Global urban vegetation exhibits divergent thermal effects: from cooling to warming as aridity increases

Urban vegetation, a key nature-based solution for mitigating heat stress, is critical as global warming, and urban heat islands amplify high temperatures in cities, affecting over half the global population. Yet, its potential warming effects remain unquantified globally, with mechanisms unclear. Using high-resolution satellite and climate data, we provide the first global assessment of vegetation's temperature regulation across 761 megacities across 105 countries, uncovering a paradox: cooling weakens in arid environments; and in 22% of cities with <1000-millimeter annual precipitation, vegetation, particularly grasslands and croplands, causes net warming. This results from lower albedo and reduced heat storage outweighing limited evapotranspiration in arid regions. During extreme heat, trees fail to cool 25% of cities, while grasslands and croplands fail in 71 and 82%, respectively, due to reduced evapotranspiration under high vapor pressure deficits and impeded canopy conductance. Climate-adaptive greening and irrigation are critical, while high-albedo surfaces may better mitigate heat in water-scarce cities. Misguided greening risks are worsening urban warming.