Stuttgart/Rastatt/Berlin, Germany, September 15, 2023. Heavy rain, heatwaves, thunderstorms and extreme snowfalls: in urban areas, climate change is particularly noticeable when extreme weather events occur, causing huge devastation and serious health risks. Climate change mitigation alone is no longer enough – cities, towns and villages have to adapt now to the changed climate. Appropriate measures for doing this are demonstrated by the city of Rastatt in the German federal state of Baden-Württemberg, and the Berlin TXL district development. Further approaches will be presented by cities, towns and villages from September 18 to 22, 2023 at the German national Climate Change Adaptation Week (Woche der Klimaanpassung), which is supported by the specialist construction and real estate consulting company Drees & Sommer SE, one of the event partners.
“Our cities and towns are not built for extreme heat or tropical downpours. The biggest problem is the high rate of soil sealing with heat-absorbing materials like concrete, asphalt and glass.
This can make cities up to ten degrees warmer than the surrounding areas, creating what are referred to as ‘urban heat islands’,” explains Associate Partner Gregor Grassl, who is an expert for sustainable urban development at Drees & Sommer. Heavy rainfall events are also becoming increasingly frequent. According to the German Insurance Association (GDV), the federation of insurers active in Germany, these caused around EUR 12.6 billion worth of damage to residential buildings alone between 2002 and 2021.1 “Apart from climate change mitigation and the associated reduction in CO2, we have to adapt our built environment to the climatic changes to avoid damage and maintain livable cities,“ says Gregor Grassl.
Climate Change Mitigation and Climate Change Adaptation: Two Sides of the Same Coin
Measures for climate change mitigation and climate change adaptation often go hand in hand. However, their implementation can involve a clash of opposing interests. An example of this is the debate as to the pros and cons of green roofs versus photovoltaic panels. “The former is a climate adaptation measure, while the latter aims to mitigate climate change. But a number of studies suggest that the two can be combined and that this in fact creates synergies. They claim that photovoltaic modules on green roofs generate higher yields because they work better through the cooling effect of the green roof,“ comments Gregor Grassl. Another example is solar villages which, by always being arranged in a south-facing direction, in some cases disrupt the all-important urban ventilation. Construction measures should therefore always be examined from different perspectives to exploit as best as possible the potential for both climate change mitigation and climate change adaptation.
10-Point Plan of the City of Rastatt: More Green, More Shadow, Less Concrete
The city of Rastatt has engaged Drees & Sommer and consulting firm alpS to help it develop a climate adaptation plan. Rastatt is in the middle of the Rhine rift valley, in one of Germany’s warmest regions. Climate data from the years 1960 to 2020 shows that temperatures in Rastatt in both summer and winter have risen by around three degrees. “The number of hot days and warm nights has increased, while rainfall in the summer months is falling. To prepare our city and its inhabitants for climate change, we and representatives of the local government have prioritized ten important measures for climate adaptation, which are to be implemented in the next five years. As Rastatt was one of the first German cities to launch a climate action plan back in 1994, and because we aim to make the city climate neutral by 2035, a number of individual measures have already commenced,“ comments Martin Schursch, Climate Protection Manager of the city of Rastatt.
They include the greening and shade creation initiative, under which 1,000 new trees are being planted in the municipal area since 2020. The package of measures also includes stepping up façade greening to provide cooling through evaporation. Martin Schursch adds: “For the ventilation of the city and in order to channel heat and pollutants outwards, we need ventilation corridors as well as green spaces and forests to generate cool and fresh air. To protect these, since 2017 every construction project has had to be preceded by a climate analysis.“ To improve water availability, the city is planning to store more water in a decentralized storage system to be released when needed, following the principle of the sponge city. Tree infiltration trenches, deep beds and cisterns are some of the methods that will be used to achieve this. Along the Rhine and Murg rivers, new retention areas are to be created for flood control.
Urban Areas as Sponges
Flat green roofs in city-center areas can also act like a sponge, but the structure of the buildings has to be designed to cope with this additional load. Water-permeable solutions such as grass pavers can take the place of soil-sealing materials, like asphalt or paving stones, although it should be borne in mind that these restrict accessibility. Large squares or skateparks can be constructed in a trough shape as multifunctional retention basins, allowing rainwater to gather there to be discharged later into the sewage system in a controlled way.
For the urban development project Berlin TXL, on the site of the former Tegel airport, Drees & Sommer has developed a biodiversity plan including rainwater management for a sustainable water supply with a flood defense system. The buildings and open spaces are designed according to the principles of a sponge city; surfaces were unsealed and a naturalistic rainwater basin incorporated into the design. Some of the rainwater absorbed can be used as service water.
Light-Colored Surfaces and Night Cooling instead of Air Conditioning
In cities, due to the heat island effect, hot days with temperatures of more than 30 degrees often go hand in hand with tropical nights during which the temperature does not fall below 20 degrees. This causes prolonged thermal heat stress on the body and poses a health risk, particularly for senior citizens, those with cardiovascular disease, and small children. To reduce the temperatures to which buildings and urban centers heat up, urban development planners are paying increasing attention to what is referred to as the ‘albedo effect’ – as in the case of the Schumacher quarter, the residential district within the Berlin TXL development project.
Light-colored and reflective surface materials with a low heat storage capacity (high albedo) can reduce the high rate of heat absorption in cities, towns and villages on hot days. The lighter the buildings and surfaces are, the less heat builds up, as shortwave radiation is reflected and the material cannot heat up. “Light-colored road surfacing is also already in use in some places,“ explains Gregor Grassl.
However, in some cases it is not possible to forego interior cooling systems completely. “There are plenty of carbon-neutral solutions available to deal with heat stress in urban areas. However, while air conditioning solutions such as split systems cool the interior, they simultaneously heat the outdoor spaces further through the rejected heat. They are also huge energy guzzlers,“ comments Gregor Grassl. Low-tech systems are therefore being used more and more as an alternative for individual buildings. This involves incorporating high thermal-mass materials in the building, which enables the interior to be cooled at night with the cooler outdoor air. “However, on tropical nights it is too warm and this principle does not work. Looking ahead, therefore, even from the current perspective sustainable buildings have to be renovated in response to climate change,“ says the expert.
While green façades lower the temperature of the façade, thus reducing the need for cooling, underfloor heating can become underfloor cooling systems in summer, and ceilings can serve as surface cooling systems through component activation. Geothermal energy can also be used for cooling as well as heating. The same applies to low-energy systems (LowEx systems). These create a local district network with a low flow temperature which helps buildings connected to it to exchange heat and cold, and which incorporates different renewable heat sources. Seasonal heat stores can also be integrated into the system; these store surplus heat captured in the summer underground for use in winter – and vice versa. These forward-looking solutions are resilient and work even on tropical nights.
About the Climate Adaption Week:
The nationwide Climate Adaptation Week will take place from 18 to 22 September. It is organized by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety and Consumer Protection (Bundesministerium für Umwelt, Naturschutz, nukleare Sicherheit und Verbraucherschutz – BMUV). The federal information centerfor climate adaptation (Zentrum KlimaAnpassung – ZKA) is responsible for design and implementation of the events.
The aim is to increase the visibility of the measures taken by cities, towns, villages, institutions and other interested parties to adapt to climate changes. Drees & Sommer supports the Climate Adaption Week as a partner. For more information, please read the information available in German language at the following link: https://zentrum-klimaanpassung.de/wdka23
1 Heavy Rain Balance: total damage of EUR 12.6 billion in Germany (gdv.de)