The termabove sea level generally refers to theheight above mean sea level (AMSL). The termAPSL 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.[2] Thisflattened spheroid, combined with localgravity anomalies, defines thegeoid of the Earth, which approximates the local mean sea level for locations in the open ocean. The geoid includes a significantdepression in the Indian Ocean, whose surface dips as much as 106 m (348 ft) below the global mean sea level (excluding minor effects such as tides and currents).[3]
Sea level measurements from 23 longtide gauge records in geologically stable environments show a rise of around 200 millimetres (7.9 in) during the 20th century (2 mm/year).
Precise determination of a "mean sea level" is difficult because of the many factors that affect sea level.[4] Instantaneous sea level varies substantially on several scales of time and space. This is because the sea is in constant motion, affected by the tides, tsunamis,wind, atmospheric pressure, local gravitational differences, temperature,salinity, and so forth. The mean sea level at a particular location may be calculated over an extended time period and used as adatum. For example, hourly measurements may be averaged over a fullMetonic 19-year lunar cycle to determine the mean sea level at an officialtide gauge.[5]
Still-water level orstill-water sea level (SWL) is the level of the sea with motions such aswind waves averaged out.[6]Then MSL implies the SWL further averaged over a period of time such that changes due to, e.g., thetides, also have zero mean.Global MSL refers to a spatial average over the entire ocean area, typically using large sets of tide gauges and/or satellite measurements.[5]
One often measures the values of MSL with respect to the land; hence a change inrelative MSL or (relative sea level) 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, or both.In the UK, theordnance datum (the 0 metres height on UK maps) is the mean sea level measured atNewlyn in Cornwall between 1915 and 1921.[7] Before 1921, thevertical datum was MSL at theVictoria Dock, Liverpool.Since the times of theRussian Empire, inRussia and its other former parts, now independent states, the sea level is measured from the zero level ofKronstadt Sea-Gauge. In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of 0.146 m (5.7 in) abovechart datum[8] and 1.304 m (4 ft 3.3 in) below the average sea level.In France, the Marégraphe in Marseille 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 atAlicante, while theEuropean Vertical Reference System is calibrated to theAmsterdam Peil elevation, which dates back to the 1690s.
Satellite altimeters have been making precise measurements of sea level since the launch ofTOPEX/Poseidon in 1992.[9] A joint mission ofNASA andCNES, TOPEX/Poseidon was followed byJason-1 in 2001 and theOcean 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 a reference datum for mean sea level (MSL). It is also used in aviation, where some heights are recorded and reported with respect to mean sea level (contrast withflight level), and in theatmospheric sciences, and inland surveying. An alternative is to base height measurements on areference ellipsoid approximating the entire Earth, which is what systems such asGPS do. In aviation, the reference ellipsoid known asWGS84 is increasingly used to define heights; however, differences up to 100 metres (328 feet) exist between this ellipsoid height and local mean sea level.[3] Another alternative is to use ageoid-based verticaldatum such asNAVD88 and the globalEGM96 (part of WGS84). Details vary in different countries.
When referring togeographic features such as mountains, on atopographic map variations in elevation are shown bycontour lines. A mountain's highest point or summit is typically illustrated with the AMSL height in metres, feet or both. In unusual cases where a land location is below sea level, such asDeath Valley, California, the elevation AMSL is negative.
It is often necessary to compare the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called thegeoid. In the absence of external forces, the local mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth'sgravitational field which, in itself, does not conform to a simple sphere or ellipsoid and exhibitsgravity anomalies such as those measured by NASA'sGRACE satellites. In reality, the geoid surface is not directly observed, even as a long-term average, due to ocean currents, air pressure variations, temperature and salinity variations, etc. The location-dependent but time-persistent separation between local mean sea level and the geoid is referred to as (mean)ocean surface topography. It varies globally in a typical range of ±1 m (3 ft).[10]
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.[11]
"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.[12]
"isostatic" refers to changes in the level of the land relative to a fixed point in the earth, possibly due to thermal buoyancy ortectonic effects, disregarding changes in the volume of water in the oceans.
