PNNL

Abstract

Urbanization is a consequence of human activity yet perturbs the surface energy balance, elevating environmental and health risks. Although the urban heat island (UHI) effect—the local warming in urban environments—is well recognized, the quantitative thermal influence of surrounding urban structure, and its coupling with background climate remains poorly constrained. Here, we present a global analytical framework that integrates a six-class urban typology, long-term climatology, and machine learning to quantify the structural thermal influences around each location and to project future urban heat across 2,213 cities. We define a city-level thermal impact of the surrounding built environments (TBE) by weighting structure-induced changes in canopy UHI associated with specific built-up types by their areal fractions. Climate and morphology jointly organize global diurnal patterns of urban heat. Climatically, cities in cold regions most frequently exhibit high daytime TBE (=?0.41°C), while arid regions dominate high nighttime TBE (=?0.39°C). Structurally, however, a universal pattern remains across climates; high daytime TBE is consistently associated with denser and taller built forms, whereas low TBE (=?0.14°C) is dominated by low-density low-rise types; these contrasts persist at night. Future projections indicate that climate change dominates TBE change in 69% of cities, whereas the Global South exhibits stronger tendencies toward morphology-driven and synergistic TBE intensification than the Global North. Our results establish the combined controls of climate and urban form on contemporary and future urban heat, highlighting the need for locally tailored adaptation strategies that target the dominant drivers—climatic, morphological, or both.