The evidence for the 27 key publications is distributed over 36 case studies in coastal (n = 8), deltaic (n = 3), estuarine (n = 10), and lagoonal (n = 6) ecosystem archetypes (Figure 3) concentrated primarily in Europe (28.6%), North America (25.7%), and Asia (22.9%). The publications principally focused on single case studies (Figure 3A) at the micro- and meso-scale, with only one publication discussing a multi-case study comparison in deltaic environments (Figure 3B). The key articles utilize a mix of terminology to describe the NBS including terms such as but not limited to, living shorelines, managed realignment, building with nature, habitat restoration, and blue carbon ecosystems. For the purpose of this analysis and to aid the comparison between key articles, the different NBS terminology in the key articles has been translated to the overarching IUCN NBS typology (Cohen-Shacham et al., 2016; Cohen-Shacham et al., 2019) including: Ecosystem Restoration, Ecosystem-based Management, Natural Infrastructure, Issue-Specific Ecosystem-Improvement, and Area-based Conservation (defined in SM4). In Figure 3C it can be observed that the majority of the NBSs considered in this review can be classified as ‘Ecological restoration’ (63%). These NBSs focus on the recovery of ecological functioning through the restoration of habitats. ‘Natural infrastructure’ (22.2%) instead focuses on NBSs to reduce erosion and enhance coastal protection through various techniques such as living shorelines and utilizing a broad range of habitats including saltmarshes and riparian areas. ‘Ecosystem-based Management’ (11.1%) focuses on measures such as the reconstruction or transplantation of seagrasses to maintain ecosystem health, resilience and ESs. Finally, ‘Issue-specific Ecosystem-Improvement’ (3.7%) refers to the use of biodiversity and ESs as part of an overall adaptation strategy to help people adapt to, mitigate or reduce adverse effects of climate change, through for instance blue carbon ecosystems. Within the 27 key articles, the habitats most frequently utilized as part of the NBS strategy include tidal and saltmarshes, seagrasses and mangroves (Figure 3D). While this focus may in part be introduced due to the phrasing of the queries (SM1), it can also be attributed to the functions and ESs they offer, which directly address the pressures experienced by the NBS as discussed in the next section.

Understanding the potential of NBSs to withstand and tackle climate related pressures through regulating ESs is essential for the incorporation of NBSs in climate adaptation. Across the 27 key publications, 78 climatic and anthropogenic pressures were identified (Figure 4). Most key publications (81.5%, n = 22) underlined the influence of multiple pressures afflicting the ecosystems archetypes. Relative sea level rise and climate change emerged as the most prominent natural pressure, appearing in 40.7% (n = 11) and 25.9% (n = 7) of key publications respectively. Anthropogenic pressures were discussed in 20 out of 27 articles, focusing primarily on coastal infrastructure and engineering (19.5%, n = 8), pollution (14.6%, n = 6), and eutrophication (12.2%, n = 5); while few key publications focused on pressures such as boating disturbances (n = 3), land use changes (n = 3), coastal population (n = 2), water quality degradation (n = 1) or hydrological changes due to ecosystem alterations (n = 1). While half of the publications noted that both climatic and anthropogenic pressures (48.1%) affecting the ecosystems and NBS, the vast majority of key publications did not explicitly discuss their interplay but treat them as contextualizing factors instead. These findings corroborate statements in the reviewed key articles including Lin et al., (2023) who stated that climatic and anthropogenic pressures are known to interact simultaneously with ecosystem dynamics; however, they are typically reported independently. Aligning the role of NBSs with the multifunctionality of restored habitats in light of multiple hazards, and explicitly integrating the synergetic behaviour of the protective benefits these interventions provide would allow for a better understanding of their functioning and improved hazard prevention as underlined by another key article (Gittman et al., 2016).

Coastal and transitional ecosystems are ranked amongst the most important habitats regarding ESs provisioning (Baptist et al., 2019). To allow for the comparability of the ESs identified in the key articles (Table 1), they have been translated into the CICES V5.1. – Common International Classification of Ecosystem Services–terminology (SM5). The prevalence of regulating ecosystem services (88.1%) within the key articles reflects the core ES of the review (SM1), which is consistent with their role in NBS climate adaptation. What can be noted in Table 1 is that the NBSs considered in estuarine ecosystems consider a wide range of regulating ESs, including flood control (6 mentions), nursery habitats and biodiversity, (6 mentions) water quality regulation (6 mentions) and sequestration (5 mentions). Coastal NBSs instead seem to be more focused on filtration and sequestration (6 mentions) and flood control (5 mentions), while lagoonal NBS highlight nursery habitats and biodiversity (5 mentions) instead.

