Near-universal recognition of the threat of human-caused global warming, tragically, has not been accompanied by comprehensive quantitative assessment of technological options for mitigating climate change. In this issue of Joule, David W. Keith and co-authors describe a process of capturing CO 2 from the air and provide a much-needed empirical cost estimate based on results from a pilot plant. Estimated costs, exceeding $100 per ton of CO 2 without including the cost of CO 2 storage, are lower than some prior estimates, yet are so high as to strongly support the need for rapid reduction of fossil fuel emissions.