Extreme sea levels and coastal flooding

Extreme sea levels have increased at most locations along the European coastline. Both observed and projected increases can be explained mainly by increases in mean local sea levels. However, extreme sea levels can be further increased by storm surges and tidal changes, particularly along the northern European coastline. In the absence of better coastal protection, the sea level rise projected for 2100 will increase the frequency of extreme coastal flooding events by a factor of 10 to more than 1 000 along most European coastlines, depending on the location and the emissions scenario.

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Figure 1. Change in the frequency of flooding events in Europe given projected sea level rise under two climate scenarios
Change in the frequency of flooding events in Europe given projected sea level rise under two climate scenarios

Increasing coastal flooding risks are threatening the achievement of a climate-resilient Europe, as aimed for by the EU strategy on adaptation to climate change.

Extreme sea levels can occur during storms and this can lead to coastal flooding in the absence of sufficient coastal protection. A 10-cm rise in sea level typically increases the frequency of flooding to a given height by a factor of approximately three. Extreme sea levels along coastlines result from a combination of factors, including increases in local mean sea levels and tidal levels, storm surge events, waves and changes in coastal morphology. Changes in mean local sea levels are the main driver of observed and projected changes in extreme sea levels along the European coastline. However, changes in wave and storm surge climate may also play a substantial role in the changes in extreme sea levels in some European regions. Storm surge levels are projected to increase along the northern European Atlantic coastline, while projections suggest a mixed picture south of 50 °N. Notable increases in high tide levels are projected for the northern part of the Irish Sea, the southern part of the North Sea and the German Bight, whereas decreases are projected for the western English Channel.

By 2100, historically 1-in-100-year coastal floods are projected to occur at least once a year along the Mediterranean and Black Sea coasts and at least once a decade along almost all remaining European coasts, even under a low emissions scenario. Under a high emissions scenario, 1-in-100-year coastal floods are expected to occur at least once a year along most European coasts.

Figure 2. Return period of current 100‐year extreme sea levels under two emissions scenarios
Return period of current 100‐year extreme sea levels under two emissions scenarios

The concurrence of high sea levels and heavy precipitation resulting in large run-off volumes may cause compound flooding in low-lying coastal areas, as was the case in the catastrophic floods in Venice in November 2019. Currently, the Mediterranean coasts are at the highest risk of compound flooding. A recent study projected that climate change will increase the risk of compound flooding along most European coastlines, with the largest increases being expected along the North Sea and Baltic Sea coasts.

The planning of coastal protection at local and regional levels requires more detailed analysis of coastal flood risks than is currently possible at the pan-European level. A comprehensive overview of relevant data sets is available from a recent output of the ECLISEA (European advances on climate services for coasts and seas) project.

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