Regional Sea Level Changes in the German Bight, North Sea, Germany Elke Meyer1, Frauke Albrecht1, Hans von Storch1, Thomas Wahl2, Ralf Weisse1 Introduction Regional sea level rise and the associated impacts represent one of the Grand Science Challenges identified by the World Climate Research Programme. To a substantial amount the regional impacts of sea level rise will be experienced by corresponding changes in the extremes. Such changes comprise contributions from relative mean sea level changes, from changes in wind wave and storm surge climate, tidal dynamics, or from local changes such as caused by coastal engineering. Mean Sea Level Changes Over the past about 110 years mean sea levels in the North Sea increased at an average rate of about 1.6 mm/year, broadly consistent with the global estimate (Figure 1). Spatial variability exists with rates being somewhat larger in the eastern part of the German Bight, broadly consistent with patterns of vertical land movements and large scale atmospheric variability. Decadal variability in the rates of sea level rise is partly associated with the nodal tidal cycle and large scale atmospheric changes. Present rates appear to be high but still comparable to those observed earlier during the last century. Figure 1: . Standard deviation from de-trended annual mean sea level time series from 30 tide gauge sites around the North Sea (colored dots) together with sea level indices for the Inner North Sea and for the English Channel derived from the individual tide gauge measurements. A low pass filtered curve is shown additionally. (Courtesy: Thomas Wahl, after Wahl et al. 2013) Extreme Sea Level Changes Extreme sea levels have increased over the past about 150 years at a rate comparable to that of global mean sea level rise. The increase can be attributed primarily to mean sea level rise while storm surges and waves show pronounced interdecadal variability but no substantial long-term trend (Figure 2). Here the upper curve broadly represents storm surge variability while the lower curve is a rough proxy for mean sea level changes. Changes in tidal dynamics over some decades of years also had some influence (Figure 3). Figure 3: Change in annual mean high (MHW) and low (MLW) water Figure 2: Annual mean high water levels (black) relative to German North Sea levels at the tide gauge Hamburg, St. Pauli. Timing of major waterworks gauge level (NN+5.00m) and linear trend (blue) in m at Cuxhaven, Germany and in the estuary is indicated by the black horizontal bars. Within the period the corresponding difference in m between 99 percentile and annual mean high shown, the mean tidal range in Hamburg increased from about 2 to 3.5 water levels (top, black) together with an 11-year running mean (top, blue). m. (Courtesy: Hamburg Port Authority) Future Developments While there is relatively high confidence in future changes of global mean sea level, regional mean sea level changes are less certain and less well explored. Future changes in extreme sea levels are highly uncertain, in particular because of the high uncertainty associated with future changes in wind wave and storm surge climate. The latter largely arises from corresponding uncertainties in wind climate change. It has contributions from different sources: different emission scenarios reﬂecting uncertainty about future socio-economic development, the range of different results produced by different models reﬂecting our imperfect knowledge, and the range of results obtained from one model using the same emission scenario reflecting natural climate variability (Figure 4). More information and details in: Albrecht, F.; Wahl, T.; Jensen, J. & Weisse, R 2011: Determining sea level change in the German Bight. Oc. Dyn. doi:10.1007/s10236-011-0462-z Wahl, T.; Haigh, I.; Woodworth, P. L.; Albrecht, F.; Dillingh, D.; Jensen, J.; Nicholls, R.; Weisse, R. & Wöppelmann, G. 2013: Observed mean sea level changes around the North Sea coastline from 1800 to present. Earth Sci. Rev., doi:10.1016/j.earscirev.2013.05.003 Weisse, R.; von Storch, H.; Niemeyer, H. & Knaack, H. 2012: Changing North Sea storm surge climate: An increasing hazard? Ocean and Coastal Management, doi:10.1016/j.ocecoaman.2011.09.005 Figure 4: Location of the tide-gauges and changes in storm surge heights in cm (middle) and frequencies (right) until 2100 derived from a Weisse, R.; Bellafiore, D.; Menendez, M.; Mendez, F.; Nicholls, R.; Umgiesser, G. & Willems, P. 2013: Changing extreme sea levels along European Coasts. Coastal Eng. doi:10.1016/j.coastaleng.2013.10.017 statistical downscaling of 17 realizations of the A1B scenario using the same climate model (dots). Distributions are shown as Box- Whisker plots with median and 25 and 75 percentiles (grey). 1 Helmholtz Zentrum Geesthacht, Institute for Coastal Research, Germany 2 University Siegen, Research Institute for Water and Environment, Germany Helmholtz-Zentrum Geesthacht • Max-Planck-Straße 1 • 21502 Geesthacht / Germany • Phone +49 (0)4152 87-0 • Fax +49 (0)4152 87-1403 • [email protected] • www.hzg.de Contact: Dr. Ralf Weisse • Phone +49 (0)4152 87-2819 • Fax +49 (0)4152 87-42819 • [email protected]