(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.

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.

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."

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.

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But they add that their models of future tree cover are characterized by high uncertainty, and don't take into account potential loss of forest for pasture or cattle raising.

Christiana Figueres, former executive secretary of the UN Climate Convention described the study as "an authoritative assessment of how much land we can and should cover with trees without impinging on food production or living areas."

Simon Lewis, professor of Global Change Science at University College London, who was not involved with the research, agreed that we need to plant new forests to meet our climate change obligations, but said that "the estimate that 900 million hectares restoration can store an additional 205 billion tons of carbon is too high and not supported by either previous studies or climate models."

He added: "The idea that climate change risks losing areas of tropical forests before 2050 is also not consistent with the results from climate models or observational studies for today's forests.

"Today, the immediate major threat to tropical forests is deforestation by people and out of control fires, not the more subtle impacts of higher temperatures."

June 28, 2019 -- a forest fire in Catalonia, Spain, burned more than 6,500 hectares of land.

Efforts underway

Tree planting is no quick climate fix. It can take decades of growth for the carbon storage to reach its full potential. A more immediate benefit can come from halting deforestation, says Crowther, which costs our planet around 15 billion trees each year.

But although tree planting on such a colossal scale faces significant challenges (not least identifying who owns the land in question, and securing the rights to plant and maintain trees there), widespread efforts are already underway.

The Australian government has announced it will plant 1 billion trees by 2030; work is underway on a "Great Green Wall" to stop the spread of the Sahara by restoring 100 million hectares of degraded land (and sequester 250 million tons of carbon), and China's anti-desertification program, also known as the "Great Green Wall," has planted more than 50 billion trees since the 1970s. The UN-endorsed Bonn Challenge aims to reforest 350 million hectares of degraded land globally by 2030.

Africa's "Great Green Wall" aims to slow down desertification.

Myles Allen, professor of geosystem science at the University of Oxford, said the study had overestimated the reduction in atmospheric carbon, and added: "Restoration of trees may be 'among the most effective strategies,' but it is very far indeed from 'the best climate change solution available,' and a long way behind reducing fossil fuel emissions to net zero.

Crowther stressed that the potential of forest restoration mustn't be at the expense of reducing carbon emissions.

"Often when we do these papers people say this will disincentivize people cutting their emissions," he said. "Obviously, we must reduce emissions as much as we can, it's a huge priority, but even if we stop emissions now there are still 300 gigatons (billion tons) in the atmosphere that will keep warming the planet, and this restoration could cut vast amounts of that."