Sea Level the level of the sea half-way between high and low-tide, 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. Sea level is ever changing in response to tidal variations, fluctuations in water temperature and salinity, air pressure, changes of season, upwelling, river discharges, etc. If all these influences are excluded, then progressive changes in sea level can be observed. Sea level therefore fluctuates in periods ranging from seconds to a year as a result of these factors. For many 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 a long period of time.

Evidently, throughout the history of the earth, sea level has changed repeatedly. A great concern has been raised, as the rising sea level would affect a vast expanse of global population, particularly the low-lying major deltaic nations of the world. Bangladesh is expected to be in the casualty list. Probable effect on demography, landuse and national income up to the year 2050 at different inundation level has been estimated. Sea level rise of 144-209 cm would be subject to inundate 16-18% of habitable land and 13-15% of current population, and up to 13% of gross domestic product (GDP) would be potentially affected. [Mohd Shamsul Alam]