Some have proposed that climate-engineering methods could be developed to offset climate change. However, whilst some of these methods, in particular a form of solar-radiation management referred to as stratospheric aerosol injection (sai), could potentially reduce the overall degree of global warming as well as some associated risks, they are also likely to redistribute some environmental risks globally. Moreover, they could give rise to new risks, raising the issue of legal responsibility for transboundary harm caused. This article examines the question of international accountability of states for an increased risk of environmental harm arising from a large-scale climate intervention using sai, and the legal consequences that would follow. Examination of the applicability of customary rules on state responsibility to sai are useful for understanding the limitations of the existing accountability framework for climate engineering, particularly in the context of global environmental problems involving risk-risk trade-offs and large uncertainties.