Priorities in coastal protection due to extreme sea levels under sea level rise

Extreme sea levels (ESLs) pose severe flood risks to coastal communities. Climate change is amplifying these risks as sea level rise (SLR) will increase the probability of given baseline ESL events. This will challenge coastal design standards relying on fixed return periods, as this assumption becomes obsolete under rapidly changing climate conditions. This study evaluates how future SLR will transform ESL return periods and compress the windows for adaptation along the German Bight at the North Sea coast.

Using data from the coastDat-2 hindcast – a high-resolution dataset of water levels and waves for the North Sea region –  we performed statistical analyses to derive return curves for regional ESLs, linking return heights to their corresponding return periods. These return curves were then combined with sea level projections from the Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) under various global warming scenarios. From this integrated analysis, we calculated two key metrics: amplification factors (AFs) and timings. The AFs quantify how much more probable a baseline event will become under future SLR conditions, whereas the timings describe the available timeframe before a specific amplification threshold is exceeded, providing valuable information about windows for adaptation planning.

Our results demonstrate that AFs across the study region increase substantially with higher warming levels, dramatically raising the probability of baseline ESL events becoming commonplace. Timings also shrink considerably under rising temperatures, highlighting the accelerating urgency for proactive adaptation measures. Importantly, we also identified significant regional variability in how coastal locations respond to SLR. Locations with lower baseline ESL return heights – associated with smaller tidal ranges and lower water level variability – experience larger amplification sooner. At these sites, a 100-year ESL event could become a 10-year event (AF = 10) within only a few decades under high warming levels, whereas this threshold will be exceeded much later elsewhere. This spatial heterogeneity emphasizes that effective adaptation strategies must be tailored to the local response to SLR rather than applying uniform, coast-wide approaches.

For practical adaptation planning, AF thresholds can be translated directly into required intervention frequencies. Establishing widely accepted thresholds is crucial for implementation: lower AF thresholds better manage residual flood risk but compress adaptation windows, potentially necessitating a paradigm shift from occasional adjustments to continuous adaptation.