CM bears some resemblance both to conventional abatement and to geoengineering. This ambiguity and CM's character as an "end-of-pipe" technology foretell intense and proper controversy over the appropriate extent of intentional human intervention1 in global systems, as well as over CM's specific benefits and risks. Yet CM has so far received inadequate attention in climate assessments and policy debates. Large-scale adoption of CM would be a major technological and social choice that would be difficult to reverse. Continued rapid technical progress in CM and continued slow political progress on abatement pose the risk that this choice will be made without adequate reflection on its implications.