There were once around 6 trillion trees on the planet; now, thanks to human civilization, there are around half that number. Reversing that trend is a key pillar in the fight against climate change. And for companies and countries aiming to use carbon offsets for net-zero emissions, planting more trees can seem like an obvious option. Trees capture carbon, and planting them is a relatively cheap way to fight climate change. But there’s also a risk: Many of those trees might not survive, and could end up adding to emissions themselves.
“I think the emphasis on planting trees is risky or even a misplaced dream unless we first put priority on reducing or halting climate change, because the trees are going to be stressed—they are getting stressed now,” says Jonathan Overpeck, a climate scientist at the University of Michigan and one of the authors of an op-ed in Science arguing that the world needs to rethink how much we rely on tree planting as a climate solution.
Vegetation is being stressed by climate change “in ways that are increasingly unprecedented,” he says. In California, hundreds of millions of trees died in the last drought, after a combination of heat, lack of rain, and attacks from pests, and then many burned in the record-breaking wildfires in 2020. In Australia, millions of acres of forest burned in 2019 and 2020, emitting more CO2 than the rest of the economy combined. Zombie fires are burning forests in Siberia now.
Even in areas where large wildfires aren’t common now, such as Michigan, climate change is killing some trees. “Part of the story is the same thing as in the west: Trees are getting stressed out by the warmth,” Overpeck says. “And because they’re stressed, they can’t fight the diseases as much.” Hotter temperatures dry out soils faster, and dry out plants. The changing climate also means that some species are no longer growing in the right place to survive.
There’s a risk, then, that trees that are newly planted in a forest might end up dying from disease or burning in a fire. “When you get these large wildfires, of course, you’re converting carbon that’s in vegetation—and often in soil that’s accumulated over hundreds of thousands of years—you’re converting that carbon to carbon dioxide in the atmosphere,” he says. “You’re also putting soot into the atmosphere, which is dark colored, and therefore also acts to warm, just like greenhouse gases. And that’s accelerating the climate change that is stressing the vegetation out.” (In other cases, tree planting projects might happen so haphazardly that even without the added stresses of climate change, the trees are unlikely to make it.)
Any trees that are planted need to be planted carefully—like a project in Paradise, California, where species have been chosen to have the best chance of surviving in a hotter, drier climate, and planted in a way to minimize the likelihood of spreading fire later. But there also needs to be much more done to protect existing forests and to study new ways to help them survive, Overpeck says. In another example in California, some trees are being thinned out in forests to reduce fire risk and then converted into biochar, a material that can be put back in the soil to sequester carbon. Companies and countries should focus on directly cutting emissions rather than relying on trees for offsets. “It’s an easy way out to plant trees instead of making the harder choice to limit greenhouse gases,” he says.
“You have a lot of folks say, ‘well, our best way to fight climate change is to plant more trees,'” he says. “Or to buy carbon offsets for these existing forests. And we’re saying, ‘whoa, wait a minute. That’s okay. But the first thing we have to do is we have to stop the damn climate change that is stressing out these forests.’ Because the carbon isn’t safe.”