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Abstract. A growing body of literature investigates the effects of solar radiation modification (SRM) on global and regional climates. Previous studies have focused on the potentials and the side effects of SRM, with little attention being given to possible deployment timescales and the levels of carbon dioxide removal required for a phase out. Here, we investigate the deployment timescales of SRM and how they are affected by different levels of mitigation, net-negative emissions (NNEs) and climate uncertainty. We generate a large dataset of 355 emission scenarios in which SRM is deployed to keep warming levels at 1.5 ∘C global mean temperature. Probabilistic climate projections from this ensemble result in a large range of plausible future warming and cooling rates that lead to various SRM deployment timescales. In all pathways consistent with extrapolated current ambition, SRM deployment would exceed 100 years even under the most optimistic assumptions regarding climate response. As soon as the temperature threshold is exceeded, neither mitigation nor NNEs or climate sensitivity alone can guarantee short deployment timescales. Since the evolution of mitigation under SRM, the availability of carbon removal technologies and the effects of climate reversibility will be mostly unknown at its initialisation time, it is impossible to predict how temporary SRM deployment would be. Any deployment of SRM therefore comes with the risk of multi-century legacies of deployment, implying multi-generational commitments of costs, risks and negative side effects of SRM and NNEs combined.

Abstract. A growing body of literature investigates the effects of Solar Radiation Modification (SRM) on global and regional climates. Previous studies have mainly focused on potentials and side-effects of SRM with little attention given to potential deployment timescales. Here, we look at a scenario that fails to achieve 1.5 °C-compatible mitigation and instead relies on SRM and Carbon Dioxide Removal (CDR) to avoid temperature rises above the threshold. Assuming SRM removes the incentive to increase mitigation beyond the currently pledged level of ambition, we assess SRM deployment lengths under three illustrative emission scenarios that follow current climate policy and are continued with varying assumptions about net-negative CDR (-11.5, -10 and -5 GtCO2yr-1). Under these assumptions, SRM would need to be deployed for around 245–315 years. We find only minor effects of SRM on the global net carbon flux decades after cessation. In total, around 976–1344 GtCO2 would need to be removed by CDR, much more than in so-called high-overshoot 1.5 °C scenarios. Our study points towards an additional risk of SRM that so far has received limited attention: Initialization and commitment to SRM would happen under the assumption that CDR can be scaled up sufficiently to allow SRM to be phased out again. In our scenarios, SRM would come with very long legacies of deployment, implying centennial commitments of costs, risks and negative side effects of SRM and CDR combined.