We gratefully acknowledge funding from the ESRC Centre for Climate Change Economics and Policy and Grantham Foundation for the Protection of the Environment and thank the organisers of the Harvard Solar Geoengineering Research Residency. The first author gratefully acknowledges support from the German Research Foundation (DFG), grant HE 7551/1. This paper has benefited substantially from discussions and comments at the following events: seminars at FEEM Milan and the Institute for Science, Innovation and Technology Oxford; the EAERE 2017 conference in Athens, Climate Engineering Conference CEC 17 in Berlin, EENR 2018 workshop in Orléans, WCERE 2018 conference in Gothenburg and the GRI workshop at LSE.