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Chapter 2 Dynamics of the Coastal Zone Chapter 2 Dynamics of the Coastal Zone James l? M. Syvitski, Nick Harvey, Eric Wolanski, William C. Burnett, Gerardo M. E. Perillo, Vivien Gornitz Contributors: Russell K. Arthurton, Henry Bokuniewicz, Janet W; Campbell, Lee Cooper, Kenneth Dunton, Shu Gao, Patrick l? Hesp, Yoshiki Saito, Joe Salisbury, Maria Snoussi, Wyss W-S. Yim climate-warming scenario will undoubtedly impact 2.1 Introduction some regions more than others. The Siberian coast is experiencing a reduction in offshore sea-ice cover, with Earth's coastline has evolved for many thousands of a associated increase in ocean fetch, leading to higher years, experiencing changes to habitat, coastal dynam- sea levels during the open-water summer and accelera- ics and the supply of sediment from the continental tion of coastal erosion. Recent studies also suggest that interior. Relative sea level has risen in some areas, but tropical and temperate coastal environments are expe- fallen elsewhere. There is an acknowledged range in riencing stormier conditions (i.e., increased numbers natural variability within a given region of the global and severity of hurricanes). Will local storm surges mag- coastal zone, within a context of longer-term geological nify the impact of a global sea-level rise, increasing risks processes. to humans and their infrastructure? Are there negative Many of the regional controls on sea level involve feedbacks to engineering options for the protection of long-term geological processes (e.g., subsidence, iso- coastal settlements? stasy), and have a profound influence on controlling Perhaps the largest impact on coastal stability is due short-term dynamics. As sea levels fluctuate, the mor- to modification to the global flux of sediment to the phology of a coastal zone will further evolve, changing coastal zone. Changes in global hydrology have modi- the boundary conditions of other coastal processes: cir- fied the timing and intensity of floods, and therefore culation, waves, tides and the storage of sediment on the effective discharge available for sediment transport. flood plains. Climate shifts have varied the contributions from melt- Human development of coastal regions has modified water (snow, ice), altered the intensity of rainfall, pristine coastlines around the globe, by deforestation, changed drainage basin water-storage capacity, and al- cultivation, changes in habitat, urbanisation, agricultural tered precipitation and evaporation rates. Human influ- impoundment and upstream changes to river flow. ences have also greatly modified downstream flow. Over Humans can also influence changes in relative sea level half of the world's rivers have seen stream-flow modi- at the local scale. For example, removal of groundwater fication through the construction of large reservoirs. and hydrocarbons from subterranean reservoirs may These and other rivers have also been impacted by wa- cause subsidence in nearby areas, with a concomitant ter withdrawal for agriculture, industry and settlements. rise in relative sea level. Our concern in LOICZ is not Our understanding of the importance of submarine just in the magnitude of change, but also in the recent groundwater discharge in the coastal zone and of its and accelerated rate of change. Our interests extend processes has improved markedly in recent years; a to whether alterations on the local level can cumula- significant impetus has been given to this understand- tively give rise to coastal zone changes of global signifi- ing by the LOICZ-associated SCOR Working Group 112. cance. The outcomes of its work are summarised in this chap- Climate warming may also contribute significantly ter. to sea level fluctuations. Predictions by the International Human migration to the coastal zone and consequent Panel on Climate Change (IPCC) suggest that sea level land-use changes have also greatly impacted the stabil- is rising globally (15 to 95 cm by 2100) as a result of the ity of our coastal areas. Human impacts on the coastal recent warming of the ocean and the melting of ice caps zone ranges from massive (e.g., reduction in wetlands, (Houghton et al. 2001). As sea levels rise, coastal desta- urbanisation) to non-existent (e.g., many polar coast- bilisation may occur due to accelerated beach erosion, lines). This chapter synthesises how climate shifts and trapping of river sediment on flood plains and increas- humans can affect and have affected our coasts on a glo- ing water residence during floods. The predicted IPCC bal scale. 