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 fluctuations 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 and signal-to-noise-maximizing pattern filtering method, the emergence timescale is assessed as an indicator for the detectability of externally-driven surface temperature trends across varying temporal spans, which signifies the predominance of externally-forced signals over internally-generated noises. We find that future slowdown in AA reduction across the extratropical Northern Hemisphere expedites the subpolar North Atlantic surface warming by intensifying the Atlantic meridional overturning circulation, enhancing the detectability of externally-driven surface temperature trends over a span of three decades. Further, such slower AA reduction over extratropical Northern Hemisphere accelerates the South Indian Ocean and Southern Ocean surface warming through positive low-cloud feedback and oceanic dynamical adjustment. It is triggered by the southward migration of the Southern Hemisphere westerlies under interhemispheric energy constraint. These AA-driven warmings exacerbate greenhouse warming, significantly enhancing the detectability of local decadal warming trends. Upholding stringent AA mitigation policies is imperative for effectively mitigating the risk of severe ocean warming.