Trees, among their many other superpowers, help absorb some of the excess carbon dioxide humans have been adding to Earth's atmosphere lately. That's a valuable service, considering we still release about 2.57 million pounds of CO2 every second, on average, and the heat-trapping gas can linger in the sky for centuries.

We know Earth needs more trees. And although we are doing far too little about climate change in general, we are planting trees — so many, in fact, that global tree cover has reportedly increased by about 7% in the last 35 years.

That's just a drop in the bucket, though, since Earth's total number of trees has fallen by 46% since the dawn of agriculture about 12,000 years ago. Today, we're mostly adding slower-growing trees at higher latitudes, which are less effective carbon absorbers, while rapidly losing trees across the tropics. In 2017 alone, for example, Earth lost about 39 million acres (15.8 million hectares) of tropical tree cover, which is like losing 40 football fields of trees every minute for a year.

Dead trees stand in a recently deforested section of the Amazon rainforest near Abunã, Brazil, in 2017. The past half century has seen about 20 percent of the Amazon vanish, according to the WWF. (Photo: Mario Tama/Getty Images)

Tropical forests are especially important for many reasons, and stopping this destruction should be a high priority for humanity. But given the huge scale of climate change, that still won't be nearly enough to avert disaster. On top of stopping deforestation, we'll need to add a lot more trees in a lot more places.

How many trees? According to the United Nations' Intergovernmental Panel on Climate Change (IPCC), adding 1 billion hectares (nearly 2.5 billion acres) of forests could help limit global warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit) above pre-industrial levels by 2050. That much warming would still be terrible, but it would be a lot better than 2 degrees Celsius (3.6 Fahrenheit).

To put that in perspective, 1 billion hectares is slightly larger than the land area of the United States. Is it even feasible to add that much forest, especially when we're already struggling to preserve the old-growth forests we have?

But trees likely won't be able to help us forever. Researchers answering the question of how much carbon dioxide trees can absorb found that they can only clean a fraction of the carbon dioxide in the atmosphere. Because we don't know how much carbon dioxide humans will create — or how trees will respond — it's unclear how much of it trees will be able to handle beyond the year 2100.

In the meantime, tree planting is still important.

Two new studies take a closer look at this issue. One looks at the possibility of planting trees virtually everywhere they could grow, estimating the maximum possible scope of reforestation in response to climate change. In the other, researchers focused on reforestation opportunities in the tropics, singling out "restoration hotspots" where newly planted forests are most likely to succeed.

The benefit of 500 billion new trees

This map shows areas that could support new forests, excluding existing forests as well as urban and agricultural areas. (Photo: Bastin et. al./Science)

This map shows areas that could support new forests, excluding existing forests as well as urban and agricultural areas. (Map: Bastin et. al./Science)

In one of the new studies, published in the journal Science, researchers tried to quantify just how many more trees the planet could support. They analyzed nearly 79,000 satellite images of Earth's land surface, then paired their tree-cover data with 10 global layers of soil and climate data to reveal areas suitable for various forest types. After they excluded existing forests, along with urban and agricultural areas, they calculated the potential habitat for newly planted trees.

It turns out Earth has more than 900 million hectares of land that could support new forests, or roughly 2.2 billion acres. If all that land actually contained forests, the study's authors found, it would hold more than 500 billion trees, which could store 205 gigatonnes of carbon (205 billion metric tons). That would be a big deal, they say, accounting for about two-thirds of all the CO2 humans have released since the start of the Industrial Revolution. Some other researchers dispute that figure, though, arguing it would account for closer to one-third of historical CO2 emissions.

"That's not to say that reforestation is not an important mitigation strategy, just to caution that like every other climate solution it's part of a larger portfolio of strategies rather than a silver bullet," climate scientist Zeke Hausfather wrote on Twitter.

Either way, this shows reforestation could be a powerful tool in mitigating climate change (not to mention the many other benefits for people and wildlife). Yet it also bypasses the logistics of such a massive effort, as the authors acknowledge. Their satellite imagery doesn't differentiate between public and private land, for instance, or identify places where development or farming might already be planned. "[W]e cannot identify how much land is truly available for restoration," they write, although they say their study suggests the IPCC's reforestation target of 1 billion hectares is "undoubtedly achievable" under the current climate.

That last caveat is worth noting. Climate change is making life harder and harder for many trees, especially in the tropics, and thus threatening their ability to help us remove our excess CO2 from the atmosphere. "We estimate that if we cannot deviate from the current trajectory, the global potential canopy cover may shrink by 223 million hectares by 2050, with the vast majority of losses occurring in the tropics," they write. "Our results highlight the opportunity of climate change mitigation through global tree restoration but also the urgent need for action."

'Restoration hotspots'

The sun sets over Bwindi Impenetrable Forest in Uganda. (Photo: Dietmar Rauscher/Shutterstock)

The other new study, published in Science Advances, takes a slightly less ambitious approach. Rather than trying to quantify the global potential for reforestation, it looks at how to maximize limited resources for undoing deforestation in the tropics. In addition to identifying places where forests could be regrown, the authors also assessed the feasibility of reforestation, considering social and economic factors that could affect the success of tree-planting efforts.

They found about 863 million hectares of restorable area for forests overall, an area roughly the size of Brazil. They also assigned a "restoration opportunity score" (ROS) to various places, and determined that about 12% of the restorable area — about 101 million hectares — meets their criteria as a "restoration hotspot." Forests in these hotspots can not only hold lots of carbon and biodiversity, but they're also more likely to thrive than in other areas.

The top six countries with the highest ROS are all in Africa, the study found: Rwanda, Uganda, Burundi, Togo, South Sudan and Madagascar.

A rainforest stream flows through Masoala National Park in Madagascar. (Photo: Dennis van de Water/Shutterstock)

The two studies used different approaches and reached different conclusions, as science writer Gabriel Popkin points out in Mongabay, but both are part of a key shift from tracking the loss of forests to mapping out their potential comeback. And while forest restoration isn't a silver bullet, this research does suggest it may be our best hope for buying ourselves more time, as an author of the Science study tells Vox.

"The point is that [reforestation is] so much more vastly powerful than anyone ever expected," says Thomas Crowther, a researcher at the Swiss university ETH Zurich. "By far, it's the top climate change solution in terms of carbon storage potential."