Sucking carbon from the sky may do little to slow climate change

Drastic Cuts in Greenhouse Gases Won’t Curb Long-Term Climate Change

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As U.S. President Barack Obama finalizes plans to cut greenhouse gas emissions today, climate researchers are wondering whether even more extreme measures will be needed—such as using machines to suck carbon dioxide from the environment. Now, a new study of these so-called carbon dioxide removal (CDR) technologies finds that the strategy would have only a minimal impact.

"You hear CDR mentioned as an option to maybe avoid dangerous climate change, but when we did the simulations you see the effect of this technique is quite small," says lead author Sabine Mathesius, a climate modeler at the Potsdam Institute for Climate Impact Research in Germany.

CDR technologies belong to a class of climate change–fighting techniques known as geoengineering. They include enhancing forests to make them absorb more CO2, building chemical plants that filter the air directly, and burning biofuels and storing the emitted carbon dioxide underground. These may sound far-fetched, but the Intergovernmental Panel on Climate Change concluded last year that reaching a key emissions goal—roughly equivalent to limiting warming to 2°C by 2100—would rely on the deployment of one or several CDR technologies. Deploying them could cost more than a trillion dollars, one expert estimates.

The new study puts the concept of CDR to the test—without getting into the specifics of which technology to use. The authors used computer simulations to figure out what would happen if engineers removed a whopping 5 gigatons of carbon dioxide from the atmosphere each year. Achieving that goal, equivalent to removing roughly half the amount of CO2 that is now emitted from manmade sources, would require a gargantuan global effort. (To visualize the scale, imagine 5000 new facilities each roughly the size of a sports stadium.)

Yet the scientists found the environmental benefits of such a massive technological campaign were surprisingly small, especially in terms of protecting the ocean from the impacts of climate change. One key impact of rising CO2 levels is seen in the pH of ocean waters—the global sea surface has been acidified by roughly 0.1 units and impacts on marine shells are beginning to show. But the experiment demonstrated that the CDR campaign had only limited effect to reverse that trend: Without the CDR the surface pH was reduced by 0.75 units by 2200; with CDR the acidification was reduced 0.7 units, the team reports online today in Nature Climate Change.

In another experiment, the scientists examined how, beginning in 2150, removing an even more whopping 25 gigatons each year would work—an effort to "go negative" and remove more CO2 from the atmosphere than humanity was dumping. That approach restored surface ocean pH, but only by 2300. "I expected CDR would have a bigger effect," Mathesius says.

The main reason: CDR removes CO2 from the sky, but it cannot act on the carbon locked in the deep ocean, which takes thousands of years to return to the surface. Simulations in the paper show that massive and immediate cuts to carbon emissions—say, by ramping up efforts along the lines of the Obama regulations—would have a more potent impact, limiting acidification to only 0.2 units by 2100.

Co-author Ken Caldeira warns that relying on CDR to the detriment of cutting emissions immediately could put humanity in a big hole. "If we emit CO2 today, and then pull it back out of the atmosphere sometime in the future, we will have a deep ocean that is hotter, more acidic, and with less oxygen than it would have been otherwise," says the geochemist, who works at the Carnegie Institute for Science in Palo Alto, California. "Our paper shows that emitting CO2 today and taking it out sometime later is not the same as never emitting it at all."

Commentators on the paper writing in Nature say the work "demonstrates that proposed technological solutions, like CDR, to the problems of global warming and ocean acidification are no substitute for reducing carbon emissions."  

The study could discourage investment in CDR technologies, such as carbon-sucking machines under development by Carbon Engineering in Calgary, Canada. There scientists are following an approach known as direct air capture (DAC). The firm's Geoff Holmes says CDR has a role despite the study’s findings. "Where other options are cheaper and quicker than DAC to mitigate emissions, let’s use them," he says, mentioning technologies that would make buildings or vehicles more energy efficient as one example. "But as we tackle economywide emissions and try to drive them to zero, there are likely to be many types of emissions where using DAC is preferable to applying a fix at each source." These could include emissions from automobiles, where collecting CO2 pollution from each car is infeasible. 

*Updated, 3 August, 3:20 p.m.:  This article has been clarified to include additional information on the scenario that removed 25 gigatons of CO2 from the atmosphere each year.