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

Climate impacts on environmental systems1970, but much less evidence of such a trend overthe entire 20th century. When the contribution fromtime mean local sea level changes and variationsin tide are removed from the recent trends, theremaining signals due to changes in storminess aremuch smaller or even no longer detectable.Additional studies are available for some Europeancoastal locations, but typically focus on more limitedspatial scales. A study that examined the trend inwater levels at 18 sites around the English Channelfound that the rates of change in extreme waterlevels were similar to the rates observed for meansea level change (Haigh et al., 2010). However, thestudy also noted sizeable variations in storm surgeheights, with the largest surge intensity occurring inthe late 1950s. This large natural variability makesit difficult to detect changes in the rate of change inwater level extremes. A similar conclusion, that thechange in annual maximum sea levels are increasingat a rate not significantly different from the observedincrease in mean sea level, was found in separateanalyses for Newlyn in the United Kingdom for theperiod 1915–2005 (Araújo and Pugh, 2008) and for73 tide gauges along the Atlantic and Mediterraneancoastlines in southern Europe (Marcos et al., 2011).In contrast, significant increases in storm surgeheight during the 20th century were found alongthe Estonian coast of the Baltic Sea (Suursaar et al.,2009).We conclude that whilst there have been detectablechanges in extreme water levels around theEuropean coastline, most of these are dominatedby changes in time mean local sea level. Thecontribution from changes in storminess is currentlysmall in most European locations and there is littleevidence that any trends can be separated fromlong-period natural variability.ProjectionsFuture projections in storm surges can be madeusing either dynamic or statistical modelling ofstorm surge behaviour driven by the output ofgeneral circulation climate models (Lowe et al.,2010). Several climate modelling studies haveprojected changes in storm surge height andfrequency for the 21st century, mostly using theSRES A1B, A2 or B2 scenarios (see Section 1.5.1).The results critically depend on the simulatedchanges in mid-latitude storms; this topic remainsa highly uncertain and rapidly evolving scientificfield. The limited number of studies that separateout any long-term climate change signal frommulti‐decadal climate variability suggests thatchanges in atmospheric storminess are likely to beless important than increases in mean local sea level.Early studies on future changes in surge magnitudein the North Sea region all identified certain areaswhere increase in surge magnitude were projected,but they did not agree over its magnitude or evenwhich regions will be affected (Lowe et al., 2001;Hulme et al., 2002; Lowe and Gregory, 2005; Wothet al., 2005; Beniston et al., 2007; Debernard andRøed, 2008). Furthermore, most of these studies havenot adequately considered that changes in variousindices of storminess over the European regionexhibit decadal and multi-decadal oscillations (Sterlet al., 2009) (Jenkins et al., 2007).Two recent studies addressed some of thedeficiencies in earlier studies by using ensemblesimulations of climate models to drive a surgemodel of the North Sea for the period 1950–2100.One study found no significant change in the 1 in10 000 year return values of storm surges along theDutch coastline during the 21st century (Sterl et al.,2009). The other study projected small changes instorm surge heights for the 21st century aroundmuch of the UK coastline. Most of these changeswere positive but they were typically much lessthan the expected increase in time mean local sealevel over the same time period (Lowe et al., 2009).However, larger increases in storm surge for thisregion during the 21st century cannot yet be ruledout.A study on the Mediterranean region projected areduction in both the number and frequency ofstorm surge events during the 21st century (Marcoset al., 2011). A study on the Baltic Sea projectedincreases in extreme sea levels over the 21st centurythat were larger than the time mean local sea-levelrise for some future scenarios simulated by someof the climate models used (Meier, 2006). Thelargest changes in storm surge height were in theGulf of Finland, Gulf of Riga and the north-easternBothnian Bay. A study on storm surges aroundthe coast of Ireland projected an increase in surgeevents on the west and east coasts but not along thesouthern coast (Wang et al., 2008). However, not allof the changes were found to have a high statisticalsignificance.At some locations, such as Hamburg, local changesin bathymetry caused by erosion, sedimentation andwaterworks can have a much larger impact thanclimate change (von Storch and Woth, 2008). Finally,recent work has shown that sea-level rise may alsochange extreme water levels by altering the tidalrange (Pickering et al., 2012).Climate change, impacts and vulnerability in Europe 2012109