1 Introduction

In order to address current unprecedented climate, environmental and economic challenges, nature-based solutions (NBS) require a different approach to policy making from what has often been the status quo. Policy making often operates in sectoral silos [1]; consequently, the breaking down of these silos may be facilitated by new multi-sectoral and integrative approaches which increase the resilience of urban areas in the face of such challenges (see [2]). In this context, innovative, adaptive policy approaches are increasingly being used by public administrations seeking to use nature as a solution in urban contexts in order to derive the full range of associated benefits (see e.g. [3]). These co-benefits potentially include increased wellbeing, liveability, biodiversity, climate change mitigation and adaptation, etc., for urban citizens; contributing to increased urban sustainability (see e.g. [4]). This paper aims to examine the governance structures from which such innovation and experimentation in policy approaches are more likely to occur. This article therefore aims to contribute to knowledge relating to the United Nations Sustainable Development Goal 11: Sustainable Cities and Communities. The analysis is based on 26 semi-structured interviews and three policy workshops (one in each case study city) with key policy makers and practitioners conducted in 2021 and 2022 (see [5] and Tables 4 and 5 in Appendix), as well as policy document analysis. The interviews and policy workshops cover three European NBS case studies in contrasting urban areas: Paris Region (France), Aarhus (Denmark) and Velika Gorica (Croatia).

‘Governance’ as a concept has been interpreted in different ways, which can lead to some confusion about its meaning. Some scholars use the term as a particular mode of government (e.g. rule by self-organising networks), while others use it as a broad concept covering a range of different modes of governing (e.g. hierarchy, market, network or combinations of these) (see e.g. [6]). In this paper, we define governance as the diffuse, networked and collaborative exercise of power across multiple actors towards shared ends. Interpreted this way, it is distinct from government and governing, which implies a hierarchical command-and-control exercise of state power to achieve stated ends [7]. In this analysis, therefore, ‘governance’ involves networks, and encompasses instances where government uses instruments and techniques that steer and guide, as well as those that command and control [8]. These governance networks can involve a wide range of stakeholders including government, semi-public organizations, businesses, civil society/non-governmental organizations, local community organizations, and citizens.

The analysis draws on the concept of governing architectures [8]: p.16. This term encompasses the way that governing systems, associated structures and governing cultures (including norms, behaviors, levels of trust, etc.) overlap and interact; and highlights that both structural and cultural aspects need to be considered jointly and in specific city/local government contexts when exploring NBS policy and practice.

The existing literature tends to be separated into either looking at policy innovation (the public administration literature e.g. [9]), generic NBS policy/implementation (the NBS literature e.g. [10], or broader concerns around sustainable development in urban areas [7, 11,12,13,14]) and there is a gap in terms of bringing these aspects together. Few research studies integrate theoretical and empirically grounded insights or cover political influences on NBS. In addition, there has been little relevant comparative research to date, with most articles taking the form of single case studies. This paper therefore makes an important contribution to the literature by presenting an empirically grounded comparative study exploring the uptake of NBS through a policy innovation lens using both theoretical insights and empirical data—and bringing in the political aspects of NBS which are relatively unexplored. The emphasis on NBS also brings a focus on tangible, multifunctional and innovative solutions on the ground that can lead to nature renewal.

2 Innovative governance for NBS: a conceptual framing

NBS appeared as a concept towards the end of the first decade of the 2000’s, introduced by the World Bank and International Union for Conservation of Nature (IUCN). NBS is rooted in climate change mitigation and adaptation, but can be understood as an umbrella term for addressing multiple policy objectives at the same time [10]. Although NBS fits broadly within the sustainable development agenda, it departs from this in its interdisciplinary focus on policy innovation and on tangible, multi-functional solutions to climate-related and environmental problems—whereas sustainable development research has often investigated and made recommendations predominantly at the level of policy and strategy [see e.g. 7]. NBS also emphasises innovative nature renewal and transformation (e.g. re-naturalising of watercourses, etc.), going beyond a tendency for sustainable development to focus on preservation or conservation of existing environments. Consequently, NBS can broadly be considered a new policy idea that enables us to look at urban sustainability issues through a new policy lens and using a new solutions-focused approach. New policy ideas must be processed through existing political structures and can therefore meet barriers or enablers in the ‘structural realities’ they encounter when stakeholders try to put them on the political agenda [9], by, for example, hostile stakeholders or ministries etc. With its focus on multiple interconnected benefits, NBS is a novel area of policy making that can challenge the status quo of policies and the vested interests of powerful stakeholders embedded therein; innovative policy approaches carry some element of risk to administrations and privileged policy networks. Sectoral silos may be thus very hostile (or indifferent) towards new policy ideas. Moreover, innovation carries an additional element of risk for administrations as there are more unknown elements that can increase the possibility of policy failure.

