Trying to Plant a Trillion Trees Won't Solve Anything

Only a monster would say no to this pitch: The best way to beat climate change —the warming of Earth caused by gases like carbon dioxide emitted by human industry, leading to rising sea levels , worsening fires and storms, drought, and disease—is simple. Plant a trillion trees. It’d be “one of the most effective carbon drawdowns to date,” said an article on the idea in the journal Science this past summer. And who doesn’t love trees , right ?Except the math turned out to be a little shady. Last month a bunch of climate scientists and ecologists piled onto that tree research in the same journal, calling out numerous errors in the first team’s calculations. At about the same time, a whole other bunch of ecologists started pushing back on the agriculture-tech startup Indigo for pitching a similar land-based carbon sequestration strategy, the “Terraton Initiative,” paying farmers to use new methods that could suck down a trillion metric tons (a teraton) of carbon. These goals are critical and the ideals are noble—who doesn’t want to stop climate change? Pretty much everyone except the US government agrees on that. It’s the numbers that are the problem.
Take the trees thing. The scientists who proposed it made careful maps of where trees grow today, all over the planet. They had a census of how many were there, combined with satellite data, all used to estimate how many potential trees could grow—and how much carbon those trees would slurp out of the atmosphere, a nontrivial calculation. There’s room for 0.9 billion hectares of new trees, they said—2.2 billion acres of tree cover, which draws down 205 metric gigatons of carbon, or 225 billion tons in US non-metric. That’s in line with the goal of keeping warming at or below 1.5 degrees, per the Intergovernmental Panel on Climate Change. World: saved!But then the bills started coming due. The team forgot that 55 percent of all historically emitted carbon got absorbed by the oceans, not the land, and so underestimated the total amount of carbon by about one half. They overestimated carbon uptake by trees, and suggested putting trees where they’ve never been, or where they’d actually make the planet hotter (by darkening planetary albedo over icy, more reflective terrain). The didn’t take into account that the ecosystems where they wanted to plant trees already sequestered carbon. And so on. “We’re not talking about small errors here. We’re talking about a huge difference in the total amount of carbon you could sequester,” says Carla Staver, an ecologist at Yale University.

The lead scientist and the person who runs the lab that produced the initial paper didn’t respond to my requests for interviews, but in response to the criticisms in Science they wrote: “We intended to highlight that we are aware of no other viable climate change solution that is quantitatively as large in terms of carbon drawdown. We did not suggest that tree restoration should be considered as the unique solution to climate change.” They broadly stuck to their numbers, or said their critics had misinterpreted or misread them. But mostly, the implication was, hey, come on! We’re all in this together.

Which is true, as far as that goes. But if scientists get it wrong, the fight could fall apart—or become an easier target for climate deniers who already see conspiracies behind every scientific warning. “All of those technical responses were written by people who believe there is a place for forest restoration in mitigating climate change and emissions,” Staver says. “It behooves us as scientists to be realistic about how much that’s going to get us. We have to make good faith estimates.”