## 4 Results and Discussion
### Population Exposure to Heatwaves in the 21 st Century
[27] also analyze the projections of heatwave characters for 1.5 \({}^{\circ}\)C and 2.0 \({}^{\circ}\)C global warmer targets, the spatial patterns of the heatwave frequency, duration and intensity are completely different from those in Figure 9. That is caused by the different heatwave definitions used in these two works. In general, the spatial pattern of HWDI, T_HW and N_HW in the 21 st century and in 1.5 \({}^{\circ}\)C and 2.0 \({}^{\circ}\)C warmer future scenarios show that the larger increase will be in the inland areas, especially over western China and the coastal region over China will experience the least increment. In theory, the heatwaves should increase with global warming and the spatial pattern should be consistent with that of temperature on a large scale, especially for the diurnal maximum temperature (TX). However, the heatwave is defined as prolonged periods of excessive heat (i.e., TX), which have special temporal character. This reason causes the discrepancy between the spatial pattern of heatwave change and the pattern of heatwave changes per \({}^{\circ}\)C warming average temperature in work of [PERSON]  
Figure 9: Spatial distribution of T_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 DME projection. Regional average of T_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**). et al. (2016) over China. In addition, the spatial pattern of heatwave trend is also similar to that of annual maximum daily maximum temperature (TXx) in [PERSON] et al. (2016) to some degree, which implies there is a higher probability for heatwaves under higher TX on an annual scale. Thus, the spatial pattern of the heatwave change trend is between that of the maximum temperature on diurnal and annual scales. In a word, the addition of a half-degree of warming will lead to a remarkable increase in intensity, frequency and total time for heatwaves. The increasing magnitude is higher over high latitude and high altitude regions than southern China under RCP4.5 and RCP8.5. Compared with 2.0 \({}^{\circ}\)C global warming, the half-degree less of warming will avoid 26.9% and 29.1% increase of HWDI, 34.7% and 39.64% for N_TW, and 35.3% and 40.10% of T_HW under RCP4.5 and RCP8.5, respectively. Population Exposure for 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C Global Warming Climate Scenarios  
Similar to the results in Figure 6, the percentage of the total population affected by heatwaves (red line) is shown on the right axis (Figure 10). Note that the total population exposure under RCP8.5 at 1.5 \({}^{\circ}\)C (1.07 billion) is lower than that under RCP4.5 (1.10 billion). However, the results illustrate that there is no notable difference between the population exposure under the two scenarios under 2.0 \({}^{\circ}\)C, and the total population affected by heatwaves is projected to reach 1.153 billion under 2.0 \({}^{\circ}\)C global warming. Under RCP4.5, holding the global warming below 1.5 \({}^{\circ}\)C instead of 2 \({}^{\circ}\)C can avoid 83 million people from suffering heatwaves, and this number will be 53 million people under RCP8.5. The percentages of the population threatened by different total days of heatwaves are shown through the bar on the left axis (Figure 10). The results in Figure 10 clearly illustrate that there is obviously a discrepancy between the two warming levels under the two RCP scenarios in terms of the ratio of the population suffering different heatwave durations. At the 1.5 \({}^{\circ}\)C warming level, people will be mainly threatened by T_HW ranging from 15 days to 25 days under RCP4.5, and the ratio of the population suffering T_HW 7 days-15 days will remarkably increase under RCP8.5 in comparison with RCP4.5.