Study areas

Four European cities were selected in this study, representing different geographical and climatic regions of the continent (Fig. 1). The selected cities are located in Northern (Helsinki), Western (Rotterdam), Central (Vienna), and Southern Europe (Athens), experiencing different climate types. According to the updated Köppen–Geiger climate classification (Kottek et al. 2006; Peel et al. 2007), Helsinki has a boreal climate, fully humid with warm summer (Dfb). The climate in Rotterdam and Vienna is classified as warm temperate, fully humid with warm summer (Cfb). Although both cities are sharing the same type of climate, Rotterdam is located near to the coast while the continental city of Vienna is situated on the foothills of the Alps. On the contrary, Athens is located in the Mediterranean climate zone and has a warm temperate climate with dry and hot summer (Csa).

Map presenting the selected cities along with coordinates, elevation, and climate type following the Köppen-Geiger climate classification (Map was produced in Google Earth Pro; Map data SIO, NOAA, US Navy, NGA, GEBCO; Image Landsat)

Besides different climate types, the considered cities demonstrate different demographic characteristics such as human population, population density and built-up area, which are important societal variables in the analysis of climate change impacts. The urban population has increased over the last 50 years in all four cities and nowadays approximates 907,000 in Helsinki, 1.8 million in Vienna, 1.9 million in Rotterdam and 3.3 million in Athens according to the Urban Centre Database of the European Commission (Florczyk et al. 2019). Helsinki presents the lowest population density (2638 inhabitants/km²) and Athens the highest (7569 inhabitants/km²) among the considered cities. While Vienna and Rotterdam have similar urban population, the population density in Vienna (4736 inhabitants/km²) is much higher compared to Rotterdam (2909 inhabitants/km²). Rotterdam has the largest built-up area (419km²), followed by Athens (326km²) and Vienna (247km²), while Helsinki has the smallest one (117km²).