Patterns and drivers of surface cooling effect of urban trees across global cities

Urban trees offer a promising strategy to mitigating rising heat stress in cities globally. However, the spatial distribution and influencing factors of the tree cooling effect across urban surfaces worldwide are not thoroughly understood. Here we quantified the surface cooling effect during the summer season across 1016 cities globally using two key metrics simultaneously - cooling intensity and cooling distance - primarily based on Landsat-8 land surface temperature data. We then investigated the impact of various drivers on the surface cooling effect, including tree attributes at the small scale (e.g., area and landscape shape index), urban characteristic at the medium scale (e.g., city population), and background climate at the large scale (e.g., air temperature and precipitation). Furthermore, the combined effects of urban and climatic contexts on the regulation of tree attributes on the surface cooling effect were examined. Our findings reveal that the global average cooling intensity is 1.67 ± 1.13 °C, while the global average cooling distance is 136.86 ± 60.44 m. Cooling intensity is mainly regulated by tree attributes (88 %), followed by background climate (9 %) and urban characteristic (3 %). The variable that explains the most variation in cooling intensity is the NDVI of trees at the small scale. Similarly, the cooling distance is primarily influenced by tree attributes (73 %), background climate (22 %), and urban characteristic (5 %). The area of trees at the small scale is the primary factor contributing to the variation in cooling distance. Additionally, the impact of tree attributes on cooling effect is jointly moderated by urban characteristic and background climate. In cities with larger populations or higher air temperatures, the area of trees and the landscape shape index exert a less pronounced impact on the cooling effect. This implies that the presence of small, widely dispersed tree cover in such urban areas can effectively provide the cooling intensity. Our study provides a crucial baseline for formulating tree cooling strategies and managing urban tree cover across global cities.