Nature-based-solutions (NbS) are defined as strategies meeting societal challenges through the utilisation of natural features or processes which simultaneously provide biodiversity and human well-being benefits. NbS examples include vegetated foreshores or mangroves for coastal surge reduction, wetlands for water retention and/or quality improvement, and tree planting for urban heat mitigation. NbS are recognised as offering the potential for adaptation (and mitigation) to climate change while strengthening biosphere integrity and societal well-being effectively.

However, widespread implementation and realisation of NbS potential remain elusive. A significant factor contributing to this is the poor understanding of associated multi-functional trade-offs, leading the literature to urge for a systems approach to enhance this understanding. Multi-functionality concerns the three main impact dimensions of NbS: social, ecological and economic. As such, this research addresses the need for comprehensive assessment and understanding of multi-functional trade-offs associated with Nature-Based Solutions (NbS) for climate change adaptation, with a specific focus on deltaic regions, and aims to explore the applicability of a system dynamic modelling (SDM) approach in this context. To arrive at the main research objective, a mixed method approach grounded in the SDM methodology was adopted, which was applied on a deltaic case-study. Specifically, the study highlighted the complex social-ecological system of the Ebro Delta in Spain, where NbS are being considered for climate change adaptation.

Initially, an extensive literature review identified knowledge gaps related to the research objective, setting the stage for a heavily iterative exploration. This iterative approach was utilized to develop a guiding meta-model capable of capturing the multi-functional dynamics associated with NbS for climate change adaptation in deltaic regions, detail this meta-model for the case study, and apply this detailed meta-model to guide the quantitative SD modelling effort. As the object of inquiry, the case-study served to refine and illustrate the exploration, while yielding contextual insights benefiting local policy.

The results suggest that the SDM methodology is well-suited to assess the multi-functional trade-offs associated with NbS for climate change adaptation in deltaic regions. Three aspects of the methodology have been clarified: Firstly, the need for the application of the SDM methodology to this context has been strongly underscored. Secondly, the SDM methodology is able to comprehensively quantify multi-functional trade-offs, while maintaining versatility for different applications (i.e. contextual and/or case-specific) and facilitating communication and learning. Thirdly, the developed meta-model is suitable to facilitate a) problem exploration and b) guide a quantitative SD modelling effort. Furthermore, the findings illustrated that the trade-offs associated with NbS under climate change depend to a large extent on the existing social, economic, and ecological structure and associated values of the system where the interventions are intended to function. As such, it is argued that an extensive understanding of this structure and its behaviour (under changing conditions) is imperative if an accurate and comprehensive assessment of NbS impacts is desired. Moreover, in the context of regional-scale NbS, policy should be cautious of simplicity. Instead, it should embrace the inherent and systemic complexity of NbS, recognizing and leveraging the wide and multi-functional solution space that spans multiple disciplines and stakeholders