Quantifying blue–green space degradation dynamics and associated thermal responses across urban expansion patterns: Insights from interpretable machine learning

Urban expansion areas have emerged as critical hotspots of blue–green space degradation and thermal intensification during rapid urbanization. However, nonlinear relationships between multidimensional blue–green space degradation and thermal intensification, including their variation across expansion patterns, remain insufficiently understood. Using 26 cities in the Yangtze River Delta Urban Agglomeration, this study characterizes urban-expansion-area blue–green spaces (UEBGSs) and their degradation from 2000 to 2020, while identifying urban expansion patterns (UEPs). By integrating TabPFN with SHAP, the study reveals nonlinear relationships between UEBGS degradation and changes in relative land surface temperature (ΔrLST), as well as pattern-specific heterogeneity. The results indicate that: (1) edge-expansion was the dominant UEP in the Yangtze River Delta, with the intensity and pathways of UEBGS degradation varying across UEPs; (2) UEBGS degradation exhibited nonlinear relationships with ΔrLST, where green-space quantity loss and biophysical-condition degradation were the dominant contributors to thermal intensification, while structural degradation played a significant role at the early stage; and (3) the relative importance of degradation indicators, sensitive response intervals, and marginal effects differed among UEPs. Specifically, edge-expansion was more sensitive to quantity degradation, infilling-expansion responded more strongly to biophysical-condition degradation, and outlying-expansion showed buffering effects at low levels of degradation but accelerated thermal intensification at higher levels. Overall, the UEBGS degradation–ΔrLST relationship is strongly pattern-dependent, with distinct response intensities and sensitive intervals across UEPs. This finding underscores the need for pattern-specific blue–green space interventions across UEPs. This study provides a framework for understanding heat-risk formation and supports targeted thermal governance in rapidly urbanizing regions.