Indicators and metrics to evaluate the effectiveness of nature-based solutions for climate risk management and adaptation: A systematic review

In recent years Nature-based Solutions (NbS) have received increasing attention in coastal areas due to their ability to counteract Hydro-Meteorological Hazards (HMHs) and adverse climate change effects through habitat restoration and the re-establishment of Ecosystem Services (ES). Regardless of the wide adoption of NbS, there remain gaps and barriers in the effective uptake and implementation. There is an urgent need to define the specifics of NbS outcomes, measures of success and appropriate evaluation metrics. To bridge this knowledge gap this review focuses on: i) the terminology of NbS applied in coastal archetypes; ii) the ecosystem services delivered; iii) the HMHs targeted by NbS; and iv) the effectiveness indicators and metrics applied to monitor the impact of NbS implementation, including the tools and technologies employed. The NbS terminology applied addresses a range of different approaches included under the umbrella term NbS, e.g., building with nature, nature-based adaptation, or mitigation, and ecosystem-based management. Yet most of the included approaches mention the provisioning of ES as part of the main objective, relying on habitats and ecosystems to provide these services. In the scope of this paper 87.1% of the included ES can be attributed to regulating services such as reduction of erosion rates, coastal protection, carbon sequestration and water quality improvement.  The ES also clearly align with the climate change hazards addressed by NbS which include, e.g., flood and erosion risk, sea level rise, eutrophication, and extreme weather. These hazards are addressed through the implementation of NbS which aim to, e.g., reduce wave energy, anticipate storm surges, achieve good ecological/environmental status of water, re-establish carbon sinks and mitigate storm risk. To evaluate the effectiveness of NbS in counteracting these hazards and mitigate the impact of climate change this work identified 28 indicators. The indicators reflect mainly habitat characteristics and ES, e.g., geomorphology, vegetation cover and composition, risk reduction, carbon sequestration, and storm surge attenuation, complemented by socio-economic indicators such as willingness to pay and stakeholder perception. They are supported by multitude of metrics, evaluated through a variety of monitoring methods encompassing historical records (to create a baseline using, e.g.,  salinity records, seagrasses, vegetation status, or habitat size), questionnaires (to evaluate stakeholder values), in-situ measurements and remote sensing (to assess change in, e.g., bed level dynamics, vegetation presence, carbon stock, bird species, and marsh surface following NbS interventions) and modelling (the impact of NbS through, e.g., UVVR, morphology, vegetated leading edge, and habitat distribution). The results of this review will support the upcoming monitoring activity of saltmarsh restoration in the Venice lagoon (Italy) as part of the REST-COAST project and will pave the way for the creation of a methodological framework to systematically evaluate NbS effectiveness under current and future climate change scenarios. The project leading to these results has received funding from the European Union’s Horizon2020 research and innovation programme under grant agreement No 101037097.