Recourse to the literature on institutions would suggest that policy-making institutions tend, though not exclusively, to avoid major changes in policy direction—with historical approaches to problems and who to engage with around them, tending to dominate processes and thus limiting the scope for more innovative policy change [15, 16]. For this reason, policy change as a norm can often be seen to be incremental [17], with policy-making institutions being disinclined to innovate. However, long periods of policy stability can be broken by appearance of new policy agendas or policy images [18], new scientific understanding and knowledge, external shocks such as economic decline, or by new strong policy networks challenging existing policy networks with new policy images [19]. We argue that NBS is a new priority on the political agenda that requires more innovative, adaptive policy approaches by public administrations seeking to use nature in urban policy contexts to derive multiple benefits for the well-being of constituents. Examples of innovative NBS approaches covered in this paper include tree and woodland planting combined with improvement of access and recreational areas (e.g. aimed at target user groups); planting and maintenance of street trees; depaving and regreening of paved areas; greening of schoolyards; water management (rewetting etc.) combined with improvement of green spaces; greening/planting/improvement of existing green areas to enhance liveability of urban environments; urban rewilding; green roofs, and renaturalising of watercourses, etc. Many of these are being designed in conjunction with local communities/user groups so that they move beyond conservation and innovate, leading to multifunctional co-benefits including climate change adaptation (counteracting flooding and/or heat effects), increased liveability, and recreational, health and wellbeing benefits. In order to be effective, NBS therefore also entail utilisation of distinct, collaborative modes of governing in their implementation. NBS also involve recognising nature (renewal) to be an integral part of governance.

This paper analyses the types of factors driving NBS policy innovation and implementation using qualitative data to create a comparative analysis (see e.g. [20]) and aims at analytical as opposed to statistical generalization (see [21, 22]). It does so through exploring influencing factors (both theoretical and empirically observed) listed in the existing literature as well as analysing whether and how these elements play out in three European NBS case studies, namely Paris Region (France), Aarhus (Denmark) and Velika Gorica (Croatia). Because this study aims to identify the main drivers/hindering factors of NBS implementation, with differences expected between cases in the qualitative data collected, and because the data shows that context affects what types of NBS are implemented, we have not provided accounts of exactly the same NBS elements for each case study. However, the analysis allows us to illustrate similarities and differences as well as general patterns.

2.1 Factors facilitating or hindering policy innovation

Based on an examination of the existing literature, we would suggest that there are three broad factors that disrupt incremental policy approaches to pave the way for policy innovation:

  • External factors outside the control of an administration such as problem events, scientific breakthroughs and other countries’/states’ policies;

  • Societal factors such as economic growth, shifting social/community values, public perception, interest groups, public distrust and socioeconomic components; all factors which drive disruption. Influencing these is likely to be difficult and time consuming; and

  • Governing factors such as governance structure, existence of policy networks, policy framing, collaboration in governance and political leadership; factors which can be modified but often will encounter political obstacles.

2.1.1 External factors

The literature on historical institutionalism suggests that more radical policy change tends to occur in reaction to external shocks impacting a relevant policy sector (see [16, 23]). Such shocks facilitate more radical change because they strongly highlight and reinforce the need for a policy change in order to address the issue—i.e., they expose a lacuna or inadequacy in current approaches thus paving the way for policy innovation. A large body of literature, for example, has explored the role of climate change-related shocks and risks, often linked to the occurrence of new scientific evidence, extreme climate impacts and events such as severe floods, droughts or heatwaves [see e.g. 24]. Such events have demonstrated vulnerability to a changing climate and thus spur more innovative policy responses [25,26,27]. The timeframe for innovation within this context is often suggested to be limited and characterized as a ‘window of opportunity’ (see [27,28,29]). This situation would suggest that if innovative action is not in train shortly after the event, then the enduring policy approaches and practices will remain entrenched. Another major external factor mentioned in the literature relates to the transfer of new and innovative policy ideas from neighbouring countries, regions or municipalities, usually termed policy diffusion (see e.g. [26, 30]). This line of reasoning is based on the premise that neighbouring polities are integrated and interdependent socially, economically, environmentally and geographically and thus share similar problems. This can result in cooperation and collaborative learning, or a race to the top as neighbouring administrations see the success or failure of each other’s approaches which can lead to similar (or avoidance of) policy innovations.

