Our world is approaching a situation where several resources are becoming scarce at the same time, e.g., energy, nutrients, water, space, while at the same time climate change is proceeding. This will cause problems even in areas where such problems may at present seem negligible. Wealth and wellbeing of coming generations will depend on our ability to adapt our economies to this challenge in the finite world we are living in. Transforming today’s cities into sustainable cities is one of the main adaptations that will be necessary. A holistic approach looking at cities from a system’s perspective is needed to achieve this goal.

Nature based solutions (NBS), especially Green Infrastructure (GI) are introduced in the urban landscape to cope with challenges cities are facing. These challenges are urban heat islands, flooding, treatment of waste- and runoff waters from different origins and food provision. NBS offer a range of ecosystem services beneficial for the environment. However, NBS are often built without considering the system’s perspective and thus NBS only fulfil a single function with little consideration of their recovery potential of waste and water or their positive symbiosis with other systems. NBS can provide an array of valuable services, such as clean water production, nutrient recovery, heavy metals retention and recovery and a broad range of plant-based materials. The ongoing urban expansion coupled with the implications of climate change will cause resource scarcity even in areas where such problems may at present seem negligible. Linear resource flows entering cities are consumed and disposed as waste, leading to the loss of valuable resources. The Circular Economy (CE) philosophy based on the 3Rs; Reduce, Reuse and Recover (EC, 2014; Winans et al. 2017), has emerged as an alternative to the wastefulness of the current linear “take-make-use-dispose” practices of urban areas.

The principle of CE is to create a closed loop for each natural or man-made product by transforming the linear resource flow into a circular flow. It targets all kinds of industrial processes and products. For the urban environment the scale of thinking shall be more global in order to address the urban metabolism as a whole, and create not only specific CE systems but an overall resource management system for the urban biosphere. Therefore, NBS show to be a sufficient way to address important problems at the local scale as such systems can be easily adapted and operated decentralised where the highest demand occurs. The highest benefits of NBS besides their technical initial purposes is the influence on urban micro-climate and recreation purposes for the inhabitants.

The aim of the COST Action “Circular City Re.Solution” is to build an interdisciplinary platform for connecting city planners, architects, system designers, circular economists, engineers and researchers from social and natural sciences that develop systems allowing cities to cope with the challenges described above. In this COST Action, the definition for a common language and understanding across disciplines are seen as crucial success factor, while CE concepts are seen as key approach and NBS or GI solutions are seen as core elements of the toolbox. This Cost Action is to encourage collaboration and research to test the hypothesis that “A circular flow system that implements NBS for managing nutrients and resources within the urban biosphere will lead to a resilient, sustainable and healthy urban environment”. The planned Action will test this hypothesis in five domains: the built environment, the urban water, the resource recovery, the urban farming, and the society.