## 2 Data and Study Area
### 1.5 \({}^{\circ}\)C and 2.0 \({}^{\circ}\)C Warmer Scenarios  
There are two approaches (i.e., the transient and the stabilized approach) to evaluate climate change impacts under the 1.5 \({}^{\circ}\)C and 2.0 \({}^{\circ}\)C warming levels. The stabilized approach is based on the simulation by the recent Half a Degree Additional Warming, Projections, Prognosis and Impacts project (HAPPI), which is designed to provide stabilized scenarios for the 1.5 and 2.0 \({}^{\circ}\)C warming worlds [18; 35; 36]. The transient method is based on transient climate states extracted from the CMIP5 archive to evaluate climate change impacts at specific global temperature targets. In terms of the difference between these two approaches, a recent study reported that the stabilized scenarios from HAPPI are largely consistent with the transient scenarios extracted from CMIP5 simulations in agricultural regions [37]. However, so far no clear conclusion has been reached.  
In this study, we chose the transient method to evaluate climate change. In order to simulate the change of heatwave at under 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C global warming targets, analyses are performed using time-slice periods following the method in previous studies ([PERSON] et al.; [PERSON] et al.; [PERSON] et al.; [PERSON] et al.) [37; 38; 39; 40]. We specifically assessed the projected change of the heatwave as well as population exposure between 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C warmer future over eight climate regions in China, which can help to quantify the potential benefit of limiting warming to 1.5 \({}^{\circ}\)C.  
According to previous studies [38; 39; 41], the spread range of the chosen 10 CMIP5 model threshold-crossing times of 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C global warming above pre-industrial levels under RCP4.5 and RCP8.5 scenarios are shown in Figure 1. The MME timing of CMIP5 models when the 20-yr averaged Global Mean Temperature (GMT) reaches 1.5 and 2.0 \({}^{\circ}\)C above the pre-industrial period is presented in Figure 1.  
The global warming would exceed the 1.5 \({}^{\circ}\)C threshold in 2031 and 2028 under the RCP4.5 and RCP8.5 scenarios, respectively. The 2 \({}^{\circ}\)C global warming will occur around 2049 and 2041 under the  
Figure 1: The box-plot of threshold-crossing times of 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C global warming above pre-industrial levels for the 10 CMIP5 models used in this study, the horizontal line within boxes and the number above the boxes represent the median result and the cross label indicates the mean result.  
RCP4.5 and RCP8.5 scenarios, respectively (Figure 1). Obviously, the time to reach the two targets under RCP8.5 is shorter than that under RCP4.5. As shown in Figure 1, there is a large difference among the ten models for threshold-crossing times of 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C warming. For both warming targets, the spread range for the 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C reaching time under RCP8.5 is narrower and more consistent in comparison with those under RCP4.5.  
Thus, according to Figure 1, 1.5 \({}^{\circ}\)C scenarios warming will be during 2021-2040 and 2018-2037 under RCP4.5 and RCP8.5, respectively, and the period of MME projection during 2041-2060 and 2030-2050 is for 2.0 \({}^{\circ}\)C warming scenarios. Although we just used 10 CMIP5 models, the MME threshold-crossing times of 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C are very close to the result based on 27 CMIP5 models [42] and the result of [PERSON] et al. by using 20 CMIP5 [43]. That means the chosen ten models have a good representation.