The melting ofglaciers at the end ofice ages results in isostaticpost-glacial rebound, when land rises after the weight of ice is removed. Conversely, older volcanic islands experiencerelative sea level rise, due to isostaticsubsidence from the weight of cooling volcanos. The subsidence of land due to the withdrawal ofgroundwater is another isostatic cause of relative sea level rise.
On 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" orzero-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 long enough that fluctuations caused bywaves andtides are smoothed out, typically a year or more. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can occur at rates similar tosea level changes (millimetres per year).
Some land movements occur because ofisostatic adjustment to the melting ofice sheets at the end of thelast ice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away theland slowly rebounds. Changes in ground-based ice volume also affect local and regional sea levels by the readjustment of thegeoid andtrue polar wander.Atmospheric pressure,ocean currents and local ocean temperature changes can affect LMSL as well.
Eustatic sea level change (global as opposed to local change) is due to change in either the volume of water in the world's oceans or the volume of theoceanic basins.[14] Two major mechanisms are currently causing eustatic sea level 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.[15]
The sea level has been rising since the end of theLast Glacial Maximum, which was around 20,000 years ago.[16] Between 1901 and 2018, the average sea level rose by 15–25 cm (6–10 in), with an increase of 2.3 mm (0.091 in) per year since the 1970s.[17]: 1216 This was faster than the sea level had ever risen over at least the past 3,000 years.[17]: 1216 The rate accelerated to 4.62 mm (0.182 in)/yr for the decade 2013–2022.[18]Climate change due to human activities is the main cause of this persistent acceleration.[19]: 5, 8 Between 1993 and 2018, meltingice sheets andglaciers accounted for 44% ofsea level rise, with another 42% resulting fromthermal expansion ofwater.[20]: 1576
Sea level rise lags behind changes in theEarth's temperature by decades, and sea level rise will therefore continue to accelerate between now and 2050 in response to warming that has already happened.[21] What happens after that depends on future humangreenhouse gas emissions. If there are very deep cuts in emissions, sea level rise would slow between 2050 and 2100. The reported factors of increase in flood hazard potential are often exceedingly large, ranging from 10 to 1000 for even modest sea-level rise scenarios of 0.5 m or less.[22] It could then reach by 2100 between 30 cm (1 ft) and 1.0 m (3+1⁄3 ft) from now and approximately 60 cm (2 ft) to130 cm (4+1⁄2 ft) from the 19th century. With high emissions it would instead accelerate further, and could rise by 50 cm (1.6 ft) or even by 1.9 m (6.2 ft) by 2100.[23][19][17]: 1302 In the long run, sea level rise would amount to 2–3 m (7–10 ft) over the next 2000 years if warming stays to its current 1.5 °C (2.7 °F) over the pre-industrial past. It would be 19–22 metres (62–72 ft) if warming peaks at 5 °C (9.0 °F).[19]: 21
Rising seas affect every coastal population on Earth.[24] This can be through flooding, higherstorm surges,king tides, and increased vulnerability totsunamis. There are many knock-on effects. They lead to loss of coastalecosystems likemangroves.Crop yields may reduce because ofincreasing salt levels inirrigation water. Damage to ports disrupts sea trade.[25][26] The sea level rise projected by 2050 will expose places currently inhabited by tens of millions of people to annual flooding. Without a sharp reduction in greenhouse gas emissions, this may increase to hundreds of millions in the latter decades of the century.[27]
Local factors liketidal range or landsubsidence will greatly affect the severity of impacts. For instance,sea level rise in the United States is likely to be two to three times greater than the global average by the end of the century.[28][29] Yet, of the 20 countries with the greatest exposure to sea level rise, twelve are inAsia, includingIndonesia,Bangladesh and the Philippines.[30] The resilience andadaptive capacity of ecosystems and countries also varies, which will result in more or less pronounced impacts.[31] The greatestimpact on human populations in the near term will occur in low-lyingCaribbean andPacific islands includingatolls. Sea level rise will make many of them uninhabitable later this century.[32]
Societies can adapt to sea level rise in multiple ways.Managed retreat,accommodating coastal change, or protecting against sea level rise through hard-construction practices likeseawalls[33] are hard approaches. There are also soft approaches such asdune rehabilitation andbeach nourishment. Sometimes these adaptation strategies go hand in hand. At other times choices must be made among different strategies.[34] Poorer nations may also struggle to implement the same approaches to adapt to sea level rise as richer states.
