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population pressure. Nicholls et al. (1999) estimated that sea-level rise alone could
lead to a loss of almost a quarter of the world's coastal wetlands by 2080; accounting
for added human impacts could increase the losses to 70 percent. However,
accounting for feedbacks among inundation, primary production, and accretion of
organic and inorganic material on marshes suggests that marsh surface elevations
may be able to keep pace with rates of sea-level rise on the low end of future
projections if sufficient sediment is available. Marsh erosion rates on the high end of
projections are likely to eliminate most existing marshes in this century (Kirwan et
al., 2010). At lower latitudes, mangroves and coral reefs offer critical protection from
storm-produced erosion to the coastal areas they fringe, and mangroves face many of
the same threats as salt marshes, and coral reef systems are even more endangered.
Coral reefs are part of the coastal marine ecosystem and are adversely impacted by
nutrients, pollution, and sediment from terrestrial runoff (Hoegh-Guldberg et al.,
2007). Globally, trapping of sediment in reservoirs and channeling of river flows by
levees and other structures has significantly reduced the natural supply of terrestrial
sediment to the coastal zone, resulting in sinking deltas and eroding coastlines
(Syvitski et al., 2009). Barrier island systems, which make up close to 10 percent of
the continental coastlines, are also highly vulnerable to the impacts of climate change
and human disturbance (see Figure 2.22).
The history of human modification of coastal environments extends back at
least several thousand years (e.g., Stanley and Warne, 1993; Weinstein et al., 2007),
including drainage of wetlands, dredging of channels, damming of rivers, mining of
sand, and coastal constructions designed to reduce wave energy and shoreline
erosion. History has shown that these kinds of modifications tend to increase the
vulnerability of coastal environments to catastrophic flooding and storm damage,
such as was witnessed during Hurricane Katrina on the Gulf Coast of the United
States. Despite the susceptibility of coastal systems to climate change, human
activities are likely to be the dominant impact on coastal systems for the foreseeable
future (Weinstein et al., 2007; McNamara and Werner, 2008; Kirwan et al., 2010).
Given the high value of coastal systems, both economic and environmental, it
is imperative that more effective strategies be found for coastal restoration,
stabilization, and adaptation. This requires an investment in fundamental science to
develop a far greater understanding of the interactions and feedbacks among
hydrodynamics, morphodynamics, ecosystem response, mitigation strategies, human
agency, and economic valuation than is presently available. For example, beach
stabilization by sand addition (beach nourishment) may have significant negative
impacts on beach ecosystems, but neither the monitoring nor the understanding of the
underlying physical and biological processes is adequate to evaluate the long-term
risks associated with this practice (Peterson and Bishop, 2005). This lack of
understanding extends to the full range of coastal environments and includes such
fundamental questions as the degree and nature of coastal protection offered by
mangroves, wetlands, reefs, and dunes (Barbier et al., 2008; Valiela and Fox, 2008;
Feagin et al., 2009, 2010; see Figure 2.22). Several recent studies have attempted to
couple models of ecogeomorphological processes with economic models (e.g.,
McNamara and Werner, 2008); to identify strategies for moving toward a more
rational assessment of, for example, the minimum level of landscape stability needed
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