Global Morphological Thresholds Governing Urban Street Tree Microclimates Across 12 Megacities
Abstract
Abstract Municipal climate adaptation plans globally champion street tree planting, frequently mandating uniform canopy quotas like the '3-30-300' guideline. Yet blanket targets overlook three-dimensional canyon geometry, orientation, and macroclimatic context. Here, coupling a six-dimensional differential-algebraic equation (Index-1 DAE) biometeorological solver with geospatial morphology across 204,267 authentic corridors (5,898 km) in 12 global megacities, we show nature-based cooling is governed by strict geometric tipping thresholds (H/W^*). While daytime shading provides net cooling along 29.6% of corridors, 46.4% of street length in dense morphological cores incurs nocturnal radiative-aerodynamic heat trapping. In warm-night metropolises (Athens, Madrid, Tokyo, Singapore), 36.6% of global corridors invert into nocturnal thermal liabilities (+ 0.38 to + 1.44 K UTCI). Under CMIP6 warming, allowable aspect ratios contract by 18.7% to 29.1% by 2080. Validated against empirical field campaigns across three continents—the European Basel BUBBLE campaign (Cfb, R² = 0.853), Australian Melbourne canyon array (Cfa, R² = 0.812 to 0.836), and North American Phoenix desert microclimate (BWh, R² = 0.783 to 0.827; grand composite R² = 0.923, RMSE = 1.93 K, N = 432)—we demonstrate nocturnal liabilities are invariant to municipal irrigation and provide an open-source QGIS plugin to operationalize shape-adaptive precision urban silviculture.
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