Global Sea-Level Rise and its Impacts on Coastal Areas
M.S. Coastal Zone Management
Richard E. Dodge
In general, global surface air temperature has increased by 0.5°C since the middle of the nineteenth century. The global warming trend has occurred concurrently with increasing in number and concentration of greenhouse gases in the atmosphere such as CO2, NO2, H2O, CH4, and Chlorofluorocarbons (CFCs).
Global atmospheric CO2 concentrations, both monthly and annually, have increased since 1955. For instance, the Mauna Loa station, Hawaii, records indicate a 12.8% increase in the mean annual CO2 concentration from 315.83 part per million by volume (ppmv) of dry air in 1959 to 356.24 ppmv in 1994 in the mid layer of the troposphere. Global atmospheric CH4 concentrations increased at the rate of 16.4 part per billion by volume per year (ppbv/yr) from a minimum value of 1557.2 ppbv in December 1980 to a maximum value of 1737.1 ppbv in December 1991. Nitrous oxide concentrations increased from about 300 ppbv to 310 ppbv within the last decade. CFC-11 and CFC-12 concentrations also increase from about 140 part per trillion (ppt) and 250 ppt in 1977 to 260 ppt and 500 ppt in 1993 respectively. Based on the increase concentration trends of greenhouse gases, the best estimate from various studies for global warming by the year 2100 range from 2 to 3°C over the 1990 levels.
From radiocarbon date estimations, the ancient relative sea level could range from the lowest level of about 150 m below the present level at about 16,000 years ago to the highest level of about 255 m above the present level at about 90 million years ago. Various studies indicated that the average global mean sea level has been rising about 1.5 mm/yr for the past century. The increasing trend of global mean sea level has a high correlation with the trend of global mean surface air temperature.
Along United States of America's coasts, sea levels have generally been rising at the rate from 1 to 3 mm/yr with some locations indicate negative trends and others indicating extreme positive trends. Negative trends indicating high land uplift rate occur in Alaskan coasts, AL (-1.6 to -17.3 mm/yr), Crescent City, CA (-0.6 mm/yr), Astoria, OR (-0.3 mm/yr), Neah Bay, WA (-1.1 mm/yr), San Island, Midway (-0.6 mm/yr), and Apra Harbor, Guam ( -1.2 mmlyr). Whereas, the extreme positive trends indicating high land subsidence rate occur in Texas coasts (3.1 to 14.0 mm/yr) and Louisiana coasts (9.7 to 11.7 mm/yr).
Global warming is a major factor controlling global sea level rise through thermal expansion of the ocean water, and the melting of glaciers and ice caps of the world. Geological processes such as sea-floor spreading, plate tectonic movements, geological faulting, sinkholes, tsunamis, subduction belts, and volcanoes contribute to global sea level change. The precise rates of land movements due to these geological processes are difficult to measure. Therefore, it is difficult to include them into global sea level change models. Land subsidence and uplift are very important factors causing sea level to change locally. El Nino causes sea level to rise significantly in the Eastern Pacific Ocean and to decline in the Western Pacific Ocean.
Permanent coastal inundation and accelerated coastal erosion are the most detrimental impact of sea-level rise on coastal areas. Changes in circulation of estuaries and lagoons, increasing salinity of estuaries and lagoons, changes in sedimentation patterns, tidal currents, increase storm surge, enhanced wave energy at shorelines, loss of wetlands, changes in ecotones and habitats, loss of sea turtle and bird nesting areas, and increased salt intrusion into ground water are some possible physical and biogeochemical impacts of sea-level rise on coastal areas. Sea level rise in mangrove ecosystems could alter succession, enhance mortality, change in benthic community structure, and reduce seedling establishment. It could cause changes in zonation, frontal regression, and landward extension of mangrove communities. Nitrogen and phosphorous concentrations could increase on a regional scale as a result of coastal flooding and soil erosion, particularly in subpolar and midlatitude, such as the Barring Sea. Pesticides that are presently retained in coastal soils could find their way into the open ocean as a result of sea level rise, which could affect coastal ecosystems. Rising in sea-level forces reefs to accordingly keep pace, fall behind or sprint, and creating characteristic external geometries and internal facies mosaics depending on the different rate between sea-level rise and coral reefs growth. The current and the future projected sea-level rise seem not to cause significant implications on coral reefs. However, indirect impact of sea-level rise such as changes is salinity, temperature, light intensity, dissolved oxygen level, and wave energy could produce some implications on coral reefs. Other consequences of sea level rise could changes biodiversity and changes in the endangered species populations.
Coastal areas experiencing physical, chemical, geological, or biological impacts due to the sea level rise also incur economic damages. For instance, the loss of 20,000 km2 coastal area in the United States would represent a financial loss of about $650 billion.
Bisman Nababan. 1998. Global Sea-Level Rise and its Impacts on Coastal Areas. Capstone. Nova Southeastern University. Retrieved from NSUWorks, . (234)
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