The following sections characterise the coastal ecosystem impacts that are anticipated to result from the climate change summarised in Figures 6.1 and Table 6.2. The summary of impacts on natural coastal systems and implications for human society (including ecosystem services) leads to the recognition of key vulnerabilities and hotspots.
6.4.1 Natural system responses to climate change drivers
188.8.131.52 Beaches, rocky shorelines and cliffed coasts
Most of the world's sandy shorelines retreated during the past century (Bird, 1985; NRC, 1990; Leatherman, 2001; Eurosion, 2004) and sea-level rise is one underlying cause (see Section 6.2.5 and Chapter 1, Section 1.3.3). One half or more of the Mississippi and Texas shorelines have eroded at average rates of 3.1 to 2.6 m/yr since the 1970s, while 90% of the Louisiana shoreline eroded at a rate of 12.0 m/yr (Morton et al., 2004). In Nigeria, retreat rates up to 30 m/yr are reported (Okude and Ademiluyi, 2006). Coastal squeeze and steepening are also widespread as illustrated along the eastern coast of the United Kingdom where 67% of the coastline experienced a landward retreat of the low-water mark over the past century (Taylor et al., 2004).
An acceleration in sea-level rise will widely exacerbate beach erosion around the globe (Brown and McLachlan, 2002), although the local response will depend on the total sediment budget (Stive et al., 2002; Cowell et al., 2003a,b). The widely cited Bruun (1962) model suggests that shoreline recession is in the range 50 to 200 times the rise in relative sea level. While supported by field data in ideal circumstances (Zhang et al., 2004), wider application of the Bruun model remains controversial (Komar, 1998; Cooper and Pilkey, 2004; Davidson-Arnott, 2005). An indirect, less-frequently examined influence of sea-level rise on the beach sediment budget is due to the infilling of coastal embayments. As sea-level rises, estuaries and lagoons attempt to maintain equilibrium by raising their bed elevation in tandem, and hence potentially act as a major sink of sand which is often derived from the open coast (van Goor et al., 2001; van Goor et al., 2003; Stive, 2004). This process can potentially cause erosion an order of magnitude or more greater than that predicted by the Bruun model (Woodworth et al., 2004), implying the potential for major coastal instability due to sea-level rise in the vicinity of tidal inlets. Several recent studies indicate that beach protection strategies and changes in the behaviour or frequency of storms can be more important than the projected acceleration of sea-level rise in determining future beach erosion rates (Ahrendt, 2001; Leont'yev, 2003). Thus there is not a simple relationship between sea-level rise and horizontal movement of the shoreline, and sediment budget approaches are most useful to assess beach response to climate change (Cowell et al., 2006).
The combined effects of beach erosion and storms can lead to the erosion or inundation of other coastal systems. For example, an increase in wave heights in coastal bays is a secondary effect of sandy barrier island erosion in Louisiana, and increased wave
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