There are various potential ways to “geoengineer” the Earth to counteract global warming. Brightening clouds, putting a haze into the stratosphere, bringing cooler water up from the ocean depths: all are being studied. There is little doubt that at least one of these methods could, if used in concert with significant emission cuts, end the warming and bring temperatures back down sooner than emission cuts can on their own. But if a nation starts geoengineering, how should others react? Would they continue to cut emissions in the face of an alternative? What if geoengineering helped in some places but made things worse in others? Worries about justice, democracy and safety mean some want research into the area stopped. In an ever-hotter world would everyone comply with such a ban?