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A Moderate Mitigation Can Significantly Delay the Emergence of Compound Hot Extremes

Compound hot extremes (ChotEs) that refer to continuous heats throughout days and nights are projected to increase, causing more serious impacts on human health than daytime or nighttime heats alone. Previous studies have focused on daytime heats, but the timing of substantial increase in ChotEs relative to natural variability, which is defined as the time of emergence (ToE) for ChotEs, remains unknown. Here we examine ToE for duration of summertime ChotEs from coupled model intercomparison project phase 6 climate model projections under two shared socioeconomic pathway scenarios (i.e., SSP245 and SSP585). We further quantify the cumulative fraction of areal and population exposed to the emergence at global and continental scales. We find that, without implementation of climate mitigation policies (i.e., SSP585), global mean ToE is around 2062 (with 16%–84% uncertainty range of 2048–2072). On the basis of the ToE for each grid cell, 80.7% (with uncertainty range of 64.2%–96.7%) of global lands will expose to the emergence by 2080. Such substantial increases in ChotEs will lead to 75.2% (66.8%–93%) of global population exposed to the emergence by the end of 21st century. A moderate mitigation (i.e., SSP245) can delay the ToE by over 14 years and, more importantly, reduce the global land areal and population exposures by 50.3% and 39.7% respectively. Regionally, northern Europe, central America and western North America benefit the most. Therefore, early action towards moderate development socioeconomic pathways can remarkably cut back the possibility of large population exposure to ChotEs and relevant impacts.