Capturing carbon dioxide from the atmosphere ("air capture") in an industrial process has been proposed as an option for stabilizing global CO 2 concentrations. Published analyses suggest these air capture systems may cost a few hundred dollars per tonne of CO 2 , making it cost competitive with mainstream CO 2 mitigation options like renewable energy, nuclear power, and carbon dioxide capture and storage from large CO 2 emitting point sources. We investigate the thermodynamic efficiencies of commercial separation systems as well as trace gas removal systems to better understand and constrain the energy requirements and costs of these air capture systems. Our empirical analyses of operating commercial processes suggest that the energetic and financial costs of capturing CO 2 from the air are likely to have been underestimated. Specifically, our analysis of existing gas separation systems suggests that, unless air capture significantly outperforms these systems, it is likely to require more than 400 kJ of work per mole of CO 2 , requiring it to be powered by CO 2 -neutral power sources in order to be CO 2 negative. We estimate that total system costs of an air capture system will be on the order of $1,000 per tonne of CO 2 , based on experience with as-built large-scale trace gas removal systems.direct air capture | gas separation economics | separation thermodynamics | concentration factor | Sherwood plot S everal researchers investigating chemical systems for capturing CO 2 from the air* have suggested that air capture could be a viable climate mitigation technology costing no more than a few hundred dollars per tonne of CO 2 avoided (1-3). It has been further argued (4) that air capture may be cost-competitive with more accepted climate change mitigation options like renewable power, nuclear power, and CO 2 capture and storage from large stationary sources (carbon capture and storage, CCS). Indeed, during visits to the Massachusetts Institute of Technology in the spring of 2009, the US President's Science Advisor, John Holdren, and Secretary of Energy, Steven Chu, each mentioned capturing carbon dioxide (CO 2 ) directly from the air as an option that may be needed for stabilizing global CO 2 concentrations and, thereby, global temperatures. To examine these claims, we have undertaken a series of analyses of the costs and energy requirements of air capture.Instead of focusing on any particular proposed air capture system, we analyze the capture of CO 2 from air, where it has a concentration of approximately 0.04%, in the context of analogous industrial separation systems. Although the minimum thermodynamic work to separate CO 2 from air is not a prohibitive burden, separation systems themselves require significantly more energy than the thermodynamic minimum. This approach, which is independent of any particular air capture system or process, is motivated on the one hand by the utility of having a generic analysis, and on the other hand by the lack of literature regarding a detailed design of a particular captu...