IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3increases in wave height were projected for most mid-latitude areasanalyzed, including the North Atlantic, North Pacific, and Southern Ocean(Christensen et al., 2007) but with low confidence due to low confidencein projected changes in mid-latitude storm tracks and intensities (seeSection 3.4.5). Several studies since then have developed wave climateprojections that provide stronger evidence for future wave climatechange. Global-scale projections of SWH were developed by Mori et al.(2010), using a 1.25° resolution wave model forced with projected windsfrom a 20-km global GCM, in which ensemble-averaged SST changesfrom the CMIP3 models provided the climate forcing. The spatial patternof projected SWH change between 1979-2004 and 2075-2100 reflectsthe changes in the forcing winds, which are generally similar to the meanwind speed changes shown in Figure 3-8. Extreme waves (measured bya spatial and temporal average of the top 10 values over the 25-yearperiod) were projected to exhibit large increases in the northern Pacific,particularly close to Japan due to an increase in strong tropical cyclonesand also the Indian Ocean despite decreases in SWH.A number of regional studies have also been completed since the AR4in which forcing conditions were obtained for a few selected emissionscenarios (typically B2 and A2, representing low-high ranges) from GCMsor RCMs. These studies provide additional evidence for positive projectedtrends in SWH and extreme waves along the western European coast(e.g., Debernard and Roed, 2008; Grabemann and Weisse, 2008) and theUK coast (Leake et al., 2007), declines in extreme wave height in theMediterranean sea (Lionello et al., 2008) and the southeast coast ofAustralia (Hemer et al., 2010), and little change along the Portuguesecoast (Andrade et al., 2007). However, considerable variation in projectionscan arise from the different climate models and scenarios used to forcewave models, which lowers the confidence in the projections. For example,along the European North Sea coast, 99th-percentile wave height overthe late 21st century relative to the late 20th century is projected toincrease by 6 to 8% by Debernard and Roed (2008) based on wavemodel simulations with forcing from several GCMs under A2, B2, andA1B greenhouse gas scenarios, whereas they are projected to increaseby up to 18% by Grabemann and Weisse (2008), who downscaled twoGCMs under A2 and B2 emission scenarios. In one region, oppositetrends in extreme waves were projected. Grabemann and Weisse (2008)project negative trends in 99th-percentile wave height along the UKNorth Sea coast, whereas Leake et al. (2007) downscaled the sameGCM for the same emission scenarios, using a different RCM, and foundpositive changes in high percentile wave heights offshore of the EastAnglia coastline. A wave projection study by Hemer et al. (2010)concluded that uncertainties arising from the method by which climatemodel winds were applied to wave model simulations (e.g., by applyingbias-correction to winds or perturbing current climate winds with windchanges derived from climate models) made a larger contribution to thespread of RCM projections than the forcing from different GCMs oremission scenarios.In summary, although post-AR4 studies are few and their regionalcoverage is limited, their findings generally support the evidencefrom earlier studies of wave climate trends. Most studies find alink between variations in waves (both SWH and extremes) andinternal climate variability. There is low confidence that therehas been an anthropogenic influence on extreme wave heights(because of insufficient literature). Despite the existence ofdownscaling studies for some regions such as the eastern NorthSea, there is overall low confidence in wave height projectionsbecause of the small number of studies, the lack of consistencyof the wind projections between models, and limitations in theirability to simulate extreme winds. However, the strong linkagesbetween wave height and winds and storminess means that it islikely that future negative or positive changes in SWH will reflectfuture changes in these parameters.3.5.5. Coastal ImpactsSevere coastal hazards such as erosion and inundation are important inthe context of disaster risk management and may be affected by climatechange through rising sea levels and changes in extreme events.Increasing sea levels will also increase the potential for saltwater intrusioninto coastal aquifers. Coastal inundation occurs during periods of extremesea levels due to storm surges and high waves, particularly whencombined with high tides. Although tropical and extratropical cyclones(Sections 3.4.4 and 3.4.5) are the most common causes of sea levelextremes, other weather events that cause persistent winds such asanticyclones and fronts can also influence coastal sea levels (Green etal., 2009; McInnes et al., 2009b). In many parts of the world, sea levelsare influenced by modes of large scale variability such as ENSO (Section3.4.2). In the western equatorial Pacific, sea levels can fluctuate up to halfa meter between ENSO phases (Church et al., 2006b) and in combinationwith extremes of the tidal cycle, can cause extensive inundation in lowlyingatoll nations even in the absence of extreme weather events(Lowe et al., 2010).Shoreline position can change from the combined effects of variousfactors such as:1) Rising mean sea levels, which cause landward recession of coastlinesmade up of erodible materials (e.g., Ranasinghe and Stive, 2009)2) Changes in coastal height due to isostatic rebound (Blewitt et al.,2010; Mitrovica et al., 2010), or sediment compaction from theremoval of oil, gas, and water (Syvitski et al., 2009)3) Changes in the frequency or severity of transient storm erosionevents (K.Q. Zhang et al., 2004)4) Changes in sediment supply to the coast (Stive et al., 2003;Nicholls et al., 2007; Tamura et al., 2010)5) Changes in wave speed due to sea level rise, which alters waverefraction, or in wave direction, which can cause realignment ofshorelines (Ranasinghe et al., 2004; Bryan et al., 2008; Tamura etal., 2010)6) The loss of natural protective structures such as coral reefs (e.g.,Sheppard et al., 2005; Gravelle and Mimura, 2008) due toincreased ocean temperatures (Hoegh-Guldberg, 1999) and oceanacidification (Bongaerts et al., 2010) or the reduction in permafrost182