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

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Vulnerability and Impacts on Natural Resources 573.2 Biota, Habitats, and Coastal Landforms that are Impacted byComplex Stressor Interactions and Subject to Nonlinear Changesand Tipping PointsCoastlines are dynamic systems subject to complex interactions of climate and non-climatestressors. When multiple stressors act in combination on an ecosystem, nonlinearinstabilities may be triggered (Burkett et al., 2005; CCSP, 2009a; Nicholls et al., 2007).Such nonlinear responses can be amplified and accelerated by synergies and positivefeedbacks that increase the alteration of the system and cause a domino-like propagationof potentially-irreversible change. Synergies and feedbacks occur when the impactof one stressor is either strengthened or weakened by variation in another, and the combinedinfluence of multiple stressors can result in nonlinear ecological changes if populationsor ecosystems are pushed beyond a critical threshold or tipping point (Harleyet al., 2006; Lubchenco & Petes, 2010). In some cases, threshold shifts can be caused bygradual climate change if that change is occurring at a rate beyond which the ecosystemcan adapt (McNeall et al., 2011). A threshold is defined as “the point at which there isan abrupt change in an ecosystem quality, property or phenomenon, or where smallchanges in one or more external conditions produce large and persistent responses in anecosystem” (CCSP, 2009b: pg. vii).Multi-stressor interactions and resulting nonlinear changes in ecosystems, which canmanifest over varying spatial and temporal scales, add further complexity (Groffmanet al., 2006). Research on identifying and predicting thresholds in ecosystems is still nascentbut is growing rapidly (Groffman et al., 2006; Scheffer et al., 2009). Both theoreticaland empirically-based examples of thresholds are on the rise and signal an increasingunderstanding of how climate and non-climate stressors interact to propel sudden shiftsin ecosystems.Wetlands: These ecosystems are vulnerable to relative rise in water levels togetherwith projected increase in storm activity in zones of significant human use.Salt marshes have considerable capacity to adjust to sea-level rise under favorable conditionsof hydrology and sediment supply (Cahoon et al., 2009). Wetlands build soilby a combination of trapping fine-grained cohesive mineral sediments carried by tidalwaters and accumulation of plant matter generated onsite. Fine-grained sediment budgetsare often poorly understood (Cahoon el al., 2009). The supply of mineral sedimentscan vary widely from high loads in deltaic and back-barrier systems to minimal inputsin extensive estuarine marshes.Increasing inundation can lead to higher rates of sedimentdeposition (Marion et al., 2009), sediment trapping, and organic matter accretion(Morris et al., 2002). However, the ability of these ecogeomorphic feedbacks to preservesalt marshes under climate change is limited. Based on simulations from five numericalmodels, Kirwan et al. (2010) concluded that coastal marshes would likely survive conservativeprojections of sea-level rise but would be vulnerable under scenarios of rapidsea-level rise linked to ice sheet melting.The regional and sub-regional marshes in North America differ significantly in theirability to build elevation. Northeastern U.S. marshes exhibit high rates of accretion,while southeastern Atlantic, Gulf of Mexico, and Pacific salt marshes exhibit lower rates
58 Coastal Impacts, Adaptation, and Vulnerabilities(Cahoon et al., 2006). Even within a single estuary, tidal marshes can be more or lesssusceptible to sea-level rise depending on local sediment availability (Stralberg et al.,2011). Marshes with low accretion rates are vulnerable to present and future sea-levelrise, with the exception of those in areas where the land surface is rising, such as on thePacific Northwest coast of the U.S.The natural supply of mineral sediments contributing to wetland accretion can besignificantly altered by human activities. The interruption of sediment supply to wetlandscaused by diking along the Mississippi River is the classic example. Organicmatter deposition can be significant in highly productive wetland systems, but decompositionprocesses moderate the relative importance of this source. Chemical componentsin saltwater increase the rate that organic matter decomposes and thus reduce thecontribution of organic sediments to soil-building processes in salt marshes compared tofreshwater systems. Disturbance to vegetation on an otherwise stable marsh could alsolead to submergence beyond depths capable of supporting emergent vegetation, with aconsequent shift to an alternative state such as subtidal open water (Kirwan et al., 2008;Marani et al., 2007). Recent vulnerability assessments carried out for the San FranciscoBay and Massachusetts Bays regions (EPA, 2012a, b) supplement these findings. Localexperts identified the potential for wetlands to drop below a threshold for sustainabilityby mid-century in both locations. In California, this could be caused by the effects of sealevelrise in combination with increased wind-driven waves and sediment starvation. InMassachusetts, wetland loss could result from changes in hydrology and land use alongwith increased nutrient runoff.Mangroves: Mangrove range will expand as minimum temperatures increase.Mangroves in the southern U.S. have been affected by lapses in freeze events and extremedrought events in recent decades. These represent a threshold change in climatepattern that may account for unprecedented mangrove expansion into subtropical saltmarshand freshwater ecosystems along the northern Gulf of Mexico. Mangrove populationshave long persisted along the coasts of Texas, Louisiana, and Florida but areundergoing more recent expansion in latitudes above the tropical Everglades region(Doyle et al., 2010; Michot et al., 2010). An abrupt increase in establishment of blackmangrove, Avicennia germinans, was observed in 2009 based on aerial surveys begunless than a decade earlier, with ground studies showing increased area and density oflocal populations since the last damaging freeze two decades ago (Michot et al., 2010).As the periodicity of freeze events lengthens, mangrove expansion is expected to proceedlandward and poleward along the northern Gulf Coast, changing the proportion ofsaltmarsh area. Recent field and mapping studies in the northern Everglades have documentedupslope migration of mangroves into tidal freshwater wetlands over the lastcentury, concomitant with historical rates of sea-level rise (Doyle et al., 2010; Krauss etal., 2011). Landscape simulation models have been used to reconstruct historical migrationand forecast expansion of mangrove systems in relation to tropical storms and sealevelrise along with decreased freeze events and increased drought frequencies underclimate change (Doyle & Girod 1997; Doyle et al., 2003, 2010).
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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 Summaryxxviiweakened by v
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Executive SummaryxxixAdaptation and
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Introduction and Context 3irrigatio
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ReferencesAbuodha, P. & Woodroffe,
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Coastal Impacts, Adaptation, and Vu
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