6. Extreme Sea Levels for Norway

• Knowledge of future extreme water levels is important for coastal management.

• There are no observations or projections of wind and wave climate that can suggest storm surge activity will change significantly in the future.

• We obtain current return heights by statistical analysis of the 22 tide gauge records along the Norwegian coast (squares).

• We use the average conditional exceedance rate (ACER) method since it allows for use of more data and is less sensitive to outliers and missing data.

• In order to obtain return heights for all coastal municipalities, time series from the tide gauges have been extrapolated to the surrounding areas, based on temporary tide gauges, oceanographic and local knowledge, and then analyzed.

• The 20, 200 and 1000-year return heights as well as confidence intervals are calculated.

BergenOslo

Tromsø

Trondheim

Kristiansand

Stavanger

3. Regional Absolute Sea Level Changes

• Regional projections from AR5/CMIP5.• Plus estimated self attraction and loading

(SAL).• Without the GIA solution used in AR5.• Interpolated to the 276 coastal municipalities.• Projections for emission scenarios RCP2.6,

RCP4.5, and RCP8.5. • Sea level change from 1986–2005 to 2041–

2060, 2081–2100, and 2100. • We use the AR5 likely range 5 to 95%

uncertainty bounds.

7. Combining Storm Surge Statistics with Sea Level Rise

• Allowances give the height an asset needs to be raised so the probability of storm surge flooding remains the same under a given sea level change.

• We adapt the framework of Hunter (2012) for calculating allowances, to the ACER method.

• With ACER the allowances become dependent on the return height of interest, but in our case differences are small.

• We make the assumption that our regional sea level projections are normally distributed, and apply the 5 and 95% uncertainty bounds.

• We emphasize that other, maybe skewed, distributions may be more appropriate.

• The allowances lie between the mean and upper 95% bound of the projected sea level changes.

• Using these allowances may not be the best choice for planning in some areas, especially when the consequences are severe.

4. Contribution from Vertical Land Motion in Norway

• Glacial isostatic adjustment (GIA) dominates vertical land motion in Norway.

• New findings for the coastal municipalities. • Now good enough spatial coverage of the

permanent Global Positioning System (GPS) stations for statistical interpolation of the observations.

• Method performs well in comparison to the GIA model.

• Gravitational effects of GIA on sea level are taken into account (0.2–0.5 mm/yr).

• Our GIA solution appears to be broadly similar to that used in AR5, but has smaller uncertainties.

• We have more confidence in our GIA solution, the values and uncertainties being essentially based upon the GPS observations.

(See also Holger Steffen’s talk Friday at 14:45)

5. Relative Sea Level Changes for Norway

• The pattern of twenty-first century relative sea level changes for Norway is governed by GIA.

• Thus, it is ocean mass changes and dynamic and steric sea level changes that will dominate the future RSL response.

• For all RCPs the mean regional relative sea level change is projected to be below the global mean.

• For all RCPs, most of Norway will experience a relative sea level rise.

• The uncertainty bounds are on the order of ±0.2 m (for RCP8.5 in 2081–2100 shown here).

Projections of 21st Century Sea Level Changes for Norway

Matthew J. R. Simpson1, J. Even Ø. Nilsen2, Oda R. Ravndal3, Kristian Breili1, Hilde Sande3, Halfdan P. Kierulf1, Holger Steffen4, Eystein Jansen5, Mark Carson6, Olav Vestøl1

Jan Even Øie Nilsen, Nansen Environmental and Remote Sensing Center, even@nersc.no, +47 465 08 614 | www.nersc.no | www.bjerknessenteret.no | www.kartverket.no/sehavniva

1. Introduction• Effective coastal management requires understanding of future local sea level

changes.• AR5 represents a fundamental step forward in assessment of sea level rise.• Regional sea level projections are for the first time provided by the IPCC.• We provide projections of relative sea level change for each of the 276 coastal

municipalities in Norway.• We use new state of the art land uplift rates for Norway, from both new GPS

observations and GIA modelling. • Preparedness is normally based on extreme value statistics of observed water

levels.• With changing mean sea levels the return heights will change accordingly. • The extreme value analysis and return heights for Norway have been reassessed.• We provide estimates for how much assets need to be raised so that the

probability of flooding by extreme events remains the same.

