Since a buildup of humanmade carbon dioxide is causing the planet to warm, why not just suck this greenhouse gas straight out of the atmosphere? That's one strategy scientists have proposed to combat climate change. But a new analysis suggests that the approach may be neither economical nor practical.
Before widespread industrial activity began spewing CO2 into the air in the mid-1700s, atmospheric concentrations of the gas were around 280 parts per million (ppm). They now exceed 390 ppm and are growing rapidly, about 2 ppm per year. Rather than reducing emissions of the greenhouse gas by shifting to carbon-neutral sources of energy—wind, solar, and nuclear power, for example—and then letting vegetation and the oceans gradually absorb CO2 in the long term, humans could lower concentrations more rapidly by actively pulling CO2 from the air, some scientists have suggested.
In such a scheme, researchers would leave large vats out in the open, filled with solutions of sodium hydroxide, potassium hydroxide, or chemicals called amines. Or, the CO2-laden air could be forcefully bubbled through such reservoirs. When CO2 in the air reacts with these solutions, it becomes trapped in carbonate-rich compounds. Scientists can later heat these compounds and release the CO2 and dispose of it, typically by injecting it into deep geologic formations beneath impermeable rock, such as natural reservoirs of oil and natural gas.
The approach is a form of carbon capture, which also includes strategies for grabbing CO2 before it gets into the air, from smokestacks, for example. Carbon capture itself is part of a suite of ideas, dubbed geoengineering , in which scientists hope to use technology to curb global warming. (Other ideas involve schemes such as seeding the seas with zillions of tiny bubbles  to reflect sunlight back into space.)
Previous studies have hinted that capturing CO2 directly from the air could cost a few hundred dollars per metric ton of CO2. At a rate of $300 per metric ton, that would total more than $10 trillion to completely counteract the estimated 33.5 billion tons of CO2 emissions generated by humans—a tremendous cost, yet one that is still economically viable. But Kurt House, a geoscientist with C12 Energy in Berkeley, California, and his colleagues suggest online today in the Proceedings of the National Academy of Sciences that slurping a ton of CO2 from the atmosphere may actually be much more expensive.
Among other techniques, the researchers estimated the costs of this form of carbon capture by comparing it with the price of scrubbing other pollutants such as oxides of sulfur and nitrogen from industrial emissions before they leave a power plant's smokestack. Although pulling CO2 from ambient air rather than a smokestack, where CO2 concentrations can be as high as 12%, would be more difficult, it is technically possible. The problem, House says, is that it's energetically as well as economically expensive to do so. Capturing CO2 once it's in the atmosphere takes about four times the energy generated by burning the fossil fuel in the first place, he notes.
Overall, just to capture CO2 would cost at least $1100 per ton , the researchers estimate. That's a total price tag of at least $33 trillion just to hold atmospheric concentrations of CO2 steady Then, once the gas is captured, even more energy must be expended to compress the gas into a liquid and then dispose of it. And unless the energy needed to drive these processes are carbon-neutral—that is, unless they produce no CO2 emissions of their own—the net result might add CO2 to the atmosphere, not reduce it.
"I agree that this [carbon-capture] process would be expensive now, and I agree that we need clean sources of energy to do this," says Robert Socolow, a physicist at Princeton University. "Today, we don't know how to do this at low cost, but there's work that can be done that might reduce costs significantly," he adds.
The analytical techniques used by House and his colleagues are perfectly sensible, but there's a lot of uncertainty associated with them, says David Keith, a physicist at Harvard University. As a result, he notes, costs to pull a ton of CO2 from the air could range anywhere from several thousand dollars to as low as $100. Still, "at this point, carbon capture [from ambient air] is a very conceptual environment," says Keith, who is also president of a start-up company developing such technology. "To really know what it costs, someone actually has to build it."
House and his colleagues note that for the near future, it's probably better to avoid releasing CO2 into the atmosphere in the first place. Between now and 2050, they say, carbon emissions can be captured—using more-developed techniques such as scrubbing the gas before it ever leaves the smokestack, for instance—for less than $300 per ton. But the best approach, many suggest, is simply to find greener sources of energy.