Sea level is the height of the surface of the ocean at any given location. Sea level is highly variable and can undergo very rapid changes due to such events as tides, tsunamis, changes in barometric pressure, wind-generated waves, and even freshwater floods.

While these events can produce changes in sea level of several meters, they are local in scale and of a very short duration, generally lasting only for hours. Mean sea level is the average, global height of the sea surface, independent of these local, short- term changes. Changes in mean sea level are on the order of a few millimeters per year.

Mean sea level at specific locations can be calculated using tide gauge records and subtracting the effects of annual changes in atmospheric pressure and long-term changes in tidal ranges, which are driven by astronomical factors (Axelrod, 2004). Changes in global mean sea level can be calculated using satellite-based radar altimetry, such as with the TOPEX/Poseidon satellite. The radar altimeter measures the height of the satellite above the ocean, based on the time it takes for a radio signal to travel from the satellite to the sea surface and back. Since the actual altitude of the satellite is known, any changes in the altimeter measurement reflect changes in the height of the sea surface itself.

On short timescales (decades to centuries), mean sea level is a function of the amount of water stored as ice in glaciers and ice sheets. As global temperature rises, less water is stored as ice, contributing to a rise in mean sea level. A rise in mean sea level in response to global warming has important societal consequences. First, such a rise contributes to a loss of land, as coastal areas are slowly inundated by water (Botterill, 2003). This is a concern for certain low-lying island nations such as the Maldives or Tuvalu. The Maldives is a nation made up of twelve hundred islands in the Indian Ocean, which has a maximum elevation of only 2.5 meters above current sea level.

Thus, the Maldivian population of approximately 380,000 people is highly vulnerable to even a slow rise in mean sea level. For other nations, a rise in mean sea level is also a concern because of increased hazards from flooding during high tides-- especially spring tides--and storms. A rise in mean sea level provides a higher baseline upon which tidal fluctuations build. According to the Fourth Assessment Report released by the Intergovernmental Panel on Climate Change, from 1993 to 2003, mean sea level rose approximately 3.1 millimeters per year.

References

1. Axelrod, Regina S., David Leonard Downie, and Norman J. Vig, eds. The Global Environment: Institutions, Law, and Policy. 2d ed. Washington, D.C.: CQ Press, 2004.

2. Botterill, Linda C., and Melanie Fisher, eds. Beyond Drought: People, Policy, and Perspectives. Collingwood, Vic.: CSIRO, 2003.

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