Rising vulnerability of compound risk inequality to ageing and extreme heatwave exposure in global cities

Continued warming trends lead to an increasing risk of exposure to extreme heatwaves, which threaten the health of urban residents, especially the ageing population. Here, we project the spatiotemporal trend of future exposure risk across 9188 global urban settlements between 2020 and 2100 under the shared socioeconomic pathway (SSP) 2-4.5 and SSP5-8.5 scenarios. Results show that urban heatwave exposure risk increases by 619% and 1740% for SSP2-4.5 and SSP5-8.5, respectively, and by 1642% to 5529% for the elderly. Notably, 69% of the elderly exposure risk comes from middle-income countries, where the increasing trend on the regional average is 1.2 times higher than that of high-income countries. There is an increasing trend towards greater concentration on large cities, especially in low- and lower-middle-income countries. In high-income countries, climate effects contribute 39% to 58% of increasing exposure for elderly individuals, whereas ageing effects play more prominent role in lower-income countries. This emphasizes the disproportionately higher heat-related burden for elderly individuals and inequitable trends in lower income countries. Understanding the vulnerable and priority regions in future heatwave exposure will inform adaptation strategies to support urban climate-resilient development.