A Structured Library of Local Climate and Energy Actions to Support Synergy-Oriented Sustainable Urban Planning

Abstract

Local governments increasingly adopt climate and energy strategies addressing both mitigation and adaptation objectives, yet these domains are often treated separately, limiting integrated planning. This study develops a structured Climate–Energy Action Library to support more coherent local decision-making. The library was constructed through a systematic review and harmonisation of actions from European Sustainable Energy and Climate Action Plans (SECAPs), international repositories, and related frameworks, resulting in a taxonomy of 171 actions grouped into thematic bundles and policy categories. The methodology enables the identification of potential synergies among measures, and revealing consistent cross-sector interaction patterns. The strongest interaction potential occurs when technical measures are combined with enabling governance actions, including policy instruments, planning frameworks, and capacity-building. Cross-sectoral synergies are evident in building retrofit programmes linked with heat-stress adaptation and in nature-based solutions contributing to mitigation, urban cooling, and ecosystem services. These findings indicate that governance and ecosystem-based measures often enhance the effectiveness of sector-specific interventions. The proposed library provides a practical analytical reference for municipalities, supporting the design and evaluation of integrated climate strategies and helping bridge the persistent separation between mitigation and adaptation in local climate governance.

1. Introduction

Strategic climate planning at the local level remains underdeveloped, particularly when municipalities are required to address long-term risks and uncertainties. Under the Covenant of Mayors (CoM), more than 11,000 European municipalities have prepared Sustainable Energy and Climate Action Plans (SECAPs) to translate national commitments into local measures [1]. However, these plans vary widely in structure, terminology, and depth of analysis, often resulting in fragmented action portfolios that are difficult to compare or prioritise. Many municipalities lack structured decision-support tools that enable systematic selection of actions capable of maximising long-term value, co-benefits, and implementation feasibility [2,3], leaving climate strategies vulnerable to short-termism and siloed thinking [4].

In response to these challenges, Bai et al. [5] call for an advanced systems approach to urban sustainability. They identify persistent barriers to systemic planning, including institutional inertia, path dependency, and inadequate decision-support systems. Importantly, several of these barriers can be alleviated through improved planning tools, transparent metrics, and clearer structuring of available urban sustainability data. Nevertheless, relatively few planning tools translate these principles into concrete and usable structures for combining local climate and energy actions.

Research examining interactions between mitigation and adaptation at the municipal level remains limited. Analysis of the Covenant of Mayors Europe reporting platform MyCovenant [1] indicates that synergies between actions remain largely implicit in local plans. This pattern is not confined to Europe. Data from the 2023 City Questionnaire [6], covering more than 1000 urban sustainability plans, show that only a small number explicitly assess interactions between mitigation and adaptation actions, with integrated approaches remaining the exception rather than the norm. These findings suggest that even cities with relatively high planning capacity often lack methodological support for integrated climate planning.

While several studies emphasise the importance of integrated climate planning, empirical analyses that explicitly explore synergies between mitigation and adaptation actions in local policy frameworks are still scarce. Existing literature increasingly highlights the need to assess potential complementarities and trade-offs when designing Sustainable Energy and Climate Action Plans (SECAPs), stressing the importance of “assessing potential contradictions and synergies to minimise conflicts and expand beneficial interactions” in joint climate strategies [7]. Other studies similarly call for stronger alignment between SECAPs and broader urban planning instruments to better exploit cross-sectoral synergies [8]. Recent studies have also explored the need to harmonise indicators across sustainability frameworks, including the alignment of Sustainable Development Goals and Sustainable Energy and Climate Action Plan monitoring systems, highlighting the growing demand for structured analytical tools to support integrated climate planning at the municipal level [9].

Despite these calls, previous studies show that integration between mitigation and adaptation in urban climate strategies remains limited and often fragmented across policy sectors [10] and that adaptation is often lagging, creating an “adaptation gap in cities” [11]. Most existing studies focus either on mitigation policies or on adaptation measures, rather than examining how different actions interact within a single municipal strategy. Empirical evidence suggests that this separation persists in practice. For example, a recent comparative analysis of 104 German cities found that only a small number of municipalities demonstrate strong performance in both mitigation and adaptation, while most cities show progress primarily in one of the two domains [12].

At the same time, research on climate governance indicates that effective local climate action often depends on coordination across multiple levels of government [13]. From a national perspective, the success of municipal climate strategies frequently relies on joint partnerships between national, regional, and local authorities, which provide regulatory frameworks, financial resources, and technical guidance [3]. Empirical case studies of urban climate planning highlight the importance of institutional coordination and multi-level governance for effective climate action [14]. Strengthening such multi-level governance arrangements can therefore play an important role in enabling more integrated mitigation and adaptation planning.

Taken together, these findings highlight the continuing fragmentation between mitigation and adaptation planning at the local level. Addressing this fragmentation represents an important first step toward identifying potential synergies between actions and enabling municipalities to design more coherent and effective climate policy portfolios.

While several initiatives provide repositories of urban climate measures, including the Covenant of Mayors databases, Climate-ADAPT, and city-network planning frameworks, these sources differ substantially in terminology, scope, and level of detail. As a result, actions are difficult to compare across cities, and systematic identification of potential synergies between measures remains limited.

This study addresses this gap by developing a structured Climate–Energy Action Library that harmonises mitigation and adaptation measures commonly used in local climate planning and organises them into a coherent classification framework.

The paper explores two research questions:

RQ1: How can mitigation and adaptation actions used in local climate and energy plans be systematically classified into a structured and transferable action library suitable for municipal planning contexts?

