We expect largest sea-level rise at Sørlandet, at the western coast of Norway, in Lofoten, and in Finnmark. These projections are adopted from the report “Sea Level Change for Norway – Past and Present Observations and Projections to 2100”, written by the Norwegian Mapping Authority in cooperation with Nansen Environmental and Remote Sensing Center. The report addresses also the combined effects of sea-level rise and extreme sea levels that arise during storm surges.
Global warming rises sea level
Increased sea level is a consequence of global warming. The most important effects are thermal expansion of seawater and melting of glaciers and ice caps. In addition, small contributions come from changes in ocean currents, air pressure, winds, and land water storage. Satellite observations from 1993 to present time prove that the global sea level in total has risen by 3.2 millimeter each year.
The height of the coastlines may also change due to vertical land motion. This is most relevant for the Norwegian coast, experiencing uplift rates of up to 5 millimeters per year. The land uplift is an effect of the ice cap that covered most of Scandinavia during the past ice age. The load from the ice was enormous, and deflected the crust downwards. Then, when the ice started to melt about 20000 years ago, the crust was allowed to rebound back to its initial state. The process is known as glacial isostatic adjustment – and it is still ongoing.
The difference between the changing height of the sea surface and the land uplift is called relative sea-level change. For planning and coastal adaption, the relative sea level change is the quantity of interest.
The change in sea level is inhomogeneous
The sea level along the Norwegian coast may rise at a different pace than the global average. For instance, we expect thermal expansion to be about 10 % larger than the global average along the Norwegian coast. Variation is also introduced because melting land ice influences on the Earth's gravity field. As an example, the Greenland and Antarctic ice sheets surround with gravitational forces that attract seawater. As the ice sheets melt, the attracting forces diminish. The effect is that the sea level will fall in the near field of the melting ice (within about 2200 km), even though the total volume of the world's ocean increases. A consequence of this effect is that the Antarctic ice sheet is of concern for Norway, because the gravitational effect of land ice loss results in an above average sea level rise in our region (as opposed to the Greenland ice sheet which will have a negligible contribution).
Projections of future sea level along the Norwegian coast
Our sea-level projections include four effects:
1) Regional changes in ocean density (thermal expansion).
2) Redistribution of water and changes in ocean circulations.
3) Mass changes due to melting land ice and associated gravitational effects.
4) Land uplift and associated gravitational effects
We employed the same climate models and ice models as used for the fifth assessment report of the IPCC to calculate the contributions 1 to 3. For contribution 4, time series from the national network of permanent Global Navigation Satellite Systems (GNSS) stations and repeated levelling were explored. Finally, the associated gravitational effects were assessed from an Earth model, describing deformations and gravity changes resulting from the mass changes predicted by the climate models.
The emission scenarios are decisive
We projected future relative sea level for three emission scenarios, i.e. RCP2.6, RCP4.5, and RCP8.5 (see additional details in "facts"). For each scenario, the projections are presented as likely ranges, illustrated for six key locations in Figure 1. Depending on your location, we expect over the period 1986-2005 to 2081-2100:
- Between -0.10 and 0.30 m for RCP2.6
- Between 0 and 0.35 m for RCP4.5
- Between 0.15 and 0.55 m for RCP8.5
Figure 1. Relative sea level projections for RCP2.6 (green), RCP4.5 (blue), and RCP8.5 (red) at (a) Oslo, (b) Stavanger, (c) Bergen, (d) Heimsjø, (e) Tromsø, and (f) Honningsvåg. The vertical bars on the right side of the panels represent the most likely relative sea level rise and spread for 2081-2100. Yellow markers indicate annual mean tide gauge observations.
Significant variations in the land-uplift rates
For each coastal municipality, the most likely relative sea-level change (the mean of the likely range) is illustrated in Figure 2.
We expect largest sea-level rise at Sørlandet, Vestlandet, in Lofoten, and Finnmark, and modest sea-level rise around Oslofjorden, Trondheimsfjorden, and parts of Nordland. The spatial pattern reflects first that the pace of the land uplift varies significantly along the Norwegian coast. In addition, due to the land uplift, the relative sea-level changes are less than the global average for the entire coast and for all emission scenarios. Still, most of the coast will experience higher sea level in 2091-2100 relative to 1986-2005.
Figure 2. Most likely relative sea level change (meter) over the period 1986-2005 to 2081-2100 for (a) RCP2.6, (b) RCP4.5, and (c) RCP8.5.
Extreme sea levels arise due to storm surges, i.e. when the combined effects of low pressure, winds, and ocean tides rise sea level up to several meters above the normal. Analysis of several decades of tide gauge observations allows so-called return heights to be estimated. Statistically, the sea will exceed these levels only once during a certain period.
In the future, the return heights will adjust as the mean sea level rise due to climate change. This implies that present return heights will be exceeded more often in the future. For instance, at Vestlandet we expect the sea level to exceed the level for the 200-year period 40 times throughout the 21st century. It is, therefore, crucial to take into account the return heights of storm surges when planning for future sea level.
The uncertainty of the projections
For each scenario, we have estimated the projections as a likely range. The ranges illustrate that the projections are not exact. Uncertainty is introduced because 1) the climate models are not able to perfectly describe the future climate on the Earth; and 2) the modelling of land uplift, glacial mass changes, and associated gravitational effects are not perfect. In addition, it is not clear which of the three emission scenarios that best describes the future.
For precautionary planning, it is natural to use the projections of RCP8.5, which implies that the emissions of greenhouse gases will continue to rise throughout the whole 21st century. Nevertheless, planners and stakeholders should bear in mind that we cannot rule out sea-level changes beyond the likely range of RCP8.5. For Norway, a low probability/high impact event is possible collapses of marine portions of Antarctica.