2.1.2 Societal factors

Strong economic growth and positive socio-economic trends are viewed as an important foundation, but not a direct driver of policy innovation [26, 27, 31]). The logic here is that innovation needs the foundation of economic stability and prosperity to occur as these conditions provide more social and economic resilience should an innovation not work as intended, with associated costs on society. Short-term costs need to be balanced against longer term benefits (as is often the case with environmental issues). Public perception has also been argued to be a key factor that determines the potential for policy innovation [25,26,27]. Of course, public perception can be linked to external factors such as, for instance, shocks related to a severe flooding event which can shift public opinions and create demands for change. More organized interest group activity is also said to positively push for and promote policy innovation. Within the mix of public interest, more organized interest groups can represent special interests to push for change through working with public authorities or through applying external pressure to build up a societal appetite for change [27, 32]. Moreover, we would argue that interest groups can facilitate change through pursuing innovative community-level approaches from the bottom up independently of public authorities.

2.1.3 Governing factors

Visionary political leadership is noted across the literature as a key condition for policy innovation [27, 31, 33]. Such leadership can help to negotiate risks associated with experimentation, broker policy change with institutions and provide financial backing and incentives. The strength of governance and leadership offers a layer of complexity because they have opposing effects. Weak or fragmented governance [27, 31, 33] can reduce policy innovation because it encounters structural obstacles [34, 35], such as multiple veto points, as is the case with the U.S. federal structure of government [27]. However, weak governance can also encourage politicians and entrepreneurial policy stakeholders to introduce new policies because they have felt sidelined in the political process [32]. There are questions over whether more or less centralized policy making has a similar level of impact on policy innovation, a key component of the governing approach. While the literature has identified the adverse effects which both of these components can impose, it has not determined in which context they either influence or limit experimentation. Broadly, those studies highlighting the importance of decentralization suggest that the flexibility that decentralized decision-making settings offer provides the space for policy innovation by local stakeholders, including ‘policy entrepreneurs’ [36, 37]. By contrast, those advocating more centralized settings suggest that the authority and accountability of the state creates conditions of stability that encourage policy innovation [27, 35, 38]. As described above, potentially, it is also important whether the decision-making (centrally or locally) is more hierarchical or more flat-structured and inclusive, which is likely to be best answered empirically (see below). The final governing factor to consider is the issue of policy failure which has been argued to be a trigger for innovation [39,40,41]. Here, the logic is that failure leads to policy learning, which in turn leads to the development of more innovative policy.

3 Methods

3.1 Case study design and selection

This research uses a case study design [33, 42], allowing us to look at city level NBS policy characteristics as well as providing insights at the operational level—the implementation of local NBS projects/initiatives (outlined in the sections below). The city case studies enable us to reflect on comparative insights across the participating cities. The case study design therefore ensures maximum comparability and compatibility of data across the individual cases (see the conceptual framework in [8]). In addition to the use of interview data (see below), we have triangulated the findings with data from three policy workshops, also carried out in the three case study sites in 2022 (see [5]), and drawn on background analysis of policy documents as well as from field visits and local knowledge of staff from the partner organisations in the case study cities.

The city case studies were selected to include a diverse range of European regions, governing architectures (e.g. the degree of centralization), and sizes of urban area. The criteria for case selection were as follows:

  • Policy institutions already engaged with NBS (or would like to);

  • Differing governance systems (in particular de-/centralization etc.);

  • Representing different European (political-historical) contexts;

  • Cities of different sizes; and

  • Different stages of adoption of NBS.

The case study selection was also influenced by practical considerations such as where the project team and partner organizations were located and considerations of access to relevant stakeholders, with partners assisting with the recruitment of participants, without which it would have been extremely difficult to carry out the research. The data collection draws on ongoing collaborative work between partner organisations and the NBS initiatives covered.

The case studies consist of: firstly, Paris Region, France (large sized urban area, relatively decentralized, Western European political/historical context); secondly, Aarhus, Denmark (medium size, most decentralized of the three, Northern European political/historical context); and thirdly, Velika Gorica, Croatia (small size, relatively centralized, Central European context, a formerly socialist country). Interviewees and workshop participants were recruited based on their involvement in case study NBS. We developed a protocol for case study selection (see [5, 8]) on NBS within each case study site. This entailed stipulations to include co-benefits, participation of stakeholders/citizens to varying types and extents; and representation of different phases of the policy cycle.

By employing a most different cases approach [42, 43], we incorporated different-sized urban municipalities with distinct cultures of governance. In this way, we are able to identify those elements relating to innovation that are consistent across the cases and therefore may be more related to the challenges and opportunities posed by NBS, from those elements that differ by case and may be more related to governance systems and cultures that are specific to the studied contexts.