RQ2: What types of potential synergies between climate actions can be identified in the planning phase, through such a structured classification, and how can these interactions support more integrated planning discussions?

By addressing these questions, the study aims to demonstrate how a structured action library can support early-stage decision-making in local climate governance by enabling systematic comparison of measures, facilitating knowledge transfer between municipalities, and highlighting opportunities for coordinated implementation of climate actions.

While interactions between climate actions may include both synergies and trade-offs, the present study focuses on the identification of potential synergies at the level of action categorisation. Trade-offs between measures are highly context dependent and typically emerge during detailed project design, when local conditions, resource constraints, and stakeholder priorities can be evaluated. At the stage of developing a harmonised action taxonomy, it is therefore methodologically more appropriate to identify areas where interactions are likely to occur, rather than attempting to predict context-specific trade-offs. The framework presented in this study supports this early-stage screening process, while a more detailed assessment of trade-offs can be undertaken in subsequent decision-support or multi-criteria evaluation stages.

The Climate–Energy Action Library was developed as a structural component of the SYNERGISE+ decision-support framework created within the PROSPECT+ project [15], under the coordination of the authors of this study. Its purpose is to provide a standardised and operational catalogue of local climate and energy actions that can be directly integrated into multi-criteria decision-analysis (MCDA) processes used by municipalities when prioritising Sustainable Energy and Climate Action Plan (SECAP) measures, as well as easily enable cities to recognise synergistic potential among actions. At the same time, the library is presented in this study as a standalone reference framework that municipalities can use to review existing action portfolios, identify potential synergies among measures, and explore additional actions even in the absence of formal MCDA-based prioritisation processes.

The remainder of this paper is structured as follows. Section 2 reviews the method used to compile, harmonise, and classify the Action Library, including the development of the synergy-recognition approach and the procedure for linking actions to climate hazards. Section 3 presents the results: an overview of the completed Action Library, the identified synergies, the distribution of actions across climate-hazard categories, and the result of empirical testing of the library and synergy system on three real EU SECAPs. Section 4 discusses the implications of these findings for local climate governance, explores how recognised synergies can support more coherent planning, and outlines a preliminary direction for future work on estimating synergistic benefits, which lies beyond the scope of this study. Section 5 concludes with opportunities for replication in other urban or regional contexts.

2. Materials and Methods

This study applies a qualitative knowledge-structuring approach to develop a harmonised library of local climate and energy actions and to identify potential interactions between measures. The methodological process consisted of four main stages, as shown in Figure 1: (1) source identification, (2) action extraction and screening, (3) harmonisation and classification, and (4) qualitative identification of potential synergies among actions.

The objective was to compile an exhaustive set of climate and energy actions documented in existing action libraries, guidance frameworks, and municipal plans, including a detailed review of European Sustainable Energy and Climate Action Plans (SECAPs) prepared under the Covenant of Mayors framework and widely used international repositories. During consolidation, duplicate or overlapping actions were harmonised and merged; however, the resulting Climate-Energy Action Library is intended to comprehensively cover the range of measures currently applied or recommended in local climate planning. While individual actions may be labelled differently across sources, all identified measures can be accommodated within the proposed classification structure.

Actions were extracted from the reviewed sources and compiled into an initial long list. During this process, actions were screened to remove duplicates, harmonised to ensure consistent terminology, and consolidated where similar measures were described using different labels across plans or repositories. Where necessary, action descriptions were simplified to capture their core intent while preserving relevance for local planning. Both mitigation and adaptation measures were included, reflecting the integrated nature of contemporary local climate planning, and actions were retained where they could be clearly associated with municipal competences and implementation responsibilities.

The consolidated set of actions was then organised into a structured taxonomy to support comparability and integrated planning. Actions were grouped according to their primary thematic focus (such as buildings, energy supply, mobility, governance, or land use), key implementation characteristics, and relevance for local decision-making processes. The classification was designed to balance conceptual coherence with practical usability, relying on categories familiar to municipal planners rather than introducing a highly granular or theory-driven typology. Although the Action Library may be expanded as new policy instruments or technologies emerge, the proposed taxonomy is designed to be sufficiently comprehensive to accommodate additional measures within existing categories.

Local climate action plans typically list projects in thematic sectors such as buildings, transport, or renewables, but these categories vary across countries and programs. Existing compilations, like the EU Covenant of Mayors measure database [16], C40’s Climate Action Planning framework [17], and national adaptation catalogues, offer valuable examples but differ in terminology and level of detail. Few provide a standardised classification that integrates mitigation and adaptation measures within a single structure. To address this, the present study developed a harmonised taxonomy that merges both domains under shared sustainability themes. Each action is described through key attributes: objective, expected impact, investment intensity, implementation scale, and climate hazard targeted.

The identification of potential synergies was based on a structured review of academic and policy literature addressing interactions between mitigation and adaptation measures in urban climate strategies. Thus, instead of determining new synergies, the study adopts and simplifies an existing synergy-recognition framework [17] to characterise relationships among actions in the newly formed library. Synergies, defined as the interaction where combined adaptation and mitigation effects exceed the sum of their separate impacts, are increasingly recognised alongside co-benefits [18,19,20,21,22]. Documented complementarities reported in the literature were translated into qualitative links between action categories within the Action Library. This approach is intentionally aligned with early-stage planning contexts, where the objective is to identify promising action combinations prior to detailed modelling or valuation.

To support adaptation-relevant planning, actions were additionally linked to climate hazards commonly addressed in local climate strategies, such as heat stress, flooding, drought, or extreme weather events. This linkage was based on the primary risk addressed by each action as described in the source documents and relevant guidance materials. The hazard linkage does not imply quantified risk reduction but serves to support planning discussions on coverage, coherence, and potential gaps in local adaptation strategies.

