Anthropogenic emissions and urbanization increase risk of compound hot extremes in cities

Heat extremes threaten the health of urban residents with particularly strong impacts from day-night sustained heat. Observation and simulation data across eastern China show increasing risks of compound events attributed to anthropogenic emissions and urbanization. Urban areas are experiencing strongly increasing hot temperature extremes. However, these urban heat events have seldom been the focus of traditional detection and attribution analysis designed for regional to global changes. Here we show that compound (day-night sustained) hot extremes are more dangerous than solely daytime or nighttime heat, especially to female and older urban residents. Urban compound hot extremes across eastern China have increased by 1.76 days per decade from 1961 to 2014 with fingerprints of urban expansion and anthropogenic emissions detected by a stepwise detection and attribution method. Their attributable fractions are estimated as 0.51 (urbanization), 1.63 (greenhouse gases) and -0.54 (other anthropogenic forcings) days per decade. Future emissions and urbanization would make these compound events two to five times more frequent (2090s versus 2010s), leading to a threefold-to-sixfold growth in urban population exposure. Our findings call for tailored adaptation planning against rapidly growing health threats from compound heat in cities.