Dynamics of the Coastal Zone

Abstract

Earth’s coastline has evolved for many thousands ofyears, experiencing changes to habitat, coastal dynam-ics and the supply of sediment from the continentalinterior. Relative sea level has risen in some areas, butfallen elsewhere. There is an acknowledged range innatural variability within a given region of the globalcoastal zone, within a context of longer-term geologicalprocesses.Many of the regional controls on sea level involvelong-term geological processes (e.g., subsidence, iso-stasy), and have a profound influence on controllingshort-term dynamics. As sea levels fluctuate, the mor-phology of a coastal zone will further evolve, changingthe boundary conditions of other coastal processes: cir-culation, waves, tides and the storage of sediment onflood plains.Human development of coastal regions has modifiedpristine coastlines around the globe, by deforestation,cultivation, changes in habitat, urbanisation, agriculturalimpoundment and upstream changes to river flow.Humans can also influence changes in relative sea levelat the local scale. For example, removal of groundwaterand hydrocarbons from subterranean reservoirs maycause subsidence in nearby areas, with a concomitantrise in relative sea level. Our concern in LOICZ is notjust in the magnitude of change, but also in the recentand accelerated rate of change. Our interests extendto whether alterations on the local level can cumula-tively give rise to coastal zone changes of global signifi-cance.Climate warming may also contribute significantlyto sea level fluctuations. Predictions by the InternationalPanel on Climate Change (IPCC) suggest that sea levelis rising globally (15 to 95 cm by 2100) as a result of therecent warming of the ocean and the melting of ice caps(Houghton et al. 2001). As sea levels rise, coastal desta-bilisation may occur due to accelerated beach erosion,trapping of river sediment on flood plains and increas-ing water residence during floods. The predicted IPCCclimate-warming scenario will undoubtedly impactsome regions more than others. The Siberian coast isexperiencing a reduction in offshore sea-ice cover, witha associated increase in ocean fetch, leading to highersea levels during the open-water summer and accelera-tion of coastal erosion. Recent studies also suggest thattropical and temperate coastal environments are expe-riencing stormier conditions (i.e., increased numbersand severity of hurricanes). Will local storm surges mag-nify the impact of a global sea-level rise, increasing risksto humans and their infrastructure? Are there negativefeedbacks to engineering options for the protection ofcoastal settlements?Perhaps the largest impact on coastal stability is dueto modification to the global flux of sediment to thecoastal zone. Changes in global hydrology have modi-fied the timing and intensity of floods, and thereforethe effective discharge available for sediment transport.Climate shifts have varied the contributions from melt-water (snow, ice), altered the intensity of rainfall,changed drainage basin water-storage capacity, and al-tered precipitation and evaporation rates. Human influ-ences have also greatly modified downstream flow. Overhalf of the world’s rivers have seen stream-flow modi-fication through the construction of large reservoirs.These and other rivers have also been impacted by wa-ter withdrawal for agriculture, industry and settlements.Our understanding of the importance of submarinegroundwater discharge in the coastal zone and of itsprocesses has improved markedly in recent years; asignificant impetus has been given to this understand-ing by the LOICZ-associated SCOR Working Group 112.The outcomes of its work are summarised in this chap-ter.Human migration to the coastal zone and consequentland-use changes have also greatly impacted the stabil-ity of our coastal areas. Human impacts on the coastalzone ranges from massive (e.g., reduction in wetlands,urbanisation) to non-existent (e.g., many polar coast-lines). This chapter synthesises how climate shifts andhumans can affect and have affected our coasts on a glo-bal scale.