Case 1: Denmark 1 - Flooding

The following is a preliminary assessment of how small remote communities on three islands in the southern part of Denmark will be affected by climate change – the main hazard in focus, in this case, is flooding from the sea. The assessment will be developed further as the project progresses. 

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The harbor area on the island Strynø during the storm surges in January 2019 (Photo: Lise Thillemann Sørensen)

A country of islands

Denmark consists of more than 440 named islands and has 8750 km of coastline. 27 islands have the characteristics that they have an all-year population below 1200 persons, no connection to the mainland, and are not privately owned. Those small islands are the focal point of this case. Many of the small islands have a low elevation over sea level. During floods, the help and support people on the islands can get from the mainland is therefore limited, as ferry traffic often gets affected by high sea levels. In a future where sea level rise is expected, the small Danish islands are therefore at particular risk of experiencing negative effects of climate change.

This case will focus especially on three islands south of Funen: Drejø, Skarø, and Birkholm (Figure 1). They all serve as year-round communities. Drejø is 4,1 km2 and has a population of 69 persons, Skarø is 2 km2 and has a population of 28 persons and Birkholm is 0,9 km2 and has a population of 11 persons. The aim is that findings from the CliCNord case study of those three islands will be transferable to some extent to the rest of the 27 small Danish islands.

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Figure 1 The location of the three islands Skarø, Drejø and Birkholm

Hazard and exposure

Historical floods

Several historical storm surges have resulted in floods on small Danish islands, as well as along the rest of the Danish coast. The most damaging within recorded history took place in November 1872. On the islands south of Funen, water levels up to three meters were reported (Colding, 1881). This historical event is often referred to when flood risk in the area is discussed. In recent times, several storm surges have also created damages to houses, infrastructure, and farmland on the islands. In November 2006 the water rose to 1.7 meters in the area, in 2017 to 1.6 meters (The Danish Coastal Authority, 2020a), and latest in 2019 to around 1.6 meters again (The Danish Coastal Authority, 2021b).

Climate change predictions

The Danish coastal authority concludes that there is a 38 % increased risk of flooding from the sea in Denmark in 2115, compared to today. Also, on average, the Danish coastline in the inner Danish waters will redraw 35 meters as a result of rising sea and stronger winds (Ministry of Environment and Food, 2016).

The Danish Metrological Institute (DMI, 2021) predicts a sea-level rise of up to one meter in the next 80 years for the sea south of Funen. They build their prediction on two different Intergovernmental Panel on Climate Change climate scenarios, depending on how successful we will be in reducing our CO2 emission globally. The first scenario (RCP4.5 where we succeed in reducing our CO2 emissions) DMI predict will give a rise in sea level between 2 and 67 cm in 2017-2100 compared to 1981-2010. For the second scenario (RCP8.5, where no reduction takes place - we continue business-as-usual) the calculated levels are 12 to 101 cm for the same period.

The storm surge in 1872 is today used as a reference point for flood risk assessments in Danish coastal areas (The Danish Coastal Authority 2018). Those historic water levels, combined with the just mentioned climate change predictions, make a three-meter flood a not unrealistic scenario for the area. Figure 2 shows the flooding that would occur on the islands with a three-meter sea-level rise.

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Figure 2 Flooding of the islands with a sea level of 3 meters over the average (Ministry of Environment of Denmark/Environmental Protection Agency, 2021)

A further problem particularly relevant for the small islands is that higher sea levels can cause saline groundwater, as an elevated sea level will move the current boundary of freshwater further into the land, which can, locally, and especially pronounced on small islands, create problems with saltwater penetration into coastal extraction wells and the need to establish new wells (Nature Agency, 2012). This is especially a worry on Birkholm with its highest point only two meters above sea level. Thus, when the expected climate change occurs, it will have major and potentially serious consequences for the populations on the Danish islands, not only during storms surges but also with a general rise in sea level. At this moment no national plans exist to respond to this.

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Omø during the storm surges in November 2006 (Photo: Ole Odsgaard)

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Strynø 2019 (Photo: Lise Thillemann Sørensen)


Though Drejø, Skarø, and Birkholm, as well as many other small Danish islands, are at high risk of flooding, they are not given any special attention from the national nor local political level. When The Danish Coastal Authority in 2018 published a report indicating the fourteen Danish areas with the highest flood risk, none of these three islands were included. The main reason is that in the national flood risk assessments, the risk is calculated on the background of how costly a flood will be for a certain area. The areas that are considered as the highest risk are the ones where the damages after a flood would be most costly.  

