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Abstract Understanding how urbanization affects compound hot extremes (CHEs) is important for the sustainable development in urban agglomeration under global warming. In this study, we investigate the changes of CHEs in summer over the Beijing‐Tianjin‐Hebei region (BTH), Yangtze River Delta (YRD) and Pearl River Delta (PRD) in China, and assess the effects of urbanization on these changes. We find that the frequency and intensity of CHEs in summer show significantly increasing trends in these agglomerations in the recent five decades, particularly in megacities such as Beijing, Shanghai, Guangzhou and Tianjin. The urbanization contributions to CHEs in the YRD and PRD were estimated to 36 ∼ 58%, while relatively small in the BTH, ranging 16 ∼ 29%. They were bonded to the unban size, expansion speed and local climate. In addition to global warming and urbanization, the strengthening continental high and enhancing Western Pacific Subtropical High and its westward displacement were favor to the increased CHEs.

Compound hot extremes (CHEs) are receiving increasing attention due to their significant impacts on human health, ecosystems, and society compared to individual hot days or nights. While previous studies have focused on the characteristics of CHEs at individual points or stations, assessments of features for regional CHEs (RCHEs), which have a specific impact area and duration, are still lacking. This study aimed to investigate the climatic characteristics of RCHEs in mainland China by applying an objective identification technique for regional extreme events based on a compound index. The results show that 379 RCHEs were identified during 1961–2020, most of the events had a duration of 5–11 d and a maximum impacted area of approximately 460 10 ^4 km ^2 . Long-duration RCHEs were found to have vigorous extreme intensity and large maximum impacted area. The middle and lower reaches of the Yangtze River were most susceptible to RCHEs, while the Yellow River Valley had the most robust positive trend of frequency for RCHEs, suggesting a significant risk of compound temperature disasters in this region. Furthermore, RCHEs in mainland China showed significant increasing trends in several aspects, such as annual frequency, integrated index, and single indices (e.g. duration, accumulated intensity, accumulated impacted area, and extreme intensity). These upward trends were accompanied by evident interdecadal variations, with low values before 1992 and high values after 1992. This study provides valuable insights into understanding and monitoring CHEs in China from the perspective of regional extremes.

Abstract Yangtze River basin (YZB) experienced record‐breaking heat in the summer of 2022. Here, we focused on daytime‐nighttime compound heat waves, and used the magnitude index that considers both duration and intensity to investigate the risk of the 2022 extreme heat. The magnitude of heatwaves in 2022 was much larger than the historical average level, which was estimated as a 1‐in‐64‐year event over 1979–2014 climate. Without mitigation efforts (SSP585), the record‐breaking heat would emerge as normal during 2050s, and would affect ∼70% of land and projected population in the basin before global mean temperature change reaches 3°C. Such an emergence could be progressively delayed and impacts could be reduced under lower warming levels. The affected area would be 60% lesser at 2°C warming, and the emergence could be avoided by limiting warming to 1.5°C. Our results call for urgent mitigation efforts for reducing the risk of compound heat extremes.