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Although solar radiation management (SRM) might play a role as an emergency geoengineering measure, its potential risks remain uncertain, and hence there are ethical and governance issues in the face of SRM's actual deployment. By using an integrated assessment model, we first present one possible methodology for evaluating the value arising from retaining an SRM option given the uncertainty of climate sensitivity, and also examine sensitivities of the option value to SRM's side effects (damages). Reflecting the governance challenges on immediate SRM deployment, we assume scenarios in which SRM could only be deployed with a limited degree of cooling (0.5°C) only after 2050, when climate sensitivity uncertainty is assumed to be resolved and only when the sensitivity is found to be high (T2x = 4°C). We conduct a cost-effectiveness analysis with constraining temperature rise as the objective. The SRM option value is originated from its rapid cooling capability that would alleviate the mitigation requirement under climate sensitivity uncertainty and thereby reduce mitigation costs. According to our estimates, the option value during 1990-2049 for a +2.4°C target (the lowest temperature target level for which there were feasible solutions in this model study) relative to preindustrial levels were in the range between $2.5 and $5.9 trillion, taking into account the maximum level of side effects shown in the existing literature. The result indicates that lower limits of the option values for temperature targets below +2.4°C would be greater than $2.5 trillion.solar radiation management | option value | climate sensitivity | uncertainty | decision tree analysis U ncertainties surrounding equilibrium climate sensitivity (T2x) (1) pose a challenge to mitigation policymaking aimed at achieving the goal of limiting the temperature rise below certain levels (e.g., +2°C relative to preindustrial levels). The inertia of temperature rise would demand more stringent mitigation policy over the period of climate sensitivity uncertainty in preparation for possible occurrence of high climate sensitivity, because it is difficult to reduce emissions sufficiently rapidly even with all of the currently available technologies of mitigation.The possibility of higher climate sensitivity (2) and the inability to instantaneously control the temperature via mitigation have sparked positive debates about solar radiation management (SRM), especially about conducting research and development (R&D) on the topic (3, 4). SRM has been regarded as a rapid (5) and relatively inexpensive geoengineering measure compared with mitigation; in this study, our cost estimates are based on albedo enhancement via stratospheric sulfur injections (Materials and Methods). However, SRM's overall costs would be higher than simply the deployment costs alone when its environmental risks (6, 7) [e.g., polar ozone depletion (4, 8), changes in precipitation patterns (9)] are factored in, although the true magnitude of all of the risks is still unknown. Moreover, SRM cann...

Adaptation has the attributes of a private good that aims to protect a country from climate change, while mitigation, which contributes to the reduction of worldwide greenhouse gas emissions, has the properties of a public good. This paper attempts to investigate how the innovation of adaptation technology by a developed country (the North) may affect a developing country (the South) as well as the North through changes of mitigation in both countries. We show that the efficacy of adaptation determines the level of both countries' mitigation, and thereby causes them to be better or worse off. For both countries to be better off, it is required that the innovation be neither very effective nor very ineffective. Furthermore, we demonstrate that by introducing an international transfer system in which the North is taxed according to the level of adaptation, both countries can be better off even if the innovation is highly effective.