Vegetation to cool cities: a synthesis based on eddy covariance measurements in European cities

Global warming and urbanization growth are accelerating and fuelling typical urban stressors including microclimate alterations with an intensification of urban heat islands (UHI). UHI areas are characterized by warmer temperatures with respect to the surrounding rural areas, affecting human health and mortality. Trees and urban green areas (UGAs) have been shown to be crucial in reducing the UHI because of the canopy transpiration-induced cooling: by turning liquid water to vapor absorbing heat energy from the surrounding environment, solar radiation is converted into latent heat flux, which lowers air temperatures surround. In this study conducted over 10 European cities we investigated if and how much UGAs impact latent heat fluxes and the related ambient air cooling, and how UGAs could be used to develop more habitable and sustainable urban environments. Specifically, the objectives of the study are to: 1) assess the impact of the green areas in cooling down the air temperature in summer months using in situ eddy covariance (EC) measurements and 2) assess the role of the environmental factors driving the latent heat fluxes and, consequently, the related cooling of urban microclimate. Results confirm that green areas within urban environments are key elements for enhancing the summer air cooling and thus the well-being of local inhabitants.