Heat in the Inner City: a Plea for More Trees and Shade

Climate change is resulting in an increasing number of hot summer days in Germany. Tropical nights – when the temperature doesn't fall below 20°C – are increasing. This is a particular burden for vulnerable groups in densely populated inner cities. This phenomenon of significantly higher temperatures in cities compared to outlying areas is called the urban heat island effect. 

A recent Guidehouse study for the German Environment Agency (UBA) has systematically investigated how cooler temperatures can be realized in inner-city neighborhoods and their buildings. The study (Nachhaltige Gebäudeklimatisierung in Europa – Konzepte zur Vermeidung von Hitzeinseln und für ein behagliches Raumklima) used microclimate simulations to investigate the potential to reduce summer heat stress in five real neighborhoods. Three neighborhoods in Germany (Hamburg, Cologne and Frankfurt am Main) and in Madrid and Tunis were selected. Thermal comfort in the outdoor space was assessed by means of the Physiological Equivalent Temperature (PET). Trees with large crowns and shading elements such as awnings and umbrellas had particularly positive effects, reducing the PET by 10 Kelvin or more in summer. Greened roofs, water spraying and light-colored building paints also improved the microclimate in the neighborhood. Overall, the measures investigated made the existing neighborhoods much more resistant to summer heat. This also had a positive effect on the temperatures in the buildings and apartments.

UBA President Dirk Messner said: "We are not at the mercy of the heat island effect. Much more greenery, especially new trees, more shading through external sunshades as well as roof and façade greening can make it more pleasant to spend time outdoors and keep temperatures down in homes. In addition to new trees, we must especially protect the mature tree population in our cities by regularly watering them in periods of persistent drought.”

The phenomenon of significantly higher temperatures in inner cities is known as the urban heat island effect. This occurs all year round and is particularly pronounced on summer nights. This is because of large-scale soil sealing and the lack of greenery in cities, which results in significantly reduced cooling through evaporation. The heating up of buildings, especially with dark surfaces such as asphalt, reduced air circulation and man-made heat sources (engine waste heat) also contribute to the formation of urban heat islands.

See the report in German for more information.