While the climate has continued to heat up over the past decades, extreme weather events such as hot summers and flood disasters have followed one another globally, people in many places are only slowly adapting to the new conditions. But a change in thinking is noticeable, also in Bonn. The city wants to be climate-neutral by 2035 and has set up a programme office to achieve this. There are numerous creative ideas and worldwide role models for the way there.

In Paris and Athens, for example, residents can already use an app to find cool places in the city during hot summers. In various US metropolises, such as Chicago, so-called "community cooling centres" have existed for years for this purpose. For the homeless in particular, such rooms could be especially important in the future, similar to heat shelters in winter. The city of Los Angeles has also been experimenting with white streets since 2017, which reflect the sunlight in heated urban areas and thus ensure milder temperatures. Summer sprinklers", as they exist in Vienna, could also help. The spray showers emit a cooling mist of water that offers passers-by a brief cooling against the heat in the city.

Development researcher Joachim von Braun, who is involved in the "Innovation and Technology for a Sustainable Future" project at Bonn University, also observes a trend towards bio-based building materials such as wood, in which climate gases can be stored. In addition, building surfaces such as roofs and façades could be used as cultivation areas. The trendsetter for vertical or urban farming is Singapore, which is increasingly relying on green spaces along building facades in its urban planning.

The fact that many such innovative climate ideas have not already been implemented in Bonn or are in the planning stage is mainly due to practical hurdles, according to Bonn's offices for environment and urban planning. For example, goals such as the creation of living space and climate protection often compete with each other, so that there can only ever be case-by-case solutions in development. Fast, comprehensive solutions or changes are therefore not foreseeable.

Effective and space-saving climate ideas are green roofs and planted facades. Especially for inner cities, additional surfaces for cooling and fine dust binding can be created in this way, Meike Rohkemper knows. The expert consultant for green roofs explains that especially green roofs with a high substrate thickness are good for the environment. In addition to its contribution to biodiversity and CO² storage, a green roof can contribute to the sponge city principle by collecting rainwater and can store up to 90 percent of the annual precipitation.

In an environment with several green facades or roofs, the evaporation of the collected water can cool down the temperature noticeably by up to 10 degrees - without air conditioning. By providing shade, green façades also provide the building itself with an insulating layer and "fulfil additional functions in terms of cooling, fine dust binding, living space and an increased amenity value", explains Rohkemper. To ensure that these functions can be fulfilled and that green facades do not dry out, an automated irrigation system is a "basic requirement for fulfilling these functions", says the expert advisor.

Despite the promising approach, there has been little evidence of green facades in Bonn so far. Even a subsidy programme of the city, in which homeowners were reimbursed up to 50 percent of the costs, could not change this much. In the first year, 43 green roofs were subsidised by the municipalities with about 67,000 euros. By extending the programme, another 150,000 euros are available for each of the next two years. In addition, the administration wants to install green roofs as standard for municipal buildings in future in combination with the solar obligation.

Whether a green roof makes sense for one's own building can be determined via the green roof register of the NRW State Office for Nature, Environment and Consumer Protection. Residents can use aerial photographs to locate their own roof, which is highlighted in colour according to its suitability. Categories such as inclination, precipitation and the thermal load situation are taken into account for the assessment. With a click on the corresponding roof area, an initial cost calculation for different green roofs can also be carried out.

Original text: Jonas Dirker

Translation: Mareike Graepel