Climate change mitigation strategy under an uncertain Solar Radiation Management possibility

Solar radiation management (SRM) could provide a fast and low-cost option to mitigate global warming, but can also incur unwanted or unexpected climatic side-effects. As these side-effects involve substantial uncertainties, the optimal role of SRM cannot be yet determined. Here, we present probabilistic emission scenarios that limit global mean temperature increase to 2 °C under uncertainty on possible future SRM deployment. Three uncertainties relating to SRM deployment are covered: the start time, intensity and possible termination. We find that the uncertain SRM option allows very little additional GHG emissions before the SRM termination risk can be excluded, and the result proved robust over different hypothetical probability assumptions for SRM deployment. An additional CO2 concentration constraint, e.g. to mitigate ocean acidification, necessitates CO2 reductions even with strong SRM; but in such case SRM renders non-CO2 reductions unnecessary. This illustrates how the framing of climatic targets and available mitigation measures affect strongly the optimal mitigation strategies. The ability of SRM to decrease emission reduction costs is diminished by the uncertainty in SRM deployment and the possible concentration constraint, and also depends heavily on the assumed emission reduction costs. By holding SRM deployment time uncertain, we also find that carrying out safeguard emission reductions and delaying SRM deployment by 10 to 20 years increases reduction costs only moderately.