Air quality impacts of stratospheric aerosol injections are likely small and mainly driven by changes in climate, not aerosol settling

Stratospheric aerosol injection (SAI) is a proposed climate intervention method to offset future global warming through increased solar reflection in the stratosphere, but its broader environmental and public health implications are yet to be thoroughly explored. We use three large ensembles of fully coupled CESM2-WACCM6 simulations to assess changes in mortality attributable to fine particulate matter (PM2.5) and surface ozone exposure (O3). Maintaining temperatures at 1.5 °C above preindustrial levels through SAI is projected to yield a modest 0.4 % (ensemble range: −1.9 % to +1.5 %) reduction in pollution-related mortality relative to middle-of-the-road climate change scenario, reflecting a 1.3 % (−2.3 % to −0.6 %) reduction in ozone-related deaths and a 0.9 % (−0.4 % to +2.1 %) increase in PM2.5-related deaths. The spread among ensemble members underscores the influence of internal variability and highlights the importance of ensemble-based analyses when assessing the potential health impacts of climate intervention strategies. We find that global PM2.5 mortality changes exhibit little sensitivity to injected sulfate amounts, with the most variability driven by precipitation-mediated changes in non-sulfate PM2.5 species (e.g., dust and secondary organic aerosols), whereas ozone-related mortality is primarily driven by surface cooling and hemispheric asymmetries in stratospheric-tropospheric exchange and ozone transport. However, our results heavily reflect the specific forcing patterns of the SAI scenarios used; our estimates are also limited by model shortcomings, including omitting the effects of aerosols in the photolysis scheme – which might limit UV-driven changes and impact surface ozone rates – or not including nitrate aerosols. Within our framework, we find that SAI impacts on pollution-related mortality are modest but regionally heterogeneous, and that the magnitude of the SAI-driven changes is smaller than the improvements expected from near-term air quality policies planned or implemented within the same time frame.