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As the international community continues to fall short on reducing emissions to avoid disastrous impacts of climate change, some scientists have called for more research into solar geoengineering (SGE) as a potential temporary fix. Others, however, have adamantly rejected the notion of considering SGE in climate policy discussions. One prominent concern with considering SGE technologies to help manage climate change is the so-called “free driver” conjecture. The prediction is that among countries with different preferences for the level of SGE, the country that prefers the most will deploy levels higher than the global optimum. This paper tests the free-driver hypothesis experimentally under different conditions and institutions. We find that aggregate deployment of SGE is inefficiently high in all settings, but slightly less so when players are heterogeneous in endowments or when aggregate deployment is determined by a best-shot technology. Despite persistent inefficiencies in SGE deployment, free-driver behavior, on average, is less extreme than the theoretical predictions. 

As international efforts to mitigate greenhouse gases continue to fall short of global targets, the scientific community increasingly debates the role of solar geoengineering in climate policy. Given the infancy of these technologies, the debate is not yet whether to deploy solar geoengineering but whether solar geoengineering deserves consideration and research funding. Looming large over this discussion is the moral hazard conjecture – normalizing solar geoengineering will decrease mitigation efforts. Using a controlled experiment of a collective-risk social dilemma that simulates the strategic decisions of heterogeneous groups to mitigate emissions and deploy solar geoengineering, we find no evidence for the moral hazard conjecture. On the contrary, when people in the experiment are given the option to deploy solar geoengineering, average investment in mitigation increases.