Spatial heterogeneity and landscape associations of urban heat risk under urban expansion in Chinese cities

Urban heat risk is shaped by the spatial interaction between thermal hazard, population exposure, and vulnerability, yet how long-term urban expansion changes the spatiotemporal distribution of heat risk remains insufficiently understood. This study examined the spatiotemporal evolution of urban heat risk in 15 Chinese cities from 1990 to 2025 using an integrated framework combining hazard, exposure, and a vulnerability index. Dynamic urban boundaries were used to distinguish old urban cores from expansion zones. Nonlinear associations between landscape structure and heat risk were examined across different development stages, and whether the growth of areas classified as high risk followed the pace of urban expansion was assessed. The urban area affected by heat risk increased 5.83-fold, whereas areas classified as high risk increased 3.77-fold, indicating that urban expansion enlarged the overall risk footprint without producing a proportional increase in areas classified as high risk. The population exposed to high thermal hazard increased substantially over the study period, highlighting the increasing concentration of thermal burden among urban populations. Within the adopted composite risk framework, contrasts between high risk and lower risk areas were greater for exposure and the adopted vulnerability index than for hazard. Impervious patch dominance and fragmentation showed stronger associations with composite heat risk in expansion zones, whereas old urban cores showed broader associations involving impervious shape complexity and green space fragmentation. These findings improve understanding of the spatial evolution of urban heat risk during urban expansion and provide insights for heat adaptation strategies tailored to different urban contexts.