Compared to individual hot days/nights, compound hot extremes that combine daytime and nighttime heat are more impactful. However, past and future changes in compound hot extremes as well as their underlying drivers and societal impacts remain poorly understood. Here we show that during 1960–2012, significant increases in Northern Hemisphere average frequency (~1.03 days decade⁻¹) and intensity (~0.28 °C decade⁻¹) of summertime compound hot extremes arise primarily from summer-mean warming. The forcing of rising greenhouse gases (GHGs) is robustly detected and largely accounts for observed trends. Observationally-constrained projections suggest an approximate eightfold increase in hemispheric-average frequency and a threefold growth in intensity of summertime compound hot extremes by 2100 (relative to 2012), given uncurbed GHG emissions. Accordingly, end-of-century population exposure to compound hot extremes is projected to be four to eight times the 2010s level, dependent on demographic and climate scenarios.

Traditional univariate analysis on weather and climate extremes failed to consider temporally compounding events and the resulting cascading impacts. A case in point is a sequence of flood and heatwave within a week, which slows recovery and amplifies damages. We show that across China, floods and heatwaves seldom occurred serially within seven days in the past, but after 2000 the probability is five‐to‐ten times higher in southern, northwestern and northeastern sectors. It is the significant increase in heatwaves that alters the clustering of independent extremes, and facilitates the emergence of sequential extremes. Typhoon‐participating sequential extremes have also increased significantly in frequency in both inland and coastal areas, with the fastest rate at around 200% decade⁻¹ registered within the 30–35°N latitudinal band. The observed increases in sequential flood‐heatwave events are discernibly stronger and more widespread than what would be expected from pure random variability, implying a detectable role of anthropogenic forcings.

Key Points: Unprecedented compound heat waves will occur on a regular basis in three urban agglomerations in China since mid‐21st century under RCP8.5 The likelihood of unprecedented compound heat waves in a 1.5°C warmer world will halve compared to 2°C warming scenario Population exposures to future unprecedented compound heat waves will increase by nearly 70, 90, and 60 million by the end of this century

When multiple extremes occur in rapid sequence, their impacts cascade to cause disproportionate damages. However, the prevalence of univariate definitions and inability to identify low‐likelihood events in short observations/simulations leave the knowledge on sequential extremes sparse. Leveraging two initial‐condition large ensembles, we project future changes in historically unprecedented sequential flood‐hot extremes in China. Results show that despite dozens of 1 in 50‐year floods and hot extremes in more than 2,000 years of historical simulations, their sequenced occurrence within a week has no historical precedent. This out‐of‐ordinary configuration is projected to be increasingly possible across China, with earlier emergence and larger frequency increases expected in Southwest and Southeast China. The direction, spatial extent, and magnitude of projected changes in unprecedented sequential extremes cannot be explained by internal variability alone, though it has the potential to modulate human‐caused changes in emergence timing and magnitude.