Given the possibility of irreversible, anthropogenic changes in the climate system, technologies such as solar radiation management (SRM) are sometimes framed as possible emergency interventions. However, little knowledge exists on the efficacy of such  deployments. To fill in this gap, we perform Community Earth System Model 2 (CESM 2) simulations of an intense warming scenario on which we impose gradual early-century SRM or rapid late-century cooling (an emergency intervention), both realised via stratospheric aerosol injection (SAI). While both scenarios cool Earth's surface, ocean responses differ drastically. Rapid cooling fails to release deep ocean heat content or restore an ailing North Atlantic deep convection but partially stabilizes the Atlantic meridional overturning circulation. In contrast, the early intervention effectively mitigates changes in all of these features. Our results suggest that slow ocean timescales impair the efficacy of some SAI emergency interventions.