Coastal processes and zones

Many coastal regions are already experiencing the effects of relative (local) sea-level rise, from a combination of climate-induced sea-level rise, geological and anthropogenic-induced land subsidence, and other local factors. A major challenge, however, is to separate the different meteorological, oceanographic, geophysical and anthropogenic processes affecting the shoreline in order to identify and isolate the contribution of global warming. An unambiguous attribution of current sea-level rise as a primary driver of shoreline change is difficult to determine at present.

Global sea level has been rising at a rate of about 1.7 to 1.8 mm/yr over the last century, with an increased rate of about 3 mm/yr during the last decade (Church et al., 2004; Holgate and Woodworth, 2004; Church and White, 2006; Bindoff et al., 2007, Section 5.5).

1.33.1 Changes in coastal geomorphology

Sea-level rise over the last 100 to 150 years is probably contributing to coastal erosion in many places, such as the East Coast of the USA, where 75% of the shoreline removed from the influence of spits, tidal inlets and engineering structures is eroding (Leatherman et al., 2000; Daniel, 2001; Zhang et al., 2004) (Table 1.4; see Table SM1.4 for observations of changes in storm surges, flood height and areas, and waves). Over the last century, 67% of the eastern coastline of the UK has retreated landward of the low-water mark (Taylor et al., 2004).

In addition to sea-level change, coastal erosion is driven by other natural factors such as wave energy, sediment supply, or local land subsidence (Stive, 2004). In Louisiana, land subsidence has led to high average rates of shoreline retreat (averaging 0.61 m/yr between 1855 and 2002, and increasing to 0.94 m/yr since 1988) (Penland et al., 2005); further erosion occurred after Hurricanes Katrina and Rita in August 2005. These two hurricanes washed away an estimated 562 km2 of coastal wetlands in Louisiana (USGS, 2006). Climate variability also affects shoreline processes, as documented by shoreline displacement in Estonia associated with increasing severe storms and high surge levels, milder winters, and reduced sea-ice cover (Orviku et al., 2003). Significant sections of glacially rebounding coastlines, which normally would be accreting, are nonetheless eroding, as for example along Hudson Bay, Canada (Beaulieu and Allard, 2003). Reduction in sea-ice cover due to milder winters has also exacerbated coastal erosion, as in the Gulf of St. Lawrence (Bernatchez and Dubois, 2004; Forbes et al., 2004). Degradation and melting of permafrost due to climate warming are also contributing to the rapid retreat of Arctic coastlines in many regions, such as the Beaufort and Laptev Sea coasts (Forbes, 2005).

Anthropogenic activities have intensified beach erosion in many parts of the world, including Fiji, Trinidad and parts of tropical Asia (Mimura and Nunn, 1998; Restrepo et al., 2002; Singh and Fouladi, 2003; Wong, 2003). Much of the observed erosion is associated with shoreline development, clearing of mangroves (Thampanya et al., 2006) and mining of beach sand and coral. Sediment starvation due to the construction of large dams upstream also contributes to coastal erosion (Frihy et al., 1996; Chen et al., 2005b; Georgiou et al., 2005; Penland et al., 2005; Syvitski et al., 2005b; Ericson et al., 2006). Pumping of groundwater and subsurface hydrocarbons also enhances land subsidence, thereby exacerbating coastal erosion (Syvitski et al., 2005a).

13.3.2 Changes in coastal wetlands

In the USA, losses in coastal wetlands have been observed in Louisiana (Boesch et al., 1994), the mid-Atlantic region (Kearney et al., 2002), and in parts of New England and New York (Hartig et al., 2002; Hartig and Gornitz, 2004), in spite of recent protective environmental regulations (Kennish, 2001). Many of these marshes have had a long history of anthropogenic modification, including dredging and filling, bulkheading and channelisation, which in turn could have contributed to sediment starvation, eutrophication and ultimately marsh submergence (Donnelly and Bertness, 2001; Bertness et al., 2002). In Europe, losses have been documented in south-east England between 1973 and 1998, although the rate of loss has slowed since 1988 (Cooper et al., 2001); elsewhere there is evidence that not all coastal wetlands are retreating, for example in Normandy, France (Haslett et al., 2003).

Although natural accretion rates of mangroves generally compensate for current rates of sea-level rise, of greater concern at present are the impacts of clearance for agriculture, aquaculture (particularly shrimp), forestry and urbanisation. At least 35% of the world's mangrove forests have been removed in the last two decades but possible sea-level rise effects were not considered (Valiela et al., 2001). In south-eastern Australia, mangrove encroachment inland into salt-marsh environments is probably related to anthropogenic causes and climate variability, rather than sea-level rise (Saintilan and Williams, 1999). Landward replacement of grassy freshwater marshes by more salt-tolerant mangroves in the south-eastern Florida Everglades since the 1940s has been attributed to the combined effects of sea-level rise and water management, resulting in lowered watertables (Ross et al., 2000).

Sea-level rise can have a larger impact on wetland ecosystems when the human land-use pressure in the coastal area is large, e.g., coasts defended by dykes and urbanisation. Wetlands disappear or become smaller when human land use makes inward movement of the ecosystem impossible (Wolters et al., 2005).

13.3.3 Changes in storm surges, flood heights and areas, and waves

The vulnerability of the coastal zone to storm surges and waves depends on land subsidence, changes in storminess, and sea-level rise (see Supplementary Material). Along the North American East Coast, although there has been no significant long-term change in storm climatology, storm-surge impacts have increased due to regional sea-level rise (Zhang et al., 2000). The U.S. Gulf Coast is particularly vulnerable to hurricane surges due to low elevation and relative sea-level rise (up to

Table 1.4. Changes in coastal processes.

Type of change

Observed changes

Period

Location

References

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