3.2 Case study descriptions

We provide here further information about the case studies, including about the implementation/operational level (local projects) to give additional context to the findings (more information is provided on these in [44communes—towns; with French communes now being grouped together for administrative purposes), Denmark has 98 municipalities, and Croatia 428 [45,46,47].

The Paris Region, France, is by far the largest European urban area among our three European cases, and the most complex, as it represents an agglomeration of 20 districts (arrondissements), subdivided into townships (cantons) and towns (communes). The City of Paris has a population of around 2.1 million [48] (2020 figure), and the greater Paris area of around 11.2 million [49] (2021 figure). Interviewees reported on a number of individual projects in Paris Region, including the Openness, Adaptation, Sensitisation, Innovation and Social Ties (OASIS) project which involved depaving/greening predominantly of schoolyards but also implemented in other public spaces (see [50])—which had strong local government support; and various other urban NBS initiatives, some government-supported (e.g. an urban depaving project (e.g. removal of concrete paving and greening of public spaces, car parks, etc. [51]); green roofs; and the project ‘Nature in the City’). Other local initiatives led by citizens or non-governmental organizations included urban planting; shared community gardens; enhanced liveability and green spaces, etc. (e.g. ‘Villages in the City’ and some associated with EU-funded projects such as the ‘Life intégré ARTISAN’ project and the ‘Nature4Cities’ project).

Aarhus is the second largest city in Denmark with a population in the Municipality of Aarhus of around 355,000 [52] (2021 figure). It is of medium size for the purposes of the project’s city case studies. Interviewees reported on a range of projects involving the municipality including forestry and tree planting projects that engaged with local stakeholders, such as schoolchildren. These include a collaboration between the municipality and private companies to create new multifunctional, landscape-based, and biodiverse forests with natural hydrology close to the city, also involving public access. Through this project, several new forests with paths, lakes, streams, open areas, and dense tree plantings were established within the municipality in recent years (e.g. Hasselager). Water projects addressing flooding/flood risk have also been implemented. This includes a project in the Åbyhøj area, where the water utility company and the municipality created surface water systems, including lakes (rainwater ponds) and small streams, with additional value and co-benefits provided by multifunctional areas such as playgrounds, recreational spaces, paths and wildflowers. In addition, health and inclusion projects involving modification/redesign of unused outdoor spaces (e.g. with disadvantaged communities and the elderly) were also reported on in the data.

Velika Gorica in Croatia is the smallest city in our study, with a population of around 61,000 for the municipality; the city itself has around half this number (based on 2021 census data) [53]. It is the sixth largest city in Croatia, and has more recently attracted increasing numbers of residents because of its proximity to Zagreb, the capital city, and particularly to Zagreb airport, as well as its size and liveability/quality of life (e.g. compared to parts of Zagreb). The history of Croatia as part of former Yugoslavia—a former socialist country—has influenced its governing architecture. Interviewees were from a range of types of organizations, including local government, university departments, a school, and local NGOs. The interviews and field visits covered a number of projects including: renaturalisation of natural river habitats/watercourses (combined with enhancements for recreation e.g. natural walkways) e.g. involving a local NGO (voluntary fire service); a green NGO centre of knowledge for permaculture/natural building methods & materials carrying out educational/awareness raising work; an eco-schools project implementing a number of environmental initiatives with schoolchildren; green/blue space enhancements and maintenance associated with a group of lakes managed for fishing and recreation by a local NGO; and a university project to build an eco-building (at planning stage). Other projects in the city relate to flooding, urban rainwater retention and green roofs [see 44].

3.3 Recruitment of interviewees

This paper draws on European data from a total of 26 interviews conducted (a mixture of in-person and virtual), of which 7 relate to Velika Gorica, Croatia; 12 to the Paris Region, France; and 7 to Aarhus, Denmark, supplemented by data from three policy workshops, one in each of the three cities (see Tables 4 and 5 in Appendix for details and [5] for further information about the composition of and recruitment of policy workshop participants). Different types of stakeholders involved in NBS policy and implementation in different roles (proximate actors, peripheral actors and outsiders—see Tables 4 and 5 in Appendix)—from key stakeholder groups including government, businesses, non-governmental organizations (NGOs), small civil society organizations, consultants, etc.—were sought for the interviews in order to give a range of views of NBS policy, governance and implementation. The local project partner organizations assisted with interviewee selection based on their knowledge of, and ongoing collaborations with for the purposes of facilitating implementation, key local actors associated with NBS. As well as obtaining suggestions for interviewees from partner organizational staff in each city, researchers also used an iterative process to aid with identification and recruitment of interviewees—asking interviewees for targeted suggestions of further key people to interview. In Aarhus, the interviewees comprised of local government policy-makers from the City of Aarhus administration. This is a limitation associated with the constraints of the research process in this case, and therefore future research would seek to include a greater range of stakeholders in order to deepen the research findings. In the Paris Region and in Velika Gorica, a greater mix of types of stakeholders indicated above were recruited. In addition, three of the interviewees in the Velika Gorica case studies were recruited from amongst the participants of the policy workshop (held in September 2022 in Velika Gorica) based on their knowledge/key role.

