The average annual temperature and the frequency and intensity of extreme events are steadily increasing as a result of climate change in Germany. This also increases the need for action to mitigate the consequences of these events. The aim of the sponge city concept is to design urban spaces in such a way that they can collect rainwater and keep it in the water cycle and, if necessary, utilise it. In the event of heavy rainfall, open spaces and urban greenery can retain and absorb water and thus counteract flooding as well as heat and drought.
Many actors from administrative bodies, science and politics are in favour of implementing the sponge city concept. The President of the German Environment Agency (UBA), Dirk Messner, said: “Designing cities with more greenery and more water not only increases their resilience to heavy rain, heat and drought, but also promotes the health and well-being of their residents and thus improves the quality of life of all sections of the population.”
More and more municipalities are already on the way to becoming sponge cities They are setting targets, adopting strategies, implementing measures or promoting implementation by private actors. Nevertheless, legal, organisational and financial obstacles still stand in the way of the sponge city concept being deployed more widely. The earlier involvement of specialised agencies and municipal companies in planning procedures, topic-related working groups or committees can reduce some of these obstacles. This can also be supported at federal level through improvements to the underlying conditions for municipal action.
The Climate Adaptation Act, which came into force on 1 July 2024, forms an important basis for this by creating a new strategic framework for the federal, state and local governments to systematically promote climate adaptation as comprehensively as possible. Among other things, the federal states are obliged to ensure that local climate adaptation concepts are drawn up at district and local authority level on the basis of risk analyses which also contain specific action plans.
In addition, the UBA is proposing a set of policy instruments in the recently published technical brochure "objectives and policy instruments for climate-resilient sponge cities". Among the most important are: Making greater use of rainwater as a local resource and anchoring this approach in the Water Resources Act through appropriate regulations; integrating into the Building Code the technical term ”green-blue infrastructure” and sponge city measures to mitigate climate risks and improve the local water balance.
The conversion to sponge cities can only succeed with the involvement of private companies and individuals Incentives are therefore required, for example through the integration of desealing (aka depaving) and greening measures for outdoor facilities and the greening of buildings in programmes for climate-resilient construction.
In combination with other instruments, long-term changes in municipal practices can be achieved. For example, improved data bases and parameters for the quantitative and qualitative provision of green-blue infrastructure at municipal level support the development of guiding strategies for implementing the sponge city concept. The EU Urban Wastewater Treatment Directive, which is currently being revised, also stipulates that local authorities should prioritise green-blue infrastructure measures in the wastewater management plans required in the future. Financial support for the implementation and evaluation of sponge city projects makes it possible to acquire and analyse local experience and solutions.
At the same time, there are still open research questions regarding the implementation of sponge cities. For example, how can good examples and small-scale approaches attract imitators and thus spread? And how can sponge city approaches better connect cities and their surrounding areas in terms of climate-adapted development? These and other questions are being investigated, for example, in the UBA project "Rethinking the New European Bauhaus".
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