Quantifying adaptation strategies: The gate index approach to extreme sea level scenarios in the western Baltic Sea

Sea level rise is a significant threat to coastal regions, requiring thorough scientific evaluations to implement effective adaptation planning. Recent incidents of damages to ships and infrastructure in harbours during storm surge events in the western Baltic Sea have highlighted the urgent need for robust adaptation strategies to sea level rise. This study utilizes regional climate simulations, employing five members of the EURO-CORDEX ensembles, to investigate sea level rise scenarios under the RCP4.5 and RCP8.5 scenarios. Using the concept of "gate index", the study quantitatively assesses the frequency and duration of potential closures of storm surge gates in harbours in response to extreme sea level rise events. The results show significant spatial differences in vulnerabilities across the region, with increased risks under the RCP8.5 scenario. The analysis also emphasises substantial uncertainties, stemming from various factors such as variations in global climate models, complexities in ocean-atmosphere interactions, potential changes in ice sheet dynamics, and uncertainties in future greenhouse gas emissions trajectories. In addition, regional factors such as local sedimentation processes, tectonic activities, and land-use changes can further amplify these uncertainties. The interplay of these multifaceted factors underscores the complex nature of projecting sea-level rise, highlighting the need for a cautious and adaptive approach in coastal planning and policy formulation. These findings provide critical insights for policymakers and practitioners engaged in coastal risk assessment and adaptation planning in the vulnerable Baltic Sea region.