Scrubbing the Skies

Pulling CO2 back out of the air might be easier than building jets and cars that don’t emit it.

National Geographic: Robert Kunzig - August 2010

Art: Splashlight. Source: Klaus Lackner, Lenfest Center for Sustainable Energy, Columbia University

Every time you drive to work, or worse yet, fly on a plane, the vehicle emits carbon dioxide that will stay in the atmosphere, warming the planet for thousands of years. Does it have to? Trees can take CO2 back out again—but even covering the planet with forests wouldn’t solve our problem, and there would be an awful lot of wood to preserve. (If allowed to rot or burn, trees release their carbon again.) Physicist Klaus Lackner thinks he has a better idea: Suck CO2 out of the air with “artificial trees” that operate a thousand times faster than real ones.

They don’t exist yet, and when they do, they probably won’t look like real trees. But in Lackner’s lab at Columbia University he and colleague Allen Wright are experimenting with bits of whitish-beige plastic that you might call artificial leaves. The plastic is a resin of the kind used to pull calcium out of water in a water softener. When Lackner and Wright impregnate that resin with sodium carbonate, it pulls carbon dioxide out of the air. The extra carbon converts the sodium carbonate to bicarbonate, or baking soda.

Art: Joe Zeff Design. Source: David Goldberg, Lamont-Doherty Earth Observatory, Columbia University

Government studies suggest there is room underground for many decades’ worth of CO2—though the idea has not been tested on the massive scale required. Click the labels above to see more about each method.
CO2 scrubbers that rely on similarly simple chemistry already recycle human exhalations in submarines and space shuttles. Devising an economic way of scrubbing the outside air, though, is harder. Lackner’s plastic offers two advantages, he says, over schemes that other labs are working on. It sponges up CO2 quickly— the porous material has lots of surface area to contact the air—and holds on to it lightly. The latter is crucial. The CO2 must be separated from the sponge for disposal, and in most schemes that step takes a lot of energy. But Lackner and Wright just rinse their plastic with water in a vacuum chamber, and the CO2 comes off.

What to do with it? Most likely, compress it to a liquid and pump it underground—the same option being studied for coal-fired power plants, which could capture CO2 at the smokestack. That’s not practical for cars and planes; there wouldn’t be room on board to store the gaseous stuff until you got to the CO2 dump. A scrubber that pulled CO2 out of the air, on the other hand, could be located anywhere—right above the most convenient dump site, say.

Another option would be to add hydrogen to the CO2 and convert it back into liquid hydrocarbons. If the energy for that came from renewable sources, engines that burned the fuel would emit no new carbon. Jet travel would become guilt free again. We could keep our cars and gas stations—no need for a whole new hydrogen- or electric-powered infrastructure. Subversive thought: We could keep our lifestyles. “That’s historically what we’ve done,” Lackner says. “We’ve run into environmental issues that seemed insurmountable— and we’ve found a solution.” One day, he says, when we’ve finally stopped the rise of CO2, we might even be able to reduce its concentration in the atmosphere, back to a level that won’t melt glaciers.