–VLM(≈GIA)

∆RSLRCP8.5Mean

Change from 1986–2005 to 2081–2100 Change from 1986–2005 to 2081–2100 For the end of the century (2081–2100)Relative to mean sea level (1996–2014)Change from 1986–2005 to 2081–2100

+ =

Affiliations and acknowledgements

1 Geodetic Institute, Norwegian Mapping Authority, 3507 Hønefoss, Norway.2 Nansen Environmental and Remote Sensing Center and Bjerknes Centre for Climate Research, Thormøhlensgt. 47, 5006 Bergen, Norway.3 Hydrographic Service, Norwegian Mapping Authority, Postboks 60, 4001 Stavanger, Norway.4 Lantmäteriet, Lantmäterigatan 2C, 801 82 Gävle, Sweden.5 Department of Earth Science at University of Bergen, and Bjerknes Centre for Climate Research, Allégt. 41, 5007 Bergen, Norway.6 Institute of Oceanography, KlimaCampus Universität Hamburg, Bundesstraße 53, 20146 Hamburg, Germany.

The report is commissioned by the Norwegian Environment Agency and under the auspices of the Norwegian Climate Service Center.

∆SSHRCP8.5Mean

200-yearReturn Heights

AllowancesRCP8.5

HAT

2. Observed Sea Level Rates in Norway• Relative sea level rates at the Norwegian tide gauge network reflect the pattern of land uplift.• South-, west-, and northern Norway is now experiencing sea level rise in spite of land uplift (red

arrows).• After correcting for land uplift, the coastal average SSH rate of change is 2 mm/yr (1960–2010).• More recently (1992–2012), the average coastal sea level change south of 66ºN is

• 3.8 mm/yr estimated from altimetry• 3.1 mm/yr estimated from tide gauges.

• These higher rates in recent decades may be a result of multidecadal (natural) variability.• The regional sea level trend budget is far from closed for Norway.• Observations point to warming ocean and melting land ice as the main contributors to the trends:

• Thermal expansion contributed between 0.5–1.0 mm/yr, dependent on location in 1960–2010 .

• Land ice melt’s contribution estimated 0.6±0.2 mm/yr in 2003–2009 (GRACE).• Salinity and atmospheric pressure contributions smaller than ± 0.5 mm/yr.

2 mm/yr(1960–2010)

∆RSLOBS

Beyond the Likely Ranges?

• There is no new evidence that allows us to quantify probabilities outside the likely range.

• However, the ice sheet contributions might have a skewed probability distribution with large values in the upper tail.

• As a demonstration we apply a skewed distribution for the contribution from Antarctic ice dynamics, fitting the upper 95% bound at 40 cm (dashed blue).

• The total global projection becomes skewed (from full to dashed red).

• Our regional sea level projections become even more skewed with 20 cm higher upper 95% bound for Bergen (lower panel).

• A proper assessment would require full probability distributions for all contributions.

RCP8.5

Time Series of Observations and Projections

• The projected time series (RCP2.6 green; RCP4.5 blue; RCP8.5 red).• Only small differences between the RCPs before 2050. • Uncertainties best represented by observed (yellow) natural variability the next few decades. • In the latter half of the century the separate projections from RCPs diverge but still large overlaps

between the likely ranges (shaded areas, and bars for 2081–2100 means).

OsloBergenTromsø

BergenOslo

Tromsø

BergenTrondheimTromsø Stavanger Kristiansand Oslo

• The likelihood of exceeding the present-day return heights can be dramatically increased with sea level rise.

• Changes in likelihood are dependent on both the projected sea level change and the statistics of the observed sea level extremes.

• For Oslo, which has a relatively small projected sea level change but has relatively large differences between the return heights, we expect only small changes in the frequency of exceedance (RCP2.6 green;

RCP4.5 blue; RCP8.5 red). • However, for Stavanger and Bergen, the

reverse is true and we therefore expect a large increase in the frequency of exceedance.

• In latter cities we expect that the present-day 200-year return height will be exceeded in ~40 of the years between 2001 and 2100.

Oslo

Bergen

More Frequent Extreme Sea Levels