3.4 Structure, framing and analysis of interviews

Semi-structured interviews were designed using a data collection and analytical coding matrix (see [5]) developed by researchers across the three European case studies. This matrix was developed based on the literature review and on the project conceptual framework to ensure compatibility and comparability of data across the case studies. It includes various categories of interest for the project—including governing structure(s), governing style; governing actors; governing procedures; stakeholder and citizen participation; innovation and experimentation, urban sustainability challenges; urban nature in urban governing; policy learning; and policy activism; with a number of additional sub-categories. Interview questions were then compiled based on these categories and adapted to each interviewee based on their area(s) of expertise. The interviews were recorded and transcribed and then translated into English using reputable online translation software. Where possible, the transcriptions were checked against the original recording and edited as necessary by a project researcher. The data was then analysed using the analytical coding matrix. The analysis was carried out using a thematic analysis approach [54, 55], combining both deductive (based on the coding matrix and the literature) and inductive analysis. The results are presented in the following section.

4 Results

This section reports the results of the analysis of the interview data relating to factors facilitating and hindering NBS policy innovation. We have summarised these factors in order to show comparative insights across the three European cities (see Tables 1, 2, 3) on NBS implementation at the operational level as well as city-level NBS policies. For each city, governance characteristics are first outlined for each case study, drawing on additional policy document analysis for the case studies. This is followed by the summary table of results covering all three case studies, and the discussion and conclusions.

Table 1 External factors facilitating NBS innovation (main drivers)
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Table 2 Societal factors facilitating NBS innovation (main drivers)
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Table 3 Governing factors facilitating and hindering NBS innovation (main drivers)
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4.1 Paris region

4.1.1 Governance characteristics

Data from the interviews and background document analysis carried out in the Paris Region showed the governing architecture of Paris Council to be relatively decentralized, with ultimate decision-making held by the Mayor of Paris and the Deputy Mayors (particularly regarding financial support); but significant NBS-related local decision-making is devolved to the district and town (commune) levels (or associations of communes). Implementation of NBS in the Paris Region (e.g. depaving, greening of schoolyards, liveability initiatives—as detailed above) involves multiple departments, professions and agencies (as well as a range of other non-governmental stakeholders). The vast majority of interviewees highlighted that the complexity and multiple hierarchies present challenges in terms of the number of actors that need to be consulted or to agree before NBS initiatives can be implemented. Most interviewees also indicated that departments tend to be highly compartmentalized/siloed, bringing additional challenges for the cross-departmental working necessary for implementing NBS (see Table 3).

4.2 Aarhus

4.2.1 Governance characteristics

Data collected in interviews underlines the high level of decentralization, with autonomy for municipal government to develop local policies within the fixed framework and regulations set by national laws. The local public administrations both develop local environmental policies and administer the environmental regulations issued by national agencies within a multi-level governance structure.

Interview data in Aarhus shows that formal organization of policy making and administration means that expertise can be highly specialized and that policy making tends to be fragmented into specialized units. This can increase the compartmentalization of policy making. However, existing cross-sector procedures, NBS projects (e.g. forestry, water/flood risk) and collaborative networks within the municipal administration were reported, which have been established in recent years to alleviate this siloing and to promote integration of urban nature across policy areas. Decision-making about urban planning and nature was reported to be strongly linked to the highest political level in the city, particularly the current political views and priorities of the Elderman (Mayor) in office and the Committee for Technical and Environmental (T&E) issues.

4.3 Velika Gorica

4.3.1 Governance characteristics

The interview data indicates governing to be quite centralized (especially for public services); with some decentralization of specific elements within this. City administrations were highlighted as having very significant influence over whether NBS projects (e.g. renaturalising watercourses, tree planting etc.) get implemented. This is due to the decentralization of national policy making to city level and at the same time, centralization of policy making within the city and the hierarchical influence of the city on policy making at local level. According to our interview data, the vast majority of decisions on NBS were reported to be made by the Mayor (or by the Mayor’s office), sometimes consulting experts or being influenced by public opinion. However, whilst the city authorities technically have the decentralized political autonomy for NBS policy innovation, this is not matched by economic decentralization, with city authorities reported to often lack sufficient discretionary funds to start or support (longer term) innovative NBS projects.

