On this new global map, huge swaths of land are dotted in green pixels. These are the areas that could potentially be recovered with forests that have disappeared, according to a new study—and in total, could help capture as much as two-thirds of the carbon that humans have pumped into the atmosphere since the industrial revolution.
Planting trees is far from the only thing that needs to happen to fight climate change. But the study makes clear exactly how much it can help. “Our research shows that it should really be considered to be a top solution that must be prioritized,” says Tom Crowther, a professor of global ecosystem ecology at ETH Zurich and one of the authors of the paper.
A UN climate report last year suggested that the world needs another billion hectares of carbon-sucking forests to have a chance of hitting the critical goal of staying under 1.5 degrees Celsius of global temperature rise. But other researchers had only rough estimates of how much space exists to plant new trees. The new study looked at thousands of photos of forests in protected areas to build a predictive model of how trees naturally grow in various climates, and then used Google Earth Engine to map out the rest of the world. Then they took out areas that are already used for farming or cities to see what was left.
“This study looks at how much it could actually contribute, and where could it contribute,” says Fred Stolle, deputy director of forests at the nonprofit World Resources Institute, who was not involved with the study. “That detail is important. It’s nice if you’re saying that you want to restore a billion hectares and you want to have a gigaton of CO2 sequestered, but this goes further on where that could happen and how much could happen.”
Another key question is how it could happen. The study identified 1.7 billion hectares of land that could feasibly return to forest (0.9 billion hectares of this would be covered in trees), an area greater than the total area of the U.S. and China combined. It’s a project of an almost unimaginable scale, and also costly.
“Restoring at the scale of the planetary need is going to be incredibly expensive,” says Niko Alexandre, who oversees the global restoration strategy at the nonprofit Conservation International. “We’re talking hundreds of billions of dollars per year to really leverage the full potential of restoration to mitigate against climate change, to secure habitat necessary for key species to kind of remain resilient under climate change, and to help communities that are living in rural areas to continue benefiting from ecosystem services that are not being generated by those natural areas. That’s hugely expensive.”
Which is not to say that it will be less hugely expensive than climate disaster, only that it will take effort to figure out how to pay for it. To be strategic, Alexandre says that governments and conservation groups can now go a step further and look at where forests have the potential to naturally regenerate without requiring any tree planting. In areas next to existing forests, for example, seeds can still naturally spread; local indigenous communities could also take extra steps to protect young trees from fires or grazing cows, but might not necessarily need to actively plant and maintain trees. “What we’re trying to do is think about how do you tap into nature’s potential to restore on its own before you start planting trees?” he says. In Brazil, organizations have calculated that almost a third of the Brazilian Atlantic Forest—where 70 million hectares of land have been degraded—could regenerate naturally.
In areas where more funding is needed, “what we’re trying to do is create a portfolio of landscapes that would be ‘bankable’ or investable, that have a variety of restoration businesses at their core that generates some sort of returns,” he says. That might mean, for example, harvesting fruits or other products from trees in the forest, or finding ways to value “ecosystem services” that forests provide, such as clean water. Other funding can come from donations and the purchase of carbon offsets: Companies increasingly want to support planting trees.
Of course, preventing deforestation is equally important. An area of tropical forest larger than Maryland disappeared in 2018 alone. “It is at least as important that we preserve what we have,” says Crowther. “Gaining a hectare of forest is useless if we lose another hectare at the same time. We need conservation and restoration.” Companies like Apple have helped fund the protection of massive forests. Others have pledged to end deforestation in their supply chains, though they’re struggling to meet those goals.
Many countries already have ambitious goals to plant trees. In Africa, a group of countries have pledged to restore 100 million hectares of forest. In Latin America, countries plan to restore another 20 million hectares. China deployed thousands of soldiers last year to plant trees covering an area roughly the size of Ireland (though that project planted single species, resulting in something that’s more like a tree farm than a real forest). But the new study found that 43% of the countries in the Bonn Challenge, a global effort to restore 350 million hectares of land by 2030, have only committed to restoring half of the land that could be restored.
The new maps will help countries better plan where to plant trees, and what kinds of trees to plant. The interactive version of the data includes suggested species for some areas. As new trees are planted, the next step is equally difficult—helping the seedlings survive and giving communities nearby a reason to keep trees standing rather than eventually chopping them down. Even a startup that is planting trees by drone recognizes that it needs to work closely with local communities for the projects to succeed. “It’s not about planting, it’s about a restoration roadmap where planting is only a little part of it,” says Stolle.