One way to counteract the rise in greenhouse gases in the atmosphere is to make use of known chemical reactions to capture carbon dioxide from the atmosphere and store it underground or use it to make something useful like hydrocarbon fuels or plastics. The process is similar to what plants do as they convert carbon dioxide into roots and leaves, but using industrial processes might make it happen faster and on a larger scale.

The question is whether this can be done cheaply enough to be worthwhile. Some have argued that the existing processes are too expensive, and that what's needed is basic research to find chemical reactions that require less energy, among other things. Meanwhile, it's better to try capturing carbon dioxide from smokestacks, where it's much more concentrated, making the capture process cheaper.

David Keith, director of the energy and environmental systems group at the University of Calgary, argues in next week's issue of Science that air capture (as capturing carbon dioxide from the atmosphere is often called) could be cheaper than people think, and that it may make more sense than other methods of offsetting carbon dioxide that are being used today.

He's careful to point out that the economics are by no means certain at this point. Indeed, he argues that what's needed now is funding for some large-scale tests to determine how much various processes really will cost. But his early research, including preliminary work with a startup company, suggests that costs for at least one part of the process--collecting carbon dioxide molecules from the air--will be cheaper than once thought, he says. What's more, such systems could be located anywhere in the world, making it possible to choose a location with low construction costs and easy access to geological structures for storing the carbon dioxide, both of which would make the process cheaper. Carbon dioxide capture from smokestacks, in contrast, must be done near existing power plants, where construction costs and the cost of transporting carbon dioxide to storage sites are higher.

Carbon dioxide capture from smokestacks will probably still have the edge, he says. But air capture may prove cheaper than other methods that are being funded now, such as putting solar panels on roofs in dark regions (Germany, for example), where payback times for the solar panels are very long.

Whether or not it makes sense to fund demonstration projects or to focus on basic research, air capture will likely play an important role in addressing climate change in the coming century. Even with strict emissions cuts in place, the carbon dioxide already in the atmosphere could cause serious problems. Air capture could provide a way to actually reduce greenhouse-gas concentrations, rather than merely stopping their growth.