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. The 2007 Fourth Assessment Report (IPCC 4) projected century-end sea levels using the Special Report on Emissions Scenarios (SRES). The six SRES 'marker' scenarios projected sea level to rise by 18 to 59 centimetres (7.1 to 23 in). Their projections were for the time period 2090'99, with the increase in level relative to average sea level over the 1980'99 period. More recent research from 2008 observed rapid declines in ice mass balance from both Greenland and Antarctica, and concluded that sea-level rise by 2100 is likely to be at least twice as large as that presented by IPCC AR4, with an upper limit of about two meters. A literature assessment published in 2010 by the US National Research Council summarized the results of more recent studies. These projections ranged from 56'200 centimetres (22'79 in), based on the same period as IPCC 4. In 2011, Rignot and others projected a rise of 32 centimetres (13 in) by 2050. Their projection included increased contributions from the Antarctic and Greenland ice sheets.
Sea level rise has various impacts on Bangladesh, a coastal country with a 710 km long coast on the Bay of Bengal. It has already affected Bangladesh through land erosion, salinity intrusion and loss in biodiversity and will cause further damage in the form of damage to infrastructures, crop failure, fisheries destruction, and loss of biodiversity. A sea level rise of 1 m will inundate 17.5% of the country's vast coastal area and flood plain zone. [Mohd Shamsul Alam]