4.4 Summary tables of three case studies

The following three Tables 1, 2, 3, divided into external, societal and governing factors, indicate the factors facilitating and hindering NBS innovation identified in the analysis. They are presented to enable comparison across the cases. The tables below show the facilitating and hindering factors that represent the main drivers for NBS innovation. Interviewee codes (numbered interviews by case study—see Appendix for further details) are provided in square brackets in the tables—Paris (P), Aarhus (A) and Velika Gorica (VG) [e.g. P12; A3; V2], supplemented by information from the policy workshops, coded as follows: Paris Region policy workshop (PPW); Aarhus policy workshop (APW); Velika Gorica policy workshop (VPW) (see [5]).

When we compare the findings across case studies, the external factors that emerged as main drivers were climate change effects (either flooding, heat effects or both) and policy diffusion—the spread of or exposure to ideas and examples from other countries (e.g. greening of schoolyards in the Paris Region). As indicated in Table 1, these were highlighted by interviewees as a factor across all three cities.

Comparing findings across case studies (see Table 2), two main societal facilitating factors were shared across all three case studies and, based on the data, judged to be important drivers of NBS innovation. These comprised shifts in public and media perceptions across multiple policy areas (climate, health and wellbeing, water management) towards favouring NBS implementation; and the city having a tradition of use of (green/blue) public spaces. Other important societal facilitating factors shared across two case studies included the role of a boundary actor and the active role of a policy entrepreneur driving NBS innovation and implementation.

Table 3 indicates the comparative results across the case studies, showing a number of governing factors shared across all three case studies, identified as main drivers. These relate to governance structures and cultures. Firstly, division of responsibility within a multi-level governance system (and the level of decentralisation of NBS policy/implementation) was cited as driving (or hindering) NBS policy and implementation according to how it plays out in terms of complexity (e.g. too much), or siloing of policy areas. Secondly, political support for and prioritization of NBS policy was reported as having significant influence—involving factors such as leadership and the role of an internal champion; how political support plays out at municipality level and in local spatial planning processes; and a willingness to experiment/fail. Thirdly, policy integration/coherence of NBS into strategy (lack of siloing of policy areas; the existence of active cross-departmental/-sectoral collaborative networks/procedures was identified as an important driver. Fourthly, policy integration in NBS implementation—a willingness to explore multifunctional solutions balancing needs of different users was an important factor in terms of NBS innovation and creating co-benefits for target user groups. Fifthly, structures, cultures and practices (including support for) citizen/public participation, consultation and engagement were reported to be a key driver for successful implementation and outcomes such as local buy-in and multifunctional benefits for user groups. Sixthly, resource and funding availability was cited as having important positive impacts (where available), with constraints cited, e.g. around coverage and longer-term funding. This included the availability of government and/or external support for a range of aspects such as local NBS initiatives, green NGOs; workforce & technical expertise; public engagement/participation on NBS; and practical maintenance of green spaces.

In addition, lobbying of interest groups/service providers was also cited in one case study (e.g. advocating for grey rather than green infrastructure solutions). Although political lobbying may well be more important than it appears to be here, particularly at national level, the data indicates that political lobbying by external actors is less of a factor at local municipality level, due to the strong established and proximal relationships in place, and the importance of internal champions and local boundary actors/policy entrepreneurs.

5 Discussion

The comparative analysis of the three case studies (see e.g. [20]) has allowed us to differentiate examples of the different types of factors (external, societal, governing) influencing NBS policy innovation, how they are driving NBS innovation in the different contexts and whether they are facilitating and/or hindering depending on the policy and implementation contexts, as outlined in the key stakeholder interviews and policy workshops. Circumstances that affected whether factors were facilitating or hindering included the underlying governance structures (level and which sectors/functions were de-/centralized) and type of policy response; as well as resource constraints. We provide a few overarching reflections below. Our conceptual framework in general allowed clear analysis of key drivers (facilitating and hindering) for NBS innovation. There were, however, one or two cases where factors could be seen to cut across all three categories (external, societal and governing), notably, questions of financial resources; or across two categories as in the case of structures and practices around citizen participation, consultation and engagement. These were amalgamated into the governing category, as ultimately they were often reliant on political decision making at local/national government level (apart from some external funding sources).

