Trees are good for all sorts of things, like providing shade for picnics and habitat for animals. But they’re also a huge part of the efforts to combat climate change by sucking carbon dioxide out of the air.
New findings were published on Thursday in Science show just how important a role they could play in climate mitigation efforts by calculating “Earth’s tree carrying capacity.” Right now there are estimated to be nearly 17 million square miles of forest cover on Earth, and there’s enough room to add another 3.5 million square miles of trees—a US-sized chunk of land—to sequester even more carbon. There’s just one slight wrinkle: Climate change could make life in certain parts of the globe inhospitable for some of those new trees, particularly in the tropics.
Despite trees being nearly everywhere, figuring out just how much tree coverage the planet has is a pretty challenging task. The US Food and Agriculture Organization defines forest as any area with more than 10 per cent tree cover. And the best way to really see just how much tree cover’s out there is using satellite data, which is exactly what the study turned to.
Using the open access software Collect Earth to gather satellite imagery, the researchers pulled 78,774 satellite snapshots of forested area. They looked specifically at protected areas and places with limited human activity to avoid including city parks, farms, and other land uses that might look like forest but aren’t in actuality. They fed all that data as well as 10 other variables chronicling the climate and soil through a model to estimate current tree cover as well as areas where tree cover could be expanded. The results show an area of the world roughly equivalent to Russia, Canada, the U.S., and Australia—or nearly a third of all land area in the world—is covered in forest.
More than 12 million square miles of land could host more forest according to the study, but given that we need that land for crops and places to live, just 3.5 million square miles of that land is actually suitable for forest cover. The top four places primed for reforestation are Russia, the US, Canada, and Australia, all developed countries and in the case of the first three, all home to a piece of the vast boreal forest that rings the northern tier of the world. Brazil and China are also on the list and together those six countries contain 50 per cent of the area where forests could grow again.
Based on what we know about forests, that would store an extra 205 gigatonnes of carbon as the trees grew to maturity. To compare, the world emitted 37.1 gigatonnes of carbon dioxide last year. The newly filled out forests would be a huge boon to absorbing new emissions and the carbon pollution we’ve committed to the atmosphere.
“The restoration of ecosystems that could support trees is our main weapon to fight climate change,” Jean-François Bastin, the study’s lead author from ETH-Zürich, told us in an email. “Restoring the potential areas available, we could store about a quarter of the current amount of carbon held in the atmosphere.”
That would help combat climate change in addition to providing other benefits from recreation to habitat restoration. The study has one caveat, though. The researchers modelled two climate scenarios—one where emissions rise rapidly and another where they peak by mid-century and start to decline—to see how habitable those areas would actually be for trees. It turns out that while the boreal forest would likely fare OK, tree cover is likely to decline in the tropics as the climate warms. The Amazon is particularly at-risk since it’s also expected to dry out. Overall, the tree carrying capacity of Earth would be lower with more warming and with it, so would the chances of staving off severe climate change consequences.
The even more dire piece of news is that Amazon deforestation rates are climbingunder the rule of far-right President Jair Bolsonaro, tropical deforestation is continuing elsewhere, and massive fires are engulfing the world’s northern forests thanks to already rising temperatures. In short, humanity is headed in the wrong direction.
But if there’s a silver lining, it’s that we know the solutions. The new study provides yet another incentive to start slashing carbon pollution now instead of later and keep the Earth’s tree carrying capacity on the up and up. And it shows where we could concentrate conservation efforts to max out the climate benefits.
