## 4 Results and Discussion
### Population Exposure to Heatwaves in the 21 st Century  
As presented in Section 4.1, China is projected to experience more long, more frequent and more intense heatwaves with global warming. However, in most of China's areas, such as the sparsely populated Northwest China and Tibetan Plateau, the heatwaves would not cause a significant negative socio-economic effect, especially for public health. According to the projected spatial distribution of T_HW on decadal, we employ China population density grid map in 2010 with a 1 km spatial resolution (DOI:10.12078/2017121101) to analyze the population exposure to heatwaves in the 21 st century. The percentage of the population affected by heatwaves (red line) is shown by the line on the right axis (Figure 6). Obviously, there is a significant increasing trend for population exposure, indicating that more and more people will be threatened by heatwaves with global warming in the 21 st century. The total population affected by heatwaves is projected to reach 1.18 billion in the middle and later part of the 21 st century. Figure 6: The projected temporal variation of total population (red line) threatened by the heatwave as well as the relative ratio of population influenced by different level of T_HW (bar) in the 21 st century under RCP4.5 (**a**) and RCP8.5 (**b**), with the 2010 China Population density map as the reference. There are some differences for the population exposure temporal variation in the early part of the 21 st century between the two scenarios. Under RCP4.5, there is a rapid increase for the population exposure from the 2000s to 2070s, then the population exposure will gradually increase and remain stable until the end of the 21 st century, while under RCP8.5, the population exposure will increase quickly to the highest level (1.18 billion) in the 2050s. The percentages of the population affected by different levels of heatwave days (T_HW) are shown by the bar on the left axis (Figure 6). Although there is no significant difference for the total population threatened by heatwavea at the later 21 st century, the ratio for population exposure under longer time heatwaves, i.e., T_HW, is projected to increase. At the end of the 21 st century, more than 50% of 1.15 billion will be threatened by heatwaves which are longer than 25 days under RCP4.5, and the domain of heatwave persistent time will be longer than 65 days under RCP8.5. In addition, the ratio for the same level of longer T_HW under RCP8.5 is higher than that with RCP4.5. In other words, there will be a larger population suffering longer duration heatwaves in the future, especially under RCP4.5. Although the population density map used in this research is based on 2010 data, the analysis could still illustrate the increase of heatwave risk and reflect the serious impact of heatwaves under global warming over China in the 21 st century. Heatwave Changes Over China Under 1.5 \({}^{\circ}\)C and 2.0 \({}^{\circ}\)C Global Warming Future Scenarios  
As shown in Figure 7, the region with the largest increase is projected to be Northwest China under 1.5 \({}^{\circ}\)C and 2.0 \({}^{\circ}\)C global warming scenarios and the least increase under the two warming levels will occur in the west part of Southwest China and the coastal area in southern China. On the regional scale, the high latitude and high altitude regions are projected to suffer a larger increase of HWDI (3.9 days-5.9 days) than southern China (2.3 days-4.1 days) in 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C warmer futures. In 1.5 \({}^{\circ}\)C warmer climate, the HWDI is projected to increase by 4.2 days and 5.8 days in Northwest China and 1.2 days and 2.0 days in the south of China under RCP4.5 and RCP8.5, respectively, while in a 2.0 \({}^{\circ}\)C warmer climate, it will increase by 4.9 days and 7.0 days in Northwest China and 3.0 days and 3.7 days relative to 1961-1990. In other words, the extra 0.5 \({}^{\circ}\)C will result in 0.7-1.2 days and 1.7-1.8 days increase in HWDI over Northwest and South China, respectively.