Anthropogenic Emissions and Urbanization Increase Risk of Compound Hot Extremes in Cities


Most areas of the world are experiencing increasing and intensifying hot extremes, which pose severe health and socio-economic impacts. Heat-related health consequences vary with the characteristics of the exposed landscape and types of hot extreme. Mortality tends to be higher in urban areas, due to greater population exposure and urban heat island effects. Despite mounting evidence of detrimental health impacts from hot days or nights due to their high intensity and/or long duration, their occurrence in close sequence—that is, a compound hot extreme—has received little attention but may bear disproportionately large health risks.
A new study published in Nature Climate Change addresses the human health impacts, drivers of observed changes, and future risk of compound (day-night sustained) hot extremes in urban eastern China. The study involves interdisciplinary multi-institute collaborations with researchers studying climate science, human health, geography and urban planning.
The research shows that compound hot extremes are more dangerous than solely daytime or nighttime heat, especially to female and older urban residents. In the significant increase of urban compound hot extremes (1.76 days per decade) in 1961–2014, the fingerprints of urbanization and anthropogenic emissions (e.g., greenhouse gases and aerosols) can be detected. 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 vs. 2010s), leading to a threefold-to-sixfold growth in urban population exposure.
"Our study reveals that the public health risks from anthropogenic increases in compound hot extremes have been increasing and will continue to increase over urban eastern China." Said Dr. WANG Jun, the first author of the study from the Institute of Atmospheric Physics, Chinese Academy of Sciences. 
According to the study, the uncovered age-specific vulnerability to compound hot extremes implies further elevated health burdens due to rapid ageing of the urban population there. Therefore, adapting to and mitigating climate change in the urban context will achieve co-benefits and synergies between reducing heat-related health risks and achieving committed net-zero emission goals.
This study was supported jointly by the National Key Research and Development Programme of China (Grant No. 2018YFC1507700) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA20020201).
Wang, J., Chen, Y*., Liao, W., He, G., Tett, F. B. S., Yan, Z., Zhai, P., Feng, J., Ma, W*., Huang, C., and Hu, Y. Anthropogenic emissions and urbanization increase risk of compound hot extremes in cities. Nat. Clim. Chang. (2021).
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