How waves slow down the amplification of extreme water levels due to sea-level rise

As sea level rises, the recurrence frequency of extreme water level events amplifies. A critical measure for coastal adaptation is the rate at which this amplification will unfold. Here, we quantify this rate in terms of the doubling time of the present-day 1-in-100 year event. Applying a conventional peak-over-threshold method to a quasi-global set of tide gauge records, we confirm results from previous studies that approximately half of the locations may expect a doubling of extreme water levels within ten years. However, these tide gauge records commonly do not capture wave set-up and run-up. Applying an advanced joint probability method, accounting for the potential co-dependence of storm surges, deep-sea wave height, and mean wave period, we quantify doubling times including the contribution of waves. For this contribution, we apply a range of parameterizations to represent different local shoreline properties. We find that waves lengthen doubling times, depending on the local shoreline properties and whether coastal planners are interested in wave set-up or run-up. Overall, we conclude that the estimate rate of amplification may be greatly overestimated if the contribution of waves to extreme water levels is not accounted for.