Heat in the inner city: a plea for more trees and shade

Messner: “We are not at the mercy of the urban heat island effect”

Stuttgart von oben, zu sehen ist die Lage in einem TalkesselClick to enlarge
Heat islands form in urban areas with heavily sealed surfaces
Source: JCG / Fotolia.com

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 study by the German Environment Agency (UBA) has systematically investigated how cooler temperatures can be realised in inner-city neighbourhoods and their buildings. 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 present 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 neighbourhoods. Three neighbourhoods 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-coloured building paints also improved the microclimate in the neighbourhood. Overall, the measures investigated made the existing neighbourhoods much more resistant to summer heat. This also had a positive effect on the temperatures in the buildings and apartments.

In addition to climate resilience, the study also examined the energy needs of the neighbourhoods. Improved insulation, shading of windows and managed ventilation can also save energy for cooling. However, only the Tunis simulation actually achieved climate neutrality while the neighbourhoods in Europe still required an energy supply from outside. This is because the household electricity demand in Europe is significantly higher and solar power generation, on the other hand, is less feasible because the storey to roof area ratio is less favourable than in Tunis. 

Although the need for air conditioning could be significantly reduced, for example by shading the exterior and the buildings, it is not possible to dispense with mechanical air conditioning in subtropical neighbourhoods without sacrificing comfort. Even in temperate zones, temperatures of more than 27°C are not unusual, especially in exposed locations (attic apartments), despite thermal insulation measures. Only the Hamburg neighbourhood did not require air conditioning to ensure comfortable temperatures at all times. Climate protection reasons suggest the use of air-conditioning systems if surface cooling systems use natural refrigerants.

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.

Umweltbundesamt Hauptsitz

Wörlitzer Platz 1
06844 Dessau-Roßlau

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 Hitzeinseln  Anpassung an den Klimawandel  Urbaner Hitzeinseleffekt  Klimaresilienz