## 4 Results and Discussion
### Projected Heatwave Changes in the 21 st Century
#### 4.1.2 Uncertainty of MME Heatwave Projections  
Box-and-whisker plots are employed to illustrate the uncertainty of MME results and the consistency of multi-model projection among the 10 models over China's climate regions. In general, the model spreads of HWDI, N_HW and T_HW under RCP8.5 are larger than that under RCP4.5 and the model spread will gradually increase in the 21 st century over all the regions, especially for RCP8.5. In addition, the consistency of GCMs projections over Northeast China and South China is relatively higher than other regions under RCP4.5 and RCP8.5 (Figure 5).  
Figure 5: _Cont._  
As shown in Figure 5, except the HWDI in the end of the 21 st century under RCP8.5, all the MME projections for the three indices over different periods are located in the 25 th-75 th quantile of multi-model projections range, which means the MME projection is a good indicator to represent the projection of the 10 CMIP5 model results.  
Note that the change of T_HW is projected to increase by 150 days at the end of this century by some models. This result implies that heatwaves are projected to occur in any season rather than just in summer. This is caused by the definition of heatwave used in the present work.  
Figure 5: Projection of HWDI (**a**), N_HW (**b**) and T_HW (**c**) anomaly over eight sub-regions during the different period in the 21 st century using ten CMIP5 models under scenario RCP4.5 (cyan color) and RCP8.5 (original color) respective to 1961–1990. The central rectangle boxes indicate the interquartile spread range from the 25 th to 75 th quantile, the MME medians are indicated by the horizontal lines within boxes, and the extreme ranges of models are indicated by whiskers.  
As mentioned in the introduction, previous works have systematically assessed the capacity of GCM from CMIP5 in reproducing the present climatology and the historical changes of heatwaves over China based on ground observations [57], NCEP reanalysis datset [14] and the \(0.5^{\circ}\times 0.5^{\circ}\) daily temperature dataset (CN05.1) over China [56; 58]. In general, the CMIP5 model could reproduce the spatial pattern of HWDI and the trend in the observed period, but there is large variation for the ability of GCM in CMIP5 and over the different regions in China. On the regional scale, for example, the results in [PERSON] et al. [58] conclude that CMIP5 tends to overestimate the HWDI over NEC, NC, SWC and underestimate the HWDI over EC in comparison with CN05.1. So far, there is no direct evaluation for the NEX-GDDP for HWDI except over Pakistan [59]. However, the ability of NEX-GDDP for daily temperature, precipitation as well as precipitation extremes over China have been evaluated over China [60; 61]. The assessment provides that NEX-GDDP largely reduces the bias of temperature near-term and long-term projections in GCMs in terms of the range of absolute values as well as the spatial distribution and extremes across China [60]. Therefore, we believe that the ability of NEX-GDDP for heatwave simulation and projection have been significantly improved in comparison with the original CMIP5. Despite all this, the evolution of NEX-GDDP for HWDI is still very meaningful and needed in the future in order to understand the possible bias in heatwave-related indices projection.