## 4 Results and Discussion  
### Projected Heatwave Changes in the 21 st Century  
#### 4.1.1 Temporal-Spatial Heatwave Changes in the 21 st Century  
Figure 3 shows the change trends of HWDI, N_HW and T_HW under the RCP4.5 and RCP8.5 scenarios during the 21 st century. Although there is a remarkable difference among the spatial patterns for the intensity, frequency and total heatwave days change rate on a decade scale, all three indices are projected to increase significantly over China under both scenarios. Overall, China is projected to experience a significant increase of HWDI (1.07 days/decade and 2.90 days/decade) and T_HW (3.78 days/decade and 14.59 days/decade) and N_HW (0.40/decade and 1.26/decade) in the 21 st century under RCP4.5 and RCP8.5. Figure 3: Spatial pattern of increase rate for HWDI (**a**,**b**), N_HW (**c**,**d**), and T_HW (**e**,**f**) during the 21 st century under RCP4.5 (left: **a**,**c**,**e**) and RCP8.5 (right: **b**,**d**,**f**) by using NEX-GDDP multi-model ensemble (MME) results. The cross hatch in the image represents that there is no significant change trend at the 0.05 significance level in this pixel. Under RCP4.5, the largest increase for HWDI will be in Northwest China and Tibetan Plateau, with more than 5 days/decade on pixel scale. Moreover, Central China also is projected to experience a larger increase for HWDI than its surrounding regions. The projection under RCP8.5 shows that the increment t of HWDI is greatest in the Tibetan Plateau with a rate of 13 days/decade, closely followed by Northwest China (5 days/decade). Similar to RCP4.5, the increase rate over Central China is higher than that in neighboring regions. Though the spatial pattern is quite similar under the two scenarios, there is a remarkable difference between the change rate of N_HW, which under RCP8.5 is much higher than that under RCP 4.5. The increase rate for N_HW is projected to reduce from 1.4/decade over Northwest China to 0.203/decade over South China along with the northwest-southeast direction over China under RCP8.5 (Figure 3d). Under RCP4.5, the largest increase for N_HW will happen in Northwest China (0.46/decade), closely followed by central China (0.42/decade) and Tibetan Plateau and (0.37/decade), and the change rate in coastal regions and Southwest China (around 0.15/decade) is lowest (Figure 3c). The spatial pattern of change rate for T_HW is also similar under two scenarios. The value of T_HW increase rate is projected to be the largest in Tibetan Plateau (3.7 days/decade and 16.7 days/decade), then next in Northwest China (4.3 days/decade and 16.3 days/decade), and Central China (3.6 days/decade and 13.6 days/decade under RCP4.5 and RCP8.5, respectively) is closely followed. In general, the increase rate for T_HW under RCP8.5 scenario is around four times larger than that under RCP4.5 scenario, which shows the great impact of higher greenhouse emissions. Figure 4 shows the temporal variation of HWDI, T_HW and N_HW over eight sub-regions relative to 1960-1991. As presented in Figure 4, the increasing trends on a regional scale for HWDI, T_HW and N_HW over China are more notable under both scenarios. In general, in contrast to the nearly linear increasing trend under RCP8.5, the increases of the three indices are projected to reach a peak in the 2080s under RCP4.5. The increasing amplitude for three indices under RCP4.5 is significantly lower than that under RCP8.5. Taking N_HW for example, the increase amplitude is in the range of 6.4-12 under RCP8.5, which is almost three times of that (1.65-3.75) under RCP4.5 in the later 21 st century. The anomalies of HWDI, T_HW and N_HW show a similar spatial pattern, i.e., the change amplitude of these three indices over the higher altitude and latitude region is larger than that over southern China except for Central China. As shown in Figure 4a, the largest change of HWDI is found in Northwest China and Tibetan Plateau, while South China will experience the least change of HWDI.