Dynamics of the Coastal Zone
Coastal Fluxes in the Anthropocene
Earth's coastline has evolved for many thousands of years, experiencing changes to habitat, coastal dynamics and the supply of sediment from the continental interior. Relative sea level has risen in some areas, but fallen elsewhere. There is an acknowledged range in natural variability within a given region of the global coastal zone, within a context of longer-term geological processes. Many of the regional controls on sea level involve long-term geological processes (e.g., subsidence,
... subsidence, isostasy), and have a profound influence on controlling short-term dynamics. As sea levels fluctuate, the morphology of a coastal zone will further evolve, changing the boundary conditions of other coastal processes: circulation, waves, tides and the storage of sediment on flood plains. Human development of coastal regions has modified pristine coastlines around the globe, by deforestation, cultivation, changes in habitat, urbanisation, agricultural impoundment and upstream changes to river flow. Humans can also influence changes in relative sea level at the local scale. For example, removal of groundwater and hydrocarbons from subterranean reservoirs may cause subsidence in nearby areas, with a concomitant rise in relative sea level. Our concern in LOICZ is not just in the magnitude of change, but also in the recent and accelerated rate of change. Our interests extend to whether alterations on the local level can cumulatively give rise to coastal zone changes of global significance. Climate warming may also contribute significantly to sea level fluctuations. Predictions by the International Panel on Climate Change (IPCC) suggest that sea level is rising globally (15 to 95 cm by 2100) as a result of the recent warming of the ocean and the melting of ice caps (Houghton et al. 2001) . As sea levels rise, coastal destabilisation may occur due to accelerated beach erosion, trapping of river sediment on flood plains and increasing water residence during floods. The predicted IPCC climate-warming scenario will undoubtedly impact some regions more than others. The Siberian coast is experiencing a reduction in offshore sea-ice cover, with a associated increase in ocean fetch, leading to higher sea levels during the open-water summer and acceleration of coastal erosion. Recent studies also suggest that tropical and temperate coastal environments are experiencing stormier conditions (i.e., increased numbers and severity of hurricanes). Will local storm surges magnify the impact of a global sea-level rise, increasing risks to humans and their infrastructure? Are there negative feedbacks to engineering options for the protection of coastal settlements? Perhaps the largest impact on coastal stability is due to modification to the global flux of sediment to the coastal zone. Changes in global hydrology have modified the timing and intensity of floods, and therefore the effective discharge available for sediment transport. Climate shifts have varied the contributions from meltwater (snow, ice), altered the intensity of rainfall, changed drainage basin water-storage capacity, and altered precipitation and evaporation rates. Human influences have also greatly modified downstream flow. Over half of the world's rivers have seen stream-flow modification through the construction of large reservoirs. These and other rivers have also been impacted by water withdrawal for agriculture, industry and settlements. Our understanding of the importance of submarine groundwater discharge in the coastal zone and of its processes has improved markedly in recent years; a significant impetus has been given to this understanding by the LOICZ-associated SCOR Working Group 112. The outcomes of its work are summarised in this chapter. Human migration to the coastal zone and consequent land-use changes have also greatly impacted the stability of our coastal areas. Human impacts on the coastal zone ranges from massive (e.g., reduction in wetlands, urbanisation) to non-existent (e.g., many polar coastlines). This chapter synthesises how climate shifts and humans can affect and have affected our coasts on a global scale.