The escalating impacts of climate change, particularly sea-level rise, pose significant threats to coastal cities globally. A substantial portion of the world's megacities, and billions of people, reside in low-lying coastal areas, placing immense pressure on local governments to develop effective adaptation strategies. Traditional engineering solutions often prove costly and may not provide the diverse benefits necessary for enhancing urban livability. Nature-based solutions (NBS), which leverage the power of ecosystems to address societal challenges, offer a promising alternative. NBS, in the form of ecosystem-based adaptation, utilize biodiversity and ecosystem services to enhance resilience to climate change. Examples include protecting existing natural systems like marshes, restoring degraded systems, or creating hybrid systems. These solutions offer numerous advantages, providing protection from sea-level rise and storms, mitigating climate change through carbon sequestration, and offering recreational opportunities and essential wildlife habitats. However, implementing NBS in urban settings presents challenges. Space limitations, coastal squeeze (lack of space for habitats to migrate upslope), reduced sediment supply, complex ownership and governance structures, regulatory hurdles, and funding constraints all complicate implementation. Environmental justice concerns, including the potential for displacement and gentrification associated with green infrastructure projects, are also paramount. The San Francisco Bay Area, and specifically San Mateo County, exemplifies these challenges, with significant assets and populations vulnerable to sea-level rise. A history of traditional shoreline engineering exists, but a growing interest in green and hybrid strategies necessitates scientific understanding of NBS effectiveness and benefits in urban environments. This study aims to address these gaps by co-developing nature-based solutions for climate adaptation planning in San Mateo County, focusing on the multiple benefits they provide compared to traditional methods.

The introduction extensively reviews existing literature on sea-level rise impacts on coastal cities, the benefits and challenges of nature-based solutions, and the complexities of implementing such solutions in urban environments. The literature supports the use of nature-based solutions for coastal protection and highlights the multiple benefits they can provide, including storm protection, carbon sequestration, recreation, and habitat provision. However, the literature also acknowledges challenges such as limited space, sediment starvation, complex governance, funding issues, and environmental justice concerns. Specific studies cited address the economic benefits of coastal habitats, the effectiveness of different nature-based measures (e.g., marshes, living shorelines), and the social implications of green infrastructure projects, particularly gentrification. The review sets the stage for the study's focus on San Mateo County and its unique challenges.

The study employed a collaborative, stakeholder-driven approach involving County staff, the San Francisco Estuary Institute (SFEI), researchers from Stanford University's Natural Capital Project, and various government and NGO stakeholders. The methodology involved several key steps: 1. **Guiding Principles:** Stakeholder workshops led to the development of six guiding principles for adaptation efforts, prioritizing nature-based solutions while considering social and economic factors. 2. **Exposure Assessment:** Using San Mateo County's Sea Change Sea Level Rise Vulnerability Assessments, the study summarized the extent of sea-level rise exposure across five Operational Landscape Units (OLUs) under three scenarios (baseline, mid-level, and high-end). 3. **Suitability Analysis:** Biophysical suitability of various nature-based adaptation measures (beach restoration, ecotone levees, tidal marsh restoration, submerged aquatic vegetation restoration, and nearshore reefs) was assessed within each OLU, drawing upon the San Francisco Bay Shoreline Adaptation Atlas. Stakeholder feedback was incorporated to refine the suitability analysis based on social and practical considerations. 4. **Scenario Development:** Three spatially explicit adaptation scenarios were co-developed with stakeholders: (1) 'What we might have done' (fully engineered shoreline), (2) 'What we are doing' (existing and planned conservation and restoration projects), and (3) 'What we could do next' (building on Scenario 2 with additional feasible nature-based features). All scenarios were designed to provide equivalent flood protection. 5. **Ecosystem Service Modeling:** The InVEST modeling software was used to quantify three ecosystem services—stormwater nutrient pollution reduction, recreation (using Flickr photo data as a proxy), and carbon sequestration—as well as the provision of habitat for Ridgway's Rail. Models considered simplifying assumptions detailed in the supplementary methods. The study compared the provision of these services under each scenario. The integration of stakeholder engagement throughout the process ensured the relevance and applicability of the findings to the local context.

The study's key findings demonstrate the substantial benefits of nature-based solutions in coastal adaptation planning. Compared to a fully engineered baseline ('What we might have done'), scenarios incorporating nature-based solutions ('What we are doing' and 'What we could do next') showed significantly improved outcomes: * **Scenario 2 ('What we are doing'):** This scenario, representing existing and planned projects, yielded five times more marsh habitat (and therefore, habitat for Ridgway's Rail), five times the carbon sequestration, and six times the stormwater pollution reduction compared to the engineered baseline. An additional 50 ha of beach was also projected. * **Scenario 3 ('What we could do next'):** This scenario, incorporating additional feasible NBS, showed even greater benefits, with up to six times the marsh area, eight times the stormwater pollution reduction, and six times the carbon sequestration compared to the baseline. This scenario also projected an additional 170 ha of beach. While recreation levels did not differ significantly across all scenarios at the county level, analysis at the OLU scale revealed differences: northern OLUs tended to benefit from beach restoration, while southern OLUs experienced reductions in recreation with increased marsh restoration. The study's spatial analysis highlighted significant variations in habitat and ecosystem services across the different OLUs, driven by geomorphic and ecological factors. The study also notes that existing marsh restoration projects often include limited public access to protect wildlife during nesting seasons. This finding underscores the importance of balancing conservation and recreational use in planning efforts.

The study's findings highlight the significant advantages of integrating nature-based solutions into sea-level rise adaptation strategies. The substantial increases in ecosystem services—stormwater pollution reduction, carbon sequestration, and habitat provision—demonstrate the multiple benefits of these approaches beyond flood protection. The collaborative, co-creation process proved invaluable in identifying shared goals, overcoming implementation barriers, and generating relevant and useful results. The study emphasizes the importance of considering spatial variations in ecosystem service provision at a finer scale than the county-level. Addressing challenges such as limitations on public access to restored marshes to protect wildlife is crucial for successful implementation and ensuring community acceptance. False dichotomies between 'gray' and 'green' solutions should be avoided; hybrid approaches that combine both are frequently most effective. The study also emphasizes the importance of considering the scale of adaptation planning to match the scale of natural processes. However, the analysis also reveals challenges in implementing more ambitious scenarios due to concerns about land-use changes and the need to balance competing interests.

This study demonstrates the significant multiple benefits of incorporating nature-based solutions into sea-level rise adaptation planning in urban coastal environments. The co-created scenarios and ecosystem service modeling provided valuable insights for decision-makers in San Mateo County. Further research should focus on more comprehensive cost-benefit analyses that include a wider range of ecosystem services and social considerations, as well as explore ways to address the potential for gentrification associated with some nature-based projects. Ongoing monitoring of implemented projects and broader community engagement will be critical for successful implementation and maximizing the benefits of nature-based solutions. This integrated approach offers a replicable model for other coastal regions facing similar challenges.

The study acknowledges several limitations. First, it assumed unchanging upland land use, which is unlikely given future sea level rise. Second, it only considered a subset of the potential ecosystem services provided by urban nature. Third, relatively simple modeling approaches were used, making simplifying assumptions. Finally, cost-benefit analysis was not performed, precluding comparisons of net benefits across scenarios. While preliminary cost estimations are available from other research, integrating costs and benefits in a comprehensive framework is an important future research direction. More research is required to understand the social impacts of nature-based projects and prevent negative consequences.