Fig. 2. (a)–(c) Daytime and (d)–(f) nighttime mean 2 m temperatures in the SSP2-4.5 scenario in the 2010s, 2040s, and 2090s. Dotted and slashed areas represent areas with temperatures more than 33 °C (first row) and 28 °C (second row) respectively. Red points indicate major cities and their names in the PRD.
Fig. 3 shows the daytime and nighttime hourly mean 2 m temperature in the 2040s and 2090s in the SSP1-2.6 and SSP5-8.5 scenarios. The spatial patterns among the scenarios do not vary considerably in the 2040s because the CO2 emission levels in all scenarios will continue to rise until the mid-century (IPCC 2021). However, the situation in SSP1-2.6 is different from those in the other two scenarios in the 2090s. The SSP1-2.6 scenario is projected to achieve carbon neutrality after 2050, resulting in a less intense heat profile in the PRD during the 2090s compared to the 2040s. This is characterized by a decrease in daytime and nighttime temperatures by -0.3 °C and -0.2 °C respectively, along with a reduction in the size of hot areas (-10% and -7% for daytime and nighttime respectively).
On the other hand, the situation is pessimistic for the worst-case scenario, SSP5-8.5. In the 2090s, the daytime and nighttime hourly mean 2 m temperature will increase to 34.6 °C and 30.3 °C, respectively. The mean state of most locations in the PRD will even exceed the current local extreme thresholds for health cautions. In the 2090s, it is projected that over 86% and 96% of land areas will experience daytime and nighttime hourly mean 2 m temperatures exceeding 33 °C and 28 °C, respectively. Only a few areas with elevated terrain height are expected to fall below these temperature thresholds.
To examine the impact of urbanization on temperature simulation, we calculated the mean temperature of persistent rural and rural-to-urban grid cells under SSP2-4.5. Grid cells other than urban and water bodies are classified as rural grid cells because they share similar characteristics of agricultural activities, natural vegetation, open spaces, and lower population densities. Persistent rural grids refer to the grid cells that remains rural from the 2010s to the 2040s, while rural-to-urban grid cells refer to the cells that transition from rural to urban between these periods. Synoptic systems mainly affect the temperature change of persistent rural grid cells from the 2040s to 2090s, while both synoptic systems and land use change affect the temperature of rural-to-urban grid cells in the same periods. The temperature difference between persistent rural and rural-to-urban grid cells in the 2040s and 2090s compared to the 2010s can be attributed to the impact of land use change on temperature simulation. There are a total of 1572 grid cells changing from rural to urban in the 2040s. The mean 2 m temperature of persistent rural grid cells in the 2010s, 2040s, and 2090s are 28.11 °C, 28.89 °C, and 29.72 °C, respectively. For rural-to-urban grid cells, the mean 2 m temperature was 28.87 °C, 30.27 °C, and 31.11 °C in same periods. The grid cells that underwent urbanization had a larger mean 2 m temperature in the 2010s than those persistent rural grids, and they will experience a greater temperature increase in the 2040s, and 2090s. The temperature increases for persistent rural grid cells and rural-to-urban grid cells in the 2040s and 2090s compared to the 2010s are 0.78 °C and 1.4 °C, and 1.62 °C and 2.24 °C, respectively. Therefore, we assume the land use change contribute to 0.84 °C in both 2040s and 2090s. The contribution percentage of land use change in the 2040s is 51.9% and will reduce to 37.5% in the 2090s.