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MIT Technology Review

“A Trillion Trees” is a great idea—that could become a dangerous climate distraction

Reforestation is critical for lots of reasons, but it’s no substitute for cutting emissions.

January 28, 2020
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TreesPhoto by Casey Horner on Unsplash

Signing on to the Trillion Tree initiative was basically the cost of admission for the global elite at this year’s World Economic Forum (well, that plus tens of thousands of dollars for the badge). In fact, tree planting was the rare issue on which even Jane Goodall and Donald Trump could get on the same page at Davos.

Meanwhile, Axios revealed last week that Congressman Bruce Westerman, an Arkansas Republican, is working on a bill dubbed the Trillion Trees Act that would set a national target for tree planting (although apparently it won’t be—and almost certainly couldn’t be—a literal trillion).

It’s great that trees are having a moment. Nations absolutely should plant and protect as many as possible—to absorb carbon dioxide from the atmosphere, provide habitat for animals, and restore fragile ecosystems.

“Trees are an important, very visible, and very socializable answer,” says Roger Aines, who leads Lawrence Livermore National Lab’s Carbon Initiative, a research program on carbon dioxide removal.

But it’s also a limited and unreliable way of addressing climate change. We have a terrible track record on carrying out reforestation efforts to date. We’d have to plant and protect a massive number of trees for decades to offset even a fraction of global emissions. And years of efforts can be nullified by droughts, wildfires, disease, or deforestation elsewhere.

Perhaps the biggest risk is that the appeal of natural-sounding solutions can delude us into thinking we’re taking more meaningful action than we really are. It “invites people to view tree planting as a substitute” for the sweeping changes required to prevent greenhouse-gas emissions from reaching the atmosphere in the first place, says Jane Flegal, a member of the adjunct faculty at Arizona State University’s School for the Future of Innovation in Society.

As we think through the role that trees could play in combating climate change, it’s crucial to consider several issues.

Time

Last week, a travel booking app called Hopper announced it will donate funds to plant four trees for every flight booked on its service.

The company estimates that an average tree sequesters just shy of a metric ton of carbon dioxide, about as much as as one passenger’s share from an average flight purchased through the app. The problem is that this would require about 40 years of tree growth. Given the varying species, climate conditions, and other factors, they predict that at four trees per passenger it’ll generally take about 25 years to offset the share of emissions from each flight.

It would be a complete delusion, then, to think these sorts of carbon offset programs make our actions immediately carbon neutral. But such thinking could encourage us to continue spewing carbon at a moment when emissions need to decline rapidly now.

Tally up every individual’s flight plus every corporation justifying business-as-usual behavior with tree planting that won’t have much of an effect for a couple of decades—assuming the trees survive that long—and you see how quickly this thinking can become a major problem.

Scale

For trees to play a major role in climate, we’d have to find the space to plant a tremendous number of them.

A report last year from the National Academies of Sciences, Engineering, and Medicine estimated that removing and sequestering 150 million metric tons per year would require converting as much as four million hectares (9.9 million acres) of land into forests that can never be harvested. That’s an area larger than Maryland.

But the US produced about 5.8 billion tons of emissions across the economy last year. Absent other climate policies, that’d suggest we need to dedicate nearly 155 million hectares (371 million acres), or well over twice the area of Texas.

The problem is, the US and most nations don’t have vast amounts of suitable land sitting around. And converting it comes at a cost to farming, food production, logging, and other uses.

Indeed, a report last week by the Committee on Climate Change concluded the United Kingdom would need to commit a fifth of its farmland to dedicated carbon storage, on top of many other efforts, for the nation to reach its target of net zero emissions by 2050.

Given land limits, economic constraints, and other factors, the National Academies study estimates the “practically achievable” amount of carbon removal from forests in the US at 250 million metric tons per year—1/23 of what we emitted last year.

Planting a trillion trees around the globe, assuming a relatively dense 2,000 trees per hectare, would require about 500 million hectares (1.2 billion acres). A hotly debated Science paper last year put the amount of land around the world that could support continuous tree cover and isn’t currently being used by humans at about 900 million hectares (2.2 billion acres).

Jesse Reynolds of the University of California, Los Angeles, questioned those figures, noting that some of the land is likely dedicated to livestock grazing, while others argued that much of it may not really be suitable for forestation efforts.

Critics also contested the broader conclusions of the study, which called tree planting “the best climate change solution available today,” arguing that the researchers significantly overestimated the amount of carbon dioxide potentially stored per hectare.

Accounting

There are inherent and possibly insurmountable challenges in accurately assessing much how much additional carbon dioxide we’re removing through forestation efforts. Studies and investigative stories have consistently found that carbon offset programs, including those established by the UN and California, have dramatically overestimated reductions from trees and invited gamesmanship among landowners.

The problem is carbon offsets are often treated as a one-for-one substitution, granting permission to emit the same level we supposedly offset. So if the estimated reductions are inflated, it can mean we end up emitting more in total than we otherwise would have.

Permanence

It’s especially odd to see so many parties embracing tree planting the same year we witnessed catastrophic fires in Australia and widespread deforestation in Brazil, Flegal notes. When trees and plants die, whether from fires or logging or simply falling down, most of the carbon trapped in their trunks, branches, and leaves simply returns to the atmosphere.

“Just shifting the stock of CO2 from the atmosphere to the land biosphere is not a permanent sequestration of emissions,” she says. “Carbon sinks can become carbon sources very quickly.”

And that’s only likely to become a bigger issue as the climate becomes harsher in the coming years. Droughts and higher temperatures are expected to strain forests and make them more susceptible to beetle infestations and major fires.

A seductive idea

Most research finds we will need to remove carbon dioxide from the air at a huge scale to prevent dangerous levels of warming. And planting trees is the cheapest and most reliable way we have for doing that at scale today. So there’s no question we need to figure out better ways to encourage, fund, monitor, and enforce forestation and preservation efforts around the globe.

But an earlier National Academies report found that trees won’t even be enough for this role, known as negative emissions, on their own. We’ll need other land-based solutions, like better ways to store carbon in soil, and a still-theoretical concept known as bioenergy with carbon capture and sequestration. And if we hope to feed a rapidly growing global population, we’ll likely need technological solutions that don’t take up a lot of land, such as direct-air-capture machines.

So yes, trees can and will need to play some role in sequestering carbon already in the atmosphere, at least for a while. But that’s all the more reason we can’t rely on trees as a stand-in for the separate monumental task of cutting emissions from our energy, transportation, and agricultural systems.

And it’s hard to read Republican’s sudden enthusiasm for tree planting as anything other than a cynical effort to dampen growing calls for the sorts of regulations and taxes required to bring about those changes.

There’s a jumble of other complicating issues to consider as well, including the high cost of forestation efforts at a large scale, the additional emissions that arise from planting and caring for trees, and the fact that tree cover can actually absorb heat and increase warming to some degree.

But the thing is, people want trees to be able to solve this problem. Natural-sounding solutions are far more appealing than technological ones. They avoid unsettling and expensive compromises like natural-gas plants with carbon capture systems, nuclear power plants, and long-distance transmission lines.

So people and publications across the political spectrum will be inclined to embrace the myth that trees will save us, and those hoping to stall or limit more effective efforts will very happily exploit it.