Projected changes in risk of heat waves throughout Belt and Road Region in the 21st century

The "Belt and Road Initiative", an important national measure undertaken during the reform and opening-up policy of China, includes the Silk Road Economic Belt and 21st Century Maritime Silk Road. The Belt and Road Initiative involves over four billion people from more than 60 countries; therefore, it is imperative to make accurate projections of future climate changes for improved mitigation planning at the national level. Extreme heat waves may have significant impacts on several aspects of natural ecosystems and society, prompting considerable concern; thus, research is necessary with respect to potential changes in the frequency and intensity of heat waves in the context of global warming. Increasing heat wave related disasters in a potentially warmer future may pose threats to the sustainable development of the Belt and Road Region (BRR)-A region with various complex climates and a weak economic foundation. Heat wave disasters that are accompanied by high temperatures and humidity can threaten human health. Traditional research on heat waves focuses on the effects of temperature; however, taking the comprehensive effect of temperature-humidity into consideration can better explain heat stress on the human body in a high-temperature environment and better measure the intensity of heat waves. Therefore, we adopted a heat wave index (a widely used indicator for the synergistic effect of temperature and humidity) developed by the National Oceanic and Atmospheric Administration to characterize the health risks associated with heat waves, as well as to further project the changes in areal and population exposure to heat waves in the 21st century at different risk levels throughout the BRR under two typical climatic-socioeconomical scenarios (SSP2-4.5 and SSP3-8.5). Our analysis was based on state-of-the-art global climate models that are a part of the Inter-Sectoral Impact Model Intercomparison Project and a population projection dataset developed by the National Center for Atmospheric Research and the City University of New York's Institute for Demographic Research. Our results indicated that the intensity and frequency of heat waves in most areas throughout the BRR showed various increasing trends. Under the SSP2-4.5 and SSP3-8.5 scenarios, the sum of the population exposure to all levels of extreme heat waves in the BRR will increase to 2.0 and 3.3 times, respectively, by the end of the 21st century compared with that of the reference period (1986-2005). Furthermore, the relative increase in population exposure will be more remarkable for heat waves associated with higher risk levels; namely, the population exposure to heat waves at low, medium, and high risk levels will increase to 1.3, 2.1, and 8.9, respectively, under SSP2-4.5 and 1.5, 2.9, and 28.2 times, respectively, under SSP3-8.5 compared with that of the reference period. Developing countries in South Asia and Southeast Asia with large populations will suffer the most prominent increase in population exposure to heat waves. Typically, climatic factors play a major role in the increase of overall population exposure and dominate the increase in population exposure to heat waves at higher risk levels. However, the relative magnitude of the contribution of climatic and demographic factors varied across the regions. Our results indicated that unlike the air temperature, changes in the heat-humidity indices were significantly greater at lower latitudes, where only slight increases in the mean may be required to substantially enhance the frequency of dangerous conditions. Future construction of the Belt and Road must establish an early warning system for high-risk areas based on risk evaluations for extreme heat wave events. Additionally, it will be necessary to strengthen the vulnerability assessments for different areas and industries in the BRR and to establish an effective technical system to cope with extreme events.