Future Slower Reduction of Anthropogenic Aerosols Enhances Extratropical Ocean Surface Warming Trends

Global surface temperature short-term trends fluctuate between cooling and fast-warming under the combined action of external forcing and internal variability, significantly influencing the detectability of near-term climate change. A key driver of these variations is anthropogenic aerosols (AAs), which have undergone a non-monotonic evolution with rapid reduction in recent decades. However, their reduction is projected to decelerate under a high carbon emission scenario, yet the impact on surface temperature trends remains unknown. Here, using initial-perturbation large ensembles, we find that future slowdown in AA reduction over Europe and North America expedites the subpolar North Atlantic surface warming by intensifying the Atlantic meridional overturning circulation. Further, it accelerates the South Indian Ocean and Southern Ocean surface warming through positive low-cloud feedback and oceanic dynamical adjustment, triggered by the poleward migration of westerlies under interhemispheric energy constraint. These AA-driven warmings exacerbate greenhouse warming, significantly enhancing the detectability of local decadal warming trends. Global surface temperature rises non-monotonically, exhibiting phases of accelerated warming, warming slowdown, and transitions between warming and cooling. Anthropogenic aerosols (AAs) are a key driver of historical trend variations, making human-induced trends more detectable. However, AAs follow a nonlinear trajectory with a rapid decline over Europe and North America in recent decades, and such reduction is projected to slow down in the future. The saturation of AA reduction signifies a diminishing proportion of anthropogenic forcings, which were previously considered unrelated to future global warming. Nevertheless, based on large ensembles of climate change simulations, we reveal that this projected slowdown in AA reduction could significantly accelerate surface warming in the subpolar North Atlantic, South Indian and Southern Oceans. This is achieved by the strengthening of Atlantic meridional overturning circulation and the coupled oceanic-atmospheric adjustments triggered by the southward shift of southern westerlies. Those AA-driven ocean surface warmings exacerbate greenhouse warming, significantly enhancing the predominance of external signals over strong internal noise and thus the detectability of regional human-induced decadal warming trends. It suggests that upholding stringent AA mitigation policies is imperative for effectively mitigating the risk of severe ocean warming. Recent rapid reduction of anthropogenic aerosols (AA) over the northern extratropics would slow down under a high carbon emission scenario Slower AA reduction accelerates subpolar North Atlantic surface warming by intensifying Atlantic meridional overturning circulation Slower AA reduction accelerates South Indian and Southern Ocean warming via positive low-cloud feedback and oceanic dynamical adjustment