Geoengineering technologies have the potential to disrupt the global ecological status quo and mount a potentially coercive threat with implications as serious as those in wartime. Several of these technologies can be deployed from the global commons, but international law provides no more than indirect guidance as to how they should be governed as a matter of international security. This paper argues that, lacking explicit scientific or legal guidance, just war theory provides a useful normative framework for restraining the use of environmental force. Modifying just war theory into “just geoengineering theory” will provide ethical standards for security decision makers as they consider whether or how geoengineering should be used. The article analyzes existing legal guidance and presents a “just geoengineering theory” for considering deliberate climate modification.