IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3to investigate storm surge changes over Europe in four regionallydownscaled GCMs including two runs with B2, one with A2, and onewith an A1B emission scenario. Despite large inter-model differences,statistically significant changes between 1961-1990 and 2071-2100consisted of decreases in the 99th percentile surge heights south ofIceland, and an 8 to 10% increase along the coastlines of the easternNorth Sea and the northwest British Isles, which occurred mainly in thewinter season. Wang et al. (2008) projected a significant increase inwintertime storm surges around Ireland except the south Irish coastover 2031-2060 relative to 1961-1990 using a downscaled GCM underan A1B scenario. Sterl et al. (2009) joined the output from an ensembleof 17 GCM (CMIP3) simulations using the A1B emissions scenario overthe model periods 1950-2000 and 2050-2100 into a single longer timeseries to estimate 10,000-year return values of surge heights along theDutch coastline. No statistically significant change in this value wasprojected for the 21st century because projected wind speed changeswere not associated with the surge-generating northerlies but rathernon-surge generating south-westerlies.Other studies have undertaken a sensitivity approach to compare therelative impact on extreme sea levels of severe weather changes andmean sea level rise. Over southeastern Australia, McInnes et al. (2009b)found that a 10% increase in wind speeds, consistent with the upperend of the range under an A1FI scenario from a multi-model ensemblefor 2070 together with an A1FI sea level rise scenario, would produceextreme sea levels that were 12 to 15% higher than those includingjust the A1FI sea level rise projection alone. Brown et al. (2010) alsoinvestigated the relative impact of sea level rise and wind speed changeon an extreme storm surge in the eastern Irish Sea. Both studiesconcluded that sea level rise rather than meteorological changes hasthe greater potential to increase extreme sea levels in these locations inthe future.The degree to which climate models (GCM or RCM) have sufficientresolution and/or internal physics to realistically capture the meteorologicalforcing responsible for storm surges is regionally dependent. For examplecurrent GCMs are unable to realistically represent tropical cyclones (seeSection 3.4.4). This has led to the use of alternative approaches forinvestigating the impact of climate change on storm surges in tropicallocations whereby large numbers of cyclones are generated usingstatistical models that govern the cyclones’ characteristics over theobserved period (e.g., McInnes et al., 2003). These models are thenperturbed to represent projected future cyclone characteristics and usedto force a hydrodynamic model. Recent studies on the tropical eastcoast of Australia reported in Harper et al. (2009) that employ theseapproaches show a relatively small impact of a 10% increase in tropicalcyclone intensity on the 1-in-100 year storm tide (the combined sea leveldue to the storm surge and tide), and mean sea level rise being foundto produce the larger contribution to changes in future 1-in-100 yearsea level extremes. However, one study that has incorporated scenariosof sea level rise in the hydrodynamic modeling of hurricane-induced sealevel extremes on the Louisiana coast found that increased coastalwater depths had a large impact on surge propagation over land,increasing storm surge heights by two to three times the sea level risescenario, particularly in wetland-fronted areas (J.M. Smith et al., 2010).To summarize, post-AR4 studies provide additional evidence thattrends in extreme coastal high water across the globe reflect theincreases in mean sea level, suggesting that mean sea level riserather than changes in storminess are largely contributing to thisincrease (although data are sparse in many regions and this lowersthe confidence in this assessment). It is therefore considered likelythat sea level rise has led to a change in extreme coastal highwater levels. It is likely that there has been an anthropogenicinfluence on increasing extreme coastal high water levels viamean sea level contributions. While changes in storminessmay contribute to changes in sea level extremes, the limitedgeographical coverage of studies to date and the uncertaintiesassociated with storminess changes overall (Sections 3.4.4 and3.4.5) mean that a general assessment of the effects of storminesschanges on storm surge is not possible at this time. On the basisof studies of observed trends in extreme coastal high waterlevels it is very likely that mean sea level rise will contribute toupward trends in the future.3.5.4. WavesSevere waves threaten the safety of coastal inhabitants and thoseinvolved in maritime activities and can damage and destroy coastaland marine infrastructure. Waves play a significant role in shaping acoastline by transporting energy from remote areas of the ocean to thecoast. Energy dissipation via wave breaking contributes to beach erosion,longshore currents, and elevated coastal sea levels through wave set-upand wave run-up. Wave properties that influence these processesinclude wave height, the wave energy directional spectrum, and period.Studies of past and future changes in wave climate to date have tendedto focus on wave height parameters such as ‘Significant Wave Height’(SWH, the average height from trough to crest of the highest one-thirdof waves) and metrics of extreme waves, such as high percentiles orwave heights above particular thresholds, although one study (Dodet etal., 2010) also examines trends in mean wave direction and peak waveperiod. It should also be noted that waves may become an increasinglyimportant factor along coastlines experiencing a decline in coastalprotection afforded by sea ice (see Sections 3.5.5 and 3.5.7).Wave climates have changed over paleoclimatic time scales. Wavemodeling using paleobathymetries over the past 12,000 years indicatesan increase in peak annual SWH of around 40% due to the increase inmean sea level, which redefines the location of the coastline, and henceprogressively extends the fetch length in most of the shelf sea regions(Neill et al., 2009). Major circulation changes that result in changes instorminess and wind climate (see Section 3.3.3) have also affectedwave climates. Evidence of enhanced storminess determined from sanddrift and dune building along the western European coast indicates thatenhanced storminess occurred over the period of the Little Ice Age180