Why Planting Trees Won’t Save Us

According to a new report this week by the Pew Research Center, Americans have finally come to an agreement about how to solve the climate crisis: by planting trees. A trillion of them. In theory, those trees will suck so much carbon out of the air that we won’t have to worry about installing solar panels or ditching the SUV for an electric car. According to the Pew report, the trillion trees solution is supported by 90 percent of both Democrats and Republicans. Contrast this, for example, with support for tougher limits on carbon emissions from power plants, which is backed by 93 percent of Democrats, but only 64 percent of Republicans.

On one hand, this near-universal support for tree-planting as a solution to the climate crisis is not surprising. Who doesn’t love trees? They are our ancient partners on this planet, and they may be far more intelligent than we know (Read Richard Powers’ magnificent novel The Overstory). And yes, the 3 trillion or so trees that already grow on the planet suck up about a third of the CO2 we dump into the atmosphere by burning fossil fuels. But the idea that we’re going to solve the climate crisis by planting a trillion trees is a particular kind of lunacy, and a great example of what happens when bad science hooks up with do-gooderism and they sleep together in a bed of political expediency.

A pair of new studies out this week show just how misplaced hopes for tree-planting have been. One study, published in Nature Sustainability, looks at how 25 years of forest subsidies in Chile have decreased biodiversity without increasing total carbon stored in above-ground biomass. A second study, in Science, raises big questions about the long-term security of carbon stored in forests, especially as those forests become increasingly vulnerable to drought, wildfires, and disease in our rapidly warming world.

The idea of planting trees as a solution to the climate crisis is nothing new. It was an important part of the 1992 Kyoto Protocol and the 2015 Paris Agreement. Many nations around the world — Germany, Canada, Kenya — have launched ambitious tree-planting projects in recent decades. Last August, volunteers in India planted 220 million trees in a single day.

Tree planting is also a key part of cap-and-trade schemes, which allow polluters to continue emitting CO2 if that CO2 can be offset (or absorbed) in other ways. In California, the cap-and-trade program has recognized 133 million tons of CO2 in benefits from forest carbon offset projects between 2013 and 2019. Skepticism, however, abounds, especially on questions about permanence: What happens to all of that carbon stored in trees if, say, a forest burns down?

Tree-planting mania began in earnest last July with a high profile paper in Science authored by Timothy Crowther, a 33-year-old assistant professor at ETH Zurich in Switzerland. Crowther and his team built models that used variables such as soil quality and other factors to suggest there was plenty of room for a trillion new trees on the planet. According to Crowther, those trees could absorb two-thirds of the CO2 that humans have added to the atmosphere in the industrial era. Tree planting, Crowther argued, is “our most effective climate change solution.”

The paper was a sensation, picked up by 700 media outlets. Crowther was profiled in Nature and Science and celebrated for his simple, elegant analysis of the world’s most urgent problem.

But the report was deeply flawed. One scientist called the paper “shockingly bad.” Science published six submissions from critics who cited substantial errors. In addition to slamming the paper for miscalculating the amount of carbon storage that can be stored in forests by a factor of 10, critics argued it favored converting grasslands and wetlands to forests and ignored how trees might affect water supplies and temperatures. “The claim that global tree restoration is our most effective climate change solution is simply incorrect scientifically and dangerously misleading,” one group wrote.

In a published response to the criticism, Crowther’s team made clear they saw tree planting as just “one of the most effective carbon drawdown solutions” and emphasized that reducing carbon emissions is critical. But they challenged other objections, arguing that disagreements about carbon storage calculations were not the result of errors but different definitions of “forest” and confusion about their methods.

In any case, the flaws in the study didn’t staunch its appeal. Marc Benioff, the billionaire CEO of Salesforce and well-known Bay Area philanthropist, latched onto it and began his own trillion-tree crusade. Thanks largely to Benioff’s enthusiasm, the World Economic Forum launched the Trillion Tree Initiative at its annual meeting in Davos earlier this year, with an endorsement by environmental rock star Jane Goodall. Big environmental and conservation groups like the World Wildlife Federation launched their own trillion-tree campaigns. Benioff also got the ear of White House advisor Jared Kushner, who passed the idea along to President Trump. Trump, who thinks climate change is a hoax perpetuated by the Chinese and has spent his entire presidency gutting protections of forests and parklands, even mentioned the virtues of tree-planting in his speech at Davos. On Earth Day, Trump planted a tree on the South Lawn (“I’ve always loved planting trees”).

