A Scenario of Storm Surge Statistics for the German Bight at the Expected Time of Doubled Atmospheric Carbon Dioxide Concentration

Abstract

Past variations of water levels at Cuxhaven, Germany (German bight), are examined, and a scenario for future changes due to expected global warming is derived. The observational record of Cuxhaven water levels features a linear upward trend in the annual mean water level of about 30 cm 100 yr⁻¹ overlaid by irregular variations due to synoptic disturbances. These irregular storm-related variations are shown to have remained mostly stationary since the beginning of observations until today. A scenario for future conditions is derived by means of a two-step downscaling approach. First, a “time slice experiment” is used to obtain a regionally disaggregated scenario for the time mean circulation for the time of expected doubling of atmospheric CO₂ concentrations. Then, an empirical downscaling model is derived, which relates intramonthly percentiles of storm-related water-level variations at Cuxhaven to variations in the monthly mean air pressure field over Europe and the northern North Atlantic. Past variations of storm-related intramonthly percentiles are well reproduced by the downscaling model so that the statistical model may be credited with skill. The combined time slice–statistical model “predicts,” for the expect time of doubled atmospheric CO₂ concentrations in the decade around 2035, an insignificant rise of the 50%, 80%, and 90% percentiles of storm-related water-level variations in Cuxhaven of less than 10 cm, which is well within the range of natural interdecadal variability. These numbers have to be added to the rise in mean sea level due to thermal expansion and other slow processes.