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Introduction to Sea Level

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Sea level is the base level considered for measuring the elevation and depth on our planet Earth. Sea level is the position of the air-sea interface. The ocean is a continuous body of water and its surface tends to seek the same level throughout the world. But, various factors such as winds, currents, river discharges, and variations in gravity and temperature, prevent the sea surface from being at a true level. Sea levels are measured in relation to the adjacent land. Just like the oceans, the land elevation also rises and falls over time. 

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Image: The graph portrays the sea-level changes since 1880

Sea Level Rise or Sea Level Change

Sea level rise is the increase in the sea level or ocean levels due to global warming. Two factors related to global warming are the primary cause of sea-level rise - the melting of ice sheets and glaciers and the expansion of seawater when it warms. Pollution is one of the main causes of global warming as carbon dioxide and other heat-trapping gases are released in the process into the atmosphere. The oceans absorb most of this heat and the ocean water becomes warmer. As a result, the ocean starts expanding, there is rising water and there is a rise in sea level. 

The glaciers and the ice sheets present in the land are also affected by global warming. Regions such as Greenland and Antarctica are covered with ice that melts in the summer (or warmer weather) and is again replenished in the winter. With an unexpected rise in temperature over the globe, these glaciers and ice sheets are experiencing a disproportionate amount of melting, and that too at a rapid rate.

Consequences of Sea Level Rise

The sea-level rise or the ocean rising is a serious threat to life on earth. Life along the coasts is at a greater threat. As the sea level rises, there could be flooding, storm surges, and heavy damage to the coastal areas. The people and the wildlife around the coasts can be displaced from their homes. The rising sea levels can contaminate the soil and the groundwater with its salt. 

Mean Sea Level

Mean sea level is the average height of the sea. It is the average level of the surface of a water body (mainly oceans) from which the heights (elevation) may be measured. Mean sea level is calculated as the average height of the sea over longer periods of time (months or years) and the shorter periods of time of tides and storm surges. The absolute mean sea level reflects the change in sea height and the relative mean sea level rise reflects the change in sea height and changes in the level of the land at a local scale. 

The changes in sea level are caused by factors such as changes in water volume and by variations in the shape of oceanic basins over geological time scales. The main factors that cause the increase in the volume of the ocean are:

  • Melting of ice sources that are on lands such as glaciers, ice caps, and ice sheets of Greenland and Antarctica.

  • The thermal expansion of seawater when it's heated up. 

  • When the water storage on land changes.

Vertical land movements are a result of natural geological and anthropogenic processes. Natural geological processes include tectonic movements and the glacial isostatic adjustment. Anthropogenic processes result in subsidence. 

Eustatic and Relative Sea-Level Changes

In Eustatic sea-level change, Eustatic came from the word “eustasy” which was coined by the Austrian geologist Edward Suess in the year 1888. It is derived from the ancient Greek words “eu” meaning well and “statikos” meaning static. As per Suess, change(rise) in ocean level and melting of ice sheets will result in global uniform mean sea level rise (change). Both mean sea level rise and solid earth surface, move vertically together and contribute to the uneven topographic and bathymetric variations. These factors which change the sea levels are called relative as the land and the ocean move together and with respect to each other. The sea-level change that is observed with respect to the land-based reference frame is the relative sea-level change.

The reconstruction of eustatic sea level at different time scales is used to study the melting of glaciers and ice sheets (mainly the ice) and the warming of water masses. The relative sea-level change is used to investigate regional or local processes. 

FAQ (Frequently Asked Questions)

1. What is the Relation Between Sea Level and the Adjacent Land?

Ans: The sea level is measured with respect to the adjacent land. The huge weight of the glaciers pushes the land downwards somewhere near sea level. This land may bounce back, known as the post-glacial rebound. 

2. Name the Lowest Place on Earth’s Surface.

Ans: The lowest place on Earth is the dead sea. It stands between the borders of Jordan and Israel and its elevation is 1312 feet below sea level. But, another fact is that if we measure the depth from the ocean floor and not the ocean water level, the Challenger Deep (located in the Pacific Ocean) is the lowest place on the Earth with an elevation of 36,200 feet below the ocean floor.

3. Name the Highest Place on Earth’s Surface.

Ans: Just like the lowest place, there are two highest places on Earth’s surface depending on the level from which their height is measured. If measured from the water level of the oceans, Mount Everest is the highest point on Earth. It stands at 29,028 feet above sea level. If the measurement is done from the ocean floor, the highest place on Earth’s surface is the peak of the volcano Mauna Kea. It is situated in the US state of Hawaii with an elevation of 33,476 feet from the ocean floor.