To the Editor

The world is struggling to reduce greenhouse-gas emissions through conventional mitigation, and initially high expectations for the United Nations Climate Change Conference (Copenhagen, 7–18 December) are seemingly being scaled back almost daily. It thus seems that brute force efforts to remove and sequester carbon dioxide, such as air capture, will come to occupy an ever greater role in climate-policy discussions. Air capture refers to the direct removal and sequestration of carbon dioxide from ambient air. Investigations of the technologies that would remove this carbon dioxide, along with the associated costs and benefits, are attracting growing attention.

In recent months, several reports have endorsed looking at air capture more closely. A comprehensive assessment concluded that there was no doubt air-capture technologies could be developed, although questions regarding cost and the subsequent sequestration of the carbon dioxide will need to be addressed2. Indeed, at least according to an idealized assessment, the costs are comparable to more conventional routes of mitigation3, and it is likely that costs will fall in the future2. Most discussion of air capture centres on chemical removal methods, but there have been other proposals for biological and geological options. A sorbent-based chemical removal method, and another, using algae to convert carbon dioxide to organic carbon, could also be viable means of carbon capture: a recent report1 concluded that such methods would be preferable to geoengineering through management of solar radiation because of the smaller risk of unintended consequences.

The primary uncertainties surrounding air capture stem more from the lack of large-scale testing rather than scientific or technical concerns4. There are no more than two dozen researchers studying air capture and only US$1.5 million of government funding available worldwide5. There is little point in debating specific details about costs or large-scale feasibility until demonstration projects are undertaken5.

There are other possible methods for air capture: natural reactions that trap carbon dioxide in volcanic rock could be enhanced7. A recent provocative study6 argues that irrigating the deserts of the Sahara and Australian outback could create forests, and thus biologically sequester most global emissions from fossil fuels for many decades or longer.

Given increasing concerns that many countries will be unable to make the emissions cuts deemed necessary by governments, air capture may become a last resort for climate mitigation. Governments should help accelerate discussion and implementation by supporting air-capture projects, especially demonstration projects.