Future Sea Level Along the Norwegian Coast

Summary and key findings of the report «Sea-Level Rise and Extremes in Norway»:

Furthermore, the rate of global sea-level rise is increasing; that is, it is accelerating. This represents a growing risk that coastal countries, including Norway, will have to adapt to.

Because of the long response times of the oceans and ice sheets to warming, today’s greenhouse gas emissions have implications for future sea-level rise over hundreds to thousands of years. Sea-level rise is a long-term challenge that will have to be managed over multiple generations. Crucially, however, we can reduce the long-term risks by acting now, and implementing rapid and deep emission cuts. Major emission cuts will rein in further sea level acceleration, and reduce the risk that thresholds for the stability of the large ice sheets in Greenland and Antarctica are crossed; which would commit us to multiple metres of sea-level rise. These increases in sea level would be permanent, that is, irreversible on human timescales, and would present a profound adaptation challenge. Furthermore, some ice sheet tipping mechanisms can potentially drive rapid sea-level rise (multiple metres over hundred year timescales).

The purpose of the report «Sea-Level Rise and Extremes in Norway» is to provide a knowledge base for policy and decision makers working with mitigation and adaptation strategies for coastal planning in Norway. Changes to sea level and sea level extremes will lead to changes in coastal impacts. These changes represent a changing risk to human and natural systems.

There are several factors that count in Norway’s favour when considering its exposure and vulnerability to sea-level rise. The coast is generally steep and rocky, and upwards vertical land motion acts against sea-level rise, meaning Norway has historically had rather stable or falling relative sea levels. Unlike other coastal countries, Norway is therefore yet to feel the impacts of sea-level rise. The danger is that this can foster a false sense of security, where the long-term risks are not understood or ignored.

The results from this report show that sea-level rise is starting to push up water levels in some parts of the country, most notably in Western and Southern Norway. Owing to global warming, Norway is transitioning from a country with on average falling or stable sea levels, to one with rising sea levels. For increasing levels of warming, sea-level rise will become faster, and more of the country will transition to relative sea-level rise.

Sea-level rise will cause flooding from sea level extremes to reach higher and further inland. Although flooding is often very localised, because of the steep topography, the sheer length and complexity of the coast means that in sum, quite a large area can be exposed. Coastal towns and cities, and a considerable amount of infrastructure, are at potential risk. Sea-level rise will also drive sharp increases in flooding frequency: A 0.1 m sea-level rise will lead to a tripling of the flood risk in many locations. This means that, unless timely adaptation measures are taken, flooding will develop into a chronic problem. 

As understanding of sea level and sea level extremes improves and evolves, our knowledge base, and therefore climate change projections, will change over time. Improved understanding will lead to better constrained projections and hopefully narrower uncertainties. Some uncertainties, however, remain difficult to quantify because the processes are not well understood (referred to as deep uncertainty and associated with ice sheet processes). This all speaks to having a flexible approach to adaptation; where you have an evolving knowledge base and need to be able to react to potential surprises from the climate system. In this regard, it is important to maintain and improve monitoring of sea level. Improved monitoring and the establishment of early warning systems, both globally and nationally, are a vital part of developing adaptation strategies and better planning for sea-level rise.

The key findings from this report are:

Sea-level observations

Because of global warming, geocentric sea level (the ocean surface) continues to rise along the Norwegian coast. Norway’s coastal average geocentric sea-level rise is 2.3 ± 0.3 mm/yr for the period 1960-2022 and 3.3 ± 0.9 mm/yr for the period 1993-2022. This agrees well with the observed global mean sea-level rise.

Vertical land uplift from glacial isostatic adjustment acts against geocentric sea-level rise to various degrees over the coastline: For the period 1960-2022, relative sea-level change from the national tide gauge network ranges from a fall around the Oslofjord and along parts of the coast of Trøndelag and Nordland (with a minimum of -2.3 mm/yr in Oslo) to a rise for parts of Western and Southern Norway (with a maximum of 1.3 mm/yr in Måløy). Around 60% of the coast experienced no significant change.

