sea level

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sea level,

the level of the sea, which serves as the datum used for measurement of land elevations and ocean depths. Theoretically, one would expect sea level to be a fixed and permanent horizontal surface on the face of the earth, and as a starting approximation, this is true. However, a number of factors operate to cause variations in sea level ranging up to several meters from place to place and to cause long-term global variations, often severe enough to cause flooding and damage to coastal zones.

Sea levels vary greatly from one location to another, i.e., between Nova Scotia and Florida sea-level heights differ at about 16 in (40 cm). Locally the levels of the surface of the world's oceans are disturbed by wind-driven waves and tidestide,
alternate and regular rise and fall of sea level in oceans and other large bodies of water. These changes are caused by the gravitational attraction of the moon and, to a lesser extent, of the sun on the earth.
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. Sea level therefore fluctuates in periods ranging from seconds to a year as a result of these factors. Thus for some purposes it is necessary to know the mean sea level (MSL) in a particular area, determined by averaging the elevations of the sea's surface as measured by mechanical tide gauges over long periods of time.

A number of other factors result in sea-level differences between one place or time and another. These may complement or counteract one another to result in a net rise or fall in mean sea level at a particular time and place. These factors include water temperature and salinity, air pressure, change of season, the amount of runoff from streams, and the amount of water stored as ice or snow on land. Characteristics of the earth also cause differences in sea level. Satellite measurements of the gravity field have shown that the earth is not a perfectly smooth sphere, making defining sea-level measurements difficult.

Worldwide, or eustatic, sea levels have changed over time, such as the last ice age when sea level was as much as 333 ft (100 m) lower in many areas than today. These changes are due to a multitude of reasons. Past transgression (or rise) or regression (or lowering) of the seas have been caused mainly by the addition or removal of water from continental ice caps. Such sea-level fluctuations may be due to changes in climateclimate,
average condition of the atmosphere near the earth's surface over a long period of time, taking into account temperature, precipitation (see rain), humidity, wind, barometric pressure, and other phenomena.
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 that cause the ice to melt or accumulate. Theories of climate and sea level change in geologic time include differences in carbon dioxide or oxygen levels, changes due to plate tectonics and earthquake activity, volcanic and asteroid activity, or changes in the sun's energy.

Since the Industrial Revolution the increase in carbon dioxide and other greenhouse gases caused by the burning of fossil fuel, deforestation of tropical rain forests, and other human activities has led to global warmingglobal warming,
the gradual increase of the temperature of the earth's lower atmosphere as a result of the increase in greenhouse gases since the Industrial Revolution. Global warming and its effects, such as more intense summer and winter storms, are also referred to as climate
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 and a resulting increase in sea level greater than that that would have been expected otherwise. Rising sea levels result in increased coastal flooding and the permanent inundation of coastal lands (in some cases threatening the existence of island nations). The economic implications of rising sea levels are magnified by the percentage of the world's population (more than one fifth) that lives near the coast, and the concentration of urban and industrial areas close to the coast.

Sea Level


the position that the surface of seas and oceans would assume if uninfluenced by tides, waves, or swells, measured as the vertical distance between the water surface and a reference point on land. Sea level is classified as instantaneous, tidal, daily mean, monthly mean, annual mean, and long-term mean.

Sea level is constantly changing under the influence of wind disturbances, tides, the heating and cooling of the ocean surface, atmospheric pressure fluctuations, precipitation and evaporation, and river and glacial runoff. Long-term mean sea level, however, is not affected by these factors; it is determined by the distribution of gravity and by the uneven spatial distribution of such hydrometeorological characteristics as water density and atmospheric pressure. Long-term mean sea level, which is constant at any given point, is used as a reference level from which land elevations are measured. In order to determine the depths of seas with minimal tidal variation, long-term mean sea level is taken as zero depth, from which depths are calculated in accordance with navigational requirements.

In the USSR, absolute elevations on the earth’s surface are measured from the long-term mean sea level of the Baltic Sea, which is read off from the zero point of the tide gage at Kronstadt.


Duvanin, A. I. Uroven’ moria. Leningrad, 1956.
Duvanin, A. I., G. P. Kalinin, and R. K. Klige. “O mnogoletnikh kolebaniiakh urovnia okeanov, nekotorykh morei i ozer.” Vestnik MGU, Seriia 5: Geografiia, 1975, no. 6.

sea level

[′sē ‚lev·əl]
The level of the surface of the ocean; especially, the mean level halfway between high and low tide, used as a standard in reckoning land elevation or sea depths.

sea level

The level of the sea, which serves as the datum used for measurement of land elevations and ocean depths. Sea levels vary greatly from one location to another. Locally, the levels of the surface of the world's oceans are disturbed by wind-driven waves and tides. The mean sea level is used as a reference height to determine a standard condition for the atmosphere and for the altitude measurement. It is measured over a nineteen-year period. It does not remain constant over the surface of the entire earth; the mean sea level at the Pacific end of the Panama Canal stands 7.8 in (20 cm) higher than at the Atlantic end.

sea level

the level of the surface of the sea with respect to the land, taken to be the mean level between high and low tide, and used as a standard base for measuring heights and depths