Geoengineering proposals have drawn growing attention as possible climate change responses. Carbon dioxide removal (CDR) would reduce greenhouse gas (GHG) concentrations in the atmosphere, whereas solar radiation management (SRM) would diminish the radiation absorbed by the Earth without reducing atmospheric GHG concentrations. Some CDR techniques have progressed beyond basic field experimentation, but the scaling up of CDR remains a difficult challenge. SRM techniques are relatively undeveloped, although scientists have proposed limited field experiments. Compared to CDR, SRM would be cheaper to implement and faster acting, but would involve greater environmental risks and uncertainties. Both types of techniques have prompted ethical concerns about public participation, the geographical and temporal distribution of risks and benefits, and the potential to undermine efforts to reduce GHG emissions. Controversy surrounding geoengineering and worries about how the technologies might develop have led to calls for governance, whether through an international agreement, a research registry, or other means.