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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]

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. Plain Language Summary: Persistent extreme high temperature occurred in Yangtze River basin (YZB) during summer 2022, leading to widespread droughts, heat‐related disease and socioeconomic losses. The physical drivers and future risk of the heat extremes have attracted extensive attention. Focusing on daytime‐nighttime compound heat waves, which have more adverse effect on human health and ecosystem, we investigated the magnitude of compound heat waves occurred in 2022, and future risk under different warming levels and emission scenarios. Observations show that the magnitude was the strongest since records began in 1979, and the return period was 64 (95% CI: 30–223) years over the 1979–2014 climate. Projections show that such unprecedented heat in observed records would emerge as normal since 2053 (2081) under nonmitigation (moderate mitigation) scenario. Moderate climate mitigation efforts could delay the time by 28 years. Compared with the 2°C global warming level, about 10% of lands and population would be avoided to normally expose to such heat at 1.5°C warming level. However, the exposures would increase by 60% from 2°C to 3°C warming level. This study suggests that mitigation actions for accomplishing 2°C warming goal and further pursuing the ambitious goal of 1.5°C can substantially reduce the exposure risk of such unprecedented heat. Key Points: The 2022 record‐breaking compound heat waves in Yangtze River basin would become normal during 2050s under a nonmitigation scenarioModerate climate mitigation efforts would delay the time of emergence by 28 yearsCompared to 3°C, limiting warming to 2°C and 1.5°C could avoid 60% and 70% of lands that are normally affected by such rare heat, respectively [ABSTRACT FROM AUTHOR]

Compound hot extremes (ChotEs) that refer to continuous heats throughout days and nights are projected to increase, causing more serious impacts on human health than daytime or nighttime heats alone. Previous studies have focused on daytime heats, but the timing of substantial increase in ChotEs relative to natural variability, which is defined as the time of emergence (ToE) for ChotEs, remains unknown. Here we examine ToE for duration of summertime ChotEs from coupled model intercomparison project phase 6 climate model projections under two shared socioeconomic pathway scenarios (i.e., SSP245 and SSP585). We further quantify the cumulative fraction of areal and population exposed to the emergence at global and continental scales. We find that, without implementation of climate mitigation policies (i.e., SSP585), global mean ToE is around 2062 (with 16%–84% uncertainty range of 2048–2072). On the basis of the ToE for each grid cell, 80.7% (with uncertainty range of 64.2%–96.7%) of global lands will expose to the emergence by 2080. Such substantial increases in ChotEs will lead to 75.2% (66.8%–93%) of global population exposed to the emergence by the end of 21st century. A moderate mitigation (i.e., SSP245) can delay the ToE by over 14 years and, more importantly, reduce the global land areal and population exposures by 50.3% and 39.7% respectively. Regionally, northern Europe, central America and western North America benefit the most. Therefore, early action towards moderate development socioeconomic pathways can remarkably cut back the possibility of large population exposure to ChotEs and relevant impacts. Plain Language Summary: Compared to daytime or nighttime hot extremes alone, compound hot extremes (ChotEs) that combining daytime and nighttime heats could lead to more serious impacts on human health and environment. Here, we quantify the time when future changes in duration of summertime ChotEs relative to current mean state will exceed the historical variability persistently (termed as time of emergence [ToE] in this study). In addition, the corresponding cumulative fractions of areal and population exposures for each year are also presented. Two future shared socioeconomic pathways (SSP) scenarios from coupled model intercomparison project phase 6 climate models, that is, SSP585—non‐mitigation and SSP245—moderate mitigation scenarios, are analyzed to address the benefits of mitigation. The results show that the global mean ToE is 2062, and about 80.7% and 75.2% of global lands and population will be exposed by 2080 under SSP585 scenario. From non‐mitigation (SSP585) to moderate mitigation (SSP245) scenarios, the risk of future ChotEs will be substantially reduced via delaying the emergence by 14 years, and reducing the areal and population exposure by 50.3% and 39.7% respectively. At continental scale, the benefits of moderate mitigation are more significant over northern Europe, central America and western North America. Key Points: The global mean time of emergence (ToE) for substantial increases in compound hot extremes (ChotEs) is 2062 under non‐mitigation scenario75.2% of global population will expose to the substantial increases in ChotEs by 2080 if no mitigation policies are implementedA moderate mitigation will substantially reduce the risk of ChotEs via delaying ToE by 14 years and reducing population exposure by 39.7% [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]

Hot temperature extremes, particularly during summer, impose severe adverse effects on human health and natural systems. Although considerable attention has been paid to individual hot days and nights, little is known about mechanisms leading to compound hot extremes that combine daytime and nighttime heat. This important type of hot extreme has occurred frequently in the populated mid‐lower reaches of the Yangtze River during the past three to four decades. Through composite analysis, dynamic–thermodynamic processes leading to the occurrence of compound hot extremes are investigated. Results show that an anomalous anticyclone, exhibiting a quasi‐barotropic structure and persisting through day and night, is a prerequisite for the occurrence of compound hot extremes in the mid‐lower reaches of the Yangtze River. The anomalous anticyclone reduces cloud cover and increases solar radiation, causing extreme daytime high temperature. Simultaneously, a descending branch within the column of the anomalous anticyclone enhances local adiabatic heating. This descending branch and resultant adiabatic heat are sustained and further strengthened by anomalous descending motions as part of vertical cells associated with positive precipitation anomalies to the north and south of the anticyclone. During compound hot extremes, intensified southerlies transport additional water vapour toward the study area, helping to establish a stable atmospheric stratification in the low troposphere. This low‐level stable moist layer plays a particularly important role to offset nighttime radiative cooling and, therefore, elevates nighttime low temperatures to form compound hot extremes. Consequently, compound hot extremes in the study area are hot‐humid in nature. [ABSTRACT FROM AUTHOR]