General Circulation Model-generated surges are analyzed with the Generalized Extreme Value distribution to study the uncertainty in surge level estimates with a return period of 104 years, derived from observational records of order hundred years. Ensemble simulations with a total length of 5336 years were generated with the KNMI General Circulation Model ECBilt, coupled with a simple surge model to transform the wind field over the North Sea to the surge level at Delfzijl (NL). The 46 estimated surge levels with a return period of 104 years, calculated from sets of 116 year each, vary between 4.5 and 17 meters, with a median of 8.5 meter. The 104-year estimate of the 118-year observational Delfzijl record (5.8 meter) fits well among these subsets, but this surge level is considerably lower than the median of the ensemble estimate. For an estimate of the 104-year return level of the surge within an uncertainty of 10%, a record length of about 103 years is required. CO2-doubling does not have a detectable influence on the mean wind speed over the North Sea in ECBilt. However, the model hints on the excitation of severe storms, with a frequency lower than once in 250 year. In ECBilt, these severe storms tend to dominate the 104-year return value of the wind speed over the North Sea.