To evaluate the applicability of the framework in realistic planning contexts, the Action Library was operationalised within the SYNERGISE+ MCDA tool [15] and tested through simulation exercises using three existing SECAPs: Maribor (Slovenia), Zagreb (Croatia), and Litoměřice (Czechia). This included expert interviews and testing with SECAP authors from Maribor and Litoměřice. Actions from these plans were mapped to the standardised action categories to understand whether all existing actions can be simply mapped into the library bundles. The testing process was performed in 2022 and involved energy and climate experts participating in the PROSPECT+ capacity-building programme, including organisations from nine European countries (Belgium, France, Greece, Austria, Slovenia, Ireland, Spain, Czechia, and Germany). When there were inconsistencies in how one single measure is defined, this was resolved through structured expert consensus among participating practitioners.

The procedure for compiling, harmonising, and classifying actions was designed to be transparent and replicable. While the Action Library reflects the specific planning context and sources used in this study, the underlying approach can be adapted by other municipalities or researchers using locally relevant climate plans and repositories.

3. Results

3.1. Library Structure and Classification

The resulting Climate–Energy Action Library comprises a consolidated set of 171 mitigation and adaptation actions identified across European Sustainable Energy and Climate Action Plans and international action repositories. The library was constructed primarily from the European Climate Adaptation Platform Climate ADAPT [23], a partnership between the European Commission and the European Environment Agency for adaptation actions, and CLIMACT Prio, a capacity-building and decision-support tool developed in cooperation between IHS Erasmus University Rotterdam, ICLEI World Secretariat and UN-HABITAT [24] for both adaptation and mitigation actions.

Following harmonisation and consolidation of 174 non-repetitive actions identified, they were organised into a structured taxonomy of 47 bundles reflecting their primary thematic focus and relevance for local climate planning.

The final Action Library comprises 174 unique measures: 58 adaptation-oriented and 116 mitigation-oriented, bundled in 47 groups of actions, where 21 of those have synergy potential (Figure 2).

It captures actions spanning six different types of sectoral actions: awareness raising, financial, infrastructure, natural, policy and technological actions (Figure 3).

The distribution of actions across policy and technological bundles reflects two fundamentally different implementation pathways in local climate governance. Technological bundles primarily consist of sector-specific infrastructure or engineering interventions, such as building retrofits, renewable energy deployment, or waste-processing technologies. These actions typically deliver direct mitigation or adaptation outcomes but require substantial capital investment and are often implemented at the project level. In contrast, policy bundles consist of regulatory instruments, planning frameworks, financial incentives, and capacity-building measures that shape the enabling environment for technological deployment. Rather than producing direct emissions reductions or adaptation outcomes, policy measures operate through institutional mechanisms that influence behavioural change, investment decisions, and long-term planning processes.

The prevalence of policy bundles in the Action Library therefore reflects the governance reality of local climate action, where municipalities often act primarily as coordinators, regulators, and facilitators rather than direct infrastructure investors. Policy instruments such as zoning regulations, procurement rules, financial incentives, and awareness programmes create the institutional conditions that allow technological measures to scale. As a result, policy bundles frequently function as enabling mechanisms that amplify the impact of sector-specific technological interventions.

The observed concentration of actions in bundles related to energy savings and waste management also reflects the policy priorities most commonly addressed in local climate strategies. Energy-efficiency measures in buildings represent one of the most mature areas of municipal climate policy, as they simultaneously address mitigation objectives, energy cost reductions, and social issues such as energy poverty. Waste-management actions similarly combine mitigation benefits through methane reduction and material recovery with broader circular-economy objectives. The large number of actions grouped within these bundles therefore indicates areas where municipalities have both established policy instruments and practical implementation experience.

Synergistic interactions between bundles emerge primarily where enabling governance measures intersect with sector-specific technological interventions. For example, energy-efficiency programmes often interact with urban planning regulations, financing mechanisms, and public-awareness initiatives that facilitate building retrofits at scale. Similarly, waste-management improvements frequently interact with public procurement policies, circular-economy strategies, and behavioural change campaigns. These interactions illustrate how synergies in local climate planning often arise not from technological coupling alone but from the combination of institutional frameworks and physical interventions.

Buildings and heating measures dominate as part of policy (14 bundles) and infrastructure (10 bundles) types of interventions. That is followed by natural ecosystems (seven) and financial (seven), technological (six), and awareness raising (three) interventions. Governance and capacity action, such as training programs or local energy agencies are least represented but often serve as enabling measures and an opportunity for synergies across all other type of interventions.

In some cases, actions were assigned to more than one intervention type. For example, low-carbon agri-environmental measures can constitute both policy and nature-based interventions. Similarly, climate-smart and conservation agriculture were grouped within the same bundle due to their shared objectives and overlapping practices but defined as separate actions based on sectoral focus rather than mitigation–adaptation categorisation. Specifically, climate-smart agriculture is classified within the agriculture sector, while analogous conservation practices implemented in urban contexts—such as green roofs, urban gardens, or park management—are categorised under urban actions. This sector-based approach departs from previous frameworks that divided actions primarily by objective (mitigation or adaptation), as sectoral integration more effectively reveals synergies, given that many measures simultaneously contribute to energy efficiency, emission reduction, and climate resilience.

The bundles were additionally divided by whether they fit into mitigation, adaptation, or both (Figure 4).

Linking actions to climate hazards shows that a significant share of measures addresses multiple risks simultaneously (

Table 1. Division of clustered actions by type, sector a, and climate impacts.

