| Summary: | Many Solar Radiation Management model experiments simulate ordered, early-onset scenarios. We investigate the undesirable scenario in which SRM is first rejected, and then implemented in a rush (``emergency brake’’). To this end, we perform a simulation in which Stratospheric Aerosol Injection (SAI) is implemented in 2080 against a background SSP5-85 scenario. The outcome is compared to a simulation where SAI starts in 2020. In both cases, SAI intensity is chosen such as to achieve a target temperature, 1.5 degrees above pre-industrial, by means of a feedback control loop.This feedback method has originally been developed for the Geoengineering Large Ensemble (GLENS; Tilmes et al., 2018) simulations and follow-ups, which used the CESM-WACCM model (i.e. including full stratospheric chemistry). We adapted the technique to work with the computationally much cheaper CESM2-CAM model (without stratospheric chemistry). The early-onset simulation stabilizes global mean surface temperature target, and the emergency brake simulation reaches the target a few years after onset. However, other climate outcomes differ. In particular, the North Atlantic Overturning Circulation (AMOC) weakens strongly before SAI onset in the emergency brake simulation and is not restored to the (also somewhat weakened) state reached in the early-onset simulation. The combination of a weakened AMOC and sudden SAI lead to a cooling below pre-industrial values in the North Atlantic region. Our results cast doubt on the viability of SAI as an emergency brake to be used “only in case we need it”: That approach might lead to persistent long-term or even irreversible climate change.
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