Assessing the escalating human-perceived heatwaves in a warming world: The case of China

With increased global warming, heatwaves are expected to become more intense, frequent, and persistent. Although the spatiotemporal characteristics of heatwaves have been extensively studied, the vast majority of these studies have solely used near-surface air temperatures, particularly daily maximum temperatures (T_max), to identify heatwaves. Given that air temperature alone proves inadequate as a metric for human heat stress. Here, using the relative threshold in conjunction with the absolute threshold and basing it on wet bulb globe temperature (WBGT), we develop a novel definition of human-perceived heatwaves. The combined effect of temperature and humidity is considered in this definition. On this basis, we quantify the climatology of and long-term changes in heatwaves in China based on homogenized in situ observations and outputs from climate models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6). The results show that the distribution of human-perceived heatwaves coincides with densely populated areas in the southeastern part of China, despite their limited spatial extent. The observed trends in human-perceived heatwaves have accelerated since the 1960s. It is now anticipated that moderate or worse human-perceived heatwaves will affect more than half of China's population. Moreover, CMIP6 climate projections suggest that the percentage of China's population exposed to historically unprecedented human-perceived heatwaves would increase rapidly in a warmer future, except for the sustainability scenario. It is noted that the increase in severe human-perceived heatwaves is more rapid than that in severe traditional T_max-based heatwaves, suggesting that the hazard of heatwaves to humans may have been underestimated by previous T_max-based studies. Our findings demonstrate the urgent need for additional planning and adaptation actions beyond the framework for short-term disaster reduction frameworks currently in place. Although we concentrated on China in this article, our method for evaluating human-perceived heatwaves is easily extended to handle comparable issues everywhere.