Last February, Arkansas Rep. Bruce Westerman, a Republican, introduced the Trillion Tree Act, a pro-logging bill masquerading as a solution to climate change. It was a naked attempt to capitalize on some of the trillion tree mojo. “This deceptive bill is the worst kind of greenwashing and a complete distraction from urgently needed reductions in fossil fuel pollution,” Randi Spivak, public lands program director at the Center for Biological Diversity, said when the bill was introduced.

But trillion trees mania rolls on. It’s a climate crisis solution gone wild.

In Chile, as one of the new studies points out, the government subsidized the replacement of native forests with profitable tree plantations. The researchers measured the full impact of the afforestation subsidies and calculated their effects on net carbon and biodiversity changes across the entire country. They found that while afforestation payments expanded the area covered by trees, they decreased the area of native forests. Because Chile’s native forests are far more carbon dense and biodiverse than plantations (think rainforest vs. Christmas tree farm), the subsidies had the perverse effect of failing to increase carbon storage, while at the same time accelerating biodiversity losses.

The new paper published this week in Science further clarifies just how risky it is to bet on trees as a big solution for the climate crisis. Yes, forests can and do suck up huge amounts of carbon, but they are also vulnerable to the ravages of the climate crisis themselves. According to the report, the biomass dynamics of nearly half the forests on the planet are strongly sensitive to what the authors call “stand-replacing disturbances.” In other words, wildfires, drought, and disease – as well as logging by lumber-hungry humans.

“When it comes to storing carbon in forests, the issue no one has really looked at is permanence,” says lead author Bill Anderegg, an assistant professor of biology at the University of Utah. “How long will the carbon be locked away for? Fifty years? A hundred years?”

The paper cites numerous examples of the impact our rapidly changing climate is already having on forests: the severe 2011–2015 drought in California killed an estimated 140 million trees and turned the state’s ecosystems from a carbon sink into a carbon source — dead and dying trees in California contributed about 600 million tons of CO2 into the atmosphere, which is about equal to 10 percent of the state’s greenhouse gas emissions over that period. Similarly, infestations of bark beetles, a voracious tree-devouring insect amped up by rising temperatures, have killed billions of trees across millions of acres of land in the past two decades and have converted large regions of the Canadian boreal forest from a carbon sink to a carbon source.

And that may be a harbinger of things to come. As Anderegg’s paper points out, if we keep burning fossil fuels and heating up the planet, the impact on forests could be so great that instead of being a climate crisis solution, forests could become a climate crisis accelerant. Earth-system model projections over the 21st century indicate that terrestrial ecosystems (the vast majority of which are forests) could sequester as much as 36.7 billion tons of CO2 a year – or, in a high CO2 emissions scenario, release as much as 22 billion tons.

In other words, trillion-tree boosters might have it all wrong: Instead of being the engine of our salvation, forests could turn out to be the engine of our demise.

This doesn’t mean planting trees is a bad idea, of course. It just means that next time you hear about a climate solution that sounds too good to be true, it probably is. The solution to the climate crisis isn’t going to depend on a simple clever idea. It requires us to stop burning fossil fuels. The trillion trees project is a form of magical thinking, as divorced from reality as the naïve hope that the COVID-19 virus would disappear with the summer heat. “There are lots of good reasons to plant trees,” Anderegg tells me on the phone from Colorado, where he was about to hike out to have a look at a mountain full of aspen that are dying as a consequence of the 2018 drought in the region. “And forests can certainly be a useful part of a climate change solution,” he explains. “But we’re not to going to plant our way out of the climate crisis.”

America being what it is today, that doesn’t mean we won’t try.