• Changes in sea level refer to fluctuations in the average level of the sea surface.
• These changes can also be called relative changes in sea level.
• When sea level rises, the land or sea surface may uplift, subside, or both can occur simultaneously.

The Major Categories of Change in Sea Level

• Eustatic changes occur when the volume of seawater changes due to factors such as global warming and melting of ice sheets (rise in sea level) or ice ages (fall in sea level)
• Tectonic changes occur due to land level alterations
• Types of tectonic changes:
  • Isostatic changes due to addition or removal of load, such as ice ages causing subsidence and apparent sea level rise, or removal of glacial ice causing land uplift and sea level fall
  • Epeirogenic movement due to broad-scale tilting of continents, causing one part to rise and the other to subside and leading to an apparent rise in sea level
  • Orogenic movement due to folding and flexuring of the lithosphere, forming mountains and causing an apparent fall in sea level.

Relevance of the study of sea-level changes

• Climate change evidence: Studying sea-level changes provides crucial evidence regarding climate change.
• Tectonic upliftment rates: Sea-level changes also help us estimate the rates of tectonic upliftment in past geological periods.
• Erosion and deposition: Sea level directly affects the rate and pattern of erosion and deposition in coastal areas.
• Industrial development: Understanding sea-level fluctuations is useful in assessing the suitability of coastal locations for industrial development.
• Land availability: Fluctuations in sea level determine the availability of land, particularly in coastal areas that are important for agricultural purposes.
• Submergence of land due to sea level rise can threaten food security and be a disaster for human civilization.
• Predicting areas at risk of being submerged undersea due to climate change can help low-lying countries build coastal embankments.
• Mapping areas at risk of storm surges and flooding requires knowledge of areas likely to be affected by future sea-level rise.
• Identifying areas at risk of submergence can help set up tidal power generation units in suitable locations.

Mechanisms of the change in sea-level

• Fluctuations of sea level have three basic mechanisms:
  • Changes in ocean water volume
  • Changes in ocean basin volume
  • Changes in the geoid (shape of the earth) due to changes in the volume of ocean water.
• If the ice in Antarctica melts, the sea level would rise by 60-75 m, and if the Greenland ice cap melts, the sea level would rise by about 5 m.
• The added load of ocean water could lead to the sinking of the ocean floor due to isostatic compensation.
• The total rise of sea level in this case would be about 40-50 m.
• However, the isostatic adjustment of the land and the ocean is still not clear due to a lack of data.
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Changes in the volume of the ocean basin

• Changes in the volume of ocean basins and sea level were important during the Mesozoic and early Cenozoic Eras.
• Causes of changes in ocean basin volume:
  • Changes in mid-oceanic ridges due to plate boundary reorganization.
  • Accumulation of sediments on the ocean floor, which can cause subsidence or upliftment.
  • Impact of orogenesis, which causes a fall in sea level due to increased ocean basin volume.
• Short-term changes in sea level occur annually and are affected by:
  • Marine water density, which is controlled by temperature and salinity.
  • Atmospheric pressure, with low pressure causing higher sea level.
  • Velocity of ocean currents, which can cause a rise in sea level on outer fringes.
  • Ice formation, which causes a fall in sea level.
  • Piling up of water along windward coasts due to air mass movement.
• Short-term global sea-level rise in the 20th century due to:
  • Global warming and thermal expansion of ocean water.
• Impact of sea-level fall:
  • Changes in base level of rivers, landform rejuvenation, extended shoreline, lengthening of rivers, death of coral reefs, and extension of ice caps.