Physical Agents of Land Loss: Relative Sea Level
Global Sea-Level Rise and Subsidence
Submergence refers to permanent flooding of the coast caused by a rise in global sea level and/or subsidence of the land. At many coastal sites, submergence is the most important factor responsible for land loss. How much land will be lost as a result of sea-level rise depends partly on how fast the water is rising. It has been estimated that each year global sea level rises about 1.8 mm as a result of a worldwide increase in water volume (Douglas, 1997). However, this value is substantially less than the total rise in relative sea level recorded at many tide gauges (Emery and Aubrey, 1991), so scientists have concluded that the remaining amount of relative sea-level rise is caused by land subsidence. At any coastal site the relative sea level includes the global sea-level component (eustasy), tectonic uplift or down warping, and at some locations subsidence that is the result of natural sediment compaction or subsidence induced by the withdrawal of subsurface fluids such as groundwater, oil, and natural gas (Morton et al., 2002). Land loss can also occur in those coastal areas that are experiencing uplift (relative fall in sea level) such as along the Pacific coast and parts of Alaska (e.g. Juneau). This is because storm surges and high waves continue to cause land loss.
Releveling surveys and tide gauge records around the world are used extensively to decipher crustal deformation and increased ocean volumes that occurred in response to atmospheric warming since the last period of continental glaciation. Although historical records of relative sea level are both spotty and brief in North America, the results show along-coast trends that demonstrate the interrelationships among vertical crustal motion, sediment compaction, and eustatic fluctuations (Clark et al., 1978; Peltier, 1986; Emery and Aubrey, 1991).
Several studies have demonstrated how unloading of thick continental ice sheets causes rebounding near the former glaciers and collapse of the surrounding forebulge as evacuated crustal material returns to the uplifted regions (Clark et al., 1978; Peltier, 1986). These crustal-response models also have been used to explain the variability among historical sea-level records and current rates of vertical motion derived from those data.
Not only is sea level rising in a relative sense at many coastal sites, statistical analyses of long-term records show that the present rates of relative sea-level rise are much greater than rates of submergence were for the past few thousand years. This discrepancy between historical and geological rates of submergence has been interpreted as evidence that atmospheric warming since the industrial revolution has caused thermal expansion of the oceans (National Research Council, 1990) and melting of mountain glaciers (Gornitz and Lebedeff, 1987) and possibly the Antarctic ice sheet.