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{{Short description|Geographical reference point from which various heights are measured}}
[[File:Israel Sea Level BW 1.JPG|thumb|upright=1.35|This marker indicating sea level is situated between [[Place:Jerusalem|Jerusalem]] and the Dead Sea.]]

'''Mean sea level''' ('''MSL''', often shortened to '''sea level''') is an [[Mean|average]] surface level of one or more among [[Astronomy:Earth|Earth]]'s coastal [[Earth:Body of water|bodies of water]] from which heights such as [[Elevation|elevation]] may be measured. The global MSL is a type of [[Vertical datum|vertical datum]]{{spaced ndash}}a standardised [[Geodetic datum|geodetic datum]]{{spaced ndash}}that is used, for example, as a [[Chart datum|chart datum]] in [[Earth:Cartography|cartography]] and [[Earth:Navigation|marine navigation]], or, in aviation, as the [[Earth:Standard sea level|standard sea level]] at which [[Physics:Atmospheric pressure|atmospheric pressure]] is measured to [[Calibration|calibrate]] altitude and, consequently, aircraft flight levels. A common and relatively straightforward mean sea-level standard is instead a long-term average of [[Earth:Tide gauge|tide gauge]] readings at a particular reference location.<ref>[https://psmsl.org/train_and_info/faqs/#1 ''What is "Mean Sea Level"?''] Liverpool, UK: [[Organization:National Oceanography Centre|National Oceanography Centre]]. Retrieved 29 January 2024.</ref>

Sea levels can be affected by many factors and are known to have varied greatly over [[Earth:Geologic time scale|geological time scale]]s. Current [[Earth:Sea level rise|sea level rise]] is mainly caused by human-induced [[Earth:Climate change|climate change]].<ref>{{Cite journal |last=USGCRP |year=2017 |title=Climate Science Special Report. Chapter 12: Sea Level Rise. Key finding 1. |url=https://science2017.globalchange.gov/chapter/12/ |language=en |pages=1–470 |access-date=2018-12-27 |website=science2017.globalchange.gov |archive-date=8 December 2019 |archive-url=https://web.archive.org/web/20191208150342/https://science2017.globalchange.gov/chapter/12/ |url-status=live }}</ref> When temperatures rise, mountain [[Earth:Glacier|glacier]]s and polar [[Earth:Ice sheet|ice sheet]]s melt, increasing the amount of water in the oceans, while the existing seawater also expands with heat. Because most of human settlement and infrastructure was built in response to a more-normalized sea level with limited expected change, populations affected by sea level rise will need to invest in climate adaptation to mitigate the worst effects or, when populations are at extreme risk, a process of [[Earth:Managed retreat|managed retreat]].<ref>{{Cite journal|last1=Nicholls|first1=Robert J.|last2=Marinova|first2=Natasha|last3=Lowe|first3=Jason A.|last4=Brown|first4=Sally|last5=Vellinga|first5=Pier|last6=Gusmão|first6=Diogo de|last7=Hinkel|first7=Jochen|last8=Tol|first8=Richard S. J.|date=2011|title=Sea-level rise and its possible impacts given a 'beyond 4°C (39.2°F)world' in the twenty-first century|journal=Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences|volume=369|issue=1934|pages=161–181|doi=10.1098/rsta.2010.0291|issn=1364-503X|pmid=21115518|bibcode=2011RSPTA.369..161N|s2cid=8238425|doi-access=free}}</ref>

The term ''above sea level'' generally refers to [[Height above sea level|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&nbsp;km (3,963.191&nbsp;mi) at the equator. It is 6,356.752&nbsp;km (3,949.903&nbsp;mi) at the poles and 6,371.001&nbsp;km (3,958.756&nbsp;mi) on average.<ref>{{cite web|title=Earth Radius by Latitude Calculator|url=https://rechneronline.de/earth-radius/|access-date=22 August 2021|archive-date=15 August 2021|archive-url=https://web.archive.org/web/20210815042828/https://rechneronline.de/earth-radius/|url-status=live}}</ref> This [[Earth:Reference ellipsoid|flattened spheroid]], combined with local gravity anomalies, defines the [[Geoid|geoid]] of the Earth, which approximates the local mean sea level for locations in the open ocean. The geoid includes a significant [[Physics:Indian Ocean Geoid Low|depression in the Indian Ocean]], whose surface dips as much as {{cvt|106|m|0}} below the global mean sea level (excluding minor effects such as tides and currents).<ref name="Geoid">{{cite journal |last1=Sreejith |first1=K.M. |last2=Rajesh |first2=S. |last3=Majumdar |first3=T.J. |last4=Rao |first4=G. Srinivasa |last5=Radhakrishna |first5=M. |last6=Krishna |first6=K.S. |last7=Rajawat |first7=A.S. |title=High-resolution residual geoid and gravity anomaly data of the northern Indian Ocean – An input to geological understanding |journal=Journal of Asian Earth Sciences |date=January 2013 |volume=62 |pages=616–626 |doi=10.1016/j.jseaes.2012.11.010|bibcode=2013JAESc..62..616S }}</ref>

