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 tides. 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 climate 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 warming 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.