Global hydroclimatic constraints on greening transitions for urban cooling

Urban green spaces can cool cities, yet the greenness level at which cooling begins remains unresolved. We analyse > 84,000 satellite observations from 711 cities worldwide to identify hydroclimate-dependent greening transitions for urban cooling. Using nonlinear mixed-effects models grounded in surface energy balance theory, we define the cooling onset (NDVI-ZC) as the greenness level at which vegetation shifts from neutral or warming effects on land surface temperature to net cooling along a smooth response curve. NDVI-ZCs vary systematically across hydroclimatic regimes, occurring at lower greenness in dry regimes and higher greenness in seasonally dry regimes, while wet regimes show persistent cooling across the observed greenness range. We demonstrate that urban cooling is constrained by hydroclimate-specific greening transitions, below which vegetation fails to deliver net surface cooling. Beyond onset, cooling responses are nonlinear and regime-dependent, indicating that effective urban cooling does not scale proportionally with vegetation cover.