The WHO counted 15,000 heat-related deaths in Europe last year. The urban heat island effect in our cities is driving temperatures in metropolitan areas to extreme levels. And it is anything but unstoppable. Read here to find out what makes the heat island effect so dangerous and what local authorities can do about it.

The summer of 2022 brought extreme temperature records in Germany. At the end of July, the German Weather Service (DWD) recorded a maximum temperature of 40.1 degrees Celsius in Hamburg-Neuwiedenthal. Hamburg was not an isolated case: temperatures rose to up to 40 degrees across the country. This made 2022 the warmest year in German weather history since records began. The average temperature in 2022 was 10.52 degrees Celsius, which is just above the average of previous years.

Causes of the heat island effect

The more densely built-up a city is, the greater the heat island effect. Dr. Astrid Zieman from TU Dresden identifies the high degree of sealing and the lack of urban green and blue as the main causes. Concrete, glass and metal store heat and only release it slowly. Asphalted surfaces and dark materials such as black, gray or red tones increase heating. In addition, the city continuously produces greenhouse gases that cover the buildings like a haze. These factors increase temperatures both during the day and at night.

Green and blue urban structures, such as planted areas and watercourses, on the other hand, have a cooling effect. Dr. Christoph Schünemann from the “HeatResilientCity” project explains that sealed streets and squares can reach surface temperatures up to 30 degrees Celsius higher than meadows. Nevertheless, many metropolitan areas do not have enough green spaces to regulate the urban climate sufficiently.

When the night brings little respite

In contrast, so-called urban green, i.e. planted areas, or urban blue, such as water-bearing structures like streams, ponds or rivers, have a cooling effect. According to physicist Dr. Christoph Schünemann from the “HeatResilientCity” project, sealed streets and squares achieve a surface temperature that is up to 30 degrees Celsius higher than that of a meadow. Green spaces therefore have the potential to cool the city. However, according to Dr. Zieman, there are not enough of them in large cities to sufficiently regulate the urban climate.

There are also other obstacles. For example, although the Elbe and the Elbe meadows in Dresden are striking urban blue and green structures, the city still faces heat problems. The reason for this lies in the air circulation – or in this case, the absence of it. This is because dense or tall buildings can block so-called air corridors. They disrupt the circulation of cooling winds. Furthermore, cool air sinks downwards. In Dresden, the slope towards the river also prevents a cool draught for the city. As a result, Saxony’s state capital also recorded a heat record of 39.2 degrees last year.

These groups are particularly at risk

The effects are serious. The Federal Environment Agency even considers the heat balance in urban areas to be the most significant effect of climate change on human health. Twenty years ago, the severe heatwave in 2003 caused around 70,000 heat-related deaths across Europe. In Germany alone, around 19,300 people died as a result of extreme heat between 2018 and 2020.

The figures show the relevance of the problem. In recent years, intensive research into the causes and effects of heat development in urban areas has therefore intensified. For example, the international study “Lancet Countdown on Health and Climate Change 2021” brought together around 38 research institutes on the topic. In their studies, they demonstrated the link between rising temperatures and health complaints. Even in healthy people, this manifests itself in exhaustion and reduced performance. For vulnerable groups, however, the consequences are far more drastic. The elderly and sick in particular are considered to be at risk. Higher temperatures and prolonged heat increase their risk of a heart attack, for example. Homeless people, young children and pregnant women also suffer particularly from the heat.

Center for allergy and environment

The burden on allergy sufferers will also increase in the future. On the one hand, the milder climate in spring means that the pollen count starts earlier. On the other hand, the Center for Allergy and Environment found that the allergen content in grasses increases due to drought stress. Both have a negative effect on the number and intensity of allergies. Finally, the increasingly high temperatures also pose indirect risks to health. For example, by changing the environmental conditions for disease vectors from the animal kingdom. The tiger mosquito, originally native to the tropics, is now also spreading in Germany.

Measures against urban heat

In view of these developments, Dr. Zieman speaks of a new normality of urban heat stress. Politicians, administrations and urban planners must respond to this. Back in 2017, the Federal Ministry for the Environment published a guideline for heat action plans, which includes both acute measures for emergencies and long-term methods for urban development. Last year, Construction Minister Klara Geywitz promised local authorities funding programs with a budget of 790 million euros.

Cold column for the homeless

According to Professor Dr. Jörn Birkmann, Head of the Institute for Spatial Planning and Development Planning (IREUS) at the University of Stuttgart, short-term measures must build on existing structures. Three fields of action are particularly important. Firstly, early risk communication is necessary in order to inform vulnerable groups and institutions, as well as civil society, of impending extreme weather events. Furthermore, free, cooled retreats should be designated. He is also in favor of expanding the supply of free drinking water in public spaces.

Concepts such as these are already regulated by national heat action plans in France, Italy, Spain, Portugal and England. In France, local town halls contact senior citizens in extreme temperatures and inform them of the help available. Cities also provide cold rooms for people whose homes heat up too much. Berlin wants to set up such cold rooms for homeless people in the future.

