Heatwave events (HWs) have become more frequent and intense due to climate change and urbanization, posing risks to human health, yet the influence of rapid temperature fluctuation on human adaptation during these events remains insufficiently explored. This study identified HWs and estimated population exposure across three major urban agglomerations in eastern China based on the Excess Heat Factor (EHF), which accounts for the superposed effect of extreme heat and human adaptability in response to rapid temperature fluctuations. From 1961 to 2022, the Beijing-Tianjin-Hebei (BTH) region and Guangdong-Hong Kong-Macao Greater Bay Area (GBA) suffered from moderate HWs with higher frequency and shorter duration, while HWs in the Yangtze River Delta (YRD) region were characterized by lower frequency and longer duration. Compared to EHF, the conventional approach that uses single temperature criteria to identify HWs tends to underestimate their intensity without accounting for the effects of sudden temperature rises on human adaptability. Based on the downscaled ensemble of 23 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6), HWs and population exposure are expected to increase across the three urban agglomerations in the near-term (2025-2035) and mid-term (2055-2065) future, with GBA experiencing the greatest rise in HW days. However, YRD will have the highest population exposure due to its large population. During the projected explosive growth of severe/extreme HW days, low and intermediate GHG emission scenarios (SSP1-2.6, SSP2-4.5) could potentially avoid 29%/45%, 28%/42% and 44%/96% of the increase in population exposure to these events across the BTH, YRD, and GBA, respectively, in the mid-term future, compared to high GHG emission scenarios (SSP3-7.0, SSP5-8.5). Further analysis reveals that the expected increase in HWs in GBA and BTH is attributable to the combined effect of intensified temperature variability and warming, while the changes in HWs in YRD are primarily driven by rising temperatures. The results emphasize the urgent need to develop resilience to HWs in a changing climate.

Heatwaves; Population exposure; Excess heat factor; NEX-GDDP-CMIP6; Urban agglomerations