Pilots can estimate height above sea level with analtimeter set to a definedbarometric 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 eitherQNH or "altimeter" and is transmitted to the pilot by radio fromair traffic control (ATC) or anautomatic 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 theinternational standard atmosphere (ISA) pressure at MSL which is 1013.25 hPa or 29.92 inHg.[35]
^abSreejith, K.M.; Rajesh, S.; Majumdar, T.J.; Rao, G. Srinivasa; Radhakrishna, M.; Krishna, K.S.; Rajawat, A.S. (January 2013). "High-resolution residual geoid and gravity anomaly data of the northern Indian Ocean – An input to geological understanding".Journal of Asian Earth Sciences.62:616–626.Bibcode:2013JAESc..62..616S.doi:10.1016/j.jseaes.2012.11.010.
^Jackson, Julia A., ed. (1997). "Relative rise in sea level".Glossary of geology (4th ed.). Alexandria, Virginia: American Geological Institute.ISBN0922152349.
^Jackson, Julia A., ed. (1997). "Eustatic".Glossary of geology (4th ed.). Alexandria, Virginia: American Geological Institute.ISBN0922152349.
^Jackson, Julia A., ed. (1997). "Steric".Glossary of geology (4th ed.). Alexandria, Virginia: American Geological Institute.ISBN0922152349.
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^abcFox-Kemper, B.;Hewitt, Helene T.; Xiao, C.; Aðalgeirsdóttir, G.; Drijfhout, S. S.; Edwards, T. L.; Golledge, N. R.; Hemer, M.; Kopp, R. E.; Krinner, G.; Mix, A. (2021). Masson-Delmotte, V.; Zhai, P.; Pirani, A.; Connors, S. L.; Péan, C.; Berger, S.; Caud, N.; Chen, Y.; Goldfarb, L. (eds.)."Chapter 9: Ocean, Cryosphere and Sea Level Change"(PDF).Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK and New York, US.Archived(PDF) from the original on 24 October 2022. Retrieved18 October 2022.
^National Academies of Sciences, Engineering, and Medicine (2011)."Synopsis".Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia. Washington, DC: The National Academies Press. p. 5.doi:10.17226/12877.ISBN978-0-309-15176-4.Archived from the original on 30 June 2023. Retrieved11 April 2022.Box SYN-1: Sustained warming could lead to severe impacts
^Bindoff, N. L.; Willebrand, J.; Artale, V.;Cazenave, A.; Gregory, J.; Gulev, S.; Hanawa, K.; Le Quéré, C.; Levitus, S.; Nojiri, Y.; Shum, C. K.; Talley, L. D.; Unnikrishnan, A. (2007)."Observations: Ocean Climate Change and Sea Level: §5.5.1: Introductory Remarks". In Solomon, S.; Qin, D.; Manning, M.; Chen, Z.; Marquis, M.; Averyt, K. B.; Tignor, M.; Miller, H. L. (eds.).Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press.ISBN978-0-521-88009-1. Archived fromthe original on 20 June 2017. Retrieved25 January 2017.
^Shaw, R., Y. Luo, T. S. Cheong, S. Abdul Halim, S. Chaturvedi, M. Hashizume, G. E. Insarov, Y. Ishikawa, M. Jafari, A. Kitoh, J. Pulhin, C. Singh, K. Vasant, and Z. Zhang, 2022:Chapter 10: AsiaArchived 2023-04-12 at theWayback Machine. InClimate Change 2022: Impacts, Adaptation and VulnerabilityArchived 2022-02-28 at theWayback Machine [H.-O. Pörtner, D. C. Roberts, M. Tignor, E. S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke, V. Möller, A. Okem, B. Rama (eds.)]. Cambridge University Press, Cambridge, UK and New York, US, pp. 1457–1579.doi:10.1017/9781009325844.012.
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