Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datum – a standardised geodetic datum – that is used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and, consequently, aircraft flight levels. A common and relatively straightforward mean sea-level standard is instead the midpoint between a mean low and mean high tide at a particular location. 
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[ citation needed] 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 above mean sea level (AMSL). The term APSL means above present sea level, comparing sea levels in the past with the level today.
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. 
Precise determination of a "mean sea level" is difficult because of the many factors that affect sea level.  Instantaneous sea level varies quite a lot on several scales of time and space. This is because the sea is in constant motion, affected by the tides, wind, atmospheric pressure, local gravitational differences, temperature, salinity, and so forth. The easiest way this may be calculated is by selecting a location and calculating the mean sea level at that point and using it as a datum. For example, a period of 19 years of hourly level observations may be averaged and used to determine the mean sea level at some measurement point.
Still-water level or still-water sea level (SWL) is the level of the sea with motions such as wind waves averaged out.  Then MSL implies the SWL further averaged over a period of time such that changes due to, e.g., the tides, also have zero mean. Global MSL refers to a spatial average over the entire ocean.
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 ordnance datum (the 0 metres height on UK maps) is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921.  Before 1921, the vertical datum was MSL at the Victoria Dock, Liverpool. Since the times of the Russian Empire, in Russia and its other former parts, now independent states, the sea level is measured from the zero level of Kronstadt Sea-Gauge. In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of 0.146 m above chart datum and 1.304 m below the average sea level.  In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collated data about the sea level. It is used for a part of continental Europe and the main part of Africa as the official sea level. Spain uses the reference to measure heights below or above sea level at Alicante, and another European vertical elevation reference (European Vertical Reference System) is to the Amsterdam Peil elevation, which dates back to the 1690s.
Satellite altimeters have been making precise measurements of sea level  since the launch of TOPEX/Poseidon in 1992. A joint mission of NASA and CNES, TOPEX/Poseidon was followed by Jason-1 in 2001 and the Ocean Surface Topography Mission on the Jason-2 satellite in 2008.
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 flight level), and in the atmospheric sciences, and land surveying. An alternative is to base height measurements on an ellipsoid of the entire Earth, which is what systems such as GPS do. In aviation, the ellipsoid known as World Geodetic System 84 is increasingly used to define heights; however, differences up to 100 metres (328 feet)[ citation needed] exist between this ellipsoid height and mean tidal height. The alternative is to use a geoid-based vertical datum such as NAVD88 and the global EGM96 (part of WGS84).
When referring to geographic features such as mountains on a topographic map, variations in elevation are shown by contour lines. The elevation of a mountain denotes the highest point or summit and is typically illustrated as a small circle on a topographic map with the AMSL height shown in metres, feet or both.
In the rare case that a location is below sea level, the elevation AMSL is negative. For one such case, see Amsterdam Airport Schiphol.
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 geoid. In a state of rest or absence of external forces, the mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's gravitational field which, in itself, does not conform to a simple sphere or ellipsoid and exhibits measurable variations such as those measured by NASA's GRACE satellites to determine mass changes in ice-sheets and aquifers. In reality, this ideal does not occur due to ocean currents, air pressure variations, temperature and salinity variations, etc., not even as a long-term average. The location-dependent, but persistent in time, separation between mean sea level and the geoid is referred to as (mean) ocean surface topography. It varies globally in a range of ± 2 m.
Several terms are used to describe the changing relationships between sea level and dry land.
The melting of glaciers at the end of ice ages is one example of eustatic sea level rise. The subsidence of land due to the withdrawal of groundwater is an isostatic cause of relative sea level rise.
Paleoclimatologists can track sea level by examining the rocks deposited along coasts that are very tectonically stable, like the east coast of North America. Areas like volcanic islands are experiencing relative sea level rise as a result of isostatic cooling of the rock which causes the land to sink.
On other planets that lack a liquid ocean, planetologists can calculate a "mean altitude" by averaging the heights of all points on the surface. This altitude, sometimes referred to as a "sea level" or zero-level elevation, serves equivalently as a reference for the height of planetary features.
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 waves and tides are smoothed out. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can be of the same order (mm/yr) as sea level changes. 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. 
