## 4 Results and Discussion
### Population Exposure to Heatwaves in the 21 st Century
Note that although South China will experience the least increase for HWDI, the difference of HWDI between 1.5 \({}^{\circ}\)C and 2.0 \({}^{\circ}\)C scenarios is remarkably larger than that in Northwest China. In order to examine the benefit of limiting global warming by 0.5 \({}^{\circ}\)C under RCP4.5 and RCP8.5 we use the avoided impact (AI) to investigate the heatwave indices intensity change between 1.5 \({}^{\circ}\)C and 2.0 \({}^{\circ}\)C scenarios on a regional scale. In comparison to the 2 \({}^{\circ}\)C warmer future, the 1.5\({}^{\circ}\) warming future will help avoid approximately 20%-40% of the increase of the intensity of heatwaves. The decreasing amplitude will differ over different climate regions. In addition, except in East China, Central China and Northwest China, the decreasing amplitudes are projected to be larger under RCP4.5 than RCP8.5. It should be noted that the results presented in Figure 7 about the spatial pattern of HWDI change are completely different from the results recently reported by [PERSON] et al. (2018), who showed that the additional half degree of warming in the 2 \({}^{\circ}\)C warmer climate will result in more longer-lasting heatwave events across southern China than other regions over China. The discrepancy among the two works is partly caused by the data used. The dataset used in [PERSON] et al. is NCAR CESM low-warming simulation work, which is the stabilized projections at 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C warming levels. In addition, the definitions of heatwaves used in the two works are also different. In terms of heatwave frequency, the change magnitude of N_HW is relatively consistent between 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C warmer climate scenarios. Under RCP4.5 and RCP8.5, the increase of N_HW over northern China is larger than that over southern China. Northwest China is the region with the largest increase magnitude of N_HW (Figure 8a-d). Figure 7: Spatial distribution of HWDI change at 1.5 \({}^{\circ}\)C (**a,b**) and 2 \({}^{\circ}\)C (**c,d**) global warming levels under scenario RCP4.5 (**a,c**) and RCP8.5 (**b,d**) relative to 1961–1990 by MME projection. Regional average of HWDI anomaly (**e**) and avoided impact between global warming of 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C above pre-industrial levels in different scenarios(**f**). On the regional scale, the high latitude and high altitude regions are projected to experience a larger increase (1.0-1.5) than southern China (0.25-1.0) in 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C warmer futures as shown in Figure 8e. In addition, 1.5 \({}^{\circ}\)C warmer climate under RCP8.5 will avoid a larger increase magnitude of N_HW in comparison with under RCP4.5 over eight regions. In general, limiting global warming by 0.5 \({}^{\circ}\)C can help avoid 28%-46% of the increases in the frequency of heatwaves over China. In general, the spatial pattern of T_HW is similar to that of HWDI (Figure 7a,b) and N_HW (Figure 8a,b). T_HW is projected to increase by 8-15 days over high latitude and high altitude regions, and by 4-8 days over southern China in 1.5 \({}^{\circ}\)C warmer futures. The additional half-degree warming will lead to an increase of T_HW ranging from 12 to 19 days for high latitude and high altitude, and 7-15 days for southern China. The change of T_HW between 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C two scenarios is similar to N_HW (Figure 9e) in terms of regional features. Under RCP4.5, holding the global warming below 1.5 \({}^{\circ}\)C instead of 2 \({}^{\circ}\)C can avoid 27%-42% of the increases in T_HW, and it will be 32%-50% for RCP8.5. Figure 8: Spatial distribution of N_HW change for 1.5 \({}^{\circ}\)C (**a,b**) and 2 \({}^{\circ}\)C (**c,d**) global warming levels under scenario RCP4.5 (**a,c**) and RCP8.5 (**b,d**) relative to 1961–1990 by MME projection. Regional average of N_HW anomaly (**e**) and avoid impact between global warming of 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C above pre-industrial levels in different scenarios (**f**). Though [PERSON] et al.