IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical Environmentor sea ice in mid- and high latitudes, which exposes soft shores tothe effects of waves and severe storms (see Section 3.5.7; Mansonand Solomon, 2007).For example, permafrost degradation and sea ice retreat may contributeto coastal erosion in Arctic regions (see Section 3.5.7).The susceptibility of coastal regions to erosion and inundation is relatedto various physical (e.g., shoreline slope), and geomorphological andecosystem attributes, and therefore may be inferred to some extentfrom broad coastal characterizations. These include the presence ofbeaches, rocky shorelines, or coasts with cliffs; deltas; back-barrierenvironments such as estuaries and lagoons; the presence of mangroves,salt marshes, or sea grasses; and shorelines flanked by coral reefs (e.g.,Nicholls et al., 2007) or by permafrost or seasonal sea ice, each of whichare characterized by different vulnerability to climate change-drivenhazards. For example, deltas are low-lying and hence generally prone toinundation, while beaches are comprised of loose particles and thereforeerodible. However, the degree to which these systems are impacted byerosion and inundation will also be influenced by other factors affectingdisaster responses. For example, reduced protection from high wavesduring severe storms could occur as a result of depleted mangroveforests or the degradation of coral reefs (e.g., Gravelle and Mimura,2008), or loss of sea ice or permafrost (e.g., Manson and Solomon, 2007);there may be a loss of ecosystem services brought about by saltwatercontamination of already limited freshwater reserves due to rising sealevels and these will amplify the risks brought about by climate change(McGranahan et al., 2007), and also reduce the resilience of coastalsettlements to disasters. Dynamical processes such as vertical landmovement also contribute to inundation potential (Haigh et al., 2009).Coastal regions may be rising or falling due to post-glacial rebound orslumping due to aquifer drawdown (Syvitski et al., 2009). Multiplecontributions to coastal flooding such as heavy rainfall and flooding incoastal catchments that coincide with elevated sea levels may also beimportant. Ecosystems such as coral reefs also play an important role inproviding material on which atolls are formed. Large-scale oceanicchanges that are particularly relevant to both coral reefs and smallisland countries are discussed in Box 3-4.As discussed in Section 3.5.3, mean sea level has risen by 120 to 130 msince the end of the last glacial maximum (Jansen et al., 2007), and thishas had a profound effect on coastline position around the world.Coastlines have also evolved over this time frame due to changes in theaction of the ocean on the coast through changes in wave climate(Neill et al., 2009) and tides (Gehrels et al., 1995), which arise from thechanging geometries of coastlines over glacial time scales and changesin storminess (e.g., Clarke and Rendell, 2009).The AR4 (Nicholls et al., 2007) reported that coasts are experiencing theadverse consequences of impacts such as increased coastal inundation,erosion, and ecosystem losses. However, attributing these changes tosea level rise is difficult due to the multiple drivers of change over the20th century (R.J. Nicholls, 2010) and the scarcity and fragmentarynature of data sets that contribute to the problem of identifying andattributing changes (e.g., Defeo et al., 2009). Since the AR4 there havebeen several new studies that examine coastline changes. In theCaribbean, the beach profiles at 200 sites across 113 beaches and eightislands were monitored on a three-monthly basis from 1985 to 2000,with most beaches found to be eroding and faster rates of erosiongenerally found on islands that had been impacted by a higher numberof hurricanes (Cambers, 2009). However, the relative importance ofanthropogenic factors, climate variability, and climate change on theeroding trends could not be separated quantitatively. In Australia,Church et al. (2008) report that despite the positive trend in sea levelsduring the 20th century, beaches have generally been free of chroniccoastal erosion, and where it has been observed it has not been possibleto unambiguously attribute it to sea level rise in the presence of otheranthropogenic activities. Webb and Kench (2010) argue that thecommonly held view of atoll nations being vulnerable to erosion mustbe reconsidered in the context of physical adjustments to the entireisland shoreline, because erosion of some sectors may be balanced byprogradation on other sectors. In their survey of 27 atoll islands acrossthree central Pacific Nations (Tuvalu, Kiribati, and Federated States ofMicronesia) over a 19- to 61-year period using photography andsatellite imagery, they found that 43% of islands remained stable and43% increased in area, with largest rates of increase in island arearanging from 0.1 to 5.6 ha per decade. Only 14% of islands studiedexhibited a net reduction in area. On islands exhibiting either no netchange or an increase in area, a larger redistribution of land area wasevident in 65% of cases, consisting of mainly a shoreline recession onthe ocean side and an elongation of the island or progradation of theshoreline on the lagoon side. Human settlements were present on 7 ofthe 27 atolls surveyed and the majority of those exhibited net accretiondue in part to coastal protection works. For a coral reef island at thenorthern end of the Great Barrier Reef, Australia, Dawson and Smithers(2010) report a 6% increase in area and 4% increase in volume between1967 and 2007 but with a net retreat on the east-southeast shorelineand advance on the western side. Chust et al. (2009) evaluated therelative contribution of local anthropogenic (non-climate change related)and sea level rise impacts on the coastal morphology and habitats inthe Basque coast, northern Spain, for the period 1954 to 2004. Theyfound that the impact from local anthropogenic influences was aboutan order of magnitude greater than that due to sea level rise over thisperiod. Increased rates of coastal erosion have also been observed since1935 in Canada’s Gulf of St. Lawrence (Forbes et al., 2004).The AR4 stated with very high confidence that the impact of climatechange on coasts is exacerbated by increased pressures on the physicalenvironment arising from human settlements in the coastal zone (Nichollset al., 2007). The small number of studies that have been completedsince the AR4 have been either unable to attribute coastline changes tospecific causes in a quantitative way or else find strong evidence fornon-climatic causes that are natural and/or anthropogenic.The AR4 reported with very high confidence that coasts will be exposedto increasing impacts, including coastal erosion, over coming decadesdue to climate change and sea level rise, both of which will be183