Heat waves, droughts and heavy rain: Germany is beginning to experience the impact of climate change. Although heat waves are not a new phenomenon, the changing climate means that they are becoming more frequent and lasting longer. This is particularly noticeable in the cities, where more and more often, the heat is becoming intolerable in summer.
The urban heat island effect
Why is it so hot in the cities? The “heat island effect” means that cities heat up especially strongly during long-lasting heat waves. There are many reasons for this. Cities have more sealed surfaces, limiting the ability of water to evaporate and cool the city. Many of the materials used, such as concrete, asphalt or metal, heat up particularly strongly.
What is more, inner-city traffic and manufacturing industries accelerate the heating effect. On some days – and especially at night – the difference in air temperature in towns versus the countryside can reach as much as 10 degrees centigrade. And not only that: the perceived temperature differential can be even higher.
For many people, high temperatures are extremely uncomfortable. In a representative survey in Germany, every second respondent said they felt worse during a heat wave than on a “normal” summer’s day. The most common complaints included problems sleeping, fatigue, headaches and dizziness.
But for many, extreme heat is more than just unpleasant: it poses a real threat to the elderly, young children and people with underlying medical conditions. It has been reported that up to 20,000 deaths attributable to heat were recorded in the over-65 age group in Germany in 2018. To put this number into context, 3,275 people died in traffic accidents in Germany during the same year.
Furthermore, air pollution is more pronounced during heat waves. Among other things, this is because heat waves are often accompanied by high air pressure systems which reduce air flow, increasing the levels of air pollution in the city. Consequently, more people report difficulty breathing and other health problems during heat waves.
This poses an important challenge for cities in Germany and Europe: they have to devise ways to protect their citizens from extreme heat as much as possible. Fortunately, a range of effective town planning measures can be deployed to achieve this. Some of these measures have undesirable side effects. For instance, some measures that help mitigate the heat are detrimental to housing construction. As is so often the case, good policy decisions depend on understanding and addressing the required trade-offs.
Nonetheless, cities have a range of measures at their disposal. At the individual level, too, there are opportunities for every city-dweller to contribute because many heat mitigation measures start at a small scale.
Heat mitigation measures
Green spaces and urban trees
Green spaces are the most effective way to combat urban heat. They measurably help lower temperatures through evaporative cooling. At the same time, they provide city-dwellers with space for leisure and recreation, as well as living spaces for animals and insects. However, urban areas are spaces of intense contestation, especially when there is a shortage of housing. Expanding green spaces without considering the consequences can therefore cause problems elsewhere.
Often, it is better to improve existing green spaces than create new ones from scratch. Not all green spaces are created equal: a mix of grass, hedges, shade-providing trees and water surfaces has a much greater effect on the urban microclimate than enormous lawn areas.
The effect of individual urban trees in public squares or along roads should also not be underestimated. Trees provide shade, improve the quality of the air, dampen noise and can reduce temperatures by up to 7°C. Urban trees should therefore be used to target heat mitigation in specific spots.
Green roofs and façades
Green roofs and façades can help create additional green spaces – either horizontally or vertically – that reduce heat sources. Traditional bitumen roofs can reach temperatures of up to 80°C in summer, heating up the surrounding environment considerably. Green roofs reach maximum temperatures of just 30°C. Green roofs and façades not only help reduce outdoor temperatures, but also keep interiors cooler. Green façades also offer an attractive side effect: they help purify the air and look aesthetically pleasing. Cities can take the lead by greening their public buildings.
Using the right materials and colors
Germany’s cities are not alone in having to deal with rising heat. They can learn a lot from countries which are exposed to higher temperatures as a matter of course. The proverbial image of Greek towns, with their white-painted stone houses, can serve as an inspiration. Apart from looking attractive, they offer concrete benefits: the white paint reflects sunlight, meaning that the houses don’t get quite as hot.
The choice of building materials also has a considerable impact on heat in cities. Materials such as wood or natural stone heat up far less than conventional building materials. Similarly, light colors absorb significantly less heat than dark ones. A simple way to reduce heat islands could be to use light-colored road surfaces, for example.
Partial unsealing of surfaces
Not surprisingly, sealed surfaces are prevalent in cities. Such surfaces absorb more heat. At the same time, they store little moisture, which otherwise would help cool the air when it evaporates. Of course, the solution cannot be to unseal urban surfaces at scale – urban infrastructure depends on sealed surfaces. But some areas do not require complete sealing. For example, pedestrian walkways and parking areas can easily be partially unsealed by using grass pavers, gapped paving or permeable concrete.
Innovative ideas are on the horizon
In addition to the measures described, a range of innovative concepts is being developed to reduce urban temperature peaks. Smart start-ups have spotted the heat problem as an opportunity and started developing effective solutions. The City trees concept entails constructing wooden towers containing moss and ventilators, as well as sensors that respond to the environment. Such smart trees can reduce the ambient temperature by up to 4°C and purify the air by as much as 275 “natural” trees would. Sun sails can help reduce extreme heat in large areas. Such installations are also a real attraction that raises the quality of stay in inner cities. Mist showers also offer quick relief during acute heat waves.
Act global on climate protection, act local on climate change mitigation
When it comes to climate change, effective action requires a global approach because it relies on globally coordinated initiatives. But the immediate impact of climate change has to be addressed locally – principally in the cities, which are the most affected and house the greatest concentrations of people.
In addition to heat waves, scientists warn that the number of torrential downpours is likely to increase. The 2021 floods in Rhineland-Palatinate and North Rhine-Westphalia gave us a first, painful sense of what to expect. The good news is that there are valuable synergies between measures that address heat and flooding. Many heat-reducing measures also help combat the effects of heavy downpours. For instance, green roofs and façades not only cool the surrounding air, they can also absorb water and reduce the load on stormwater drains.
Climate change means that intelligent town planning is even more important than before. From a liberal perspective, the objective is to future-proof urban infrastructure in response to the changing climate and thereby ensure that cities remain desirable places to live and work. Innovative ideas are a critical component of effective response and mitigation strategies. Our study “The Future of the Inner City“, produced in cooperation with the Fraunhofer IAO, provides some insight into what the future might hold.