The best way to keep climate change in check is by replanting trees on destroyed forest areas the size of the United States, scientists said on Thursday, as doing so would capture two-thirds of man-made planet-warming emissions.
Tom Crowther, a professor at the Crowther Lab, a research group based at ETH Zurich, urged hundreds of thousands of people who skipped school and work this year in a growing movement for climate action to start planting trees or funding such efforts.
"Every other climate change solution requires that we all change our behaviour, or we need some top-down decision from a politician who may or may not believe in climate change, or it's a scientific discovery we don't yet have," he told the Thomson Reuters Foundation.
"This one is not only our most powerful solution - it's one that every single one of us can get involved with," he said.
While planting trees to suck up a quarter of carbon in the atmosphere, including human and natural sources, is an ambitious goal, Crowther said it could be achieved if all members of the public concerned about global warming got behind the drive.
Crowther Lab scientists on Thursday published what they said was the first study looking at how many trees the world can support, where they could be grown, and how much carbon they could store.
The best places to reforest are in the tropics because of the fast rate trees grow there, but replanting land can be done in most countries and even at home, Crowther added.
The Crowther Lab website has a global map with advice on which types of trees and how many can be grown in gardens.
The tropics lost 12 million hectares (29 million acres) of tree cover in 2018 - much due to fires, land-clearing for farming and mining - the fourth-highest annual loss since records began in 2001, according to monitoring service Global Forest Watch.
Environmentalists say looking after existing forests and restoring damaged ones prevents flooding, stores carbon, limits climate change and protects biodiversity.
Worldwide, there are about 5.5 billion hectares of forest.
The study, published in the journal Science, analysed the maximum amount of carbon that could be captured if all available degraded forest areas were replanted and allowed to mature.
Forests could be regrown on 1.7 billion-1.8 billion hectares of denuded areas that are no longer in use, adding 1.4 billion hectares if cropland and urban areas were included, it said.
More than half the potential to restore trees is found in Russia, the United States, Canada, Australia, Brazil and China.
Many countries have already made reforestation commitments under the Bonn Challenge, agreed by nations in Germany in 2011.
That effort calls for 350 million hectares of degraded land worldwide to be restored by 2030.
Crowther said about 40 per cent of countries worldwide had proposed to restore an area "vastly under" what was possible, while 10 per cent were aiming to add more trees than they could support.
The new data would help them refine their targets, he said.