The rapid rise in interest in geoengineering the climate as a response to global warming presents a clear and significant challenge to environmental ethics. The paper articulates what I call the 'presumptive argument' against geoengineering from environmental ethics, a presumption strong enough to make geoengineering almost 'unthinkable' from within that tradition. Two rationales for suspending that presumption are next considered. One of them is a 'lesser evil' argument, the other makes connections between the presumptive argument, ecofacism, and the anthropocentrism/non-anthropocentrism debate. (...) The discussion is designed to prompt reflection on how environmental ethicists should orient themselves to the rapidly moving geoengineering debate and what they should think about the moral significance of the earth's large-scale biogeochemical processes compared to the moral significance of individuals, species, and ecosystems. (shrink)