Type of Intervention	Affected Sectors	Climate Impacts
Awareness	Buildings (Buil)	Droughts (Dr)
Financial	DRR	Extreme Temperatures (ET)
Infrastructure	Energy (En)	Flooding (Fl)
Natural	Natural Ecosystem (Nat)	Ice & Snow (I&S)
Policy	Transport (Tr)	Sea Level Rise (SLR)
Technological	Water & Waste (WW)	Storms (St)
		Water Scarcity (WS)

). Heat stress and flooding emerge as the most frequently targeted hazards, followed by drought and extreme weather events. Many mitigation actions—particularly in buildings and urban form—are associated with co-benefits for adaptation, reinforcing the relevance of integrated planning approaches. This hazard linkage highlights areas where local strategies may achieve broader risk coverage through coordinated action design rather than isolated measures.

3.2. The Library Clusters

The action bundle library used in this study is presented in

Table 2. Overview of Climate–Energy Action Library bundles originating from [15] and the associated Synergy mechanisms. Note: The type of actions included in each bundle is indicated after the bundle name as mitigation (M), adaptation (A), or energy poverty (EPov).

Bundle Name	Synergy Mechanisms Note: Empty Columns Indicate There Is No Direct Inter-Bundle Synergy Potential for That Group of Actions.
Awareness raising actions (3 bundles):
Awareness campaigns for behavioural change (M, A)	An enabling mechanism: Enables simultaneous promotion of mitigation and adaptation behaviours (energy, water, waste, greening). Synergies occur when behavioural measures are aligned with subsidised actions and targeted across sectors.
Capacity building on climate change adaptation and mitigation (M, A)
Motivational campaigns about climate-smart urban agriculture (urban green actions, new green spaces incl. green roofs) (M, A)
Financial interventions (7 bundles):
Carbon tax of production, distribution, or consumption of non-renewable energy (M)
Economic incentives for behavioural change or private sector’s climate innovation, adaptation and improved environmental quality (M, A)
Feed-in-tariffs, subsidies, tax reductions and loans for renewable energy systems (RES) and purchasing energy saving (EE) equipment (M)
Insurance as risk management tool (incl. weather derivatives) (A)
Introducing congestion charges and pricing, road charges and tolls (M)	Synergies depend on alignment with urban energy and mobility planning and reinforcement through awareness measures targeting the same behaviours.
Subsidies for low-carbon transport (bike to work program, rail transport subsidies, carpooling) (M)	Synergies arise when embedded in integrated urban energy and infrastructure planning and when coordinated with large infrastructure adaptation and innovation uptake.
Subsidies to alleviate energy poverty (M, A, EPov)
Infrastructural interventions (10 bundles):
Bioenergy and hydrogen in transport (i.e., switching garbage collection vehicles to biofuels, or at farm level) (RES) (M)
Building refurbishment incl. roof, solid walls and window insulation, and additional climate proofing against excessive heat (i.e., green rooftops, shade systems, roof albedo enhancement or cool roofs) (M, A)	High synergy potential through co-implementation of energy efficiency, renewable energy, and climate adaptation during renovation cycles, reducing costs and improving resilience.
Carbon capture, transport, utilisation and storage (CCS) in cities (M)
Combined energy (other than RES)—co-firing of biomass and wastes, combined heat and power (CHP) systems, or district heating/cooling networks (M)
Electric transport/mobility (EV charging stations at home and public, bicycle lanes and parking) (M)
Improving adaptability of large infrastructure (i.e., airports, energy distribution and transmission infrastructure, groundwater management) (A)	Synergies arise when combined with carbon pricing, efficient energy systems (e.g., CHP, district heating/cooling), and integrated waste and wastewater infrastructure, supporting resource efficiency and system resilience.
Municipal street and traffic lighting retrofit programme (M)
Retrofitting and replacing inefficient vehicle fleet (M)
Waste and wastewater structural projects, incl. decentralised systems for water-sewerage–energy infrastructure and deployment of recycling and composting infrastructure (M)	Enables synergies across water, energy, and waste systems through resource recovery, improved distribution efficiency, and demand management integration.
Other large RES incl. hydropower, geothermal, wind power, ground mount solar, waste to energy biomass plants (electricity generation only) (M)	Synergies with urban agriculture and waste systems through use of organic residues and by-products, supporting local energy generation, decentralisation, and resource efficiency.
(Note: All nature conservation and protection actions offer synergies among one another. For example, coastal zone management can include sustainable soil and land management practices, or investing in climate-smart urban agriculture such as xeriscaping could be beneficial at dry coastal zones.)
Coastal zone management (beach nourishment, flood barriers, cliff stabilisation, restoration of wetlands, retreat form high-risk areas) (A)	Synergies occur when integrated with land-use planning, density management, accessibility, and low-carbon mobility strategies.
Conservation/low-carbon agriculture (innovative crop management, manure spreading and storage, precision agriculture, nitrogen balance) (A, M)	Strong synergies with water management through reuse, efficiency improvements, and demand management, supporting resilience and resource optimisation.
Investing in climate-smart urban agriculture (urban green actions, new green spaces incl. green roofs, xeriscaping) (A, M)	Connects waste, energy, and water systems through biomass use, soil enrichment, urban cooling, water reuse, and integration into urban planning frameworks.
River and floodplain management (establishment/restoration of riparian buffers, flood barriers, dunes) (A)	Synergies arise through integration with other nature-based and land-use measures. Riparian buffers support soil management and water retention, while complementing coastal protection and enhancing ecosystem resilience. Links with climate-smart urban agriculture improve water efficiency and adaptation outcomes.
Sustainable soil and land management (afforestation, reforestation, agroforestry) (A, M)	Synergies arise when combined with land-use planning, density optimisation, accessibility, and low-carbon mobility integration.
Water reuse, restrictions and water rationing (A)	Enhances water efficiency and resilience through integration with reuse systems, urban planning, and behavioural measures.
Water-sensitive forest management (A)	Synergies arise through integration with land-use and water management measures. It enhances water retention, reduces runoff, and supports soil conservation, while reinforcing ecosystem resilience. When combined with river basin management and nature-based solutions, it contributes to flood regulation and drought mitigation.
Policy interventions (14 bundles):
Adaptive management/functional connectivity of natural habitats & ecological networks (A)
Crises and disaster management systems and plans (incl. fire and flood) (A)
Developing greenhouse gas inventory (M)
Establishment of early warning systems/plans (A)
Heat health action plans and response to heatwaves (incl. heat mapping, thermal imaging, using water to cope with heatwaves) (A)
Integrated urban energy systems—incl. traffic and road management plan (adaptation solutions such as floating or elevated roads) (M, A)	Enables cross-sectoral synergies across energy, transport, and buildings through demand management, decentralisation, and integrated system planning.
Integration of adaptation in nature-based plans (coastal zone management, land use, drought and water conservation plans) (A)	Synergies arise through integration with land-use planning, density, accessibility, and low-carbon mobility strategies.
Management plans for coastal areas and aquaculture (i.e., risk-based zoning/siting for marine aquaculture, diversification of fisheries and aquaculture products and systems) (A)
Plan to alleviate energy poverty (M, EPov)
Prescription of quota system for renewable energy production and renewable obligation for households/SMEs (M)
Stricter standards and regulations for building new and refurbishing existing buildings (M, A)	High synergy potential through integrated implementation of energy efficiency, renewable energy, and climate adaptation in building design and renovation.
Sustainable urban waste management and recycling and composting initiatives (M)	Synergies depend on alignment with infrastructure planning and integration with decentralised waste and resource recovery systems.
Urban green infrastructure plans and nature-based solutions (i.e., tree planting and creation of green spaces) (M, A)	Enables cross-sectoral synergies across water, energy, and urban systems through integrated planning and resource management.
Water sensitive urban and building design (A)	Enhances synergies when integrated with broader urban planning, resource efficiency, and water management strategies.
Technological interventions (6 bundles):
Enhanced Waste & Wastewater Resource Recovery (M, A)
Meters/detectors for intelligent lighting systems (i.e., automatization of building lighting, or street lighting part-night operation, trimming and remote monitoring) (M)
Reduction/savings of electricity and fuel—other than RES and structural refurbishment (i.e., EE, phasing out inefficient technologies, setting up a building management system) (M, A)
RES in buildings—PV, solar thermal, energy for cooking/heating incl. condensing boilers, biomass briquettes, heat pumps, geothermal (M)	Synergies achieved when combined with energy-efficiency improvements, building management systems, and climate adaptation measures.
Technical innovation in transport and modal shift (electrified transport) (M)	Synergies depend on alignment with awareness measures, behavioural change, and financial incentives supporting adoption.
Use of remote sensing in climate change adaptation (A)

