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GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 33 (1983), Pages 361-366

Rates and Frequencies of Sea-Level Changes: A Review with an Application to Predict Future Sea Levels in Louisiana

Dag Nummedal (1)

ABSTRACT

The relative elevation of sea and land has been changing throughout time in response to two fundamentally different groups of factors operating globally and locally. (1) Global factors include changes in the volume of the ocean basins due to variable sea floor spreading rates, oceanic sedimentation, continental accretion, and the opening and closing of marginal seas. Furthermore, the mass of oceanic water has changed in response to glaciations, and the specific volume of the water is temperature dependent. (2) Local factors influencing relative sea level at any measurement station include subsidence of continental margins, fault displacements, compaction due to dewatering of sediments, and a range of atmospheric factors.

This review has identified nine groups of factors which control relative sea level. These factors operate at distinctly different time scales ranging from 108 years (sea floor spreading) to hours (storms). These same groups of factors also have characteristic rates of sea-level change, ranging from 5 x 10-4 cm/year for sea floor spreading to 30 cm/year for seasonal effects due to continental run-off and steric expansion of seawater.

As one application of the data in this review an attempt has been made to predict the trend of relative sea level along the coast of Louisiana for the coming decades. Currently, the global (eustatic) sea level appears to be rising at a rate of 1.2 mm per year. The local rate of land surface sinking along the central Louisiana coast appears to be about 9 mm per year.

Based on linear extrapolation of current trends one would predict that local sea level on the Louisiana coast in the year 2020 would stand about 40 cm higher than the present.

A linear extrapolation of current trends is probably too conservative. Climatic modeling strongly suggests accelerated global warming due to the greenhouse effects of increasing atmospheric CO2. As a consequence, the global (eustatic) rate of sea-level rise is expected to increase due to steric expansion of the seawater and continued melting of land-based polar ice caps. For the next 40 years eustatic sea level may rise at a rate of 9mm/year. On this basis one would predict that local sea level on the Louisiana coast in the year 2020 would stand about 72 cm higher than the present. Such a rise would result in catastrophic inundation of coastal lowlands.

Global warming also would increase tropical storm frequencies and the extent of coastal storm tide inundation.

The economic impact on south Louisiana due to local sea-level rise is already severe and it is likely to increase in magnitude. It is imperative that plans for coastal development and protection consider these long-term trends.


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