Anthropogenic Climate Change and Urbanization Exacerbate Risk of Hybrid Heat Extremes in China

Dry- and wet-bulb temperature (Td and Tw) are usually to define heatwaves (HWs) which have been enhanced under anthropogenic climate change (ACC) and urbanization. However, responses of various types of HWs (i.e., dry HWs, only high Td; humid HWs, only high Tw; hybrid HWs, both high Td and Tw; total HWs, high Td or Tw), to ACC and urbanization remain unknown. In this study, both observations and simulations show significantly increasing occurrence probability of total HWs over China during 1971-2020, whereas this increase is mainly reflected in hybrid HWs, followed by dry HWs and humid HWs. 68.2%-93.0% of the observed increases in the above four types of HWs can be attributed to ACC; on the other hand, urbanization tends to suppress humid HWs but enhance dry HWs, as a result of contributing to the increase of hybrid HWs by 10.9%. Under future ACC, total HWs are projected to be more frequent as expected, which is mainly sourced from the increasing hybrid HWs because dry/humid HWs are projected to be steady/downward. As a consequence, urban population exposure to ACC-induced total HWs would remarkably increase to 83.55 billion person-days by the 2090s, 89.5% of which can be attributed to hybrid HWs. Urbanization would amplify this population exposure of ACC-induced hybrid HWs from 74.79 billion person-days to 110.9 billion person-days. Our results underscore the importance of improving understanding of hybrid HWs in urban areas and developing targeted adaptation planning on a warmer planet. Under global warming and urbanization, increases in heatwaves (HWs) characterized by air temperature and humidity, have profound adverse effects on human health and socio-economic well-being. In this study, HWs are divided into four types: humid HWs (dominated by high air humidity), dry HWs (dominated by high air temperature), hybrid HWs (both high temperature and high humid), and total HWs (encompassing all the three above types), to investigate changes in HWs under different temperature and humidity conditions and their responses to anthropogenic climate change (ACC) and urbanization. Both observations and simulations show that occurrence probability of the four types of HWs increased over China in recent decades, of which ACC contributes to 68.2%-93.0% of intensification of HWs. The increasing occurrence probability of total HWs is mainly reflected in hybrid HWs, followed by dry HWs and humid HWs. Urbanization tends to inhibit (promote) rise in humid HWs (dry HWs), resulting in an increase in hybrid HWs. Under urban expansion and global warming, urban population in China will be more exposed to extremes of humid and hot conditions (hybrid and total HWs), especially in densely populated and rapidly urbanizing areas. Historical increasing likelihood of total heatwaves (HWs) over China is dominated by that of hybrid HWs Urbanization tends to suppress (enhance) humid (dry) HWs and hence increase likelihood of hybrid HWs under anthropogenic climate change Future anthropogenic warming and urban expansion jointly lead to remarkable increase of urban population exposure to hybrid HWs