Spatiotemporal Evolution of Urban Blue-Green Spaces and Evaluation of Their Thermal Environmental Benefits in Beijing

Urban blue-green spaces play an important role in mitigating thermal environmental stress, yet their long-term configuration and relative thermal environmental benefits remain insufficiently understood at the metropolitan scale. This study examined Beijing from 2000 to 2020 by integrating Landsat time-series imagery, land-cover data, landscape metrics, land surface temperature retrieval, Geodetector analysis, and a configuration-oriented Blue-Green Environmental Benefit Index (BGEBI). The results showed that Beijing’s blue-green spaces experienced three stages: rapid decline during 2000–2003, gradual recovery during 2004–2012, and rapid expansion during 2013–2020. Spatially, low-temperature zones were mainly concentrated in the northwestern ecological conservation areas, whereas high-temperature zones were mainly distributed in the southeastern core and plain areas. Green-space landscape indicators, especially forest-related metrics, showed stronger explanatory associations with LST spatial heterogeneity than most wetland-related indicators at the metropolitan scale. The BGEBI results indicated an overall increase in relative thermal environmental benefits from 2000 to 2020, with high-value areas mainly located in the northwestern and central-western mountainous regions and low-value areas mainly distributed in southeastern urbanized areas. These findings suggest that blue-green space planning in high-density megacities should pay greater attention to landscape configuration, spatial connectivity, and scale-sensitive management. The proposed BGEBI framework provides a relative spatial-prioritization tool for identifying areas where blue-green configuration optimization may support thermal-environment improvement.