Introduction to Sea Level
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.
FAQs on Sea Level
1. What is sea level and why is its measurement important for geographers?
Sea level is the average height of the Earth's oceans, used as the primary reference point or datum for measuring land elevation (altitude) and ocean depth (bathymetry). Its measurement is crucial for several reasons:
Cartography: It helps in creating accurate topographical maps showing heights of mountains and depths of oceans.
Climate Science: Tracking changes in sea level is a key indicator of global warming and climate change.
Coastal Management: Understanding sea level is vital for planning and protecting coastal communities from flooding and erosion.
2. What are the primary causes of global sea-level rise?
The two main factors contributing to the rise in global mean sea level, primarily driven by climate change, are:
Thermal Expansion: As the ocean water warms up from absorbing heat from the atmosphere, it expands, taking up more space. This is the largest contributor to current sea-level rise.
Melting of Land-Based Ice: Meltwater from glaciers, ice caps, and the massive ice sheets of Greenland and Antarctica flows into the oceans, adding to their total volume.
3. How is Mean Sea Level (MSL) used as a reference point in India?
In India, Mean Sea Level (MSL) is used as the standard datum for all vertical height measurements. For decades, the MSL established at the port of Mumbai was the official benchmark. All elevations on topographical maps, railway lines, and in civil engineering projects, such as the height of dams or bridges, are specified as being a certain number of metres 'above MSL'. This ensures a uniform standard of height measurement across the country.
4. What is the difference between the highest point measured from sea level and the tallest mountain from its base?
This illustrates the importance of the reference point:
Highest Point from Sea Level: Mount Everest is the highest point on Earth, with its peak at 8,848.86 metres above sea level.
Tallest Mountain from Base: Mauna Kea in Hawaii is the tallest mountain when measured from its base on the ocean floor to its peak. While its summit is only 4,207 metres above sea level, its total height from the seafloor is over 10,000 metres, making it taller than Mount Everest in total structure.
5. Why is the melting of land glaciers a major concern for sea-level rise, but the melting of sea ice is not?
This is due to a key difference in where the ice is located. Glaciers and ice sheets are on land. When they melt, they add a new volume of water to the ocean that was not there before, directly causing the sea level to rise. In contrast, sea ice is already floating in the ocean. According to Archimedes' principle, it already displaces a volume of water equal to its total mass. When it melts, it turns into a volume of water that is almost exactly the same as the volume it was displacing, resulting in a negligible change to the overall sea level.
6. What are some of the major consequences of rising sea levels for coastal regions?
Rising sea levels have severe consequences for coastal environments and communities. Key examples include:
Increased Coastal Flooding: Higher sea levels lead to more frequent and extensive flooding, especially during high tides and storm surges.
Coastal Erosion: The shoreline is worn away more aggressively, causing land loss and threatening coastal infrastructure.
Saltwater Intrusion: Saline water from the sea can move into freshwater rivers and underground aquifers, contaminating drinking water and harming agriculture.
Habitat Destruction: Coastal ecosystems like mangroves and wetlands, which serve as crucial habitats and natural defences, are destroyed.
7. How can the sea level appear to fall in certain regions even when the global average is rising?
Local sea level can appear to fall in specific areas due to a process called isostatic adjustment or post-glacial rebound. During the last Ice Age, the immense weight of ice sheets compressed the landmass beneath them. As this ice melted, the land began to slowly rise back up. In regions like Scandinavia and parts of Canada, the land is still rising faster than the global sea level is, creating a relative drop in the local sea level against the rising land.
8. Is the surface of the ocean perfectly flat and at the same level everywhere?
No, the ocean's surface is not perfectly flat. It has permanent 'hills' and 'valleys' due to several factors. The most significant is the Earth's uneven gravitational field; areas with denser rock on the ocean floor exert a stronger gravitational pull, drawing more water towards them and creating a 'hill' on the surface that can be several metres high. Ocean currents, wind patterns, and variations in water temperature and salinity also cause regional differences in sea surface height.
