In fact, it may well prove that another similar material works better to directly capture CO2 from the air. TerraLeaf is working on using chlorophyllin—the chlorophyll from a plant turned into a salt by adding sodium ions and copper—paired with an electrically conducting polymer to form a membrane that can pull CO2 (or other greenhouse gases) from air and form carbon-based chemicals, and potentially even fuels. Harvard University scientist David Keith and his team are working on making a machine capable of pulling CO2 from the air using liquid sodium hydroxide, also known as lye, and then reheating it to release the CO2, allowing for continuous operation. This is how breathable air is re-created on submarines and spaceships, after all. And there are yet more options from groups such as Climeworks, the Georgia Institute of Technology, and the University of Southern California.
But Lackner's polymer may be hard to beat on price. The resin, made by Dow and known as Marathon MSA, finds use in food processing and water purification, among other applications, and costs just $2.50 per kilogram, according to Lackner. They are still working on their first roll of the material after nearly a decade of experimentation.
Yet, therein lies what may prove the ultimate challenge of such direct air capture: cost. On the one end, no one is willing to pay to suck CO2 out of the air at present and, on the other, few are willing to pay for CO2 either to be stored or used. An estimate from the American Physical Society (pdf) suggested that such air capture might cost roughly $600 per metric ton of CO2 captured, which makes even Lackner's bulky resin “trees” into the Tesla Roadster of emission reductions. Capturing a gas that makes up just 0.04 percent of the air may prove too energy intensive and too expensive to sustain.
Then again, given the climate change impacts already seen at present greenhouse gas concentrations, the world may not have much choice. Lackner and colleagues argued in a paper published last July in Proceedings of the National Academy of Sciences that air capture may prove the only way to deal with greenhouse gas emissions from transportation, all those tailpipes and engine exhausts on the world's millions of cars, airplanes and other vehicles. "Given the enormity of the global climate challenge, we think this [air capture research and development] needs to be scaled up urgently," the research team wrote. Already, the $25-million Virgin Earth Challenge Prize for CO2-reduction technologies has identified Lackner's work, along with four other air capture schemes, for award consideration. The other possibilities include biochar and biofuels with CO2 capture as well as efforts to enhance natural processes that capture CO2 like rock weathering and vegetation regrowth.
Human civilization shows little inclination to reduce CO2 the cheapest industrial way: cut greenhouse gas pollution from the biggest, most concentrated sources, such as coal-fired power plants or oil refineries. Placing carbon capture at these industrial facilities is significantly cheaper than general air capture and reduces emissions from the single largest sources of CO2. As Princeton University mechanical engineer Robert Socolow argues: it makes little sense to capture CO2 from the air until these sources of pollution have been eliminated.
The ultimate problem may be that the technology cannot work overnight. Even if Lackner was able to deploy his millions of artificial trees employing this resin, it would take decades at least to restore pre-industrial atmospheric concentrations of CO2. And then the question becomes: What is the appropriate concentration of CO2 in the atmosphere, and who decides? In the end it might be easier to just stop pumping CO2 into the air in the first place and let photosynthetic plants handle the rest.