Mean sea level (MSL, often shortened to sea level) is an
Sea levels can be affected by many factors and are known to have varied greatly over geological time scales. Current sea level rise is mainly caused by human-induced climate change. When temperatures rise, mountain glaciers and the polar ice caps melt, increasing the amount of water in water bodies. Because most of human settlement and infrastructure was built in response to a more normalized sea level with limited expected change, populations affected by climate change in connection to sea level rise will need to invest in climate adaptation to mitigate the worst effects or when populations are in extreme risk, a process of managed retreat.
The term above sea level generally refers to
Earth's radius at sea level is 6,378.137 km (3,963.191 mi) at the equator. It is 6,356.752 km (3,949.903 mi) at the poles and 6,371.001 km (3,958.756 mi) on average. This variation from a perfect sphere is the geoid of the Earth. It causes a significant depression in the Indian Ocean, about 1,200 km (746 mi) southwest of India, where the surface reaches a depth of 106 m (348 ft) below the global mean sea level.
Precise determination of a "mean sea level" is difficult because of the many factors that affect sea level.
Still-water level or still-water sea level (SWL) is the level of the sea with motions such as
One often measures the values of MSL in respect to the land; hence a change in relative MSL can result from a real change in sea level, or from a change in the height of the land on which the tide gauge operates. In the UK, the
Satellite altimeters have been making precise measurements of sea level
Height above mean sea level
Height above mean sea level (AMSL) is the elevation (on the ground) or altitude (in the air) of an object, relative to the average sea level datum. It is also used in aviation, where some heights are recorded and reported with respect to mean sea level (MSL) (contrast with
When referring to
Difficulties in use
To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the
Several terms are used to describe the changing relationships between sea level and dry land.
- "relative" means change relative to a fixed point in the sediment pile.
- "eustatic" refers to global changes in sea level relative to a fixed point, such as the centre of the earth, for example as a result of melting ice-caps.
- "steric" refers to global changes in sea level due to thermal expansion and salinity variations.
- "isostatic" refers to changes in the level of the land relative to a fixed point in the earth, possibly due to thermal buoyancy or tectonic effects; it implies no change in the volume of water in the oceans.[dubious ]
On planets that lack a liquid ocean,
Local and eustatic
Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time (such as a month or a year) long enough that fluctuations caused by
Some land movements occur because of isostatic adjustment of the mantle to the melting of ice sheets at the end of the last ice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away the land slowly rebounds. Changes in ground-based ice volume also affect local and regional sea levels by the readjustment of the geoid and true polar wander. Atmospheric pressure, ocean currents and local ocean temperature changes can affect LMSL as well.
Eustatic sea level change (as opposed to local change) results in an alteration to the global sea levels due to changes in either the volume of water in the world's oceans or net changes in the volume of the oceanic basins.
Short-term and periodic changes
There are many factors which can produce short-term (a few minutes to 14 months) changes in sea level. Two major mechanisms are causing sea level to rise. First, shrinking land ice, such as mountain glaciers and polar ice sheets, is releasing water into the oceans. Second, as ocean temperatures rise, the warmer water expands.
|Periodic sea level changes|
|Diurnal and semidiurnal astronomical tides||12–24 h P||0.2–10+ m|
|Rotational variations (Chandler wobble)||14-month P|
|Meteorological and oceanographic fluctuations|
|Atmospheric pressure||Hours to months||−0.7 to 1.3 m|
|Winds (storm surges)||1–5 days||Up to 5 m|
precipitation(may also follow long-term pattern)
|Days to weeks|
|Ocean surface topography (changes in water density and currents)||Days to weeks||Up to 1 m|
|6 mo every 5–10 yr||Up to 0.6 m|
|Seasonal water balance among oceans (Atlantic, Pacific, Indian)|
|Seasonal variations in slope of water surface|
|River runoff/floods||2 months||1 m|
|Seasonal water density changes (temperature and salinity)||6 months||0.2 m|
|Seiches (standing waves)||Minutes to hours||Up to 2 m|
|Tsunamis (generate catastrophic long-period waves)||Hours||Up to 10 m|
|Abrupt change in land level||Minutes||Up to 10 m|
Between 1901 and 2018, the average global sea level rose by 15–25 cm (6–10 in), or an average of 1–2 mm per year.
Rising seas ultimately impact every coastal and island population on Earth.
At the same time, local factors like
Pilots can estimate height above sea level with an
- Above ground level– Height measured with respect to the underlying ground surface
- Before Present – Time scale used mainly in archaeology, geology, and other scientific disciplines
- Chart datum – Level of water from which depths displayed on a nautical chart are measured
- Extreme points of Earth– List of extreme geographical points and other geophysical records on Earth
- Geopotential height – Type of altitude above mean sea level
- Height above average terrain – Height based on large area surrounding object; often used in U.S. for antenna towers
- List of places on land with elevations below sea level
- Raised beach, also known as Marine terrace – Emergent coastal landform
- Meltwater pulse 1A – Period of rapid post-glacial sea level rise
- Metres above the Adriatic – elevation measure
- Amsterdam Ordnance Datum, also known as Normaal Amsterdams Peil – Vertical datum
- Normal height
- Normalhöhennull – Vertical datum used in Germany
- Normalnull – Outdated official vertical datum used in Germany
- North West Shelf Operational Oceanographic System – Facility that monitors physical, sedimentological and ecological variables for the North Sea area
- Ordnance datum – Vertical datum used as the basis for deriving altitudes on maps (UK and Ireland)
- Orthometric height – Altitude above geoid or mean sea level
- Sea level equation– Rise of land masses after glacial period
- Sea level drop
- Vertical datum – Reference surface for vertical positions
- World Geodetic System – Geodetic reference system
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