Sea-level projections

Projections based on IPCC AR6 show Norway’s coastal average relative sea-level change for 2100, relative to the period 1995-2014, will range from 0.13 m (likely -0.12 to 0.41 m) for the very low emissions scenario (SSP1-1.9)  to 0.46 m (likely 0.21 to 0.79 m) for the very high emissions scenario (SSP5-8.5). This rise is between 40% and 70% lower than the projected global average; a difference that can be explained by ongoing vertical land motion in Norway and the close proximity of Greenland and Arctic glaciers. (Medium confidence projections given with median values and likely ranges.)

Projected local sea-level change will deviate from Norway’s coastal average largely because of geographical differences in vertical land motion. 

Table 3.1 from Chapter 3: Projected relative sea-level changes for 2100, relative to the period 1995-2014. Median values (50%) and likely ranges (17-83%; the central two-thirds of the probability distribution) are given for the medium confidence projections and for a selection of emission scenarios. For the very high emissions scenario SSP5-8.5, a low-likelihood high-impact storyline of rapid ice-sheet mass loss is shown (the 83rd and 95th percentiles of the low confidence projections). Units are in metres.

For the low emissions scenario (SSP1-2.6), projections show large parts of Western and Southern Norway, as well as a small part of Northern Norway will likely experience relative sea-level rise for 2100. For the remaining two-thirds of the coast, sea levels have a chance of being kept stable under SSP1-2.6. For scenarios with higher greenhouse gas emissions than SSP1-2.6, a majority of the coast will likely experience relative sea-level rise for 2100. (Medium confidence projections.)

For 2150, projections show coastal average relative sea-level change for Norway will range from 0.16 m (-0.24 to 0.61 m) under SSP1-1.9 to 0.73 m (0.25 to 1.38 m) under SSP5-8.5. (Medium confidence projections given with median values and 17-83% ranges.)

For the very high emissions scenario (SSP5-8.5), and a low-likelihood high-impact storyline of rapid ice-sheet mass loss, coastal average relative sea-level change for Norway could approach between 1 and 1.5 m by 2100. Some locations along the coast, notably Stavanger and Bergen, could experience close to 2 m (Table 3.1). Shortly after 2100, massive ice loss from Antarctica could rapidly increase Norway’s coastal average sea level to between 4.5 and 5 m by 2150. This is a storyline that cannot be ruled out and is particularly relevant for users with low risk tolerance. (Low confidence projections given with 83rd and 95th percentiles.)

Due to limited scientific understanding, we cannot rule out that rapid ice mass loss from Antarctica could also be triggered by emission scenarios below SSP5-8.5. However, processes that can drive rapid ice-sheet mass loss are unlikely to be significant for SSP1-1.9 or SSP1-2.6, at least within the timeframe of 2100. (Low confidence projections.)

For 2300, projections show coastal average sea-level change for Norway will range from 0.35 m (-0.75 to 1.4 m) under SSP1-2.6 to 4.15 m (0.4 to 16 m) under SSP5-8.5. (Low confidence projections given with median values and 17-83% ranges.)

Future flood risk due to projected sea-level rise

Sea-level rise will increase flood risk in Norway by pushing up the height of sea level extremes, which will reach higher and further inland.

Small height differences (0.3 to 0.6 m depending on location) separate the 1-in-200 year extreme still water level and the once-a-year event. This shows that, in some areas of the coast, only a few decimeters of sea-level rise are required to drive a 200-fold increase in flooding frequency. Sea-level rise will therefore cause the height of historically rare extreme sea level events to be reached annually or more frequently in the future.

There are large differences in the timing and extent of flooding frequency changes depending on projected sea level. For higher emission scenarios, and thus faster and larger future sea-level rises, flooding frequency increases occur earlier and are more widespread. Western and Southern Norway will experience increases in flooding frequency first.

Storm surge and wave projections

Projected changes to sea level extremes are primarily determined by the projected mean sea-level change. Changes to the strength and frequency of storms are of secondary importance. While projections show that the mean wind speed for the Norwegian coast may decrease, the variance can get larger. This suggests that some of the most extreme wave and storm surge events will become more severe in future. However, there is low confidence in these projected changes.

Projections indicate that the wave climate in the Barents Sea and along the coasts of Northern Norway will become more severe. This is a result of Arctic sea ice retreat and hence increased fetch, which means waves can build up over a larger stretch of water.