Urban–rural gradients in cooling efficiency trends of tree covers across global cities

Abstract

Understanding how cooling efficiency (CE) trends of tree covers respond to background climate change and local urbanization is essential for optimizing urban greening strategies. However, previous studies have largely focused on city-wide CE trends, often with a limited number of cities or with limited geographic scope. Consequently, the spatial heterogeneity in CE trends across urban–rural gradients, and their underlying drivers remain poorly understood, especially across cities worldwide. Here we quantified summer daytime CE trends from 2003 to 2020 across six urban–rural gradients (i.e., urban cores, new towns, urban fringes, suburbs, rural fringes, and rural backgrounds) in more than 5,000 global cities, utilizing MODIS-derived land surface temperature and tree cover data. We observed an inverse-V shape in CE trends along urban–rural gradient, with peak values in urban fringes (0.10 ± 0.01 °C/%/century, mean ± one standard error), followed by new towns, cores, and three rural gradients. The trends in CE were generally positive across most climate zones, yet arid regions exhibited a decline (−0.06 ± 0.06 °C/%/century). CE trends strengthened with increasing city size in urban fringes, yet they decreased in cores and new towns. Using a LightGBM-SHAP algorithm, we found that the macro-scale background climate dominated the CE trends in urban areas (43%), whereas micro-scale local surface properties emerged as the primary contributors in rural areas (48%). Our findings provide critical insights into the spatial heterogeneity of CE trends of urban tree covers at a global scale.