Global decline in urban blue-green cooling efficiency driven by vapor pressure deficit

As global warming intensifies urban heatwave risk, blue-green spaces have become critical regulators of urban thermal environments. Yet previous studies have focused mainly on temperature responses, and the extent to which atmospheric moisture demand, represented by vapor pressure deficit (VPD), shapes blue-green-space cooling across climate regions remains insufficiently quantified. Using multi-source remote-sensing and meteorological data for 225 large cities worldwide, this study quantifies the global relationships between VPD and urban blue-green-space cooling and examines the associated vegetation and surface-energy pathways. The findings show that urban summer land surface temperature and VPD increased concurrently from 2000 to 2024, while the relationships between VPD and cooling intensity were nonlinear and differed among climate regions. Piecewise structural equation models did not support a universal pathway in which higher VPD uniformly reduced latent heat flux and increased sensible heat flux. Instead, the direct and indirect associations among VPD, vegetation condition, surface energy exchange, and land surface temperature varied with climate background. In arid cities, VPD showed the largest model-implied total effect on land surface temperature. Vegetation- and latent-heat-mediated pathways were more evident in temperate and cold cities. This study elucidates the critical role of VPD in shaping the urban thermal environment, emphasizing the necessity of differentiated blue-green infrastructure configurations and adaptive heatwave response strategies based on the hydrothermal conditions of different climate zones. It provides a scientific basis for alleviating urban heat stress and enhancing urban climate resilience.