Changing North Sea storm surge climate: An increasing hazard?

Extreme sea levels provide a substantial hazard for low lying coastal areas in the Southern North Sea. They are caused by a combination of different factors such as high astronomical tides, a large-scale rise of the sea surface caused by high wind speeds and low atmospheric pressure (usually referred to as storm surges), or extreme wind-generated waves (sea states) caused by high wind speeds in atmospheric low pressure systems; that is extra-tropical storms. Long-term changes in any of these factors may substantially alter the hazard associated with extreme sea levels. Moreover, any long-term change in mean sea level such as observed over the past 100 years or as associated with future anthropogenic climate change will have an impact as it shifts the entire distribution of sea levels towards higher values; that is, it changes the baseline upon which storm induced sea levels have to be added. Moreover, in shallow waters non-linear interaction effects may occur. Here we review the present knowledge about long-term changes in any of these factors. We show that storm activity in the area underwent considerable variations on time scales of decades and longer, but that no clear long-term trend could be identified. Similar findings are obtained for long-term changes in the storm surge and wave climate. Mean sea level has increased in the Southern North Sea over the past centuries. Correspondingly an increase in extreme sea levels is found. For the future most projections point towards a moderate increase in storm activity in the area with corresponding changes in storm surge and wave climate. These changes will add to the expected future increase in mean sea level, leading to an increased hazard from extreme sea levels. The latter may have consequences for safety, especially in the low lying coastal areas in the Southern North Sea. Consequences for coastal protection and alternative strategies are discussed.