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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. Plain Language Summary: Compound extreme hot events have a great adverse impacts on the natural environment and human society in urban region. Here, we show that frequency and intensity of compound extreme hot events in summer have significantly increased in the three major urban agglomerations during the recent 50 years. These changes have been more intensive in the downtown areas of megacities such as Beijing, Shanghai, Guangzhou and Tianjin. We estimated the urbanization contributions accounting for 36 ∼ 58% in Yangtze River Delta and Pearl River Delta, 16 ∼ 29% in the Beijing‐Tianjin‐Hebei region. Furthermore, the strengthening continental high and Western Pacific high were favor to the increase of compound extreme hot events. It is urgent to take effective measures to reduce the risk of compound extreme hot events in urban agglomeration. Key Points: Compound hot extremes in summer have significantly increased in the three major urban agglomerations of China, particularly in megacitiesUrbanization contribution to the increased compound hot extremes accounts for 16 ∼ 58%, which is larger than to the increased hot daysRobustly intensified and westward extended Western Pacific Subtropical High may be bonded to the increased compound hot extremes [ABSTRACT FROM AUTHOR]

Induced by numerous environmental and anthropogenic factors, the frequency of compound hot extremes (CoHot) is increasing globally. However, there is a lack of a comprehensive understanding of the relative importance of different factors in the change of CoHot, as most studies focused solely on individual factors. We systematically attribute the long-term trends of summertime CoHot in China to 24 factors including climate background, climate change, urbanization (including impervious area expansion and three-dimensional urban morphology), and vegetation change, using partial least square regression (PLSR) and variable importance projections for analysis. Cross-evaluation results based on 2259 meteorological stations indicate that the PLSR model accurately simulates the trends of CoHot frequency, duration, and intensity, with a relative error of 4–20% from national to individual station scales. A warm and humid climate background, rising average summertime temperatures, impervious area expansions, high building volume densities, low building heights, and vegetation wilting are the main factors that favoring the increasing and intensifying trends of CoHot. Although summertime warming contributes the most, the significance of urbanization and vegetation greening has been growing in recent years. Urbanization's contribution increases from 9.3% on a national scale to over 40% in urban centers, mainly due to increased building volume density rather than expansions of impervious area. In northern China, vegetation greening has reduced the local increasing trends in frequency/duration of CoHot by 20% to 40%, and such mitigation effects are primarily induced by increased vegetation cover. Our results suggest that, reducing building volume density and promoting vegetation greening can mitigate the increasing risk of CoHot under global warming. • Compound hot extremes (CoHot) changes in China were attributed to 24 factors. • A warm and humid climate background favors the increase of CoHot frequency/duration. • Urbanization's contribution increased from 9.3% to >40% from national to grid scales. • Building volume density is more vital than impervious area fraction and building height. • Vegetation greening reduced regional CoHot frequency by 20–40% in northern China. [ABSTRACT FROM AUTHOR]

Worldwide urbanization has resulted in more hot extremes in urban areas than surrounding rural areas. Most studies focused on the contribution of urbanization to daytime hot extremes (DayHot), while the contribution to compound hot extremes (CoHot) that considers the concurrence of both daytime and nighttime heat is less understood. Here, we quantify the urbanization contributions across the world by comparing the characteristics of hot extremes between urban and rural areas from 3,401 meteorological stations classified by 30‐m resolution impervious area data. We find more significant increases in frequency, duration, and intensity of CoHot over most urban stations, where urbanization contributes to 12.5%, 15.4%, and 16.7% of the increases, respectively. Urbanization also contributes to the increase in frequency of DayHot, but has weaker impacts on the duration and intensity of Dayhot. This study implies that urbanization effects on the characteristics of hot extremes might be underestimated if only considers DayHot. Plain Language Summary: Compound hot extremes (CoHot) are defined as the concurrence of daytime and nighttime heat, and have more adverse impacts on human health than daytime hot extremes (DayHot) alone. The adverse impacts might increase due to urbanization, which caused higher temperatures and more hot extremes in urban areas than the surrounding rural areas. However, most studies focused on daytime hot extremes, and urbanization contributions to compound hot extremes compared to daytime hot extremes are not known. Here, we calculate the frequency, duration, and intensity of compound and daytime hot extremes over 3,401 meteorological observation stations during 1971–2014, and quantify the urbanization contributions by comparing the results over urban and rural stations. We found that both urban and rural CoHot are becoming longer, more frequent and severe over most stations across the world, but the increasing trends are more prominent over most urban areas. Urbanization contributions to the frequency of DayHot are significant and similar to CoHot. However, compared to compound hot extremes, urbanization effects on the duration and intensity of DayHot are much smaller or even undetectable. This suggests that urbanization results in more persistent and severe hot extremes that could be overlooked by only focusing on DayHot. Key Points: Global urbanization increases the frequency of compound hot extremes that considers both daytime and nighttime heat by 12.5%Urbanization also significantly increases duration and intensity for compound hot extremes, but much smaller for daytime hot extremesThe influence of urbanization on compound hot extremes accelerates in recent decades due to rapid growth in impervious areas [ABSTRACT FROM AUTHOR]