In 2020, a larger risk assessment was made for the Danish coast. The coast was divided into sections and reports were made for each section. For areas with flood risk, it is either recommended to mitigate or accept the risk of flooding. In the section covering the case area, it is recommended to accept the risk both in a short term (20 years) and a long term (100 years) perspective (The Danish Coastal Authority, 2020b). The reason is that the risk assessment further shows that not many valuables exist on the islands. The map, Figure 3, is from the same report and illustrates the yearly cost of flooding in 2070. The cost referrer to the price of repairing flood-related damages on infrastructure and properties, as well as the cost of a flood for local businesses.

Figure 3 Flood risk illustrated by yearly cost. The map shows, that on the islands the risk will be “very low”, meaning that the calculated cost of flooding, will be 10-25.000 DKK (1350-3350 EUR) per year per cell (100 meter x 100 meter) in 2070.

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As the risk assessments build on the concrete physical value of an area, several other aspects are not included e.g., social and cultural aspects. For the people living on the islands, those types of risk assessments are therefore not a good illustration of the losses they would experience in form of losing their homes if the flood risk is not responded to. One of the motivations behind the CliCNord project is that other values than purely economic exist in having living communities also further away from the cities and this is not only for the people living there but also for the broader society.

To summarize, small Danish islands face growing challenges because of rising sea levels. The risk of floods is only growing with climate change. At the same time, the islands are not priorities as part of the national climate change adaptation focus, because of the small population and lack of valuables.

Challenges and solutions

A challenge related to vulnerability on many small Danish islands is that they are not individual municipalities. This is the case for Birkholm which is part of Ærø municipality and Skarø and Drejø which are part of Svendborg municipality. This means they are a part of municipalities that have a long coastline in general. The islands, therefore, might not be the biggest concern for those municipalities when a flood occurs. There are relatively small values on the islands compared to the main cities in the municipality. Therefore, the main resources will first and foremost go to the more densely populated areas. This is also the case when it comes to mitigating flood risk. Consequently, the islands have a tradition of being self-sufficient to an extent when it comes to emergency management. Several actions are already known and tried out, both in relation to mitigation, preparing for, responding to, and recovering from floods on the small islands (Figure 4).

Strynø 2019 (Photo: Lise Thillemann Sørensen)

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Figure 4 The islands´ actions to mitigate, prepare for, respond to, and recover from floods


  • Colding, A (1881). Nogle Undersøgelser over Stormen over Nord- og Mellem-Europa af 12te-14de November 1872 og over den derved fremkaldte Vandflod over Østersøen [Some Studies on the Storm over Northern and Central Europe of 12th-14th November 1872 and on the Watercourse Crescent Over the Baltic Sea]. Vidensk. Selsk. Skr. 6. Række, naturvidenskabelig og Mat. Afd. 245–304.

  • Danish Metrological Institute (DMI) (2021, April 15). Klimaatlas [Climate Atlas].

  • Ministry of Environment of Denmark/Environmental Protection Agency (2021, May 5).

  • Ministry of Environment and Food (2016). Kystanalyse [Coastal analysis].

  • Nature Agency (2012). Kortlægning af klimaforandringer - muligheder og barrierer for handling [Mapping climate change - opportunities and barriers to action]. Naturstyrelsen - Task Force for Klimatilpasning. ISBN 978-87-7279-608-6

  • The Danish Coastal Authority (2018). Revurdering og ajourføring af risikoområder foroversvømmelse fra hav og vandløb. Oversvømmelsesdirektivet. Anden planperiode. [Reassesment and update of flood risk areas from the sea and rivers. EU floods directive. Second period.]

  • The Danish Coastal Authority (2020a). Kystplanlægger [Coastal planner].

  • The Danish Coastal Authority (2020b). Rapport for strategistrækning F2.19.01

  • The Danish Coastal Authority (2021a, May 5). Kystplanlægger [Coastal planner].

  • The Danish Coastal Authority (2021b, June 14). Vandstandsmålinger.