Evolution of Outdoor Thermal Stress Under Heatwaves: Comparative Analysis Based on Multiple Indices Over Mainland China

Abstract As global warming intensifies, heatwaves (HWs) occur with increasing frequency. However, the extent of the synergistic interaction between outdoor thermal stress ( TS ) and HWs, as well as their climatic drivers, is yet to be fully understood. This study examines the spatiotemporal characteristics and climatic determinants of TS across China from 1990 to 2017, utilizing historical meteorological data and multiple composite thermal stress indices. Results reveal no significant trend in TS during HWs, whereas an increasing trend is observed during non‐HW conditions. Despite smaller temperature variability, southern regions experience greater increases in thermal sensation than northern regions during HWs, as small changes in absolute temperature correspond to large increases in temperature percentiles. Moreover, TS responds differently to climatic background, as hotter regions tend to experience a smaller increase in TS . In dry areas, the increase in TS intensifies with increased precipitation, while it exhibits a decreased trend in transitional areas and remains stable in wet areas. These changes in TS are primarily driven by the changes in air temperature ( T a ) across different climate contexts. Notably, the intensification of the urban heat island intensity (UHII) reduced the increase in TS in urban areas during HWs. Various thermal stress indices respond differently to the climatic drivers. As a widely used metric, wet bulb globe temperature ( T w ) exhibits the smallest increase with rising T a , potentially indicating an underestimation of TS . These findings provide critical insights into the spatiotemporal dynamics of TS over China for developing heat adaptation strategies to mitigate the TS during HWs.