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The relative elevation of sea and land has been changing through time in response to two fundamentally different groups of factors. Global factors include changes in the volume of the ocean basins owing to tectonic processes and changes in the total amount of ocean water due to glaciation. Local factors include subsidence of continental margins and compaction of recent sediments. Over this century, global sea level (eustatic) appears to have been rising at a rate of 1.2 mm per year. Along the south-central Louisiana coast the land surface appears to be sinking at a rate of about 8 mm per year.
Recent global climatic modeling suggests strongly that we are about to enter a period of rapidly accelerating warming due to increased amounts of carbon dioxide in the atmosphere. As a consequence, eustatic sea level rise is predicted to accelerate because of both steric expansion of the ocean water and continued melting of polar ice caps. For the next 40 years the eustatic sea level rise may average 10 mm per year. The local relative sea level in coastal Louisiana would therefore rise at about twice its present rate over this time period. At this rate, local sea level will, in the year 2020, stand some 70 to 75 cm (2.3 to 2.5 ft) higher than at present.
The numbers presented above are average values for the Louisiana coastal plain. Local variability in subsidence rate appears to be related to the thickness of Holocene sediments. The highest rates of subsidence are found in the modern Mississippi (birdfoot) delta and in coastal Terrebonne Parish above the late Pleistocene Mississippi trench; in both areas the Holocene section is in excess of 200 m (650 ft) thick.
The high rate of local sea level rise along the Louisiana coast makes it imperative that plans for coastal development and protection consider the long-term consequences of sea level change.
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