Fig. 9. (a), (b), (c) Spatial plot of mean summer daytime heatwave temperature (HWT) in the 2010s, 2040s, and 2090s. (d), (e) Spatial plot of the differences in the mean summer daytime heatwave temperature between the 2040s and 2010s and those between the 2090s and 2010s. All figures correspond to the SSP2-4.5 scenario.
The nighttime heatwave metrics were also evaluated because consecutive hot nights may pose a more significant threat to human health than very hot days. Lower temperatures at night typically allow people to recover from the daytime heat, and the occurrence of hot nights can reduce this effect, which may lead to excess death tolls (Kovats and Hajat 2008). Moreover, if consecutive very hot days occur with consecutive hot nights, the health impacts may be more severe than those associated with solely consecutive hot days or consecutive hot nights (Thomas et al. 2020). Among the human population, females and the elderly are more vulnerable to extremely hot weather (Wang et al. 2021). Fig. 10 displays the spatial patterns of the number of very hot nights (HNF) across different decades. In the 2010s (Fig. 10 (a)) rural areas had fewer than 10 very hot night. As urbanization continues to expand, more regions are expected to experience more frequent hot nights in the future. The spatial pattern of HNF difference (Fig. 10 (d)) highlights urbanization as a contributor, with more rural grid cells becoming urbanized and hot spots. For the PRD, the spatial mean difference in the HNF is 21.7 and 50.9 nights in the 2040s and 2090s, respectively, considerably larger than the mean difference in the HDF (14.6 and 28.7 days in the 2040s and 2090s, respectively). This increase will make it challenging for people to recover from the daytime heat at night. Compared with the spatial pattern of the HDF increase, which is concentrated in coastal areas, the HNF increase is more significant and spatially dispersed, covering most of the land in the PRD.