40 CHAPTER 2 . Dynamics of the Coastal Zone The following is an overview of the current state of knowl- 2.2 Impacts of Local, Regional and Global edge" on the coastal im~actsof sea-level fluctuations. Sea-level Fluctuations 2.2.1 Processes and Mechanisms: Coastal Dynamics 2.2.1.2 Reconstruction of Past Changes in the Coastal Zone 2.2.1 .I lmpacts of Local, Regional and Global Sea-level Fluctuations Current and predicted sea-level changes need to be con- sidered in their geological context. The LOICZ Science In 1993 the LOICZ Science Plan (Holligan and de Boois Plan notes the necessity of developing historical recon- 1993) commented on the problems of accurate measure- structions of sea-level change "... in order to examine ment of sea-level change using tide-gauge data, and the the responses of particular coastal systems to relatively need for more sensitive measurement techniques. The large changes in external forcing that might occur in the Plan also stated that "... spatial variations in local sea future ..." and that "... many of these analyses directly level must be known in order to predict large-scale changes complement studies of the IGBP project on Past Global to coastal systems." (Holligan and de Boois 1993, p. 24), Changes (PAGES)." (Holligan and de Boois 1993). noting that local sea-level changes are affected by: Sea-level studies from a LOICZ perspective need to be placed in the context of PAGES sea-level studies for tectonics, three reasons: climatic change impacts on winds, waves and ocean circulation, and 1. Past sea-level changes provide a perspective on the coastal subsidence, often exacerbated by human im- cyclical nature of sea levels and the extent to which pacts, which can produce landlsea-level change rates current and predicted sea-level changes are pertur- higher than those of global sea-level rise. bations from natural cycles. 2. Sea-level response following the last glacial maximum Despite the use of more accurate global sea-level meas- of 20 ooo years ago (20 ka) varied around the globe urement techniques over the last ten years, such as satel- according to the coastal adjustment to the post-gla- lite altimetry and geodetic levelling (see Nerem and Mitch- cia1 redistribution of water and ice, and differential urn 2001) and the refined global viscoelastic analysis of loading of the lithosphere. glacio-isostatic adjustment (GIA; see Peltier 2001, 2oo2), 3. Very recent geological evidence of sea levels during there remains a need for better understanding of existing the last 2 ooo years provides insights into subtle re- local sea-level change data and acquisition of new data. gional climate change linked to oceanic fluctuations. B~UNHESI MATUYAMA I'l'l'I'i 0 0.2 0.4 0.6 0.8 1 .O Million years BP Fig. 2.1. Sea-level rise. Coastal barriers correlated with oxygen isotope changes - 800 ooo years BP (from Belperio 1995) ,acts of Local, Regional and Global Sea-level Fluctuations 41 2.2.1.2.1 Cycles of Sea-level Change - This perturbation from normal cycles confirms the Glacial-interglacial Cycles over need for predictions of global warming and associated 70 000 years BP sea-level rise estimates as outlined by the IPCC (Hough- ton et al. 1996,2001). Thus the geological evidence dem- Evidence of the cyclical nature of sea-level changes was onstrates a well-defined periodicity in climate-related discovered following the identification of global tempera- sea-level changes. The enhanced greenhouse gas concen- ture fluctuations deduced from oxygen isotope ('80/'60) trations in the current interglacial indicate the potential ratios of planktonic foraminifera from deep-sea sedi- for a sea-level response different from the previous inter- ments (Emiliani 1955). These ratios allowed interpreta- glacial periods. These findings have contributed to the tion of global temperature changes over time and dem- IGBP findings that Earth systems are currently operat- onstrated that a quasi-periodic cycling of climate re- ing in a "non-analogue" state (Steffen et al. 2003). curred more frequently than the four major glaciations Further geological evidence of sea-level changes recognised from previous northern hemisphere strati- across an entire glacial-interglacial cycle comes from the graphic studies. One good example of coastal change coral record from Barbados (Bender et al. 1979, Broecker correlated with the oxygen isotope record across a 1979, Gallup et al. 1994, Bard et al. iggo, Blanchon and number of glacial-interglacial cycles is from south-east- Eisenhauer 2001) and from the detailed record of coral ern Australia, where a series of stranded dune barriers terraces preserved on the rapidly uplifting coast of the associated with high sea-level stands has been preserved Huon Peninsula in New Guinea. Reworking of the Huon across a slowly uplifting coastal plain. The barriers, dat- Peninsula data has provided a close correlation with the ing back to 800 ooo years BP (Huntley et al. 1993, i994), oxygen isotope records (Chappell et al. 1996; Fig. 2.3). record coastal sedimentation linked to at least lo inter- glacial high stands of sea level (Belperio 1995; Fig. 2.1). 2.2.1.2.2 Postglacial Sea-level Response - Detailed climatic reconstructions for the last four the Last 20 000 Years major glacial cycles come from the Vostok ice-core (Petit et al. iggg), which has provided new data on the cyclical Geophysical modelling of the surface of Earth has dem- nature of global climate for glacial-interglacial cycles.
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