Recent scientific advances have introduced the possibility of engineering the climate system to lower ambient temperatures without lowering greenhouse gas concentrations. This possibility has created an intense debate given the ethical, moral and scientific questions it raises. This paper examines the economic issues introduced when geoengineering becomes available in a standard model where strategic interaction leads to suboptimal mitigation. Geoengineering introduces the possibility of technical substitution away from mitigation, but it also affects the strategic interaction across countries: mitigation decisions directly affect geoengineering decisions. With similar countries, I find these strategic effects create greater incentives for free-riding on mitigation, but with asymmetric countries, the prospect of geoengineering can induce inefficiently high levels of mitigation.