According to current forecasts, hot and humid weather is expected in Budapest at the end of July, with heat records likely to be broken. In parts of Southern Europe, the 40-degree mark has already been exceeded. May 2025 was also the second warmest May worldwide, accompanied by a severe spring drought across northwest Europe. The effects are felt particularly strongly by densely built-up inner cities - with their sealed surfaces and lack of shade provision, they heat up quickly. The minimal provision of urban greenery is often not sufficient to regulate this heat, which means it continues to build up in these areas. “On so-called tropical nights, when temperatures no longer fall below 20 degrees and heat islands form, even night-time ventilation hardly provides any cooling. Passive cooling strategies, therefore, such as external shading, green facades and roofs, and use of energy-storing building materials, are all crucial in maintaining living areas at a pleasant temperature," explains Gregor Grassl, Associate Partner and Head of Green Urban Development at the global consulting firm Drees & Sommer.
Targeted measures are not only able to reduce heat stress in cities, but can also improve air quality, increase energy efficiency and promote biodiversity. According to Gregor Grassl, there are already practical solutions available to reduce extreme temperatures in cities. These measures can significantly improve people's quality of life and at the same time make cities more resilient to the consequences of the climate crisis.
The cities of Düren, Dormagen and Rastatt in Germany, for example, demonstrate the ways in which this can be implemented particularly effectively. Drees & Sommer has already supported these cities in adaptations to deal with extreme weather and in sustainable urban development. Based on their experience from these and other climate adaptation projects, the urban development experts at Drees & Sommer have derived the following recommendations as important tools to combat urban heat islands:
1. Provision of shade
The simplest and most cost-effective way to protect streets and open spaces from the heat is to use trees or other elements that can provide shade. Simple solutions such as use of bus-stop roofs or provision of benches in the shade give people spaces to rest when they are experiencing a high level of exertion. “The tree population in cities is extremely important. Trees not only provide shade, they also cool the air through evaporation,” explains Gregor Grassl. In Rastatt, there are around 1,000 newly planted trees in the city area to ensure cooler temperatures in summer. At the same time, these trees will absorbCO2 and pollutants, and will produce oxygen, so they will improve air quality.
2. Cooling with plants: use of greener facades, unsealed floors
The unsealing of surfaces plays a crucial role for the microclimate in cities. “Squares and paths do not always have to be surfaced in asphalt. Gravel surfaces, such as those found in beer gardens, or grass pavers in parking lots, are suitable alternatives because they reduce the heat effect and are often more cost-effective than asphalt," explains Grassl.
In addition to trees and green spaces, the greening of facades is also an effective way to improve the localized climate in cities. It often takes significant time to implement these greening measures, but there are also other faster solutions to alleviate heat stress. In Dormagen, free drinking water fountains were installed in busy places and in the pedestrian zone. These are intended not only to encourage drinking, but also to prevent heat-related illnesses and thus protect the health of city dwellers.
3. Use of reflective and bright materials
In addition to more green spaces, bright, reflective materials can counteract heat in cities. These can reduce excessive heat radiation on hot days. “In urban planning, this is called the albedo effect,” says Grassl. Shortwave radiation is reflected and the material does not heat up. The results achieved by the albedo effect are particularly positive in densely built-up areas with large roof areas. The surfaces best suited to this strategy are light-colored concrete surfaces, paving made of concrete or natural stone, or gravel-bound surfaces. A combination of rough surfaces, porous materials and lighter colors in surface coatings also ensures lower surface temperatures and higher thermal storage capacity. If necessary, existing surfaces can be made brighter now by applying a light-colored paint or coating.
4. Use of low-energy cooling solutions during the night
Many people don’t realize the extent to which air conditioning systems further intensify the heat island effect. “Air conditioning systems such as split units are particularly problematic because they operate precisely when it is hot. Although they cool the interior, at the same time they are releasing waste heat, which contributes to the heating up of the exterior environment. This creates a vicious circle in which more and more cooling is required,” explains Grassl.
It would be better to rely on low-tech systems in buildings. A lot of thermal storage mass is installed in the building to cool it at night using the outside air. During the day, windows and doors remain closed. “If it gets too warm outside at night, this principle no longer works. In future development, energy-saving and sustainable buildings will also have to be renovated due to climate change,” notes Grassl. As an alternative to natural cooling, underfloor heating can be used relatively easily as a way to cool floors in summer. One possibility is cooling of the water circuit at night and dissipation of the heat from the interior to the outside. It is also possible to use blankets as a cooling surface.
At neighborhood level, so-called low-energy networks are useful, as these can be used for both heating and cooling. This works as follows: Ideally, water is used for cooling in the summer and therefore becomes heated. This heated water is then stored. In winter, the warm heated water is used for heating and therefore it cools down again. Potentially, this is a process that even has a positive effect on the overall energy balance in summer.
5. Building vertically instead of horizontally
“High-rise buildings shade each other and protect the apartments from overheating. For this to work, the window area must not exceed 40 percent. Glass palaces are expensive in terms of energy consumption, both in summer and winter, because glass offers poor insulation," explains Gregor Grassl.
Another advantage: high-rise buildings create turbulence and updrafts. This contributes to better ventilation for neighborhoods. When used in a purposeful way, these contribute to cooling and are comparable to natural landscape elements such as rivers that, as well as cooling by means of the water itself, always function as fresh air corridors and, due to their motion, as ventilation zones.
Climate adaptation is becoming a focal point of attention across Europe
More and more cities and regions in Europe are recognizing the need for early implementation of adaptations to deal with the effects of the climate crisis. In addition to national programs, European initiatives such as the EU Climate Adaptation Strategy, the promotion of nature-based solutions and financial support through Cohesion Funds and Horizon programs are also driving practical development projects forward.
Successful projects such as the “Oasis Schoolyards” in Paris, where asphalt schoolyards were transformed into green, public cooling zones with water installations, demonstrate that climate adaptation has long been a reality in European cities. In Vienna, provision of temporary “cool streets” with mobile trees, mist showers and seating areas leads to noticeable cooling and a better quality of life in the neighborhood during the summer months. And in Rotterdam, the “Water Square Benthemplein” combines clever water management with heat protection: The square serves as a recreational area during dry periods and absorbs large amounts of water during heavy rain, while the air is cooled by the surrounding green spaces.
Project examples supported by Drees & Sommer:
Climate Adaptation Concept for Cochem-Zell: The district is particularly affected by the consequences of climate change due to its location on the Moselle River. After long dry periods, there are often heavy rains and flooding.
Climate Simulation at the Clemens Site in Wiesbaden: In a previously unique approach, Drees & Sommer simulated various models and analyzed how the microclimate and thermal comfort of the area’s residents could be predicted.
Climate Adaptation Concept for Rastatt: The city has developed a 10-point plan that will be implemented in the coming years. More greenery, more shade, and less concrete are intended to cool the city down.
Climate Adaptation Concept for Dormagen: Based on a vulnerability analysis, a climate adaptation concept was developed.
Drees & Sommer is also working with strategic platforms such as BABLE Smart Cities and the New European Bauhaus to make innovative solutions available to cities and regions in Europe more quickly.