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|
|Evaporation and 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|
|El Niño/ southern oscillation||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 1–2 mm per year.  This rate is increasing; sea levels are now rising at a rate of 3.7 mm (0.146 inches) per year.  Human-caused climate change is predominantly the cause, as it constantly heats (and thus expands) the ocean and melts land-based ice sheets and glaciers.  Between 1993 and 2018, thermal expansion of water contributed 42% to sea level rise (SLR); melting of temperate glaciers contributed 21%; Greenland contributed 15%; and Antarctica contributed 8%. : 1576 Because sea level rise lags changes in Earth temperature, it will continue to accelerate between now and 2050 purely in response to already-occurring warming;  whether it continues to accelerate after that depends on human greenhouse gas emissions. If global warming is limited to 1.5 °C (2.7 °F), then sea level rise does not accelerate, but it would still amount to 2–3 m (7–10 ft) over the next 2000 years, while 2–6 m (7–20 ft) would occur if the warming peaks at 2 °C (3.6 °F) and 19–22 metres (62–72 ft) if it peaks at 5 °C (9.0 °F). : 21
Rising seas pose both a direct risk of flooding unprotected areas and indirect threats of higher storm surges, king tides, and tsunamis. They are also associated with second-order effects such as loss of coastal ecosystems like mangroves, losses in crop production due to freshwater salinization of irrigation water, and the disruption of sea trade due to damaged ports.    Globally, just the projected sea level rise by 2050 will expose places currently inhabited by tens of millions of people to annual flooding. This may increase to hundreds of millions in the latter decades of the century if greenhouse gas emissions are not reduced drastically.  While slow increases in sea level may allow time for adaptation, such as building sea walls,  the passage of time can also increase the number of people at risk, as many coastal areas have large population growth. Later in the century, millions more would be affected in cities such as Miami, Rio de Janeiro, Osaka and Shanghai under the warming of 3 °C (5.4 °F), which is close to the current trajectory.  
While the rise in sea levels ultimately impacts every coastal and island population on Earth,   it does not occur uniformly due to local factors like tides, currents, storms, tectonic effects and land subsidence. Moreover, the differences in resilience and adaptive capacity of ecosystems, sectors, and countries again mean that the impacts will be highly variable.  For instance, sea level rise along US coasts (and along the US East Coast in particular) is already higher than the global average, and it is expected to be 2 to 3 times greater than the global average by the end of the century.   Yet Asia will be the region where sea level rise would impact the most people. Eight Asian countries— Bangladesh, China, India, Indonesia, Japan, the Philippines, Thailand and Vietnam—account for 70% of the global population exposed to sea level rise and land subsidence. Out of the 20 countries with the greatest exposure to sea level rise, 12 are in Asia.  Areas not directly exposed to rising sea levels could still be effected by issues such as large scale migrations and economic disruption. Finally, the greatest near-term impact on human populations will occur in the low-lying Caribbean and Pacific islands—many of those would be rendered uninhabitable by sea level rise later this century. Societies can adapt to sea level rise in three ways: by managed retreat, by accommodating coastal change, or by protecting against sea level rise through hard-construction practices like seawalls or soft approaches such as dune rehabilitation and beach nourishment. Sometimes these adaptation strategies go hand in hand; at other times choices must be made among different strategies.  A managed retreat strategy is difficult if an area's population is quickly increasing: this is a particularly acute problem for Africa, where the population of low-lying coastal areas is projected to increase by around 100 million people within the next 40 years.  Poorer nations may also struggle to implement the same approaches to adapt to sea level rise as richer states, and sea level rise at some locations may be compounded by other environmental issues, such as subsidence in so-called sinking cities.  Coastal ecosystems typically adapt to rising sea levels by moving inland; but may not always be able to do so, due to natural or artificial barriers. 
Pilots can estimate height above sea level with an altimeter set to a defined barometric pressure. Generally, the pressure used to set the altimeter is the barometric pressure that would exist at MSL in the region being flown over. This pressure is referred to as either QNH or "altimeter" and is transmitted to the pilot by radio from air traffic control (ATC) or an automatic terminal information service (ATIS). Since the terrain elevation is also referenced to MSL, the pilot can estimate height above ground by subtracting the terrain altitude from the altimeter reading. Aviation charts are divided into boxes and the maximum terrain altitude from MSL in each box is clearly indicated. Once above the transition altitude, the altimeter is set to the international standard atmosphere (ISA) pressure at MSL which is 1013.25 hPa or 29.92 inHg. 
This corresponds to a mean sea-level rise of about 7.5 cm over the whole altimetry period. More importantly, the GMSL curve shows a net acceleration, estimated to be at 0.08mm/yr2.
Box SYN-1: Sustained warming could lead to severe impacts