Flooded streets, dry parks, heat deaths - the consequences of extreme weather have also become increasingly noticeable in German cities in recent years. For decades, experts have been warning about rising temperatures and drought caused by climate change, but also about more storms and floods. This can have devastating consequences, especially in cities.
The asphalt or concrete surfaces in cities store heat particularly well, explains Jörn Birkmann, head of the Institute for Spatial Planning and Development Planning at the University of Stuttgart. At the same time, rainwater cannot or only with difficulty seep away. According to the Federal Environment Agency, such sealed areas make up around 45 percent of the settlement and traffic areas in Germany.
Using the keyword “climate resilience,” experts like Birkmann are investigating how cities can better prepare themselves for the consequences of the climate crisis. "It's not just about the question of where the next heavy rain will be or how strong the next heat wave will be, but also about how you can prepare citizens and sensitive infrastructure for such events and secure certain functions in an emergency," Birkmann told the German press agency (dpa).
Thousands of deaths from heat wave
It is important to learn from past extreme weather events - such as the flood disaster in the Ahr Valley or heat waves in France. At least 184 people died in the floods in the Ahr Valley in 2021, and in France in 2003, thousands of people died as a result of an extreme heat wave.
“Water can not only be the problem in progressive urban planning, but can also help and solve problems,” says Roland Müller, head of the Environmental and Biotechnological Center at the Helmholtz Center for Environmental Research (UFZ). An “urban water transition” is therefore needed. Cities would have to store water when it is in abundance - i.e. during rain or heavy rain - in order to then use it when it is scarce. Cities that take this approach are often referred to as sponge cities.
Through a so-called blue-green infrastructure, cities could be unsealed and the natural drainage behavior of rainwater could be imitated, says Müller. "The most well-known are probably the green roofs, but they also include facade greening, newly designed inner courtyards or very old modules of urban water management such as trough and trench systems." In the latter, rainwater is collected in a pit and channeled into an underground storage facility. What is relatively new is that trees are also being planted in Rigolen.
Cities have to adapt
In Leipzig, the “Leipziger BlauGrün” model project, which Müller leads, is working on just such solutions for city districts. “Right from the start, we tried to clearly focus on the water perspective,” says Müller. It is important to bring urban actors such as city planning offices, environmental offices and municipal waterworks together with science. Müller sees one of the biggest challenges on the path to climate resilience in the complexity of the matter: “You have to bring together cross-sector expertise and plan in an integrated manner,” says the expert.
For a classic medium-sized city, the sponge city approach is the right one to deal with massive amounts of water, but not sufficient, says Birkmann. "In many cases, it's also about ensuring that when there are large volumes of water, the water can flow through the city without causing major damage" - for example through appropriate drains on the streets.
According to researchers, without climate adaptation, cities and municipalities will face various problems. So-called underground heat islands, for example, can cause ground deformations that have a negative impact on the performance of buildings and infrastructure, as a recent study using Chicago as an example showed.
Shading as the most effective measure
“From a certain temperature change it also becomes dangerous to health,” says Birkmann. According to health experts, the number of heat deaths is likely to rise year after year as a result of global warming. According to a study, there will have been more than 60,000 heat-related deaths in Europe in the summer of 2022 - more than 8,000 of them in Germany.
In order to counteract the risk of heat, adaptation measures such as air conditioning or cooled rooms are important, says Birkmann. “But they certainly have the disadvantage, similar to the corona pandemic, that they are very small and do not guarantee a real quality of life in the cities.” So that vulnerable groups do not have to spend the entire summer at home or in the air-conditioned city library, entire districts or cities need to be climate adapted.
Measurements and urban climate simulations have proven that shading is the most effective measure to reduce heat stress outdoors, explains Astrid Ziemann, meteorologist at the Technical University of Dresden. "For city dwellers, the perceived temperature under a tree decreases by over 10 degrees." Extensive, undeveloped meadow areas and ventilation corridors could also provide cool air in urban areas.
Climate adaptation is particularly urgent where vulnerable groups are located. “We need to introduce resilience and adaptation standards that ensure that heat and heavy rain precautions are taken into account in the future when building important infrastructure such as schools, hospitals and retirement homes,” says Birkmann. "Because if there is an extreme event with damage in these areas, it is not unlikely that people will die."