The role of dynamic urban expansion in shaping thermal environments: Evidence from China's 2000 cities in past decades

Against the backdrop of global warming and rapid urban expansion, urban residents face escalating thermal risks, characterized by intensified urban heat island effects and increased heatwave frequency. Urban expansion, driven by rapid changes in spatial patterns and land use, exacerbates the urban heat island effect through increased impervious surfaces and reduced green spaces, elevating local temperatures and adversely impacting residents' health. Analyzing the spatiotemporal patterns and thermal impacts of urban expansion is therefore essential for addressing climate change and fostering livable cities. In this study, we systematically investigate the spatiotemporal characteristics of dynamic urban expansion and thermal environment evolution using data from nearly 2,000 cities across China (1985–2022) and employs machine learning models to uncover their interaction mechanisms. First, annual 30-meter resolution land use data were used to extract urban boundary dynamics and analyze spatiotemporal patterns of expansion modes, inter-city equivalent distances, and landscape indices. Second, daily 1-kilometer resolution temperature data (1961–2022) were utilized to calculate annual heatwave indicators (e.g., frequency, intensity, and duration) using the Excess Heat Factor (EHF), enabling the assessment of urban heatwave evolution. Notably, urban reference background zones were established, and regression methods were applied to isolate the independent effects of urban expansion on heatwave indicators. Finally, we implement interpretable machine learning frameworks with SHAP analysis to quantify urbanization's contribution to thermal risk amplification. Our study addresses a critical gap in large-scale urban research, providing a scientific basis for mitigating climate change and urban thermal risks. Our findings offer robust decision-making support for urban planning, contributing to sustainable urban development goals.