Abstract: Based on four dynamical downscaling simulations by the regional climate model RegCM4 (i.e., CdR, EdR, HdR, and MdR) and population projection dataset under SSP2-RCP4.5, the characteristics of the cluster high temperature events (CHTEs) in China under 1.5℃ and 2℃ global warming were projected. The multi-model ensemble (MME) mean projection indicates the annual frequency increases by 31% and 44% respectively at the 1.5℃ and 2℃ warmer worlds compared with the reference period, and the extreme CHTE increases by 4.2 and 6.8 times respectively. Besides, the proportion of the CHTE indicators such as the cumulative intensity, duration, frequency falling into ‘large value range’ is expected to increase substantially. Compared with the CHTE under the 2℃ warming, a significant decrease of CHTE metrics across the country is projected at the 1.5℃ target, with remarkable regional differences. The frequency of CHTE decreases gradually from north to south. The duration decreases significantly by more than 6 days in Xinjiang and south of the Yangtze River (national averaged value is 0.2 days). The cumulative intensity decreases by more than 20℃ in the east-central China, and by more than 50℃ in the eastern part of Xinjiang (national averaged value is 0.6℃). In addition, under global warming of 1.5℃ and 2℃, MME projects that a widespread increase can be clearly found in the population affected by the CHTEs, with a significant decrease in the Inner Mongolia region and an increase in the central and eastern regions. The national population affected by the CHTEs increases by 1.4 times and 1.8 times, respectively. The urban population affected by the CHTEs increases more significantly (by 2.9 times and 3.8 times respectively), particularly with the most significant increase in the Beijing-Tianjin-Hebei region, Yangtze River Delta, Pearl River Delta, and Central China. The intensity exposure shows a significant increasing trend (by 2.2 and 5.2 times respectively) under the 1.5℃ and 2℃ global warming, the same as the comprehensive exposure (by 1.2 and 1.8 times respectively). By contrast, the CHTE intensity exposure and comprehensive exposure on the urban population increase by up to 10 and 4 times respectively under the 2℃ global warming.

Key words: Cluster high temperature event (CHTE), Global warming target, Regional climate model, Population exposure, Ensemble projection