== Measurement ==
[[File:Recent Sea Level Rise.png|thumb|Sea level measurements from 23 long tide gauge records in geologically stable environments show a rise of around {{convert|200|mm|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.<ref name=USNRC32>US National Research Council, ''Bulletin of the National Research Council 1932'' page 270</ref> 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, [[Earth:Wind|wind]], atmospheric pressure, local gravitational differences, temperature, [[Earth:Salinity|salinity]], and so forth. The mean sea level at a particular location may be calculated over an extended time period and used as a datum. For example, hourly measurements may be averaged over a full Metonic 19-year lunar cycle to determine the mean sea level at an official [[Earth:Tide gauge|tide gauge]].<ref name=Terms>{{cite journal |last1=Gregory |first1=Jonathan M. |last2=Griffies |first2=Stephen M. |last3=Hughes |first3=Chris W. |last4=Lowe |first4=Jason A. |display-authors=0 |title=Concepts and Terminology for Sea Level: Mean, Variability and Change, Both Local and Global |journal=Surveys in Geophysics |volume=40 |pages=1251–1289 |date=29 April 2019 |doi=10.1007/s10712-019-09525-z |doi-access=free}}</ref>

''Still-water level'' or ''still-water sea level'' (SWL) is the level of the sea with motions such as wind waves averaged out.<ref>{{Cite web|url=http://glossary.ametsoc.org/wiki/Still-water_level|title=Still-water level - AMS Glossary|website=glossary.ametsoc.org|access-date=10 December 2018|archive-date=10 December 2018|archive-url=https://web.archive.org/web/20181210110949/http://glossary.ametsoc.org/wiki/Still-water_level|url-status=live}}</ref>
Then MSL implies the SWL further averaged over a period of time such that changes due to, e.g., the [[Astronomy:Tide|tide]]s, 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.<ref name=Terms/>

One often measures the values of MSL with 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.<ref>{{Cite web|url=https://www.ordnancesurvey.co.uk/benchmarks/|title=Ordnance Survey Benchmark locator|access-date=2021-12-21|archive-date=27 December 2021|archive-url=https://web.archive.org/web/20211227142006/https://www.ordnancesurvey.co.uk/benchmarks/|url-status=live}}</ref> Before 1921, the [[Vertical datum|vertical datum]] was MSL at the Victoria Dock, Liverpool.
Since the times of the [[Place:Russian Empire|Russian Empire]], in {{wipe|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|chart datum]]<ref>[https://www.hko.gov.hk/en/tide/enotes.htm "Tide: Notes"], Hong Kong Observatory. {{Webarchive|url=https://web.archive.org/web/20220927095742/https://www.hko.gov.hk/en/tide/enotes.htm |date=27 September 2022}}.</ref> 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. {{wipe|Spain}} uses the reference to measure heights below or above sea level at Alicante, while the European Vertical Reference System is calibrated to the [[Amsterdam Ordnance Datum|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.<ref>{{cite journal|title=Evaluating models of sea state bias in satellite altimetry|date=1994|doi=10.1029/94JC00478|bibcode=1994JGR....9912581G|volume=99|issue=C6|journal=Journal of Geophysical Research|page=12581|last1=Glazman|first1=Roman E|last2=Greysukh|first2=Alexander|last3=Zlotnicki|first3=Victor}}</ref> A joint mission of [[Organization:NASA|NASA]] and [[Organization:CNES|CNES]], TOPEX/Poseidon was followed by [[Earth:Jason-1|Jason-1]] in 2001 and the [[Earth:Ocean Surface Topography Mission|Ocean Surface Topography Mission]] on the Jason-2 satellite in 2008.

