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In best-case reforestation scenario, trees could remove most of the carbon humans have added to the atmosphere

A study finds that close to a trillion trees could potentially be planted on Earth—enough to sequester more than 200 billion tons of carbon. But environmental change on this scale is no easy task.

ByKatherine J. WuNOVA NextNOVA Next

Adding about a trillion trees back to natural forestland could help remove most of the carbon human activity has put into the atmosphere. Waking up to that reality tomorrow would be like rewinding the climate clock by nearly a century. Image Credit: Stewart Watson, iStock

If a tree falls in the forest, but someone sticks around to replant it, does it still make an impact in the fight against climate change?

The answer, it seems, is yes. And, according to new research published today in the journal Science, that’s exactly the tack we humans should take. The study, which presents a global map of degraded lands that could naturally support new trees, suggests that the best case scenario for forest restoration would remove more than 200 billion tons of carbon from the atmosphere—enough to single-handedly offset two decades worth of global emissions produced at the current rate.

That kind of change doesn’t come easy, or fast. The researchers’ proposal, which calls for the seeding of approximately 1 trillion trees, is ambitious to say the least. And when it comes to rejiggering the ecological landscape on such a grand scale, there’s a whole range of social, political, and economic considerations to take into account.

“This is a very rigorous and timely approach...that highlights where [forest] restoration can and should be happening,” says Robin Chazdon, a forest ecologist at the University of Connecticut who authored a commentary on the study. But, she adds, while “it’s good and important to know the potential...we have to [do the work to] make the potential the reality.”

If enacted, though, global attempts at reforestation might constitute one of the most powerful strategies available to combat the climate crisis, the researchers say. Real-world obstacles aside, fully implementing this initiative could dwarf the projected impacts of other mitigation methods, like shifting a good chunk of our energy needs to wind power, which, in plausible scenarios, would only account for about 20 billion tons of atmospheric carbon.

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The new study builds on a long line of research championing the transformative powers of forest restoration, which capitalizes on one of the biggest perks of greenspace: Due to photosynthesis, plants are living sponges for carbon, which they can guzzle from the air and store over the long term. But this paper is the first to put hard numbers to the world’s arboreal carrying capacity, the number of trees the Earth can sustainably support.

Using a global dataset of ground-based measurements and satellite images, a team of researchers led by Jean-Francois Bastin and Thomas Crowther of ETH-Zürich sussed out the environmental conditions most conducive to tree growth in protected forest areas around the world. They then used these criteria, which factored in things like soil quality and climate, to scour the rest of the planet for regions where saplings might have a shot at taking root.

The numbers were staggering. While more than 21 billion acres around the world—roughly two-thirds of Earth’s land—could naturally harbor forest (as opposed to other kinds of natural ecosystems, like grasslands), less than 65 percent of that space actually does. By the study’s calculations, that leaves almost 8 billion acres for possible planting. At roughly 160 trees per acre, that’s about 1.3 trillion trees that could exist, but don’t.


Top: A map showing the world's potential for tree cover. Bottom: The same map with already-existing trees removed, showing what's available for restoration. The number of trees that could be added number over 1 trillion; packed together, they could blanket a region the size of the United States. Image Credit: Courtesy of Jean-Francois Bastin et al., Science, 2019

Not all 8 billion acres the researchers identified are readily available, however, as nearly half of them are taken up by cropland or urban area. The remainder exists in the form of degraded land, where human activity has reduced a habitat’s ecological productivity. In other words, places that would naturally support forests if given the chance. It’s here that the researchers found the potential for approximately 700 billion new trees, which, once fully grown, could collectively sequester 225 billion tons of carbon.

According to the study, these arboreal additions would gobble up around two-thirds of the 300 billion tons of carbon currently bopping around the atmosphere as a result of human activity, reducing the global burden to levels we haven’t seen since the mid-1900s. Waking up to that reality tomorrow would be like rewinding the climate clock by nearly a century.

“We were absolutely amazed as the results came out,” Crowther says. “It genuinely blew my mind.”

