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When global climate change came onto domestic and international policy agendas in the late 1980s, only two types of response were initially considered: reducing emissions by improving efficiencies or switching to lower or non-carbon energy sources; and adapting to the anticipated changes. Since that time the agenda of potential responses has been progressively expanded, principally by adding various ways to intervene in the global carbon cycle or the climate to break the connection between emissions of greenhouse gases and the resultant climate changes. Three types of these “intervening” responses are now, to varying degrees, present in policy debate: biological sequestration of carbon in forests or soils; point-source carbon capture from fossil fuels or combustion gases, followed by sequestration in stable reservoirs; and various forms of albedo modification or other direct manipulation of the climate system, collectively called geoengineering. In this issue of Climatic Change,Keith, Ha-Duong, and Stolaroff propose that one additional intervening option should be considered: capturing CO2 directly from the atmosphere, then sequestering it in the same reservoirs as would be used for carbon captured from point sources. They argue that air capture, like a conventional backstop energy technology, can provide an essentially unlimited quantity of mitigation at constant, high marginal cost. But because air capture would be completely uncoupled from the energy system, it would have two key advantages over any prior mitigation technology. First, air capture would take place in free-standing dedicated plants, and so would offer complete flexibility in siting, timing, and scale. Second, air capture would not be bound to any particular emissions stream, and so could be conducted at large enough scale to make any enterprise, nation, or human civilization as a whole, a net remover of carbon from atmosphere, rather than a contributor to it.