Italy’s public drinking fountains

In Italy, on the other hand, many cities already have public drinking fountains. In Rome alone, there are 2,500 of the so-called Nasoni. Germany is also slowly catching up. As a result of changes to the Water Resources Act, the provision of drinking water in public places has been considered a public service since January 12 of this year. This means that local authorities will be able to install more drinking fountains in parks and pedestrian zones in future.

Urban development and long-term adjustments

Long-term adaptations in urban development are essential. Scientists and architects are calling for more greenery in the city. Stephan Lenzen, President of the bdla, sees more open spaces and green infrastructure as an essential building block for a better urban climate. Trees have a particularly high cooling capacity: an 80-year-old lime tree can cool as much as 200 refrigerators.

In addition to planting new trees, it is important to preserve existing stands. A study conducted by BOKU Vienna in 2019 shows that trees with a high crown density can reduce the perceived temperature by up to 18 degrees Celsius in summer and up to ten degrees Celsius in winter.

Cooling effect of up to two degrees

These are factors that vertical greenery cannot achieve to this extent. Nevertheless, the BOKU researchers were also able to observe positive microclimatic effects in relation to greenery on buildings. Depending on the material composition of the façade surfaces, they recorded cooling of very high temperatures by up to around 30 degrees Celsius in full sunlight. According to the scientists of the “HeatResilientCity” project, this still leads to a cooling effect of up to two degrees in the interior of unrenovated old buildings. Although vertical greening is associated with increased maintenance and cost-intensive acquisition, numerous experts are in favor of any form of climate adaptation.

Recreational and retreat areas in the cities

Clima Grün GmbH in Bolzano has been researching how roofs can be greened and the benefits of doing so since 2002. They have discovered that roofs are among the warmest parts of the city. Depending on the material, they heat up to 80 degrees Celsius in the summer months. Green roofs not only reduce this effect. Green roofs also contribute to evaporative cooling and generate their own microclimate. This is achieved by the plants absorbing water and evaporating it again in direct sunlight. A Germany-wide study from 2021 entitled “Evapotranspiration Measurements and Assessment of Driving Factors: A Comparison of Different Green Roof Systems during Summer in Germany” showed an average reduction in air temperature of 1.34 degrees Celsius due to green roofs. In addition to reducing the heat island effect, green roofs also contribute to air quality. According to the Federal Environment Agency, one square meter of green roof binds up to five kilograms of CO2 per year and filters around 0.2 kilograms of suspended particles from the air. A team led by scientist Dr. Ngoc Cuong Nguyen from the Massachusetts Institute of Technology found that some species are particularly suitable for this purpose. Spices and aromatic plants contributed more to air purification than grasses. Green roofs are not only useful from an ecological point of view. Depending on the type and intensity of the green roof, it creates recreational and retreat areas in increasingly dense cities.

Why architects should know about the sponge city

With regard to the urban climate, the term “sponge city” is currently on everyone’s lips. It means that the city literally functions like a sponge. Instead of discharging rainwater into the sewage system, it should seep away, evaporate or be stored on site. This primarily involves unsealing surfaces. Furthermore, swales and cisterns can hold rainwater and release it back into the environment during dry periods. In addition, underground infiltration trenches delay the seepage of water into the ground. At best, rainwater is not lost in the sewage network but remains in the natural water cycle. This benefits plants and the urban climate.

Berlin, for example, has recognized the importance of this and is now anchoring the topic of sponge cities in all areas of urban planning. By 2024, 400,000 square meters of storage space for rainwater is to be created in the city. Already quite well known: The Schumacher Quartier construction project on the site of the former Tegel airport also follows the principles of the sponge city. A total of 180 evaporation beds and infiltration systems are to guarantee an evaporation rate of 60 percent. The capital has set up its own rainwater agency to implement the strategy. This provides an important transfer between findings from research and implementation in practice.

In addition to the massive expansion of green and blue infrastructures, experts continue to recommend adjustments to architecture. For example, reflective and highly reflective facades must be avoided in future. Dark surfaces on roadways and roofs also need to be questioned. While they store heat, light-colored facades and surfaces reflect sunlight and therefore heat up less.

My friend, the tree

The experts’ suggestions show that there are many clever and practical approaches that can counteract the heat in the city. Emergency measures such as improved communication, the installation of cold rooms and an increased supply of drinking water in public spaces can help in an acute situation. However, researchers and architects agree that only long-term urban redevelopment can solve the problem in the long term. The meteorologist Dr. Zieman calls for “trees, trees, trees”. The bdla advocates more open spaces, more infiltration areas and more green roofs and façades. So many concepts are already in place. However, Andrea Gebhard, landscape architect and President of the DAB, also points to the responsibility of politicians. Climate-friendly construction must be given greater priority in the new legislative period. Other experts also see national and local responsibility in equal measure. By joining forces, green and blue infrastructures can be promoted and the heat island effect in cities counteracted.

More on this topic in G+L 06/23

Published as part of the international Beat the Heat initiative.