Within the key articles, these ESs present a dichotomy, where on the one hand the services present the objectives of numerous NBS; while on the other hand the ESs are the manner in which effectiveness is monitored. Thus, the evaluation of NBSs effectiveness in the key articles blend performance and impact indicators. Where performance indicators focus on process-based ESs delivery, while impact indicators examine a range of broader environmental changes to gauge the NBSs success. For example, the ESs 2.1.1.2 Filtration/sequestration/storage/accumulation by micro-organisms, algae, plants, and animals (59.3%) is evaluated in terms of performance through the carbon sequestration rate (e.g., key article Lanceman et al., 2022). While at the same time this NBS effectiveness evaluation can be complemented by monitoring impact indicators that can be used to forecast the potential of carbon abatement such as environmental variables and biogeochemical properties (e.g., key articles Duarte de Paula Costa et al., 2023; Hurst et al., 2022). Similarly, the ES 2.2.5.2 Regulation of the chemical condition of salt waters by living processes, discussed in 55.6% of the key articles, can be assessed through plant and sediment parameters (e.g., McGlathery et al., 2012) that quantify the removal of nutrients (e.g., Bonometto et al., 2019). Furthermore, the ESs 2.2.1.3 Hydrological cycle and water flow regulation (59.3%) and ES 2.2.1.1 Control of erosion rates (40.7%) are often evaluated in terms of performance, considering the site-specific natural processes underpinning ES delivery, such as wave height reduction (Castagno et al., 2022) and shoreline stabilization (Sterckx et al., 2019). Both of which critically depend on environmental characteristics such as vegetation to stimulate accretion, reduce erosion, and start positive feedback processes, e.g., lower the hydrodynamic load and thus increase sedimentation and reduce erosion potential. Which in turn leads to the use of vegetation characteristics as an impact indicators by some key articles, including Baptist et al. (2019), McGlathery et al. (2012), and Willemsen et al. (2022).

Within this review the indicators identified used to measure effectiveness can predominantly be categorized as performance or impact indicators (Figure 5). Performance indicators highlight the ESs provisioning of NBSs, these were frequently assessed through metrics related to nutrient (N, P) and carbon (C) provisioning benefits - such as carbon sequestration- or through measures quantifying the reduced exposure to erosion and flooding Impact indicators instead focus on physical, hydrological, climatic, and biological parameters, thereby characterizing how NBS influence underlying environmental processes of the system. Various key articles, attribute these parameters into two categories: functional attributes focusing on ecological attributes such as vegetation characteristics, and structural characteristics considering geomorphic and physical attributes of the ecosystem such as slope or topography. In addition, few of the key articles evaluate impact in terms of the suitability of the site for NBSs after its implementation. Finally, impact can also be considered as the balance between the cost and benefit of the NBS. Moreover, as illustrated in Figure 5, the monitoring approaches reported in the reviewed articles predominantly relied on in-situ measurements (n = 21). These were often complemented by modelling efforts (n = 6) using tools such as InVEST, SWAN, GIS, or BlueCAM. Additional methods included the use of remote sensing and aerial photography (n = 3), as well as desk-based analyses of historical datasets (n = 3).

A closer examination reveals that impact indicators were considered by 48.1% of the key articles (n = 13), are effective and practical measures of the condition of an ecosystem, as well as an effective tool for monitoring change (Lewis and Henkel, 2016), and include indicators such salt marsh extent (Baptist et al., 2019), vegetation cover (Willemsen et al., 2022), seagrass density and coverage (Lewis and Henkel, 2016), or avian species richness, abundance and community structure (Canales-Delgadillo et al., 2019). At the same time, a significant portion of the key articles (29.6%, n = 8) displays a reliance on performance evaluations including the monitoring of process-based ESs including carbon sequestration. Whereas 22.2% of the cases combine both approaches to evaluate NBS. Changes in ecosystem state and condition, i.e., environmental and ecological indicators expressing information about the habitat utilized, its use and functioning, are tightly linked to changes in ESs supply (Watson et al., 2022). To be able to effectively create salt marshes for their ESs provisioning, such as coastal protection and carbon sequestration, it is key to quantify morpho-dynamics and vegetation presence through active monitoring, as it will provide insight in the stability of the system and its sediment erosion/accretion (Willemsen et al., 2022).