When we compare across the case studies, in terms of external factors facilitating NBS innovation, as noted by [10], the increased prevalence and severity of climate change effects (such as extreme flooding events and heat effects) was underlined in all the case studies as a driver of NBS innovation, although the particular issues highlighted varied because of differing climatic and environmental conditions. Policy diffusion was also identified as a key external driver—exposure to examples of NBS innovation from other countries. In terms of societal factors, shifts in public and media perception around climate change effects; and a tradition of use of public spaces by citizens were perceived as important driving factors facilitating NBS innovation for all the case studies, with the data suggesting non-governmental policy entrepreneurs and boundary actors to be important drivers for two out of the three case studies.

Regarding governing influences, and applying the findings of [27, 31] on climate policy to NBS, the analysis highlighted the importance of political factors in fostering NBS innovation, notably the influence of political support for and prioritisation of NBS, and political leadership from the Mayor/Mayor’s office across all three case studies, despite differences in governing architectures and cultures and of institutional logics of operation (see [56, 57]). Policy coherence with and integration into government strategy (see [58]), and leadership, e.g. the presence of an internal champion (see e.g. [59]), were also highlighted as facilitating innovation across all the case studies. Structures, cultures and practices of citizen participation, consultation and engagement with the full range of stakeholders and a willingness to explore multifunctional solutions were also identified as key facilitating drivers towards NBS innovation and uptake across all the case studies.

A few key differences were also identified. In the Paris Region there was more evidence of citizens and local NGOs developing their own NBS initiatives, based on tackling external factors and priorities such as increasing the liveability of urban areas, biodiversity, health and wellbeing, and climate adaptation, without government assistance (or only later on seeking such assistance). Whereas, in Velika Gorica the barriers to local groups doing this (e.g. funding, structural/cultural) were reported to be greater. However, further research would be beneficial on this point, ensuring data is collected from a wide range of stakeholders in all case studies to deepen the findings, particularly in relation to Aarhus. In contrast with the other case studies, the external factors in the Paris Region represented more incremental drivers (e.g. gradual increase in heat effects) rather than a sudden shock (e.g. flooding event).

In a similar vein to the Paris Region, the Aarhus data indicated that Aarhus citizens (and, in some cases, local enterprises and utility companies), in collaboration with the municipality, are playing an active role as participants in innovative NBS initiatives. Significant citizen participation and consultation as part of government and externally funded initiatives (e.g. in forestry, tree planting and water projects) was reported to be leading to innovation—particularly in terms of encouraging inclusion, creating multifunctional spaces and increasing biodiversity and quality of green/blue spaces. This was reported to also bring increased co-benefits (e.g. social, health and wellbeing). Nonetheless, it is important to note here as well that these perspectives arise mainly from municipality employees’ accounts and would benefit from further data collection to deepen the findings.

The Aarhus data indicates that local government policy makers were actively engaging with various external policy factors related to NBS—including climate adaptation (especially flooding-related), biodiversity and health and inclusion. The data indicates that extreme flooding events have had a significant effect on the policy landscape and prioritization of these issues in Aarhus, and that this can be traced through to implementation. As in the Paris Region, coherence with and integration into the government strategy were a key factor; with more of a sense that local government staff were (and had to be) skilled in noticing and appealing to shifting policy priorities and agendas in the political landscape. The data suggests that internal champions (in this case within local government) were also important drivers for innovation consistent with the literature (see e.g. [59]).

The Velika Gorica data again indicates key governing drivers. Interviewees cited a few examples of public participation leading to NBS innovation, e.g. in green space design. In contrast with the other case studies, however, there was less recognition or attention to public participation as a driver for NBS implementation and innovation (despite emphasis on public consultation for the local spatial plan), and this could be linked to the more centralized nature of governance in the city (a possible example of policy learning/implementation in the shadow of hierarchy—see [60]). Innovation seemed also to be more constrained by funding limitations, both from government and external sources e.g. EU, at times creating dependence and/or patchy geographical implementation. In Velika Gorica, the local spatial plan embraced NBS but had been subject to long delays and funding shortages. External policy factors (both incremental and sudden) such as increased heat/climate effects and localized flooding were highlighted as important drivers towards NBS implementation.

Similarly to the argument in [9] in the context of international policy coordination but applying it to the NBS context, our analysis also identified a number of barriers that are hindering the uptake of NBS across the case study sites. The key hindering factors reported were predominantly in the governing category. The number of hindering factors reported was generally lower than reported facilitating factors across the cases, which may be a reflection of the relatively advanced nature of many of the initiatives, meaning that hindering factors have been circumvented or resolved. Notably in this respect, more hindering factors were reported for Velika Gorica, in which initiatives around NBS are at a far earlier stage of development than the other two cases.

