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The possibility of overshooting global emissions targets has triggered a de-bate about the role of solar geoengineering (SGE)—using technologies to reflect solar radiation away from Earth—in managing climate change. One major concern is thatSGE technologies are relatively cheap and could potentially be deployed by a single country (the “free driver”). We develop a model to analyze how opportunities to de-ploy SGE impact global abatement and the effectiveness of international environ-mental agreements (IEAs). We show that noncooperative abatement may increase or decrease under the threat of SGE, depending on how damaging the free driver’s level of deployment is to others. When free-driver externalities are significant, other countries have additional incentives to abate—called anti-driver incentives—to reduce the free driver’s deployment. We also show that compared to a world withoutSGE opportunities, stable IEAs can be large (small) if anti-driver incentives are relatively strong (weak). 

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. 

Given the lack of progress on climate change mitigation, some scientists have proposed solar geoengineering as a means to manage climate change at least temporarily. One main concern with such a risky technological solution, however, is that it may create a “moral hazard” problem by crowding out efforts to reduce emissions. We investigate the potential for a risky technological solution to crowd out mitigation with theory and experiments. In a collective-risk social dilemma, players strategically act to cooperate when there is an opportunity to deploy a risky technology to help protect themselves from impending damages. In contrast to the moral hazard conjecture, the empirical results suggest that the threat of solar geoengineering can lead to an increase in cooperative behavior. 

This data supports the numeric example found in Section 5 of the manuscript. 

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. 

As the prospect of average global warming exceeding 1.5°C becomes increasingly likely, interest in supplementing mitigation and adaptation with solar geoengineering (SG) responses will almost certainly rise. For example stratospheric aerosol injection to cool the planet could offset some of the warming for a given accumulation of atmospheric greenhouse gases ( 1 ). However, the physical and social science literature on SG remains modest compared with mitigation and adaptation. We outline three research themes for advancing policy-relevant social science related to SG: (i) SG costs, benefits, risks, and uncertainty; (ii) the political economy of SG deployment; and (iii) SG’s role in a climate strategy portfolio.