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.