Are European Cities Getting Warmer? Investigating the Urban Heat Island Phenomenon in Europe from 1981-2018 through the Use of NOAA-AVHRR Data

The Urban Heat Island (UHI) effect is one of the most prominent signs of human impact on the Earth system. This phenomenon has significantly altered the energy exchange between land surfaces and the atmosphere which has led to several negative impacts on the quality of city life in terms of air and water quality, energy consumption, vegetation growth and human health. European cities are particularly vulnerable to UHI because of their high degree of urbanization, as illustrated by the 2003 heatwave which claimed thousands of lives. However, the existing body of research mostly concentrates on local-scale and/or short-term analyses, which leaves long-term effects at continental scale poorly understood. Within the TIMELINE project of the Earth Observation Center (EOC) of the German Aerospace Center (DLR), a consistent AVHRR Land Surface Temperature (LST) product has been generated, which is employed in this study to determine the intensity of the surface UHI (SUHI) over Europe for the period 1981−2018. Specifically, the objective is to analyze the long-term SUHI trends and corresponding changes over European cities, as well as to gain insights on their relationships with different variables, like climate, land use and land cover (change), vegetation indices and day-night temperature differences. First results point towards a remarkable increase in both SUHI spatial extent and intensity across the entire continent

Growth and cooling potential of urban trees across different levels of imperviousness

12 Pág.[EN] City centers experience higher temperatures than surrounding suburban and rural areas due to the urban heat island effect (UHI) (Oke, 1988). Previous studies have observed localized hot and cool spots, large intra-urban temperature differences, and variations of UHI intensity within the city's boundaries (Buyantuyev and Wu, 2010; Heusinkveld et al., 2014). These variations are directly linked to various urban factors, such as land cover, surface characteristics, building size and shape, material properties, anthropogenic heat generated from vehicles, building systems, and industrial activities (Smithers et al., 2018). The primary cause of UHI is the modification in the surface energy balance due to changes in land cover types in urban areas (Oke, 1973). Coupled with climate change, UHI poses a serious threat to the quality of life of the population in urban areas (Iungman et al., 2023). Thus, implementing effective urban heat mitigation strategies is essential for to promote the adaptive capacity of our cities (Pauleit et al., 2020).We thank the Deutsche Forschungsgemeinschaft (DFG) for funding the project “Impact of trees on the urban microclimate under climate change: Mechanisms and ecosystem services of urban tree species in temperate, Mediterranean and arid major cities” (grant number PR 292/21–1 and PA 2626/3–1). We also thank Ms. Alejandra Zelada, Mr. Amjad Hijazin and Mr. Alaa Amer for their help in the field work. We thank the municipality of Munich for the permission to measure the trees and extract tree cores.Peer reviewe