Finally, as mentioned by McGlathery et al. (2012) to achieve a better understanding of NBSs it is necessary to establish mechanistic studies and continuous monitoring. In order to address this, multiple key articles (48.1%, n = 13) evaluate NBSs performance by measuring the ES provisioning of a specific NBS against a desired threshold or evaluate impact by assessing the environmental changes resulting from the NBS implementation (e.g., Baptist et al., 2019; Hurst et al., 2022; Spencer et al., 2008; Willemsen et al., 2022). A pair-wise comparison between the NBS and a control site was the most frequent (n = 9), e.g., comparing environmental impact indicators for habitat suitability for nekton at natural and hardened shorelines (Bilkovic and Mitchell, 2013) or comparing habitat characteristics - e.g., species richness, abundance and percentage coverage, and water quality - between transplanted and established eelgrass beds (Lewis and Henkel, 2016). Other key publications focused on estimating the performance of NBSs by evaluating changes before and after their implementation, e.g., salt marsh growth before or after the implementation of a mud motor (Baptist et al., 2019) or the impact of an NBS on salt marsh vegetation before and after managed realignment (Spencer et al., 2008).

A shared understanding of NBSs’ effectiveness evaluation is essential for meaningful comparisons and robust adoption of nature-based adaptation. While effectiveness appears as a prevalent term in NBSs discourse (Chausson et al., 2020), a closer examination often reveals a stronger emphasis on performance or impact. This trend might reflect the respective ease of measuring the context-specific functioning of NBSs and their tangible environmental effects, compared to the long-term goals devoid of spatial context associated to effectiveness. As such it could be noted that performance focuses on a quantifiable metric, often evaluating single processes under site-specific conditions (Seddon et al., 2020). Impact instead assesses a broader set of long-term effects on environmental, social, and economic conditions related not only to the objective of the restoration, i.e., ESs provisioning. This conceptualization of impacts, in a causal manner, would support a nuanced, nonlinear view of the interactions taking place in complex socio-ecological systems (Dumitru et al., 2020). This tendency, to focus on performance and impact, as a way to quantify the effectiveness of NBS is mirrored in the articles evaluated in this review.

Most of the key articles focused on a single micro- or meso-scale study site. This aligns with the findings of Saunders et al. (2020) indicating that the majority of coastal restoration projects cover less than 1 ha. The small scale of case studies relates to the experimental nature of the sites, as locations to test new adaptation approaches, simultaneously limiting the risk and resources involved (Buuren et al., 2018; DeAngelis et al., 2020; Saunders et al., 2020). While this small scale may hamper the broader uptake of the NBSs, this barrier may be overcome with evidence-supported adaptation recommendations (Hansen et al., 2023) quantifying degradation levels, target states, and pace and scale needed to achieve this target (Sánchez-Arcilla et al., 2022). The growing body of research, actively supported by the European Union through initiatives like Horizon2020 and projects such as MaCoBios (macobios.eu), WaterLANDS (waterlands.eu), and REST-COAST (rest-coast.eu), presents a significant opportunity to solidify the evidence underpinning the effectiveness of NBS for climate adaptation, particularly in coastal environments. Simultaneously, most articles considered singular restoration objectives such as re-establishing carbon stores and sinks (Greiner et al., 2013), or pollution reduction (Martín et al., 2020), or erosion protection (Gittman et al., 2016). The evaluation of these singular objectives relied on performance indicators (e.g., carbon accumulation, pollutant removal, sediment fluxes, wave dissipation, etc.), neglect complex interactions including the omission of synergies and trade-offs between ESs (Kabisch et al., 2016; Sowińska-Świerkosz and García, 2021), that are fundamental to NBSs and their holistic evaluation. This focus could be partially driven by the goal drives the design of restoration projects (Thom et al., 2005), the type of NBS and challenges to (Sowińska-Świerkosz and García, 2021), the complexity surrounding NBSs’ influence on multiple ESs provisioning (Xu et al., 2023), and subsequent risk reduction (Zhou et al., 2024).

Indicators help assess NBSs, their effects and systematic evaluation by simplifying the complexity of ecosystems (Dai et al., 2024; Kabisch et al., 2016; Sowińska-Świerkosz and García, 2021). This review align with previous studies, indicating that NBS performance and impact indicators mostly focus on environmental, hydrological, climatic, and ecological parameters, neglecting those related to e.g., economic feasibility, balancing trade-offs, or adaptive management (Châles et al., 2023; Cohen-Shacham et al., 2019; Giordano et al., 2020). This narrow focus aligns with the scope of this review and its aim to evaluate how NBSs address climate adaptation through ESs provision, hazard mitigation and risk reduction. Which in turn is reflected in the evaluation of NBSs impacts through the measurement and description of physical environmental parameters (Raymond et al., 2017), and performance in terms of regulating ESs focusing on ecological and hydrological processes (Krauze and Wagner, 2019).

