Sucking Carbon Out of the Air: Not Ready for Prime Time

If we don't cut back on carbon emissions, there's a Plan B to stave off climate change. Known as geoengineering, it would use a variety of technologies to do one of two things: reduce the amount of sunlight reaching Earth's surface, or pull excess carbon dioxide (CO2), which is a key global warming agent, from the air. A number of labs and companies are working on the latter, known generically as Direct Air Capture, or DAC, and they've gotten a fair amount of attention for the concept.

But an exhaustive new report from the American Physical Society, a two-year project carried out by 13 scientists and engineers from the academic world, industry, and government, suggests that DAC won't play much of a role in dealing with the planet's carbon problem for decades, at least. “Our report doesn't slam the door on these technologies,” says lead author Robert Socolow, a professor of Mechanical and Aerospace Engineering at Princeton and co-director of the Carbon Mitigation Initiative, a collaboration between the Princeton Environmental Institute and oil-giant BP.

What Socolow and his co-authors do say is that it's much more efficient to prevent CO2 from entering the atmosphere in the first place — either by switching away from fossil fuels like coal and natural gas in favor of low-carbon energy sources, such as wind, solar and the usual gang of suspects, or by scrubbing CO2 from emissions sources. The CO2 coming out of a coal plant's smokestack, for example, is 300 times more concentrated than what's floating around in the atmosphere generally. That being the case, the report estimates that it would cost upward of seven times more to capture the CO2 after it has escaped into the air than it would to trap it before it leaves the smokestack. (In either case, the CO2 has to go somewhere. One leading idea is to liquefy it and pump it deep underground. Another is to turn the CO2 back into hydrocarbon fuel.)

This calculus would change, of course, if the technology for sucking carbon out of the air became a lot less expensive — something its proponents argue is pretty much inevitable, given that it's still in the experimental stage. And while the need to prevent emissions is currently more urgent, DAC technology might become a priority if and when our emissions are reduced significantly. By the time that could realistically happen, there would be much more CO2 in the atmosphere than there is today, and way more than there was before the industrial revolution. Once excess CO2 gets into the atmosphere, it takes a very long time for natural processes to remove it.

At that point, DAC could be the only reasonable way to draw down the excess CO2 and dial back the effects of climate change. But it would take a while. The report's authors calculated that under any reasonable scenario, air capture could draw down CO2 levels by 50 parts per million (ppm) over a century of operation. To put that into perspective, currently there is about 393 ppm of CO2 in the atmosphere, and that number is projected to rise to at least 450 ppm by the end of the century. Their calculation was very much oversimplified, but even so, at that rate, even if we stopped adding CO2 to the air immediately and deployed DAC devices all over the world tomorrow — neither of which is remotely possible — it would still take more than 200 years to return the atmospheric concentration of CO2 to pre-industrial levels.

Still, says Socolow, “direct air capture could certainly play a role down the road. It's an interesting approach; maybe it's not promising now, but I think it will produce lots of ideas.”