Climate change is expected to entail a mean sea level rise of between 0.3m and 1m in the seas around Denmark within this century.

On the basis of research so far, the Danish Meteorological Institute estimates the sea level around Denmark could rise between 0.3m and 1m within this century due to climate change. Because isostatic uplift and wind also play a role, this will mean that the experienced water level could rise between 0.1m and 1.2m depending on the location.

The existing scientific basis does not allow the Danish Meteorological Institute to set an upper limit for water level rises along the Danish coastline.  The Danish Meteorological Institute therefore recommends that even higher rises be included in risk assessments.

Expected  sea level rise	2050	2100
Mean water level rise	0.15m - 0.45m	0.3m - 1m
Local conditions
Isostatic uplift	- (0.05 - 0.1)m	- (0.1 - 0.2)m
Wind	0m - 0.1m	0m - 0.3m
Estimate, total	0.05m - 0.5m	0.1m - 1.2m

UN climate panel estimate
In its fourth assessment report, the UN climate panel (IPCC) predicts a global water level increase of 0.2m - 0.6m up to 2100. The majority of this increase will be due to the thermal expansion of the water. The rest of the increase will be due to ice caps and glaciers melting.

Future rapid changes in the movement of ice are not included in the UN’s estimate.  New observations in Greenland and Antarctica indicate that dynamic processes in connection with the movement of the ice increase the vulnerability of the ice cap to global warming and lead to greater sea level rises.  The Danish Meteorological Institute has included this in its assessment above. Understanding of these processes is still only limited. The assessment of the Danish Meteorological Institute corresponds to the conclusions at the international conference Global Risks, Challenges and Decisions, held at Copenhagen on 10-12 March 2009.

Regional differences
Significant regional differences in water level increases may occur due to changes in sea currents and gravitation. These have not yet been mapped in detail, but they do contribute somewhat and have therefore been included in the assessment. 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.

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 0.4m and 1.2m. 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, resulting in a maximum increase of 5cm-10cm in water level in the event of a 50-100-year event, up to 2100.

An example
What is the expected maximum water level in the event of a 50-year event in the coastal town of Hirtshals in Northern Jutland in 2050?

The water level in the event of a 50-year event today             1.44m
plus the expected mean sea level increase             0.15m - 0.45m
minus isostatic uplift                                                                     - 0.1m
plus wind                                                                                        + 0.1m
Total                                                                                   1.59m - 1.89m

For a similar calculation for the town of Esbjerg on the Jutland west coast, the isostatic uplift should be set at 5cm.

Trend in water levels in the seas around Denmark
For the long-term trend in water levels, a calculation of the mean of water level measurements over a longer period is required.

Below are 19 years of running mean for water levels at nine measurement stations throughout Denmark. The average increase rate for the period 1901-2000 is also stated. The differences between stations are due primarily to isostatic uplift.

Changes in measured water levels in the seas around Denmark over the last 100 years

Due to the isostatic uplift the measured water level in the northern part of Denmark has fallen slightly over the last hundred years, whereas the measured water level has increased in the southern part of Denmark, where the land is rising only slightly. The mean value for the years 1960-1990 has been deducted. (Source: Madsen, 2009).

Sea level in the longer term
Estimates of water level rises further in the future depend entirely on our assumptions about future carbon emissions. There is great uncertainty associated with this, however the Danish Meteorological Institute deems that the increase in 2200 will more than double compared with 2100.