Urbanization and Climate Change Shape Urban Latent Heat Flux: Three Critical Thresholds

Urban latent heat flux (LE)—the energy released through evapotranspiration—plays a crucial role in regulating urban climate and enhancing resilience to climate extremes. By analyzing vegetation LE trends across 2,252 global cities from 2001 to 2022, we found significant regional disparities: vegetation LE increased significantly in 40.3% of cities, with notably higher proportions in developing regions (56.6%) than in developed ones (21.4%). Atmospheric conditions were the dominant global driver of vegetation LE trends (51.9%), especially in developed cities, whereas vegetation-related surface biophysical factors played a stronger role in developing regions. To better understand the nonlinear dynamics governing these relationships, we reconstructed annual LE (1990–2022) using a validated remote sensing model driven by Landsat and ERA5 datasets in eight representative cities (New York, Paris, London, Sydney, Beijing, Tokyo, Hong Kong-Shenzhen, and Shanghai). Grid-based spatial analysis revealed critical thresholds in the vegetation-to-built-up ratio, delineating three distinct developmental stages: a saturation stage, where additional vegetation offered negligible cooling benefits; an intermediate stage, characterized by progressive increases in LE and enhanced urban climate resilience; and a critical stage of rapid LE decline, wherein modest vegetation gains significantly improved urban thermal conditions. Our findings identify essential thresholds and offer practical guidelines to strategically manage urban vegetation, optimize LE fluxes, and strengthen climate adaptation in cities worldwide.