Urban Forests as Main Regulator of the Evaporative Cooling Effect in Cities

Higher temperatures in urban areas expose a large fraction of the human population to potentially dangerous heat stress. Green spaces are promoted worldwide as local and city‐scale cooling strategies but the amount, type, and functioning of vegetation in cities lack quantification and their interaction with urban climate in different settings remains a matter of debate. Here we use state‐of‐the‐art remote sensing data from 145 city clusters to disentangle the drivers of surface urban heat islands (SUHI) intensity and quantify urban‐rural differences in vegetation cover, species composition, and evaporative cooling. We show that nighttime SUHIs are affected mostly by abiotic factors, while daytime SUHIs are highly correlated with vegetation characteristics and the wetness of the background climate. Magnitude and seasonality of daytime SUHIs are controlled by urban‐rural differences in plant transpiration and leaf area, which explain the dependence of SUHIs on wetness conditions. Leaf area differences are caused primarily by changes in vegetation type and a loss of in‐city forested areas, highlighting the importance of maintaining “natural reserves” as a sustainable heat mitigation policy. Plain Language Summary More than half of the world's population currently lives in cities. Large cities are exposed to higher temperatures than their surrounding rural areas, a phenomenon known as the Urban Heat Island (UHI). Greening our cities has been proposed as an effective Urban Heat mitigation strategy. However, a detailed global scale quantification of the effect of urban vegetation to urban microclimate remains an open debate. The reason for that has been traditionally been the lack of global scale data needed to describe the urban form, and how plants operate within in. In this study, we used the last generation satellite data to quantify all those factors. Utilizing those data, we found that urban vegetation is the most important factor regulating UHI intensities globally. Most importantly we found that the type of vegetation (urban forest or urban grasslands) play a major role in explaining the development or UHIs. Key Points Urban vegetation explains the largest fraction of the surface urban heat island (UHI) variability Urban vegetation type explains the dependence of surface UHI intensity to background climate Maintaining natural reserves in cities effectively mitigates UHIs