Future sea levels

Climate change is expected to cause the mean water level around Denmark to rise. The best estimate for the different scenarios is between 0.3m and 0.6m within this century. However, the figures for the water level rise are uncertain and there is a particular risk that rises will become more intense.

 

On the basis of existing research, the Danish Meteorological Institute assesses the best estimate of sea level rises around Denmark to be 0.3-0.6m from 1986-2005 to 2081-2100, depending on the scenario. However, there is much uncertainty about how the sea and ice will react to climate change, and the full interval, including the uncertainty, is therefore 0.1-1.2m within this century (see the table below). Isostatic uplift will also play a part, and this means that the observed water level could be up to 0.2m lower, depending on where you are in Denmark.

 

The graph above shows the absolute mean water level around Denmark in metres for the years 1900-2100. The grey-shaded curve for the years 1900-2012 shows the observed, annual mean water level measured by Danish water gauges, adjusted for isostatic uplift. The blue thin curve for the years 2012-2100 shows the IPCC's best estimate of mean water level in the North Sea for the RCP4.5 scenario, and the light blue shadow indicates the uncertainty for this scenario. The dotted line shows the Danish Meteorological Institute's estimate of an upper limit for water level rises for use in uncertainty calculations. To the right of the figure are shown the mean value and uncertainties for the period 2081-2100 for the four IPCC scenarios as well as for BACC's assessment of the A1B scenario. The dotted line here shows the upper estimate for this period by the Danish Meteorological Institute.

The IPCC's estimate
In its fifth assessment report, the IPCC predicts a global water level rise in the magnitude of 0.26-0.82m up to 2081-2100. This increase is due partly to thermal expansion and partly to melting ice sheets and glaciers. The interval covers different greenhouse gas scenarios, however it also covers large uncertainties for the individual scenarios. Uncertainty is stated as the probable interval, i.e. that there is at least 66% probability the water level will be within this interval. The IPCC does not assess the risk of higher water level rises, but it states that the only significant source of figures that are significantly higher would be a possible collapse of the ice sheet on West Antarctica, and that this could contribute with several decimetres.

Significant regional differences in water level increases will occur due to changes in sea currents and gravitation. Gravitational changes mean that melting ice from Antarctica will lead to a greater rise in the water level around Denmark than a similar melting of ice in Greenland. The regional differences have been included in the assessments and they mean that the IPCC's assessment for the same period of the water level rise around Denmark is 0.1-0.9m.

 

Alternative assessments
There are other assessments of future increases than the IPCC's. For Denmark, the analysis carried out by Aslak Grindsted from the University of Copenhagen (BACC II, 2015) is particularly relevant. According to this analysis, the rises in water level will be greater than stated by the IPCC, i.e. around 20%, however with greater upward uncertainty. This is attributable, in particular, to a different estimate of the melting of the large ice sheets.

 

Upper estimate
With help from Aslak Grindsted from the University of Copenhagen, the Danish Meteorological Institute has prepared an upper estimate (95% percentile) of water level rises for use in uncertainty assessments. The estimate should be understood as an expert assessment for a high-emissions scenario, corresponding more or less to RCP8.5. The estimate is a 1.2m change from 1986-2005 to 2081-2100, and 1.4m for 1986-2005 to 2100.

Isostatic uplift counteracts sea level rise
The water level rise is counteracted by a general isostatic uplift since the last ice age. DTU Space - National Space Institute calculates isostatic uplift in Denmark using e.g. modern GPS measurements. The results show that the northern-most part of Denmark has seen a land uplift of around 15cm-20cm in this century. For the southern-most part of Denmark, the land has risen 5cm-10cm. In Northern Jutland, where the land is rising the most, this will be experienced as a fall in water level until the climate-change-induced rise becomes more significant.

The water level varies
The water level in the oceans is never constant. The tide causes the sea to fall and rise hour by hour. In Denmark, water levels and waves are higher in winter, because the winter season is generally windier. When certain combinations of tidal water, wind and weather conditions occur, particularly high water levels may be reached. These are called storm surges.

Statistics of high water levels
The water level is measured continuously at 60 locations throughout Denmark.  The Danish Coastal Authority has prepared statistics for each measuring station, which show the frequency of certain water levels. These statistics show the water level that is statistically likely to occur only once in every 50 years.  A so-called 50-year water level.

Water level increase and storm surges
When the mean sea level rises, the maximum water level in the event of a storm surge can be expected to rise similarly or faster. The best estimate of the water level in the event of a 50-year event in future will therefore be: the water level in the event of a 50-year event today plus the expected sea level rise adjusted for isostatic uplift and wind. Other local conditions and e.g. tide, however, may also affect future storm-surge water levels. This has not yet been examined in more detail.

For the west coast of Jutland, within this century, the maximum water level in the event of extreme storm surges will increase by between 0.3m and 1.7m. This is due to general water level increases of 0.3m, adjusted for isostatic uplift and increased wind speeds.

In Danish coastal waters, the wind is likely to be less important during storm surges and the maximum water level in the event of storm surge is expected to be 0.0 -1.4m. 
 
An example
How do storm-surge heights change with increasing water levels in the RCP4.5 scenario?

For example, in an RCP4.5 climate scenario, the expected maximum water level in the event of a 50-year event in the coastal town of Hirtshals in Northern Jutland in 2050 is calculated at:

The water level in the event of a 50-year event today: 1.44m
plus the expected mean sea level increase: 0.1m - 0.4m
minus isostatic uplift: -  0.1m
plus wind: + 0.1m

Total: 1.5m - 1.8m

 

For a similar calculation for the town of Esbjerg on the Jutland west coast, the isostatic uplift should be set at 0.02m.

Observed water level around Denmark over the past 100 years
The observed water level is increasing along the coast throughout Denmark, except for in Northern Jutland. Differences in the observed water level are due primarily to isostatic uplift. When adjusted for this, all monitoring stations with long time series show that the water level has gone up by an average of 1.7-2.2(±0.3)mm annually around Denmark. This figure is very close to the global average.

 

Sea levels in the long term
Estimates of sea level rises further into the future depend entirely on our estimate of future CO2 emissions. There is very large uncertainty about this, however the Danish Meteorological Institute assesses that in 2200 the increase will have more than doubled relative to 2100 for the upper scenarios, while the rate of increase will tail off for the scenarios with low CO2 emissions.