Intensifying drought threatens the heat-regulating capacity of urban vegetation

Urban vegetation is critical in regulating urban heat, yet the stability of this regulatory effect under drought has not been well measured at a global scale. Using satellite observations during 2003–2024 across 2911 cities, we found that vegetation greening reduced urban land surface temperature (LST) with the strongest global mean daytime reduction of 0.405°C in summer and the weakest of 0.066°C in winter. Nighttime effects were weaker, averaging 0.049°C in summer and 0.008°C in winter. Critically, vegetation-related cooling exhibited pronounced nonlinear and threshold-limited responses to drought. During daytime in spring, summer, and autumn, cooling was generally stronger under wetter conditions but declined after crossing drought thresholds, occurring at Palmer Drought Severity Index (PDSI) values of −0.925, −0.827, and −0.754, respectively. Beyond these thresholds, cooling declined rapidly, with the drought-cooling relationship shifting from positive to negative, and the steepest post-threshold change observed in summer, where the slope changed from 0.201 to −0.205°C per unit PDSI. At night, threshold responses also occurred but with weaker magnitudes, with PDSI thresholds ranging from −0.943 in spring to −2.057 in winter. Statistical pathway analysis showed that drought-LST associations shifted across thresholds, consistent with expected drought-related changes in estimated evapotranspiration and constrained by vegetation shading and seasonal radiation-moisture conditions. Together, these results demonstrate that the heat-regulating capacity of urban vegetation is not linear or unlimited, but constrained by interacting water and energy conditions. This highlights the need for awareness of thresholds and water-constraints when using urban vegetation to mitigate warming temperatures under climate change.