Coastal Impacts, Adaptation, and Vulnerabilities - Climate ...

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Physical Climate Forces 49by sea ice (Eicken et al., 2005). Such ice-mediated export of nearshore sediments alsoplays a role in enhancing the impacts of wave action that drives erosion and thawing ofcoastal permafrost, amplified by recent changes in the length of the open water season(Overeem et al., 2011). At the same time, formation of nearshore ice well in advance ofthe appearance of the ice pack can provide protection to the coastline through the formationof ice berms in the surf zone (Atkinson, 2011). The complicated interplay betweenthese different factors that serve to amplify or dampen the effects of climate change onice action in the nearshore zone is at present not well understood. For the entire Arcticregion, sea-ice area and thickness are decreasing, with summer ice-extent reductions onthe order of 12 percent per decade for the past three decades (Stroeve et al., 2011). Atpresent, projections from climate models suggest that the Arctic Ocean may be ice-freeduring summer by about 2040 (Wang & Overland, 2009). However, even a substantiallyreduced summer ice cover will have major implications for all the processes discussedabove.An important feature of northern ecosystems is their capacity to store large quantitiesof methane and carbon dioxide in the frozen ground of the tundra, boreal forest,and near-coastal regions. Methane emission in the Arctic is dependent, in part, on thechanges in area of wetland and lakes in the North Slope of Alaska during the snow-freeseason. Ground thaw due to climate warming will increase the formation of thaw-inducedwetlands, enhancing release of greenhouse gases, which will exacerbate existingglobal warming trends. Current Arctic contributions to global methane emissions aresmall, however recent increasing trends have been noted (Bloom et al., 2010). Methaneemission had been thought to be a warm-season phenomenon in the Arctic because atmosphericmethane is unusually oxidized in the soil during winter season; however, recentfield measurements indicate that tussock tundra, which covers more than 6 millionsquare kilometres of the Northern Hemisphere, is an unexpected winter methane sourcein tundra and boreal forest ecozones (Kim et al., 2007). This indicates winter methaneemission should not be overlooked in estimation of regional or global methane budgets.Finally, large reserves of methane are found in nearshore permafrost (Shakhova et al.,2008), which is susceptible to disturbance if coastal temperatures warm; Siberian fielddata suggests strong methane release in the near-coastal marine region (Shakhova andSemiletov, 2007).Impact of Climate Change on Coastal ProcessesCoastal processes and erosion in Arctic settings differ from coastal processes in sub-arcticsettings in that both thermal and mechanical processes are important. In the Arctic,coastal sediments are normally locked in place by ice, and thawing of the ice is necessarybefore mechanical processes such as waves, currents, and wind can transport the sediments.As well, shore fast, bottom fast, and sea ice often protect the coastal zone fromwave action. For example, coastal villages on the Chukchi Sea such as Shishmarev andKivalina were historically protected from the brunt of large late-fall storms due to thepresence of nearshore and sea ice, but these villages have been subject in the last decadeto the full brunt of these storm waves and surge flooding due to diminished sea-ice coveragein the late fall.Two process sequences are responsible for much of the coastal erosion in Arctic
50 Coastal Impacts, Adaptation, and VulnerabilitiesAlaska: (1) niche erosion followed by block collapse and (2) thaw slumping. The term“niche erosion” refers to the cutting of a niche at the base of the coastal bluff. Niche erosion/blockcollapse is a four-process sequence (Figure 2-12).Figure 2-12 Conceptual model of the processesresponsible for niche-erosion/block-collapse inArctic Alaska. Source: Ravens et al., 2012.Storms raise water levels and allow the sea to directly contact the base of the bluff.Waves and currents thermally and mechanically erode a niche at the base of the bluff.The niche grows until the overburden exceeds the bluff strength and block collapse results.The fallen block is then eroded thermally and mechanically by waves and currents.Niche erosion/block collapse is the dominant erosion mechanism in locationsdominated by coastal bluffs. Beach survey measurements (Jones et al., 2009a, b; Mars &Houseknecht, 2007) and modeling (Ravens et al., 2012) have documented the rapid andaccelerating coastal retreat in locations where this erosion sequence is dominant. Ravenset al. (2012) indicate that the main driver in the acceleration of erosion is increasingnearshore water temperature, which weakens the permafrost sediments. Recent observationshave shown significant erosion even in the absence of storms.Thaw slumping is predominant in coastal bluffs with significant coarse material. Theterm “thaw slumping” refers to the slumping or sloughing of the bluff face followingthaw due to radiation or convective heating. Bluff thaw slumping is the predominanterosion mechanism in locations with significant coarse material such as Barter Island inthe Beaufort Sea. Bluff erosion in these areas leaves a significant lag deposit that heightensthe beach before the bluff and reduces the frequency of niche erosion.Three main aspects of climate change in Arctic Alaska directly affect coastal erosion:1) increased temporal and spatial extent of open water and reduced sea ice concentrations,2) increased water temperatures, and 3) increased air temperatures. The niche erosion/blockcollapse sequence will accelerate due to factors 1, 2, and 3. Increased watertemperature, increased wave height due to more open water, and increased temporalextent of open water (factors 1 and 2) will intensify and prolong the niche erosion process.Also, these process changes along with increasing air temperatures (factor 3) willaccelerate the erosion of fallen blocks. Hence, continued increasing erosion rates due to
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National Climate Assessment Regiona
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© 2012 The National Oceanic and At
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Coastal Impacts, Adaptation,and Vul
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Chapter 3Lead Author: Carlton H. He
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ContentsKey TermsAcronymsCommunicat
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4.4 Human Health Impacts and Implic
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Key TermsxixExposure 3 - The nature
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AcronymsxxiNAO - North Atlantic Osc
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Executive Summaryxxvffffffffffffffo
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Executive Summaryxxviiweakened by v
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ReferencesAbuodha, P. & Woodroffe,
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