Long-term trends and risk regulation of heat exposure in megacities: seasonal dynamics and determinants

Accelerating urbanization intensifies urban heat exposure and extends its influence beyond summer to other seasons. However, fine-scale evidence on seasonal thermal dynamics and associated population exposure remains limited. By integrating MODIS daily land surface temperature (LST) with population density, we developed a four-quadrant framework to characterize the seasonal and spatial patterns of LST and Heat Exposure Index (HEI) in Shanghai, the commercial and financial center of mainland China. Results revealed a significant warming trend in Shanghai’s LST, with an average increase of 0.087 °C·yr^{− 1}, most prominently in spring (0.175 °C·yr^{− 1}). While urban new towns and rural areas experienced rapid warming, urban cores showed significant cooling trends (Slope_LST = -0.10 °C·yr^{− 1}), likely associated with green infrastructure and urban renewal policies. Seasonally, summer exhibited the highest heat exposure risk (HEI = 6.84), with over 82% of the city classified as high-risk areas. Additionally, approximately 11% of the city maintained at high-risk in both spring and autumn, with 77.00% and 73.85% of these high-risk zones concentrated in urban cores. The quadrant analysis identified 13.5% of the city as “crisis zones”, primarily located in rapidly developing urban new towns, characterized by high and escalating risk that warrants integrated interventions. Driver analysis showed that population density was the overwhelmingly dominant factor, explaining 68.35% of the spatial variability in HEI, thereby establishing a “population-first” perspective for heat exposure assessment. These findings highlight the central role of human activity in shaping heat exposure and underscore the need for spatially targeted and adaptive strategies to enhance thermal resilience.