=== {{Anchor|Above mean sea level|AMSL}} Height above mean sea level === <!-- Other articles link here. -->
{{Main|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 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 with flight level), and in the [[Physics:Atmospheric sciences|atmospheric sciences]], and in land surveying. An alternative is to base height measurements on a [[Earth:Reference ellipsoid|reference ellipsoid]] approximating the entire Earth, which is what systems such as [[Engineering:GPS|GPS]] do. In aviation, the reference ellipsoid known as WGS84 is increasingly used to define heights; however, differences up to {{convert|100|m|0|abbr=off}} exist between this ellipsoid height and local mean sea level.<ref name="Geoid"/> Another alternative is to use a [[Geoid|geoid]]-based vertical datum such as [[North American Vertical Datum of 1988|NAVD88]] and the global [[Earth:EGM96|EGM96]] (part of WGS84). Details vary in different countries.

When referring to geographic features such as mountains, on a [[Earth:Topographic map|topographic map]] variations in elevation are shown by [[Contour line|contour line]]s. 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 as Death Valley, California, the elevation AMSL is negative.

==== Difficulties in use ====
[[File:Geoida.svg|thumb|{{ordered list |Ocean |[[Earth:Reference ellipsoid|Reference ellipsoid]] |Local plumb line |Continent |[[Geoid|Geoid]]}}]]

It is often necessary to compare the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the [[Geoid|geoid]]. In the absence of external forces, the local 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 gravity anomalies such as those measured by NASA's GRACE 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) [[Earth:Ocean surface topography|ocean surface topography]]. It varies globally in a range of ±{{nbsp}}2{{nbsp}}m.

== Dry land ==
[[File:BadwaterSL.JPG|thumb|upright=0.85|Sea level sign seen on cliff (circled in red) at Badwater Basin, Death Valley National Park]]

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.<ref>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology |date=1997 |location=Alexandria, Virginia |publisher=American Geological Institute |isbn=0922152349 |edition=4th |chapter=Relative rise in sea level}}</ref>
* "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.<ref>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology |date=1997 |location=Alexandria, Virginia |publisher=American Geological Institute |isbn=0922152349 |edition=4th |chapter=Eustatic}}</ref>
* "steric" refers to global changes in sea level due to [[Physics:Thermal expansion|thermal expansion]] and [[Earth:Salinity|salinity]] variations.<ref>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology |date=1997 |location=Alexandria, Virginia |publisher=American Geological Institute |isbn=0922152349 |edition=4th |chapter=Steric}}</ref>
* "isostatic" refers to changes in the level of the land relative to a fixed point in the earth, possibly due to thermal buoyancy or [[Earth:Tectonics|tectonic]] effects, disregarding changes in the volume of water in the oceans.

The melting of [[Earth:Glacier|glacier]]s at the end of [[Earth:Ice age|ice age]]s results in isostatic [[Earth:Post-glacial rebound|post-glacial rebound]], when land rises after the weight of ice is removed. Conversely, older volcanic islands experience relative sea level rise, due to isostatic [[Subsidence|subsidence]] from the weight of cooling volcanos. The subsidence of land due to the withdrawal of [[Earth:Groundwater|groundwater]] 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" or [[Zero-level elevation|zero-level elevation]], serves equivalently as a reference for the height of planetary features.

== {{anchor|Change}} Change ==

=== Local and eustatic ===

[[File:Mass balance atmospheric circulation.png|thumb|[[Physics:Water cycle|Water cycle]]s between ocean, atmosphere and glaciers]]

'''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 by waves and [[Astronomy:Tide|tide]]s 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 to [[Earth:Sea level rise|sea level changes]] (millimetres per year).

Some land movements occur because of [[Earth:Isostasy|isostatic]] adjustment to the melting of [[Earth:Ice sheet|ice sheet]]s at the end of the [[Earth:Last Glacial Period|last ice age]]. The weight of the ice sheet depresses the underlying land, and when the ice melts away the [[Earth:Post-glacial rebound|land slowly rebounds]]. Changes in ground-based ice volume also affect local and regional sea levels by the readjustment of the [[Geoid|geoid]] and [[Earth:True polar wander|true polar wander]]. [[Physics:Atmospheric pressure|Atmospheric pressure]], [[Earth:Ocean current|ocean current]]s 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 the [[Earth:Oceanic basin|oceanic basin]]s.<ref name="Oilfield Glossary">{{cite web|url=http://www.glossary.oilfield.slb.com/Display.cfm?Term=eustatic%20sea%20level|title=Eustatic sea level|work=Oilfield Glossary|publisher=Schlumberger Limited|access-date=10 June 2011|archive-date=2 November 2011|archive-url=https://web.archive.org/web/20111102190720/http://www.glossary.oilfield.slb.com/Display.cfm?Term=eustatic%20sea%20level|url-status=live}}</ref> 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.<ref>{{Cite web|url=http://www.climatehotmap.org/global-warming-effects/sea-level.html|title=Global Warming Effects on Sea Level|website=www.climatehotmap.org|access-date=2016-12-02|archive-date=20 November 2016|archive-url=https://web.archive.org/web/20161120171530/http://www.climatehotmap.org/global-warming-effects/sea-level.html|url-status=live}}</ref>

