Cooling Intensity of Urban Blue–Green Spaces and Its Driving Mechanisms in the Yangtze River Delta Urban Agglomeration

As global warming intensifies, urban heat stress is increasing. Urban blue–green spaces (UBGSs) reduce heat exposure, and refined planning can enhance cooling in space-limited cities. Yet drivers and mechanisms at the urban-agglomeration scale remain unclear. This study quantified the cooling intensity (CI) of 162 UBGS across 27 cities in the Yangtze River Delta using summer 2020 land surface temperature data (June–August). CI is defined as the average temperature difference (ΔLST) between the UBGS and its surrounding buffer. Extreme gradient boosting (XGBoost) and Shapley additive explanations (SHAP) methods were applied to analyze the driving factors. Key findings include (1) the top five factors by SHAP value: Area (0.38), buffer NDBI (0.16), lake/river landscape shape index (0.07), buffer tree height (0.06), and buffer population (0.06). (2) CI rises with Area only up to 50 hm2 and then plateaus at ~6 °C. Buffer NDBI strengthens CI, with faster gains when buffer NDBI < −0.2. Water-shape effects are stepwise, and a lake/river shape index of 5–8 delivers near-optimal cooling. Interactions indicate stronger cooling in large parks with high water coverage and in sites combining complex water edges with dominant grassland patches. These findings support structure-focused UBGS design and targeted mitigation in dense urban areas.