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Extreme sea levels have increased at many locations along European coastlines, due to increases in mean local sea levels. These ongoing increases will amplify the frequency of 1-in-100 years historical extreme events, by exposing most locations to critical conditions already with a sea level rise value above 10cm. In the absence of better coastal protection, the projected sea level rise would increase the frequency of extreme high coastal water levels by a factor of 10 in most European coastlines prior to 2050, with differences depending on the location and the future climate scenario.
Figure 1. Requested sea level rise for an amplification factor of 10 and 100 times
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 10cm rise in sea level typically increases the frequency of flooding to a given height by a factor of approximately 10. 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 local flood protection standards. Changes in mean local sea levels are the main driver of observed and projected changes in extreme sea levels along the coastline. The melting of glaciers and icesheets along with ocean thermal expansion being the most relevant processes impacting Europe’s seas.
The historically 1-in-100-year coastal floods are projected to increase by a factor of 10 before the year 2050 in many locations along the Mediterranean and Atlantic coasts, and along almost all remaining European coasts by the end of the century, under a low emissions scenario (CMIP6: Coupled Model Intercomparison Project Phase 6, SSP: Shared Socioeconomic Pathways). Under a high emissions scenario, 1-in-100-year coastal floods are expected to occur at least once a year along most European coasts before the year 2050, with some exception due to the already available flood protection measures.
Figure 2. Timing of occurrence of a 10 times amplification of the 1-in-100 years historical extreme in CMIP6 future projections
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 the output of the ECLISEA (European advances on climate services for coastal services) project.