Comparing across the three cases the common hindering factors cited by interviewees were all in the governing category, which would imply that public authorities have some agency in overcoming these. These included issues with enforcement of planning legislation or planning conditions, e.g. inadequate conditions or enforcement of targets for green space in new developments (mentioned across all three cases but particularly in Velika Gorica); external funding availability (Aarhus and Velika Gorica) (similar to the arguments in Massey et al. [26]), varying perspectives amongst different levels of government creating tensions (Aarhus and Paris Region), policy silos (all three, but particularly Paris Region and Velika Gorica) [1]. Also, the level of centralization came up as a hindering factor in two cases (Aarhus and Velika Gorica), but for different reasons. For Aarhus, the issue was that decentralization of policy making from national to local level provided a situation in which there was greater autonomy at local level in implementing NBS but uptake was more of a patchwork and not so well coordinated. On the other hand, in Velika Gorica the issue was too much political centralization at city level without appropriate decentralization of funding, providing little space for more localized bottom-up initiatives. This apparent contradiction is also found in the literature as discussed above (e.g. [27, 37]).

Other barriers were more specific to the context of the case studies. For instance, the size and complexity (geographically, politically and socially) of the Paris Region made it difficult to roll out NBS widely. In Velika Gorica, conflicting perspectives among key stakeholders (residents/citizens) were reported to make it hard to push forward NBS strategies in the municipality, such as street trees (see also [61]). In a similar but distinct vein, lobbying (e.g. from commercial contractors) was reported as hindering development of NBS initiatives in the Paris Region.

6 Conclusion

This paper has presented a comparative study exploring the uptake of NBS through a policy innovation lens using theoretical insights and empirical data—and bringing in political drivers of NBS uptake which have hitherto been relatively unexplored. Although NBS fits broadly within the sustainable development agenda, we have seen that it departs from this in its interdisciplinary focus on policy innovation and on tangible, multi-functional solutions to climate-related and environmental problems, rather than at the level of policy and strategy as in much of the work on sustainable development [see e.g. 7]. The focus on NBS also enabled an emphasis on innovative nature renewal and associated urban transformation, rather than preservation or conservation of existing environments.

The analysis yielded a number of key factors facilitating and hindering NBS innovation occurring across the three case studies, with political factors particularly prominent. Notably, despite the differences in scale, levels of centralization, governance structures and cultures in the European cases, there was a marked similarity in terms of the influence of the political level—particularly the Mayor/Mayor’s office—in NBS decision-making and release of funds for NBS implementation. All three cases also highlighted the importance of policy coherence of NBS initiatives with government policy strategy; and of the willingness by policy makers/local government officials to engage with and appeal strategically to political decision-makers regarding external policy drivers, e.g. climate adaptation and health. It is also interesting to note that they key hindering factors for NBS implementation across the three case studies were all in the governing category. Our cases also showed key differences that led to differing NBS outcomes, such as the size and complexity of urban area, the effectiveness of enforcement of planning regulations, and the extent to which the governing architecture facilitated citizens and local NGOs developing their own NBS initiatives.

The data for the case studies (see also [29]) supports the view that in terms of yielding increased benefits (e.g. multi-functionality, greater biodiversity) of green/blue spaces and sustainability of urban areas, public/citizen participation and consultation can add significant value, provided they are coherent with and can be integrated into the strategic and target-oriented approach of (local) government departments. The analysis indicates that where citizen participation and consultation were built into NBS projects, this brought both innovation and additionality in terms of the design, quality, biodiversity and or multi-functionality of green/blue spaces—whether this was greening of schoolyards, redesign of under-used green spaces or river restoration. This is consistent with a recent meta-study of stakeholder participation which identified power delegation (where participants were able to influence decision-making) as the most stable predictor of strong environmental outputs [62]. The increased multi-functionality in turn was reported to bring a greater range of co-benefits for local people in terms of climate adaptation, health and wellbeing and social benefits. We note that our data collection was limited by time and practical considerations exacerbated by delays due to Covid-19 restrictions, which impacted on the ability to secure the cooperation of a range of willing stakeholders/policy makers as interviewees within the project timeframe. However, every effort was made to recruit the key/expert stakeholders at city/local level, through knowledge of partner organisations of the local context, in order to maximise the validity of the findings.

The data also shows that a range of external drivers, both incremental (e.g. climate change) and sudden (e.g. extreme flooding events) are providing impetus to NBS implementation and innovation to varying extents and in varying directions. Therefore, our findings show that stakeholder participation and innovation/experimentation may not necessarily lead to desired outcomes where there are strong external (top-down) policy drivers.