Spatiotemporal variations of heat events and vulnerability assessment in the Chengdu-Chongqing urban agglomeration

Extreme heat events have intensified globally under climate change, posing severe health and socioeconomic risks that are further amplified in densely urbanized inland basins. Urban heat vulnerability research in inland polycentric regions remains limited, particularly regarding the drivers of vulnerability, seasonal dynamics, and long-term temporal changes of heat events. This study addresses these gaps by examining the Chengdu–Chongqing urban agglomeration (CCUA) from 1981 to 2020, integrating meteorological, socioeconomic, and environmental datasets. We first introduce the identification of first and last occurrence dates (FOD/LOD) of high-temperature and sweltering days to capture the seasonal extension of compound hot events. A multidimensional Heat Vulnerability Index (HVI) is then constructed using a hybrid weighting approach that combines the Analytic Hierarchy Process and entropy method across 12 exposure, sensitivity, and adaptive capacity indicators. Results indicate a significant intensification of extreme heat over the study period, with long-term upward trends in high-temperature and sweltering days, including notable increases in the 1990 s and after 2010. An east–west gradient in event intensity is observed, alongside a polarized vulnerability pattern characterized by high HVI values in Chengdu and Chongqing and low levels in peripheral ecological cities such as Ya’an and Leshan. Contribution analysis highlights exposure as the dominant driver of vulnerability (median 50 %), followed by sensitivity (30 %), while adaptive capacity remains insufficient to offset risks. Validation against hospital visit data (r = 0.7498, p < 0.001; R2 = 0.7632) confirms the reliability of the assessment. By integrating event-based diagnostics, temporal trend analysis, hybrid weighting, and health outcome validation, this study provides methodological advances and new evidence on the mechanisms of heat vulnerability, offering differentiated adaptation strategies for inland polycentric urban regions.