Resource depletion, climate change and degradation of ecosystems are challenges faced by cities worldwide and will increase if cities do not adapt. In order to tackle those challenges, it is necessary to transform our cities into sustainable systems using a holistic approach. One element in achieving this transition is the implementation of nature-based solutions (NBS). NBS can provide a range of ecosystem services beneficial for the urban biosphere such as regulation of micro-climates, flood prevention, water treatment, food provision and more. However, most NBS are implemented serving only one single purpose. Adopting the concept of circular economy by combining different types of services and returning resources to the city, would increase the benefits gained for urban areas. The COST Action Circular City aims to establish a network testing 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’. In this paper we introduce the COST Action Circular City by describing its main objectives and aims. The paper also serves as introduction to the review papers of the Action's five Working Groups in this Special Issue.

Water in the city is typically exploited in a linear process, in which most of it is polluted, treated, and discharged; during this process, valuable nutrients are lost in the treatment process instead of being cycled back and used in urban agriculture or green space. The purpose of this paper is to advance a new paradigm to close water cycles in cities via the implementation of nature-based solutions units (NBS_u), with a particular focus on building greening elements, such as green roofs (GRs) and vertical greening systems (VGS). The hypothesis is that such “circular systems” can provide substantial ecosystem services and minimize environmental degradation. Our method is twofold: we first examine these systems from a life-cycle point of view, assessing not only the inputs of conventional and alternative materials, but the ongoing input of water that is required for irrigation. Secondly, the evapotranspiration performance of VGS in Copenhagen, Berlin, Lisbon, Rome, Istanbul, and Tel Aviv, cities with different climatic, architectural, and sociocultural contexts have been simulated using a verticalized ET₀ approach, assessing rainwater runoff and greywater as irrigation resources. The water cycling performance of VGS in the mentioned cities would be sufficient at recycling 44% (Lisbon) to 100% (Berlin, Istanbul) of all accruing rainwater roof–runoff, if water shortages in dry months are bridged by greywater. Then, 27–53% of the greywater accruing in a building could be managed on its greened surface. In conclusion, we address the gaps in the current knowledge and policies identified in the different stages of analyses, such as the lack of comprehensive life cycle assessment studies that quantify the complete “water footprint” of building greening systems.

The current European Union biodiversity strategy is failing to reach its targets aimed at halting biodiversity loss by 2020, and eyes are already set at the post-2020 strategy. The European Commission is encouraging the active role of citizens in achieving policy objectives in the coming years. In this paper, we explore ways citizens discuss their priorities regarding biodiversity and abilities to influence environmental problems at individual, collective and policy levels. We also examine how the citizen discussions resonate with scientific environmental priorities and how researchers see the role of citizens in policy processes and harmonising citizen and scientific knowledge. To pursue the citizen voices, an expert working group acting as knowledge brokers, facilitated a series of citizen workshops in seven European locations and a reflective researcher workshop in Belgium. Based on the results, participants identified many concrete and value-related measures to stop environmental degradation. The environmental priorities differed between citizens and scientists, but not irreconcilably; rather, they complemented one another. Both groups stressed environmentally minded attitudes in individuals and policy. Displaying diversity of perspectives was regarded as positive and adding legitimacy. Improving methods for balanced encounters among science and society is central for participation to become more than rhetoric in the EU.

Nature-based solutions (NBS) are frequently implemented without taking the system’s perspective into account and with the main focus on technical and economic issues of implementation. This study was conducted to test the hypothesis on the potential synergistic effects between circularity and NBS to holistically tackle urban challenges. The main objective is to establish preliminary insights on the obstacles and perspectives of NBS integration and implementation, through a questionnaire set up by the network of experts gathered within the COST Action CA17133 Circular City. The following research questions arise: (i) what differences exist in the level of NBS application according to the variance of engaged countries; and (ii) what are the main obstacles and perspectives for the NBS implementation in order to holistically tackle urban challenges, enhancing the sustainable connection among urban environment, nature, and human well-being. To go beyond the current state-of-the-art and reflect on the research conducted within the Circular City Action, this study aims to open a multi-geographical academic dialogue across Europe and beyond and to move towards a holistic approach to circular cities. Accordingly, this study is: (1) multi-geographical and context-based, providing input for thirty-three EU countries and four non-EU countries to give an overview of the main obstacles and perspectives of NBS implementation, and (2) approach-directed, aiming to formulate a holistic approach to deal with societal challenges. This document intends to provide qualitative and quantitative insight into the potentials and obstacles of NBS implementation in Europe, as well as to motivate further discussion and research to achieve holistic and sustainable cities.

Globally, there is clear evidence that unsustainable urbanisation and climate change are pressing challenges for our systems. Nature-based Solutions are starting to be considered as a mechanism to help underpin and tackle societal and global challenges such as biodiversity loss, ecosystem depletion, resource use or human and ecological well-being. Nevertheless, still disciplines are working separately and enabling the co-design of Nature-based Solutions to reach sustainable urban planning in cities is far to be considered for climate adaptation and climate neutrality in cities. Therefore, the study intends to overcome those research gaps mentioned. On the way to tackle those issues, the paper frames the necessity to align science, policy and society goals to reach a sustainable future and bring sectors together to ensure and help build an inclusive, healthy and a resilient world. The methodology is based on a systematic review process where we explore the state of the art on the matter. This paper intends to open the discussion of a holistic, systemic and comprehensive approach to mainstreaming Nature-based Solutions  and  presents a novel pathway for transdisciplinary climate and environmental planning action. A novel conceptualization; socio-ecological and environmental-economic framework for Nature-based Solutions action plan with defined key principles to enable the mainstreaming of nature-based solutions into policies and governance. The study recommends and proposes specific nature-based solutions strategies to underpin the lack of coherence that sometimes shows in some approaches when designing and planning cities, implementing policies for sustainable urban planning and design, facilitating ecosystem restoration and human well-being. To reach an environmentally, socially, economically, locally, ecologically and politically sustainable, circular and resilient Europe by 2030 to help deliver the global policy agendas and the European Green Deal and its strategies.