, and the full list of individual actions and their corresponding bundle is available as Supplementary File S1 and described in detail in Appendix B of the PROSPECT+ decision-making guide [15], which forms the methodological basis of this work.

Table 2 is adapted from the original SYNERGISE+ decision-making framework, with restructuring and consolidation applied to align the content with the analytical scope of this study. Specifically, the table reorganises the action bundles by type of intervention and systematically links them to relevant sectors and climate impacts. In doing so, it provides a synthesised overview of the Climate–Energy Action Library and supports interpretation of how different types of measures contribute to mitigation, adaptation, and energy poverty objectives across multiple domains.

Most bundles comprise two actions, with three bundles containing eight actions and only two exceeding ten. The largest bundles are Bundle 31, reduction or savings of electricity and fuel (excluding renewable energy and structural refurbishment), which includes 20 actions, and Bundle 39, sustainable urban waste management, which includes 11 actions. Among the 14 adaptation-only bundles, seven are policy-oriented and four involve interventions in the natural ecosystem, while one each represent technology and infrastructure-based interventions. Of the 19 mitigation-only bundles, eight are infrastructure-oriented, with the remaining types evenly distributed across categories.

The action bundle framework employed in this study originates from the PROSPECT project, where it was developed under the coordination of the authors as a practical decision-support tool. Building on this foundation, aiming to support the identification of synergistic interactions during the screening of existing Sustainable Energy and Climate Action Plans (SECAPs), a set of predefined synergy mechanisms was developed for each action bundle. These mechanisms served as an analytical guide to systematically assess whether and how individual measures contribute to cross-sectoral, mitigation–adaptation, and socio-economic co-benefits, and the present study advances the framework by introducing a formalised synergy-oriented analytical layer (Table 2) and integrating it within the SYNERGISE+ index, thereby enabling systematic identification of cross-sectoral interactions.

Each bundle represents a set of actions with inherent synergies due to shared objectives. Beyond these internal interactions, Table 2 highlights potential synergies between different bundles, revealing recurring mechanisms that can amplify mitigation, adaptation, and resource-efficiency outcomes.

Patterns in the table show that infrastructural and technological interventions often interact across multiple sectors, creating leverage for integrated implementation. Behavioural and financial measures are mutually reinforcing, suggesting that awareness campaigns and capacity building achieve greater impact when aligned with incentives or subsidies. Nature-based interventions form a self-reinforcing group, enhancing resilience and resource management, while policy and governance bundles provide the enabling conditions necessary for other synergies to materialise. Recognising these patterns can guide the prioritisation and coordination of local sustainability actions.

