KM-I-2: Sea levels | Umweltbundesamt

KM-I-2: Sea levels

The calculated mean of water levels measured for 19 years at selected gauging stations in the North Sea and Baltic Sea illustrate the sea level rise. Measurements from gauging stations in the North Sea and the Baltic Sea show that water levels have been rising significantly. For coastal regions, in particular estuaries and coastal lowlands, rising sea levels signify an increasing risk of exposure to storm surges which can lead to flooding.

Continuously rising water levels in North Sea and Baltic Sea

Owing to climate change, the glaciers and ice shields at the poles are melting. They feed great amounts of freshwater into the seas. At the same time rising water temperatures make the seawater expand. Consequently, this leads to a global rise of sea levels. For this century, the 6th IPCC Progress Report published in 2021 projects a probable bandwidth for the global rise of sea levels – regardless of any potential scenarios of human action – by 0.3 to 1.0 metre. Nevertheless, these values reflect only those processes that are well understood and thus can be modelled. For example, the processes of ice melting in the Antarctic have so far not allowed adequate modelling. However, it is conceivable that by 2100 they might lead to an additional increase in sea levels by more than a metre. As far as German coasts are concerned, it is expected that values would hardly deviate from the global means mentioned above^{78 78}.

Various factors of influence such as the redistribution of masses in the ocean, wind conditions and air-pressure conditions might either increase or decrease the global development. Furthermore, vertical land movements have a decisive influence on the absolute sea level increase at a specific location: During the last glacial period massive ice shields covered the coastal areas of the North Sea and the Baltic Sea. Their enormous weight made the land masses sink. Ten thousand years ago, the ice melt triggered an isostatic equalising movement which has been continuing to this day. These factors are responsible for the fact that the increase in sea levels differs both regionally and locally.

The North Sea is characterised by the tidal system. This is the reason why the indicator shows the mean tidal sea level (MTmw) for the gauging station concerned. By contrast, the Baltic Sea – owing to its geographical location and tenuous links with the oceans – is subject to a comparatively weak tidal influence; that is why in this case the annual theoretical mean water levels (MW) are relevant. For these coefficients of water level, a low-pass filter was used to calculate a ⁠moving average⁠ across 19 years.

All gauges examined show that the mean (mid-tide) water levels have, statistically speaking, risen significantly in the course of the past 60 to 180 years. Analogous to the development of the increase in the global sea level⁷⁹, the trend analysis carried out for the gauging station at Cuxhaven Steubenhöft on the North Sea coast, shows a secular trend (statistically significant square trend) that started roughly towards the end of the 19th century, accompanied by variability which in each case covers several decades. From the beginning of records to roughly the end of the 1880s, the sea level measured at this gauge dropped at first, before a continuous increase was observed. The water levels measured at the Wittdün gauge were slightly regressive latterly. This can possibly be attributed to morphological changes in the gauging station’s environment.

Coastal regions, in particular estuaries and lowlands – some of which are even below sea level – can be exposed to increasing risks from rising sea levels and associated storm surges that can result in flooding (cf- Indicator KM-I-3). Another consequence of rising sea levels is progressive coastal erosion which affects sandy coastal zones, thus also affecting sections of cliffscapes (cf. Indicator KM-I-4).

In the German Bight and adjacent estuaries the increase in sea level also affects the tidal dynamics (tidal water levels and current velocities). For example, modified tidal dynamics change the sediment transport in mudflats and estuaries. Higher current velocities at high tide (compared to ebb-tide velocities) carry more sediment into those areas. The extent to which mudflats grow in line with the increase in sea level, depends – among other factors – on the availability of sediments. There are still some uncertainties. The same is true for the small islands known as the Halligen. They too increase in size owing to sediment input from inundation. However, if the sea levels rises faster in future, the Halligen islands and the salt marshes will be seriously at risk unless targeted protective measures are taken⁸⁰. In estuaries – which are also used as shipping lanes, for instance as access routes to the port of Hamburg via the Elbe – greater input of sediments might lead to the need for increasing maintenance efforts, if the decrease in the depth of water owing to increased sediment inputs outweighs the increase in water depth resulting from the rising sea level. Besides, it is important to remember that marine-engineering measures can also strongly influence these processes.

The uncertainties of projections regarding increases in the sea level combined with the impacts of regional and localised changes, make a targeted collection of data on the development of water levels on the coasts indispensible. These factors are equally important as benchmarks for measures required in coastal protection and in respect of other coastal structures that have to be continuously adapted to new challenges (cf. Indicator KM-R-2).

^{78 - IPCC – Intergovernmental Panel on Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2391 pp. doi:10.1017/9781009157896.}

^{Cf. Sea Level Projection Tool – NASA Sea Level Change Portal: https://sealevel.nasa.gov/ipcc-ar6-sea-level-projection-tool}

^{79 - Walker J. S., Kopp R. E., Little C. M., Horton B. P. 2022: Timing of emergence of modern rates of sea-level rise by 1863. Nature communications, 13(1), 966.}

^{80 - WWF Deutschland – World Wide Fund For Nature (Hg.) 2018: Land unter im Wattenmeer – Auswirkungen des Meeresspiegelanstiegs auf die Natur der Halligen und Möglichkeiten zur Anpassung. Husum/Berlin, 85 pp. https://www.wwf.de/fileadmin/fm-wwf/Publikationen-PDF/WWF-Halligstudie.pdf}