There are a few caveats, though. One of the biggest is timing: Mature, carbon-hungry trees don’t spring up overnight. “These projections kind of assume that, from day one, the forest is back,” Chazdon says. “But the process of reforestation takes several decades, even in ideal conditions.”

Given the accelerating pace of climate change, the long lag in payout is even more reason to act fast, Bastin says. If humankind doesn’t deviate from its current emissions trajectory, nearly a quarter of the land available for restoration will be gone by 2050. “The race [against climate change] is one I’m not sure we’re going to win,” he says. “The sooner [reforestation] happens, the better.”


A map showing where how the world's tree cover is expected to change by 2050. If emissions continue to rise, up to a quarter of the land available for reforestation will disappear. Image Credit: Courtesy of Jean-Francois Bastin et al., Science, 2019

The process also wouldn’t come cheap. At roughly 30 cents a sapling, planting the trees themselves is actually a good deal, Bastin says. But there are other costs to consider, including those incurred by labor, logistics, and maintenance of burgeoning forests. And the amount of planning and foresight required for forest restoration can’t be overstated. Not all trees are created equal, and planting species that are invasive, or ill-suited for a particular habitat could backfire, in some cases even hastening further degradation, Chazdon says.

Additionally, before trees could be planted in most parts of the world, environmental researchers would need to determine the cultural and economic impacts of reforestation, says Salima Mahamoudou, a research associate with the World Resources Institute who was not involved in the study. She points out that collaborations should include experts in other fields, as well as those living in and around the team’s targeted regions. “Forest restoration is a people issue, just like climate change is a people issue,” she says. “We need to start involving the people who will be impacted.”

An interdisciplinary approach would be especially useful for navigating issues of land ownership and land-use planning, says Daniela Miteva, an environmental economist at Ohio State University who was not involved in the study. Just because a forest can be restored somewhere, doesn’t mean it should, she says, especially if it might better serve the local community as a lucrative space for agriculture, for instance. And in many of the areas where reforestation could occur, farmers aren’t well compensated for giving up land or changing their practices.


Mata Atlântica (the Atlantic Forest), a South American forest in Brazil. Currently, about 500 billion tons of carbon is contained in the world's existing vegetation. Image Credit: FG Trade, iStock

But replenishing the world’s forests isn’t an all or nothing solution: Even shy of a trillion trees, every sapling planted makes a difference.

Such efforts are already well underway, Crowther points out, highlighting the effort of the Trillion Trees Campaign, aptly named for its goal of planting a trillion trees worldwide. (As of this writing, the organization is at 13.6 billion.)

Additionally, 59 countries, states, and organizations are currently pledged to the Bonn Challenge, a United Nations-endorsed initiative working to restore 850 million acres of deforested and degraded land by 2030. Among them are the United States and Brazil, which, according to the new study, are two of the six countries with the most restoration potential (the other four being Russia, Canada, Australia, and China). Even though the tropics remain the region where reforestation would probably still get you the biggest bang for your bark in terms of decarbonizing the atmosphere, Bastin says, the study suggests that a wide range of countries could have a role to play in the years to come.

Miteva stresses that, as powerful as reforestation could be, it’s no silver bullet—and we’ll need all the help we can get from other, more immediate mitigation tactics. Part of that includes safeguarding the vegetation we already have, which currently have 500 billion tons of carbon under lock and key. In this neck of the woods, Miteva says, “you don’t have to wait for the trees to grow.”

But excluding the promise of reforestation would be, in a sense, missing the forest for the trees. After all, living, growing forests are the climate gift that keeps on giving. Once mature, an acre of trees could soak up more than two tons of carbon per year—and that adds up fast, Crowther says.

And trees, of course, are good for more than storing carbon. In many parts of the world, careful and conscientious reforestation could have ripple effects on everything from ecosystem biodiversity to food and water security, replenishing local resources and boosting economic profits.

“I don’t want climate change solutions to be a burden on us all,” Crowther says. “We need to find positive solutions...and this is a nice, simple, and enjoyable one that can have a meaningful, tangible impact on us all.”

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