6.2.4 Thresholds in the behaviour of coastal systems
Dynamic coastal systems often show complex, non-linear morphological responses to change (Dronkers, 2005). Erosion, transport and deposition of sediment often involve significant time-lags (Brunsden, 2001), and the morphological evolution of sedimentary coasts is the outcome of counteracting transport processes of sediment supply versus removal. A shoreline may adopt an equilibrium, in profile or plan form, where these processes are in balance. However, external factors, such as storms, often induce morphodynamic change away from an equilibrium state. Climate change and sea-level rise affect sediment transport in complex ways and abrupt, non-linear changes may occur as thresholds are crossed (Alley et al., 2003). If sea level rises slowly, the balance between sediment supply and morphological adjustment can be maintained if a saltmarsh accretes, or a lagoon infills, at the same rate. An acceleration in the rate of sea-level rise may mean that morphology cannot keep up, particularly where the supply of sediment is limited, as for example when coastal floodplains are inundated after natural levees or artificial embankments are overtopped. Exceeding the critical sea-level thresholds can initiate an irreversible process of drowning, and other geomorphological and ecological responses follow abrupt changes of inundation and salinity (Williams et al., 1999; Doyle et al., 2003; Burkett et al., 2005). Widespread submergence is expected in the case of the coast of the Wadden Sea if the rate of relative sea-level rise exceeds 10 mm/yr (van Goor et al., 2003). For each coastal system the critical threshold will have a specific value, depending on hydrodynamic and sedimentary characteristics. Abrupt and persistent flooding occurs in coastal Argentina when landward winds (sudestadas) and/or heavy rainfall coincide with storm surges (Canziani and Gimenez, 2002; Codignotto, 2004a), further emphasising non-linearities between several interacting factors. Better understanding of thresholds in, and non-linear behaviour of, coastal systems will enhance the ability of managers and engineers to plan more effective coastal protection strategies, including the placement of coastal buildings, infrastructure and defences.
6.2.5 Observed effects of climate change on coastal systems
Trenberth et al. (2007) and Bindoff et al. (2007) observed a number of important climate change-related effects relevant to coastal zones. Rising CO2 concentrations have lowered ocean surface pH by 0.1 unit since 1750, although to date no significant impacts on coastal ecosystems have been identified. Recent trend analyses indicate that tropical cyclones have increased in intensity (see Section 6.3.2). Global sea levels rose at 1.7 ± 0.5 mm/yr through the 20th century, while global mean sea surface temperatures have risen about 0.6°C since 1950, with associated atmospheric warming in coastal areas (Bindoff et al., 2007).
Many coasts are experiencing erosion and ecosystem losses (Sections 6.2.1 and 6.4.1), but few studies have unambiguously quantified the relationships between observed coastal land loss and the rate of sea-level rise (Zhang et al., 2004; Gibbons and Nicholls, 2006). Coastal erosion is observed on many shorelines around the world, but it usually remains unclear to what extent these losses are associated with relative sea-level rise due to subsidence, and other human drivers of land loss, and to what extent they result from global warming (Hansom, 2001; Jackson et al., 2002; Burkett et al., 2005; Wolters et al., 2005) (see Chapter 1, Section 1.3.3). Long-term ecological studies of rocky shore communities indicate adjustments apparently coinciding with climatic trends (Hawkins et al., 2003). However, for mid-latitudinal coastal systems it is often difficult to discriminate the extent to which such changes are a part of natural variability; and the clearest evidence of the impact of climate change on coasts over the past few decades comes from high and low latitudes, particularly polar coasts and tropical reefs.
There is evidence for a series of adverse impacts on polar coasts, although warmer conditions in high latitudes can have positive effects, such as longer tourist seasons and improved navigability (see Chapter 15, Section 18.104.22.168). Traditional knowledge also points to widespread coastal change across the North American Arctic from the Northwest Territories, Yukon and Alaska in the west to Nunavut in the east (Fox, 2003). Reduced sea-ice cover means a greater potential for wave generation where the coast is exposed (Johannessen et al., 2002; Forbes, 2005; Kont et al., 2007). Moreover, relative sea-level rise on low-relief, easily eroded, shores leads to rapid retreat, accentuated by melting of permafrost that binds coastal sediments, warmer ground temperatures, enhanced thaw, and subsidence associated with the melting of massive ground ice, as recorded at sites in Arctic Canada (Forbes et al., 2004b; Manson et al., 2006), northern USA (Smith, 2002b; Lestak et al., 2004) and northern Russia (Koreysha et al., 2002; Nikiforov et al., 2003; Ogorodov, 2003). Mid-latitude coasts with seasonal sea ice may also respond to reduced ice cover; ice extent has diminished over recent decades in the Bering and Baltic Seas (ARAG, 1999; Jevrejeva et al., 2004) and possibly in the Gulf of St. Lawrence (Forbes et al., 2002).
Global warming poses a threat to coral reefs, particularly any increase in sea surface temperature (SST). The synergistic effects of various other pressures, particularly human impacts such as over-fishing, appear to be exacerbating the thermal stresses on reef systems and, at least on a local scale, exceeding the thresholds beyond which coral is replaced by other organisms (Buddemeier et al., 2004). These impacts and their likely consequences are considered in Box 6.1, the threat posed by ocean acidification is examined in Chapter 4, Section 4.4.9, the impact of multiple stresses is examined in Box 16.2, and the example of the Great Barrier Reef, where decreases in coral cover could have major negative impacts on tourism, is described in Chapter 11, Section 11.6.