And exactly how much carbon they remove can vary quite a bit based on variables like the health of the vegetation. “One of the major risks of some of these biology-based proposals is that an assumption gets made that you can easily equate X number of trees to X million tones of carbon without actually looking at what kinds of trees they are, and where they’ re being planted,” says Cox. The amount of captured carbon might end up being negligible. “You have a lot of trees, which is brilliant. You haven’t necessarily got the climate benefits.”
Another technique known as bioenergy with carbon capture and storage, or BECCS, also relies on a monocrop, usually fast-growing grasses. In this case, the vegetation is burned to produce energy, and the resulting emissions are sequestered underground. But it also comes with its own set of dubious side effects—it would require vast tracts of crops, and huge amounts of water, to make a dent in atmospheric carbon concentrations: A paper that published last month found that in the US alone, scaling up BECCS would expose 130 million Americans to water stress by 2100.
But in a global climate gone bonkers, there are even risks to restoring forests to their former glory, because that glory is increasingly perilous. Supercharged wildfires are now obliterating forests, instead of gently resetting ecosystems to make way for new growth. If you spend a lot of time and money restoring one of these forests to sequester carbon, and then it burns, all of that carbon goes right back into the atmosphere. Or if a given country’s political regime changes, and goes from supporting reforestation to inforestation, you’d have the same problem. Just look at what’s happening in the Amazon.
“I would argue that many proposals for land-based removals could be risky,” says Cox. “Because you’ve got a very, very high risk that either the carbon removal doesn’t happen in the first place, or that it happens, but then in 10 years’ time is reversed.”
The Dreaded “Moral Hazard”
Researchers have developed a way to mimic natural carbon sequestration with a technique called direct air capture, or DAC. These machines suck in air, pass it over membranes to remove the carbon dioxide, and pump it underground, locking it away forever. The tide may be shifting towards DAC in the US. Last month, the Biden administration threw in $3.5 billion to back direct air capture. (That comes five years after a California congressman introduced a bill that would fund the research of geoengineering, but it never went anywhere.)
But this, too, faces two big issues. The first is that DAC exists at nowhere near the scale needed to make a dent in excess atmospheric carbon. One plant that came online in Iceland last year is only capturing the equivalent emissions of 870 cars. A 2021 study calculated that it would take an investment of 1 to 2 percent of global gross domestic product to capture 2.3 gigatons of COtwo a year by 2050—and that’s only a fraction of current annual emissions, which are around 40 gigatons. “There is the risk that we cannot scale and deploy fast enough,” says Benjamin Sovacool, who studies the risks of geoengineering at Aarhus University in Denmark. “It’s looking like the rate at which we’d have to deploy these is unlike any previous energy transition we’ve had, because the scale is so immense.”
The second issue is one of “moral hazard,” or the temptation to lean on DAC as a crutch, instead of doing what’s necessary: dramatically slashing greenhouse gas emissions. If a nation’s leaders anticipate being able to worry to remove emissions via DAC, they don’t need to about cutting those emissions in the first place. It’s like waiting for an antiviral miracle—except the requisite dose doesn’t yet exist.
There’s a chance that the extreme and desperate nature of geoengineering might do the opposite—instead of encouraging complacency or a reliance on last-minute technology fixes, it may alarm the public enough that they’ll start to treat climate change like an emergency. But, says Sovacool, “politicians might be even more susceptible to the moral hazard, because they’re only thinking in the present terms. They’ll gladly push as much to future generations as they can.”