The extensive focus on environmental parameters can be associated with conceptualization of the relation between NBSs and ESs (Dumitru et al., 2020), and indicates the evaluation of NBS performance. This relationship is often expressed through site-specific information that allows for the evaluation of a variety of environmental indicators which might fluctuate in space and time, representing key parameters and processes that function as proxies for the habitats affected by the NBS’s implementation. Nevertheless, the explicit quantification of ESs is essential to assess how NBSs influence the local ecosystem and community, especially in the face of climate change (Baustian et al., 2020), whereas evidencing the link between ecosystem conditions and ESs provisioning justifies the effort and cost of maintaining and restoring such coastal and transitional ecosystems (Watson et al., 2022). However, information on how to measure the ESs provisioning by coastal and transitional ecosystems is limited (O’Leary et al., 2023), ESs are not sufficiently incorporated into risk planning (Ruckelshaus et al., 2020), and the causal relation between NBSs and ESs is often not quantified (Kabisch et al., 2016). A further investigation of these relationships, learning from the identified indicators, metrics, and established approaches, is needed to support the creation of a comprehensive evidence-based for coastal and transitional NBSs performance.

Monitoring the success of NBSs, and understanding the performance and impact indicators that can be used to evaluate effectiveness, provides essential tools for adapting their design and implementation (Dumitru et al., 2020). Monitoring should be a transversal process across the NBS stages, e.g., design, planning, implementation, strategically devised as a platform to gather evidence on NBSs’ performance and impact once specific actions are deployed (Gonzalez-Ollauri et al., 2021). Through the monitoring of multiple indicators a more complete picture of habitat functioning will appear, while long term monitoring will demonstrate whether these NBSs reach their objectives (Bilkovic and Mitchell, 2013). While currently there is no standard protocol for monitoring coastal NBSs, monitoring a variety of indicators would allow for the investigation of various ecosystem processes and a more in-depth insight in the ESs provided by these habitats. Thereby, creating a comprehensive picture of the trajectory NBSs follow to reach their full potential, and how these interventions compare to their natural equivalents. At the same time, by streamlining the metrics between regions, albeit with some flexibility for site-specific indicators, would allow for the comparability of NBSs at different scale, and under different socio-economic and climatic conditions.

Currently, the effectiveness of NBSs, measured through performance and impact indicators, is often considered in a static manner (Giordano et al., 2020), with their long-term effectiveness assessed as if they are immutable and unaffected by change (Calliari et al., 2019). As underlined in the reviewed articles (e.g., Willemsen et al., 2022), future research on time scales surpassing years and single decades is needed to assess the influence of climate change and allow for NBSs to reach their full potential (Raymond et al., 2017; Sowińska-Świerkosz and García, 2022). In part this ought to be supported by continuous monitoring of performance and impact indicators, allowing for a complete picture of the effectiveness of NBSs when implemented in the coastal and transitional ecosystems. While at the same time allowing for the inspection of the observed lag time needed for NBSs to reach their full effectiveness and ESs provisioning potential. Continuous monitoring would allow for the exploration of ecosystem dynamics, and the evaluation of processes occurring within relatively short periods of time. Through these observations, climate change strategies can take into account changing habitat suitability and identify opportunities for future NBS upscaling (Pittman et al., 2022). Another challenge with creating a strong evidence-base is to develop a balance between in-situ experiments providing bio-geophysical information, numerical modelling to evaluate the effectiveness of these NBSs while taking into consideration climate change and multi-hazards (Gonzalez-Ollauri et al., 2023; Kumar et al., 2021a) and, as indicated in the reviewed articles (e.g., McGlathery et al., 2012), create a better understanding of the synergic behaviour between ecosystem components and ESs. In this setting, the strongest benefit of numerical models is their capability to test NBSs potential through evaluating effectiveness indicators against different developmental stages, in different configuration, and considering different climate and management scenarios (Kumar et al., 2021b). Therefore, modelling the implementation of NBSs addresses the limitation mentioned in this review by Spencer et al. (2008), and can be used as knowledge base for developing scalability frameworks to move beyond the small-scale. In the past few years, machine learning (ML) and artificial intelligence (AI), alongside numerical models, have transformed from a theoretical concept to a practical tool. These advancements in climate big data processing support the identification of much more comprehensive future climate change scenarios and intelligent early warning systems (Leal Filho et al., 2022). Framed in this way the exploitation of new data sources for the evaluation of effectiveness can facilitate the adoption of NBSs for climate adaptation (Ruckelshaus et al., 2020), act as enablers to support the up- and out-scaling of NBSs in coastal and transitional environments (Sánchez-Arcilla et al., 2022), and help decisionmakers better defining climate risks and adaptation options.