3.3. Integration of Synergy Recognition

Each of the 47 action bundles contains a structured description and, where applicable, highlights: (1) intra-bundle synergies, where multiple actions grouped in the same bundle reinforce each other, and (2) cross-bundle synergies, which suggest valuable pairings between actions from different bundles (

Table 3. Summary of synergy types identified in SYNERGISE+, including definitions, practical examples, and implications for integrated planning.

Synergy Type	Definition	Example	Planning Implication
Intra-Bundle Synergy	Synergies occurring between actions grouped within the same action bundle in SYNERGISE+, often due to functional or sequential complementarity.	Building insulation and green roofs within the ‘Energy Efficiency in Buildings’ bundle.	Encourages holistic consideration of actions within a single intervention area to maximise returns.
Cross-Bundle Synergy	Synergies occurring between actions in different bundles, where coordinated implementation enhances performance, reduces costs, or improves feasibility.	Passive building design from one bundle and smart metering systems from another bundle.	Promotes cross-sector coordination and scheduling to capture system-wide efficiencies and shared benefits.

).

This information is not intended to be reviewed exhaustively but rather serves as a dynamic lookup feature to support decision-makers when considering a specific action. For each bundle, links to detailed descriptions and synergy suggestions are provided via the online SYNERGISE+ interface.

At the action level, approximately 70% of all actions exhibit at least one synergy opportunity. These include synergies between counterpart actions within the same bundle as well as inter-bundle synergies, where a mitigation action is strengthened by an adaptation action applied to the same asset (e.g., a building, energy system, or public space). Additionally, intra-bundle synergies—typically between infrastructural and behavioural components—generate substantial cross-dimensional benefits. For example, combining energy awareness campaigns with technical retrofits enhances both the effectiveness and persistence of energy savings.

All but ten bundles have more than one action in a bundle, which means there are 37 bundles with intra-synergy potential. Cross-bundle synergy potential (Figure 5) was identified in 21 out of 47 bundles, with the highest occurrence in policy interventions (seven out of 14), followed by nature-based interventions (five out of seven), and infrastructural interventions (four out of 10). This pattern reflects the enabling nature of policy measures and the multifunctional benefits of nature-based solutions, both of which tend to influence multiple sectors and dimensions simultaneously.

The results therefore suggest that synergies in local climate strategies emerge primarily at the interface between governance instruments and sector-specific technological interventions.

To provide an initial validation of the Action Library, the framework was applied to three European cities with existing Sustainable Energy and Climate Action Plans (SECAPs), as shown in

Table 4. Application of the Climate–Energy Action Library to selected SECAPs.

City	Country	Number of SECAP Actions	Share of Actions with Identified Synergy Potential (%)
Maribor	Slovenia	57	81
Zagreb	Croatia	57	84
Litoměřice	Czechia	48	81

: Maribor (Slovenia) [25], Zagreb (Croatia) [26], and Litoměřice (Czechia) [27]. The analysed SECAPs included 57 actions in Maribor, 57 in Zagreb, and 48 in Litoměřice.

Across all three cases, a high proportion of actions demonstrated potential for synergy when assessed using the library structure. Specifically, 81% of actions in Maribor, 84% in Zagreb, and 81% in Litoměřice were identified as having at least one potential interaction with other actions. These findings suggest that local climate measures are predominantly embedded within interconnected action portfolios rather than functioning as isolated interventions.

The qualitative synergy analysis of all library actions identifies interactions across multiple categories of actions. Synergies are most observed between technical measures and governance or enabling actions, as well as between mitigation and adaptation measures. Frequently occurring combinations include actions related to urban greening, building retrofits, mobility planning, policy instruments, and planning frameworks.

3.4. Operational Example: Estimating Potential Synergies Between Actions

To demonstrate how the Action Library could support prioritisation exercises, this study introduces a simplified framework for estimating potential synergies between climate actions. The framework is presented as an illustrative decision-support concept rather than a validated assessment method. Its purpose is to show how joint implementation of actions could be explored in terms of potential cost savings and performance improvements.

The proposed formulation synthesises concepts from cost–benefit analysis (CBA), multi-criteria decision analysis (MCDA), and systems thinking. The cost savings component follows marginal cost comparison approaches commonly used in life-cycle costing and public infrastructure planning [28]. The performance gain component reflects insights from systems theory and MCDA literature, where the combined implementation of actions may generate effects that exceed the sum of their individual contributions [18,29].

To account for the value of non-monetary co-benefits, the framework introduces a shadow pricing coefficient (θ), consistent with practices in climate economics and integrated assessment modelling [30,31,32]. The resulting formulation offers a flexible structure that can be adapted for expert evaluation, scenario analysis, or participatory planning processes.

This framework does not claim to provide exact savings values but serves as a strategic orientation tool, enabling local governments and planners to prioritise synergistic investments. Estimating the financial, environmental, or social value of synergies requires a more detailed, context-specific cost–benefit analysis that considers

• Temporal alignment of actions (e.g., sequencing infrastructure upgrades to maximise compatibility).
• Cost-efficiency gains, such as shared labour, materials, or administrative procedures.
• Systemic interactions, where one action enhances the effectiveness of another (e.g., stormwater capture improving insulation performance).
• Avoided costs, such as reduced vulnerability or deferred infrastructure investment.

