This section provides key insights into the ways in which coastal systems are presently changing, as context for assessing the impacts of, and early effects attributable to, climate change.
Coasts are dynamic systems, undergoing adjustments of form and process (termed morphodynamics) at different time and space scales in response to geomorphological and oceanographical factors (Cowell et al., 2003a,b). Human activity exerts additional pressures that may dominate over natural processes. Often models of coastal behaviour are based on palaeoenvironmental reconstructions at millennial scales and/or process studies at sub-annual scales (Rodriguez et al., 2001; Storms et al., 2002; Stolper et al., 2005). Adapting to global climate change, however, requires insight into processes at decadal to century scales, at which understanding is least developed (de Groot, 1999; Donnelly et al., 2004).
Coastal landforms, affected by short-term perturbations such as storms, generally return to their pre-disturbance morphology, implying a simple, morphodynamic equilibrium. Many coasts undergo continual adjustment towards a dynamic equilibrium, often adopting different 'states' in response to varying wave energy and sediment supply (Woodroffe, 2003). Coasts respond to altered conditions external to the system, such as storm events, or changes triggered by internal thresholds that cannot be predicted on the basis of external stimuli. This natural variability of coasts can make it difficult to identify the impacts of climate change. For example, most beaches worldwide show evidence of recent erosion but sea-level rise is not necessarily the primary driver. Erosion can result from other factors, such as altered wind patterns (Pirazzoli et al., 2004; Regnauld et al., 2004), offshore bathymetric changes (Cooper and Navas, 2004), or reduced fluvial sediment input (Sections 6.2.4 and 126.96.36.199). A major challenge is determining whether observed changes have resulted from alteration in external factors (such as climate change), exceeding an internal threshold (such as a delta distributary switching to a new location), or short-term disturbance within natural climate variability (such as a storm).
Climate-related ocean-atmosphere oscillations can lead to coastal changes (Viles and Goudie, 2003). One of the most prominent is the El Nino-Southern Oscillation (ENSO) phenomenon, an interaction between pronounced temperature anomalies and sea-level pressure gradients in the equatorial Pacific Ocean, with an average periodicity of 2 to 7 years. Recent research has shown that dominant wind patterns and storminess associated with ENSO may perturb coastal dynamics, influencing (1) beach morphodynamics in eastern Australia (Ranasinghe et al., 2004; Short and Trembanis, 2004), mid-Pacific (Solomon and Forbes, 1999) and Oregon (Allan et al., 2003); (2) cliff retreat in California (Storlazzi and Griggs, 2000); and (3) groundwater levels in mangrove ecosystems in Micronesia (Drexler, 2001) and Australia (Rogers et al., 2005). Coral bleaching and mortality appear related to the frequency and intensity of ENSO events in the Indo-Pacific region, which may alter as a component of climate change (Box 6.1), becoming more widespread because of global warming (Stone et al., 1999). It is likely that coasts also respond to longer term variations; for instance, a relationship with the Pacific Decadal Oscillation (PDO) is indicated by monitoring of a south-east Australian beach for more than 30 years (McLean and Shen, 2006). Correlations between the North Atlantic Oscillation (NAO) and storm frequency imply similar periodic influences on Atlantic coasts (Tsimplis et al., 2005, 2006), and the Indian Ocean Dipole (IOD) may drive similar periodic fluctuations on coasts around the Indian Ocean (Saji et al., 1999).
6.2.2 Increasing human utilisation of the coastal zone
Few of the world's coastlines are now beyond the influence of human pressures, although not all coasts are inhabited (Buddemeier et al., 2002). Utilisation of the coast increased dramatically during the 20th century, a trend that seems certain to continue through the 21st century (Section 6.3.1). Coastal population growth in many of the world's deltas, barrier islands and estuaries has led to widespread conversion of natural coastal landscapes to agriculture, aquaculture, silviculture, as well as industrial and residential uses (Valiela, 2006). It has been estimated that 23% of the world's population lives both within 100 km distance of the coast and <100 m above sea level, and population densities in coastal regions are about three times higher than the global average (Small and Nicholls, 2003) (see also Box 6.6). The attractiveness of the coast has resulted in disproportionately rapid expansion of economic activity, settlements, urban centres and tourist resorts. Migration of people to coastal regions is common in both developed and developing nations. Sixty percent of the world's 39 metropolises with a population of over 5 million are located within 100 km of the coast, including 12 of the world's 16 cities with populations greater than 10 million. Rapid urbanisation has many consequences: for example, enlargement of natural coastal inlets and dredging of waterways for navigation, port facilities, and pipelines exacerbate saltwater intrusion into surface and ground waters. Increasing shoreline retreat and risk of flooding of coastal cities in Thailand (Durongdej, 2001; Saito, 2001), India (Mohanti, 2000), Vietnam (Thanh et al., 2004) and the United States (Scavia et al., 2002) have been attributed to degradation of coastal ecosystems by human activities, illustrating a widespread trend.
The direct impacts of human activities on the coastal zone have been more significant over the past century than impacts that can be directly attributed to observed climate change (Scavia et al., 2002; Lotze et al., 2006). The major direct impacts include drainage of coastal wetlands, deforestation and reclamation, and discharge of sewage, fertilisers and contaminants into coastal waters. Extractive activities include sand mining and hydrocarbon production, harvests of fisheries and other living resources, introductions of invasive species and construction of seawalls and other structures. Engineering structures, such as damming, channelisation and diversions of coastal waterways, harden the coast, change circulation patterns and alter freshwater, sediment and nutrient delivery. Natural systems are often directly or indirectly altered, even by soft engineering solutions, such as beach nourishment and foredune construction (Nordstrom, 2000; Hamm and Stive, 2002). Ecosystem services on the coast are often disrupted by human activities. For example, tropical and subtropical mangrove forests and temperate saltmarshes provide goods and services (they accumulate and transform nutrients, attenuate waves and storms, bind sediments and support rich ecological communities), which are reduced by large-scale ecosystem conversion for agriculture, industrial and urban development, and aquaculture (Section 6.4.2).
External terrestrial influences have led to substantial environmental stresses on coastal and nearshore marine habitats (Sahagian, 2000; Saito, 2001; NRC, 2004; Crossland et al., 2005). As a consequence of activities outside the coastal zone, natural ecosystems (particularly within the catchments draining to the coast) have been fragmented and the downstream flow of water, sediment and nutrients has been disrupted (Nilsson et al., 2005; Section 188.8.131.52). Land-use change, particularly deforestation, and hydrological modifications have had downstream impacts, in addition to localised development on the coast. Erosion in the catchment has increased river sediment load; for example, suspended loads in the Huanghe (Yellow) River have increased 2 to 10 times over the past 2000 years (Jiongxin, 2003). In contrast, damming and channelisation have greatly reduced the supply of sediments to the coast on other rivers through retention of sediment in dams (Syvitski et al., 2005). This effect will likely dominate during the 21st century (Section 6.4.1).
Coasts can be affected by external marine influences (Figure 6.1). Waves generated by storms over the oceans reach the coast as swell; there are also more extreme, but infrequent, high-energy swells generated remotely (Vassie et al., 2004). Tsunamis are still rarer, but can be particularly devastating (Bryant, 2001). Ocean currents modify coastal environments through their influence on heat transfer, with both ecological and geomorphological consequences. Sea ice has physical impacts, and its presence or absence influences whether or not waves reach the coast (Jaagus, 2006). Other external influences include atmospheric inputs, such as dust (Shinn et al., 2000), and invasive species.
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 184.108.40.206). 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.
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