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.