Sea-Level Rise: Causes, Impacts and Scenarios for Change

The study of sea-level changes (SLCs) and of their link to coastal geomorphology are well established, as is reflected in an extensive research and wider publications literature. People have long recognized that ancient marine shorelines and beach sediments occur inland, well away from present day coasts. The causes of SLCs center upon a series of primary controls resulting from the operation of an integrated atmosphere-ocean system. These include, changes in the total water volume of the oceans over time, temperature effects on ocean water expansion, alterations in the shape of the ocean basins from Earth crustal movements and changes in their accommodation space through sediment accumulation. Satellite surveys also show that the topography of the oceans and shelf seas are influenced by gravitational controls, which define the Earth geoid, with up to 180m difference in inter-regional sea surface heights. In the Quaternary, past glaciations have driven sea levels (SLs) to c.−120m gmsl and created radically different coastal geomorphology and landscapes. The significance of SLCs today lies in establishing an understanding and the accurate projection of how SLs will change in the future. Of particular importance will be the impact of future land-based ice melting, together with the temperature-driven steric changes of ocean waters, as well as the operation of factors of geomorphic inheritance in influencing the patterns of coastal change under SLR. Climate warming during the 21st century will result in the melting of the Earth’s remaining ice masses, with global mean sea levels (gmsl) likely to rise at rates of c.4-5mm/year by 2050, reaching levels of 0.4-0.9m by 2100 under differing IPCC global warming scenarios. These projected changes may, however, be much higher as the uncertainty elements in the different time-lagged controls to SLCs come into operation, e.g., of the increased releases of water from the world’s permafrost zones, or of rates of ice melting. These may drive sea levels to >+2-6m gmsl. The impacts of these movements will potentially result in initial losses of at least 30% of coastal wetlands globally and an increasing ‘squeeze’ of people and biological systems into the re-organizing coastal zone.