Scenarios for North Sea and Baltic Sea

Figure 1: Spatial distribution of the number of projections for which the climate change signals of the 30-year mean of the annual 99th percentile storm surge (left) and
significant wave heights (right) for 2071-2100 relative to 1961-1990 have a positive sign. These spatial distributions are based on four water level and ten wave projections. Map: Hereon/ Nikolaus Groll

Long-term changes of storms, storm surges and sea state and a rise of mean sea level in the North Sea as they may occur with anthropogenic climate change can endanger the safety of the low lying coastal areas and can affect the off- and onshore activities.
To assess possible impact of changed atmospheric conditions on water level and sea state a variety of future climate projections has been analyzed with respect to corresponding reference simulations. The atmospheric forcing for these four storm surge and ten sea state projections and respective reference simulations originates from regionalized future global climate projections in which different models, initial states and development scenarios (included in the Forth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC)) were incorporated.

Figure 2: Thirty-year running means of the annual 99th percentile significant wave height exemplarily for a location seaward of the island of Sylt (German Bight) relative to the corresponding mean for 1961-1990 for four wave projections. Graph: Hereon/ Nikolaus Groll

Towards the end of the 21st century the pattern and the magnitude of the meteorologically (wind and sea surface pressure) induced changes in storm surge and wave heights vary between the four surge as well as between the ten wave projections (comparing 2071-2100 to 1961-1990). The surge projections indicate an increase of severe surge heights in the southeastern North Sea and the German Bight locally exceeding 10% whereas there are only comparably small changes in the other parts of the North Sea (Figure 1). Nine to ten wave projections show an increase of severe wave heights (99th percentile) in the southeastern and eastern North Sea and more than half of the projections show a decrease in the western and northwestern North Sea. The changes range between about ±15%.

Within the 21st century variations on multi-decadal time scales can be seen in time series of surge and wave heights for chosen areas. These multi-decadal variations are in the same order of magnitude as the changes towards the end of the century. Furthermore, frequency and intensity of extreme events show also strong multi-decadal variability and the highest events must not occur at the end of the century (Figure 2). This emphasizes the importance of the internal climate variability which is superimposed on anthropogenic climate changes.