Urban climate: how cities become heat islands and what we can do about it

– Source: Johanna Lindner with dpa


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Due to dense development and other factors, cities have a slightly different climate than surrounding areas. The urban climate not only causes flowers to bloom earlier, it becomes a problem in the summer.

Most of the people in Germany live in cities. In 2020, around three-quarters of Germany’s total population lived in cities. These are becoming larger and more densely built. This also has consequences for the climate in metropolitan areas.

The term urban climate refers to properties in urban areas that are heavily influenced by people. These differ in densely built up metropolitan areas from rural areas. The climate in cities is mainly influenced by the waterproofing of large areas, with mostly asphalted or built-up areas, the lack of vegetation, high levels of development and traffic emissions.

heat island town

Climate change is having a particular impact on cities and their residents. This has already become a problem during heatwaves in recent years. This is what is known as the heat island effect. Due to the current development of most cities, much more heat is stored in metropolitan areas than in rural areas. So it stays warmer at night. On some midsummer nights, the values ​​are sometimes nearly 10 degrees warmer than the surrounding area. You must have heard it yourself. If you walk on a paved road or in a densely built-up area in the summer heat and then go to a park, you will immediately notice the difference.

The phenomenon of the heat island effect was described by Luke Howard in 1818 in “The Climate of London”. He had already noticed that air temperatures in London were higher than in the surrounding area, especially at night.

At a glance – factors that promote heat in the city:

  • Dense development and sealed surfaces
  • Sources of heat from traffic and industry
  • Buildings that accumulate heat and do not reflect it
  • Too little vegetation (trees and plants)
  • Poor air exchange due to tall buildings
  • Few areas of water

The plants in the city bloom earlier

This also takes effect outside the summer. For example, a study published in the journal Nature Ecology and Evolution in 2019 showed that in cities, leaves and flowers sprout earlier and fruits ripen earlier in spring. In autumn, the leaves fall from the trees later than in the surrounding area. According to the study, vegetation in urban areas begins on average one to three days earlier with the phenological phase of spring than in the surrounding areas.

However, the urban-rural differences in the annual course of the plant’s activities were lower than assumed based on the temperature differences. In addition, other city factors such as light and air pollution, soil conditions and pollinators also affect the annual rhythm of urban plants.

Approaches to a solution: green areas against heat

In view of the continuous warming of the climate, something must be done in the cities. Otherwise, more and more heat deaths in urban agglomerations are to be feared every summer. The possibilities are the extension of green areas, the planting of facades, reflective facades, free air channels or more water surfaces. You can see more about this in the video at the beginning of the article.

In cities, green areas with trees provide a greater cooling effect than green areas without trees due to their evaporation. He demonstrated this in a study conducted by a team led by the German geoecologist Jonas Schwaab of the University of ETH Zurich. In urban green areas with lawns, meadows or flowers, the cooling effect is two to four times lower than in areas with trees. “Parks with trees have a significantly greater cooling effect during the day across Europe than parks without trees,” said Schwaab.

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