For each of the 21 identified synergistic bundles, savings could be estimated through scenario modelling, stakeholder interviews, or life-cycle costing techniques. Key indicators for estimating synergy savings may include

• A reduction in total implementation costs (compared to sequential or stand-alone deployment);
• A decrease in project delivery time;
• Enhanced performance outcomes (e.g., energy savings and resilience);
• An increase in institutional or public support due to multi-benefit framing.

A proposed calculation for estimating synergy savings includes three formulas:

• Cost savings, which represents the reduction in cost when actions are implemented together rather than separately. It can also include qualitative factors, such as increased public acceptance, faster implementation, etc.;

$$S_C=(C_A+C_B)-C_{AB}$$

(1)

where

• $S_C$ = Cost savings from joint implementation;
• $C_A$ = Cost of implementing action A independently;
• $C_B$ = Cost of implementing action B independently;
• $C_{AB}$ = Cost of implementing actions A and B jointly (bundled).

2.

Performance gain, which captures the added performance (e.g., efficiency, resilience, and emission reductions) resulting from the synergistic interaction.

$$S_p=P_{AB}-(P_A+P_B)$$

(2)

where

• $S_p$ = Performance gain from synergy;
• $P_A$ = Performance score or impact value of action A, across all five dimensions;
• $P_B$ = Performance score or impact value of action B, across all five dimensions;
• $P_{AB}$ = Performance score of joint implementation of A and B.

3.

Total synergy savings, which is a combination of cost savings combined with performance gain multiplied by a coefficient converting performance gains to monetary value (based on valuation of energy saved, emissions reduced, resilience increased, etc.).

$$S_t=S_c+(\theta \times S_P)$$

(3)

where

• $S_t$ = Total synergy savings (monetised);
• $S_c$ = Cost savings from joint implementation;
• $S_P$ = Performance gain from synergy;
• $\theta$ = A coefficient converting performance gains to monetary value (based on valuation of energy saved, emissions reduced, resilience increased, etc.).

An example application of the synergy calculation formulas, if insulating a building costs €30,000 and installing a green roof costs €40,000 separately, but jointly costs €60,000, is as follows:

$$S_c=(\mathrm{30,000}+\mathrm{40,000})-\mathrm{60,000}=\mathrm{€}\mathrm{10,000}$$

(4)

If combined implementation improves energy efficiency by 20% more than the sum of both individually

$$S_p=0.20$$

(5)

Assuming €1000 value per 1% of added efficiency

$$S_t=\mathrm{10,000}+\mathrm{10,000}\times 0.20=\mathrm{€}\mathrm{10,200}$$

(6)

4. Discussion

4.1. Interpretation of Results

The development of the Climate–Energy Action Library reveals the extent to which local climate planning measures are currently dispersed across multiple sectoral and thematic frameworks. The consolidation of over 170 measures into a smaller set of bundled actions highlights the high degree of conceptual overlap between mitigation and adaptation interventions in municipal planning practice. This observation reinforces findings in the literature that many urban sustainability actions simultaneously influence multiple climate objectives, even when they are reported separately in planning documents.

The structured classification also illustrates the importance of sectoral organisation for understanding interactions between actions. While many policy frameworks distinguish measures primarily according to mitigation or adaptation objectives, the analysis suggests that grouping actions by sector may better capture operational relationships between interventions. Actions implemented within the same urban system—such as buildings, mobility, or natural ecosystems—often share infrastructure, governance structures, or implementation pathways, increasing the likelihood of interaction effects.

The qualitative synergy analysis indicates that interactions frequently emerge when technical measures are combined with enabling governance actions. Policy instruments, planning frameworks, and capacity-building initiatives appear to function as system-level facilitators that support the implementation of sector-specific interventions. Similarly, nature-based solutions demonstrate strong cross-sectoral linkages, reflecting their capacity to influence climate mitigation, adaptation, and ecosystem services simultaneously.

These findings can be interpreted in direct relation to the research questions guiding this study. First, the results confirm that local climate actions can be systematically structured into a coherent taxonomy that enables comparison across sectors and policy domains. Second, the analysis demonstrates that interactions between actions are not random but follow identifiable patterns, particularly where technical interventions are supported by enabling governance measures. Third, the empirical application presented in Section 3 supports these observations, showing that a large majority of actions (over 80% across all tested cases) exhibit potential for interaction within local policy portfolios. Together, these findings indicate that local climate strategies are inherently interconnected and that a structured action library can serve as an effective entry point for identifying synergies and improving planning coherence.

Taken together, these findings suggest that the effectiveness of local climate strategies may depend not only on the selection of individual measures but also on the coherence of the overall action portfolio. Recognising potential relationships between actions can therefore support more integrated planning approaches and help municipalities move beyond fragmented sectoral responses to climate change.

4.2. Positioning the Findings Within Existing Literature

The findings contribute to a growing body of literature examining the integration of mitigation and adaptation in urban climate strategies. Previous research has highlighted that local climate policies often remain fragmented across sectors and governance domains, with limited coordination between mitigation and adaptation actions [10,33]. The structured Action Library developed in this study provides one possible mechanism for addressing this fragmentation by organising commonly implemented municipal measures into a harmonised framework that makes potential interactions between actions more visible. Unlike previous studies that primarily conceptualise integration, this study operationalises it through a structured and transferable action framework.

Empirical studies of European municipalities similarly show uneven progress between mitigation and adaptation efforts. For example, analyses of urban climate policies have found that many cities demonstrate strong activity in either mitigation or adaptation, but only a small number show balanced advancement in both domains [12]. The patterns observed in the Action Library reinforce this observation, suggesting that municipal climate strategies often evolve through sector-specific initiatives rather than integrated action portfolios.

