6.4 Key future impacts and vulnerabilities
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
126.96.36.199 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 heights have enhanced erosion rates of bay shorelines, tidal creeks and adjacent wetlands (Stone and McBride, 1998; Stone et al., 2003). The impacts of accelerated sea-level rise on gravel beaches have received less attention than sandy beaches. These systems are threatened by sea-level rise (Orford et al., 2001, 2003; Chadwick et al., 2005), even under high accretion rates (Codignotto et al., 2001). The persistence of gravel and cobble-boulder beaches will also be influenced by storms, tectonic events and other factors that build and reshape these highly dynamic shorelines (Orford et al., 2001).
Since the TAR, monitoring, modelling and process-oriented research have revealed some important differences in cliff vulnerability and the mechanics by which groundwater, wave climate and other climate factors influence cliff erosion patterns and rates. Hard rock cliffs have a relatively high resistance to erosion, while cliffs formed in softer lithologies are likely to retreat more rapidly in the future due to increased toe erosion resulting from sea-level rise (Cooper and Jay, 2002). Cliff failure and retreat may be amplified in many areas by increased precipitation and higher groundwater levels: examples include UK, Argentina and France (Hosking and McInnes, 2002; Codignotto, 2004b; Pierre and Lahousse, 2006). Relationships between cliff retreat, freeze-thaw cycles and air temperature records have also been described (Hutchinson, 1998). Hence, four physical features of climate change – temperature, precipitation, sea level and wave climate – can affect the stability of soft rock cliffs.
Soft rock cliff retreat is usually episodic with many metres of cliff top retreat occurring locally in a single event, followed by relative quiescence for significant periods (Brunsden, 2001; Eurosion, 2004). Considerable progress has been made in the long-term prediction of cliff-top, shore profile and plan-shape evolution of soft rock coastlines by simulating the relevant physical processes and their interactions (Hall et al., 2002; Trenhaile, 2002, 2004). An application of the SCAPE (Soft Cliff and Platform Erosion) model (Dickson et al., 2005; Walkden and Hall, 2005) to part of Norfolk, UK has indicated that rates of cliff retreat are sensitive to sea-level rise, changes in wave conditions and sediment supply via longshore transport. For soft cliff areas with limited beach development, there appears to be a simple relationship between long-term cliff retreat and the rate of sea-level rise (Walkden and Dickson, 2006), allowing useful predictions for planning purposes.