Global risk of precipitation deficits for tree-based cooling in cities

Trees are key sources of cooling in natural and urban ecosystems across the globe (1–3). Increasing tree cover is one popular nature-based approach to reducing excessive heat in cities where the majority of the world’s population lives (4–6). The cooling effects of trees arise from both shading and evapotranspiration and have been widely demonstrated with on-the-ground and remotely sensed data (1,7,8). However, cooling from evapotranspiration depends on water availability, which may become more scarce in many regions due to climate change (9). We evaluated urban tree cooling and water sustainability in 2,021 global cities by comparing summer evapotranspiration with incoming precipitation under current and increased canopy conditions. Mild increases in urban canopy evenness can effectively reduce surface temperature without increasing summer precipitation deficit in most cities. However, projected climate change impacts may expose 40% of cities to summer precipitation deficits given the characteristics of their current canopies. Our results demonstrate the use of tree cover for mitigating urban heat will likely require climate adaptation strategies that increase the drought tolerance of urban forests in many regions.