In this context, the identification of potential synergies between actions contributes to ongoing discussions about integrated climate planning. Previous work has emphasised the importance of recognising complementarities and trade-offs between urban climate measures, particularly in relation to co-benefits across energy, mobility, and ecosystem systems [18]. By systematically mapping potential interactions between actions, the Action Library offers a practical reference that may support municipalities in identifying opportunities for more coordinated policy portfolios.

These findings therefore address the study’s research objective of identifying how mitigation and adaptation actions can be systematically organised and analysed to support more integrated climate planning at the municipal level.

4.3. Policy Implications for Local Climate Planning

The Climate–Energy Action Library provides a practical reference framework for municipalities with limited analytical and modelling capacity. By standardising action descriptions, linking measures to climate hazards, and highlighting potential interactions, the library helps bridge the gap between high-level climate commitments and implementable local actions. It also facilitates knowledge transfer between cities by enabling a comparison not only of individual measures but of broader action portfolios and their synergy potential.

The findings suggest that integrated action bundles may provide a useful entry point for municipalities revising or updating Sustainable Energy and Climate Action Plans, particularly where resources for detailed assessment are constrained. Recognising potential synergies can support coordinated implementation of measures across sectors [19]. This suggests that cities can transition from fragmented to integrated strategies by prioritising combinations of governance instruments and sectoral interventions rather than isolated measures. For example, combining building retrofit programmes with heat-stress adaptation interventions can simultaneously address energy efficiency, energy poverty, and climate resilience.

More broadly, the analysis indicates that governance and enabling measures—such as policy instruments, planning frameworks, and capacity-building initiatives—often act as catalysts that amplify the effectiveness of technical interventions [10,11]. Nature-based solutions also demonstrate strong cross-sectoral effects, contributing simultaneously to mitigation, adaptation, and ecosystem services [34]. These patterns highlight the importance of integrating institutional and ecological approaches alongside technical solutions in local sustainability planning.

Linking actions to climate hazards supports integrated planning by enabling municipalities to visualise how action portfolios address multiple risks simultaneously [35]. This implicit hazard–action mapping can improve coordination between mitigation, adaptation, and disaster risk management teams and supports alignment with national and European reporting frameworks. Overall, the findings suggest that structured action libraries combined with qualitative synergy recognition can strengthen coherence, transparency, and learning in local climate and energy planning processes.

4.4. Limitations and Future Research

This study presents an initial attempt to structure a harmonised library of local climate and energy actions and to explore potential synergies between them. Several limitations should be acknowledged. First, the Action Library was developed through qualitative synthesis of existing planning documents and action repositories. Although efforts were made to harmonise terminology and consolidate overlapping measures, the classification inevitably involves a degree of interpretative judgement. Different planning contexts or governance systems may therefore require adaptation of the taxonomy.

Second, the identification of synergies is conceptual and exploratory. The analysis focuses on recognising relationships between actions that were already present in the literature [18], rather than quantifying their magnitude. In practice, the extent of synergy effects will depend on local conditions, implementation design, and institutional capacity. Empirical validation through case studies or modelling approaches would therefore be necessary to estimate the real-world impacts of combined measures. Additionally, because the analysis is based on literature-reported interactions rather than empirical project evaluations, the identified synergies should be interpreted as potential interaction pathways rather than quantified performance outcomes and should be updated as the new research on synergies emerges.

Third, the proposed synergy calculation framework is presented as an illustrative decision-support concept rather than a fully operational assessment model. The formulation highlights how cost efficiencies and performance gains could be considered jointly, but it requires further refinement, calibration, and testing with empirical data and the inclusion of trade-offs. Such trade-offs are typically context-specific and depend on local environmental conditions, governance arrangements, financial constraints, and stakeholder preferences. Assessing them therefore requires more detailed project-level information than is available at the stage of Action Library development.

Future research could extend this work in several directions. Comparative analyses of SECAPs or other municipal climate strategies could apply the Action Library to evaluate the distribution of measures across sectors and climate hazards. Case studies could also examine how integrated action portfolios perform in practice and whether anticipated synergies materialise during implementation. Finally, the library could be further developed through the inclusion of trade-offs and through participatory processes involving municipal practitioners and policy experts, strengthening its usability as a practical tool for integrated climate and energy planning.

5. Conclusions

This study demonstrates that a structured taxonomy of local climate actions reveals consistent and policy-relevant patterns of interaction across sectors. The empirical application to three European cities shows that a large majority of actions (81–84%) exhibit potential synergies, indicating that local climate measures are predominantly embedded within interconnected policy portfolios. The analysis further identifies that the strongest interaction potential emerges when technical interventions are combined with enabling governance measures, while nature-based and building-related actions consistently provide cross-sectoral benefits. An illustrative framework for estimating potential synergy benefits was also presented to demonstrate how such relationships could inform future prioritisation exercises. This calculation approach should be understood as an exploratory analytical framework designed to support early-stage prioritisation discussions and to identify action combinations that merit deeper, context-specific assessment. Rather than providing a definitive valuation method, the framework demonstrates how structured action taxonomies and interaction mapping can inform the systematic analysis of integrated mitigation–adaptation strategies at the municipal level.

Taken together, these contributions provide an initial step toward more integrated and transparent climate–energy planning at the municipal level. By clarifying the relationships between actions, sectors, and climate hazards, the Action Library offers a practical reference that can support both the design and the ex-post analysis of local climate strategies, including those developed under initiatives such as the Covenant of Mayors.

Future research should prioritise empirical validation of synergy effects and integration of trade-offs into decision-support tools.