Physical <strong>Climate</strong> Forces 45precipitation <strong>and</strong> increasing frequency of heavy precipitation (IPCC AR4 WGII, 2007;IPCC, 2011). Changes in precipitation regime could influence permafrost temperature,which has generally been increasing in Alaska (Derksen & Brown, 2011; Osterkamp,2007; Smith et al., 2010), thus affecting soil water content (Muskett & Romanovsky, 2011)<strong>and</strong> presumably the stability of permafrost in coastal regions (Buonaiuto et al., 2010).Changes in Storm TracksAccording to the recently released summary of the IPCC report on extreme events(IPCC, 2011: pg. 5), “It is likely that there has been a poleward shift in the main Northern<strong>and</strong> Southern Hemisphere extra-tropical storm tracks.” As a result, some regions will experienceincreased or decreased storm frequency due to the poleward shift of the tracks.According to a recent assessment report on tropical cyclones <strong>and</strong> climate change (Knutsonet al., 2010: pg. 2), there is “low confidence in projected changes in tropical cyclonegenesis location, tracks, duration, <strong>and</strong> areas of impact.” Existing model projections donot show dramatic large-scale changes in these features.DroughtsAn increase in the proportion of l<strong>and</strong> area in drought since the 1970s is reported byBurke et al. (2006). Dai et al. (2004) reported increases in the area in both drought <strong>and</strong>wet areas in the conterminous U.S. Trends are highly variable among regions <strong>and</strong> areattributed to both ENSO-induced decreases in precipitation <strong>and</strong> to warming-induced increasesin evaporation. The changes are consistent with increasing risk of more frequent<strong>and</strong> more intense drought over some regions (Dai et al., 2004). During 1967 to 2006, themean duration of prolonged dry episodes, 1 month or longer in the eastern U.S. <strong>and</strong>2 months or longer in the southwestern U.S., has significantly increased (Groisman &Knight, 2008). Increasing drought in some coastal areas could lead to water shortages<strong>and</strong> soil moisture deficits, called agricultural drought, leading to reduced crop yield,greater risk of wildfire, <strong>and</strong> greater susceptibility to some pests (Hatfield et al., 2008).In some regions drought will likely be compounded by higher rates of evapotranspiration,which could result in increased groundwater withdrawals because of higher waterdem<strong>and</strong>s (Hatfield et al., 2008). Cumulatively, these climatic changes, sea-level rise, <strong>and</strong>human adaptations could result in depletion of coastal aquifers <strong>and</strong> saltwater intrusion(Conrads et al., 2010).Heavy Rainfall <strong>and</strong> Floods<strong>Climate</strong> change has the potential to substantially affect risk of flooding <strong>and</strong> associatedimpacts to human health, infrastructure, <strong>and</strong> agriculture. <strong>Coastal</strong> U.S. cities have farlower populations at risk than cities in Southeast Asia, but New York <strong>and</strong> Miami rankhighly in assets exposed to risks from storm <strong>and</strong> flood damage (Hanson et al., 2011).Floods can cause population- <strong>and</strong> community-level changes in ecosystems superimposedon a background of more gradual trends (Thibault & Brown, 2008). Saltmarshes,mangroves, <strong>and</strong> coral reefs are expected to be particularly vulnerable to impacts of extremeevents associated with major coastal storms (Bertness & Ewanchuk, 2002; Fischlinet al., 2007; Hughes et al., 2003). Heavier rainfall, combined with sea-level rise <strong>and</strong> storm
46 <strong>Coastal</strong> <strong>Impacts</strong>, <strong>Adaptation</strong>, <strong>and</strong> <strong>Vulnerabilities</strong>surge, is expected to substantially increase the frequency of flooding in major metropolitanareas in the U.S. northeast in the 21st century (Kirschen et al., 2008) <strong>and</strong> in California(Moser & Tribbia, 2006).Flooding <strong>and</strong> erosion are significant problems for many Native American villages inAlaska from the combined effects of sea-level rise, loss of protective sea ice (ACIA, 2005;Polyak et al., 2009), major storms, heavy inl<strong>and</strong> rainfall that causes rivers to flood downstream,<strong>and</strong> accelerated melting of snow <strong>and</strong> ice (GAO, 2003, 2009).2.7 Temperature Change <strong>Impacts</strong> with a Focus on AlaskaTemperature TrendsIn Alaska, temperature increases melt permafrost <strong>and</strong> sea ice, which will exacerbate acceleratingcoastal erosion rates, especially on the North <strong>and</strong> West Coast.Temperature increases affect coasts due to sea-level rise associated with ocean thermalexpansion <strong>and</strong> terrestrial ice melt (see section 2.2). In addition to multi-year trends,water temperature variations occur seasonally. Changes can be small <strong>and</strong> difficult todetect (Willis et al., 2008), but they do contribute to overall coastal water level <strong>and</strong>, whencombined with other factors, could contribute to increased water levels. Biological activity(section 3.1) in coastal regions is likely to be adversely affected by temperatureincreases (Vaquer-Sunyer & Duarte, 2011). When combined with other stressors, coastalmarine productivity could become increasingly threatened.Much of the ocean off of the U.S. West Coast is an upwelling zone. The eastern PacificOcean circulation is manifested in this region as the southward-flowing Californiacurrent. The southward movement of an ocean current along a western coastal marginresults in surface waters moving away from the coast, which draws cool water up fromdeeper waters. Altering the strength of the ocean circulation will alter the upwellingregime; this linkage to atmospheric circulation represents the primary mechanism foraltering West Coast ocean temperature. Currently, the response of this circulation regimeto projected climate forcing is not well known. For most West Coast areas, datasuggest no change or a decrease in sea surface temperatures, measured by season. Onlyin southern California are increasing trends observed, <strong>and</strong> only during the strongestseason, autumn, <strong>and</strong> the weaker season, winter (Pardo et al., 2011); however, decreasingcoastal ocean temperatures could result in decreased regional precipitation by reducingevaporation <strong>and</strong>, in the same manner as a temperature increase, could act to stress themarine biota. At the same time, a cooler ocean surface could act to reduce the intensity ofcoastal storms. Finally, cooler waters could also lead to an increase in the occurrence offog, which impacts the transportation sector.The nearshore waters of the U.S. Atlantic <strong>and</strong> Gulf of Mexico coasts are much warmerwaters than the waters of the U.S. West Coast. Like the West Coast, ocean temperaturesalong the U.S. East Coast are largely a function of ocean circulation patterns, in this case,the Gulf Stream. In the Atlantic, a primary response to a change in ocean temperaturesis a change in hurricane formation <strong>and</strong> trajectory. Warmer waters favor hurricane formation,longevity, <strong>and</strong> power, but recent work suggests that atmospheric responses towarm waters of the “Atlantic Warm Pool,” which include the Gulf of Mexico, Caribbean
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ContentsKey TermsAcronymsCommunicat
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ReferencesAbuodha, P. & Woodroffe,
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