a. Mean state of future thermal environment in different scenarios
1) Decadal mean 2 m temperature in the PRD
CMIP6-driven WRF hourly output data were used to calculate the mean 2 m temperature over the 2010s, 2040s, and 2090s in different scenarios. Simulations for the 2010s in the SSP2-4.5 scenario were treated as the baseline for comparison.
Fig. 2 shows the spatial pattern of the hourly mean 2 m temperature in different decades in the SSP2-4.5 scenario in the daytime (11:00 AM–4:00 PM, Local Standard Time (LST)) and nighttime (1:00 AM–6:00 AM, LST). The spatial mean values are summarized in the Table 2. SSP2-4.5 is the medium pathway of future GHG emissions, assuming the historical trend remains unchanged in the future. At present, the hourly mean 2 m temperature in the daytime over the land area of the PRD is 31.5 °C. This temperature is expected to increase by 0.7 °C and 1.5 °C in the 2040s and 2090s, respectively. The nighttime hourly mean 2 m temperature in the 2010s is 26.9 °C and is expected to increase by 0.9 °C and 1.8 °C in the 2040s and 2090s, respectively. The rate of increase of the nighttime temperature is higher than that of the daytime temperature. The larger increase in the nighttime temperature may be caused by the increase in absorbed downward longwave radiation and anthropogenic heat. The diurnal temperature range in the 2040s is 0.1 °C lower than that in the 2010s, while in the 2090s it is 0.3 °C lower than in the 2010s, which can increase the prevalence of heat-related diseases. The hottest locations in the PRD in both daytime and nighttime were Guangzhou and Foshan in the 2010s, but the hot area (denoted by dots) is expected to considerably expand in the future decades. The extent and intensity of this expansion will become highly severe by the 2090s, with 60% of the land area reaching a mean hourly temperature of more than 33 °C in the daytime and even more regions (79%) reaching a nighttime mean hourly temperature of more than 28 °C. Compared with the 2010s, the average spatial coverage of this hot area in the 2040s is 4 and 8 times larger in the daytime and nighttime, respectively. For the 2090s, the extent is 8 and 13 times larger in the daytime and nighttime, respectively.