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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]

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]

• Compound hot extremes did not exacerbate tree growth across 70% of the forests. • Compound hot extremes have started to exacerbate tree growth decline in forests in dry regions. • Growth reduction by compound hot extremes showed a significant increase over the past decades. Under climate warming, the adverse effects of daytime or nighttime hot extremes on forest productivity have been widely reported. However, the impact of compound hot extremes (i.e., combination of daytime and nighttime hot extremes) on tree growth remains unclear. Combining long-term and large-scale tree-ring width and remote-sensing datasets between 1949 and 2020, we examined and compared the effects of individual and compound hot extremes on tree growth and forest productivity in the Northern Hemisphere. Here we showed that compound hot extremes did not exacerbate tree growth and productivity decline compared with individual daytime hot extremes across 70% of the forested area. However, we observed a stronger adverse effect of compound hot extremes on tree growth and productivity compared with individual daytime hot extremes in dry regions. Due to the increased severity and frequency of compound hot extremes, we predicted that compound hot extremes may impose a greater threat to forest productivity and global carbon cycling as compared to individual hot extremes under future climate change. Our results call for effective policies and actions to mitigate the rapidly growing adverse impact of compound hot extremes on forest ecosystems and carbon cycling. [Display omitted] Compound hot extremes generally exacerbate tree growth decline compared with daytime hot extremes in forests in dry regions but not in humid regions across the Northern Hemisphere, possibly due to the counterbalancing effects of daytime and nighttime hot extremes on water availability. [ABSTRACT FROM AUTHOR]