Climate change, impacts and vulnerability in Europe ... - MemoFin.fr

Climate impacts on environmental systems3.2.3 Storm surgesRelevanceA storm surge is a temporary deviation in sea waterlevel from that of the astronomical tide caused bychanges in air pressure and winds. Most concern iscentred on positive surge events where the surgeadds to the tidal level and increases the risk ofcoastal flooding by extreme water levels. Changes inthe climatology of extreme water levels may resultfrom changes in time mean local sea level (i.e. thelocal sea level relative to land averaged over a year),changes in storm surge characteristics, or changes intides. Here the focus will be on changes in the stormsurge characteristics, which are closely linked tochanges in the characteristics of atmospheric storms,including the frequency, track and intensity of thestorms. The height of surges is also strongly affectedby regional and local-scale geographical features,such as the shape of the coastline. Typically, thehighest water levels are found on the rising limb ofthe tide (Horsburgh and Wilson, 2007). The biggestsurge events typically occur during the wintermonths in Europe.The most obvious impact of extreme sea levels isflooding (Horsburgh et al., 2010). The most wellknown coastal flooding event in Europe in livingmemory occurred in 1953 due to a combination of asevere storm surge and a high spring tide. The eventcaused in excess of 2 000 deaths in Belgium, theNetherlands and the United Kingdom, and damagedor destroyed more than 40 000 buildings. Currentlyaround 200 million people live in the coastal zone inEurope, and insurable losses due to coastal floodingare likely to rise during the 21st century, at leastfor the North Sea region (Gaslikova et al., 2011). Inaddition to the direct impact of flooding, increasesin the frequency of storm surges can also exacerbateother coastal problems, such as erosion, salt waterintrusion, migration or river flooding.Past trendsProducing a clear picture of either past changesor future projections of storm surges for the entireEuropean coast line is a challenging task becauseof the impact of local topographical features on thesurge events. Whilst there are numerous studiesfor the North Sea coastline, fewer are available forthe Mediterranean and Baltic Seas, although thissituation is starting to improve. The uncertaintyin future projections of storm surges remains highand is ultimately linked to the uncertainty in futuremid‐latitude storminess changes (see Section 2.2).This is an area where current scientificunderstanding is advancing quickly, with someof the latest climate models simulating significantdifferences in mid-latitude storm development,evolution and movement (Scaife et al., 2011)compared to the generation of climate models usedin current studies of future storm surges.The most comprehensive global studies of trendsin extreme coastal sea level and storm surgesexamined trends from hourly tide gauge recordsat least for the period since 1970, and for earlierperiods of the 20th century for some locations(Woodworth and Blackman, 2004; Menéndez andWoodworth, 2010). The results show that changesin extreme water levels tend to be dominated bythe change in the time mean local sea level. In thenorth‐west European region there is clear evidenceof widespread increase in sea level extremes sinceKey messages: 3.2.3 Storm surges• Several large storm surge events have caused loss of life and damage to property in Europe during thepast century. The most notable event occurred in 1953 when more than 2 000 people were killed, andthere was massive damage to property around the coastline of the southern North Sea.• There is strong evidence that extreme coastal water levels have increased at many locations around theEuropean coastline. However, this appears to be predominantly due to increases in time mean local sealevel at most locations rather than to changes in storm activity.• Large natural variability in extreme coastal sea levels makes detecting long-term changes in trendsdifficult in the absence of good quality long observational records.• Multi-decadal projections of changes in storms and storm surges for the European region currentlyhave high uncertainty. The most recent studies indicate that increases in extreme coastal waterlevels will likely be dominated by increases in local relative mean sea level, with changes in themeteorologically‐driven surge component being less important at most locations.108 Climate change, impacts and vulnerability in Europe 2012