=== {{anchor|Short term and periodic changes}}Short-term and periodic changes ===
[[File:Global sea levels during the last Ice Age.jpg|thumb|The [[Earth:Last Glacial Period|Last Glacial Period]] caused a much lower global sea level.]]
[[File:Glaciers and Sea Level Rise (8742463970).jpg|thumb|Warming temperatures and melting glaciers are currently raising the sea level.]]

Many factors can produce short-term changes in sea level, typically within a few metres, in timeframes ranging from minutes to months:

{|
|-
! colspan="3" style="background:lightgrey;"| Periodic sea level changes
|-
| Diurnal and semidiurnal astronomical tides || 12–24 h P || 0.1–10+ m
|-
| Long-period tides || 2-week to 1-year P || <0.1 m
|-
| Pole tides ([[Earth:Chandler wobble|Chandler wobble]]) || 14-month P || 5 mm
|-
! colspan="3" style="background:lightgrey;"| Meteorological and oceanographic fluctuations
|-
| Atmospheric pressure || Hours to months || −0.7 to 1.3 m
|-
| Winds ([[Physics:Storm surge|storm surge]]s) || 1–5 days || Up to 5 m
|-
| [[Physics:Evaporation|Evaporation]] and precipitation (may also follow long-term pattern) || Days to weeks || <0.1m
|-
| Ocean surface [[Earth:Topography|topography]] (changes in water [[Physics:Density|density]] and currents) || Days to weeks || Up to 1 m
|-
| [[Earth:El Niño|El Niño]]/southern oscillation || 6 mo every 5–10 yr || Up to 0.6 m
|-
! colspan="3" style="background:lightgrey;"| Seasonal variations
|-
| Seasonal water balance among oceans (Atlantic, Pacific, Indian) || 6 months || &nbsp;
|-
| Seasonal variations in slope of water surface || 6 months || &nbsp;
|-
| River runoff/floods || 2 months || 1 m
|-
| Seasonal water density changes (temperature and [[Earth:Salinity|salinity]]) || 6 months || 0.2 m
|-
! colspan="3" style="background:lightgrey;"| Seiches
|-
| [[Physics:Seiche|Seiche]]s (standing waves) || Minutes to hours || Up to 2 m
|-
! colspan="3" style="background:lightgrey;"| Earthquakes
|-
| [[Tsunami]]s (catastrophic long-period waves) || Hours ||  0.1–10+ m
|-
| Abrupt change in land level || Minutes || Up to 10 m
|}

=== Recent changes ===
{{Further|Earth:Ocean heat content|Earth:Effects of climate change on oceans}}
{{excerpt|Sea level rise}}

== Aviation ==

Pilots can estimate height above sea level with an [[Engineering:Altimeter|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 [[Engineering:Air traffic control|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.<ref>US Federal Aviation Administration, Code of Federal Regulations [http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFar.nsf/3276afbe72d00920852566c700670189/da37f1d83828491d852566cf00615210!OpenDocument Sec. 91.121] {{Webarchive|url=https://web.archive.org/web/20090426130539/http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFar.nsf/3276afbe72d00920852566c700670189/da37f1d83828491d852566cf00615210!OpenDocument |date=26 April 2009 }}</ref>

== See also ==
* {{annotated link|Amsterdam Ordnance Datum|aka=''Normaal Amsterdams Peil''}}
* {{annotated link|Earth:Before Present}}
* {{annotated link|Chart datum}}
* {{annotated link|Earth:Extreme points of Earth}}
* {{annotated link|Geopotential height}}
* {{annotated link|Height above average terrain}}
* {{annotated link|Earth:Meltwater pulse 1A}}
* {{annotated link|Metres above the Adriatic}}
* {{annotated link|Normal height}}
* {{annotated link|Normalhöhennull|''Normalhöhennull''}}
* {{annotated link|Normalnull|''Normalnull''}}
* {{annotated link|Earth:North West Shelf Operational Oceanographic System}}
* {{annotated link|Ordnance datum}} (UK and Ireland)
* {{annotated link|Orthometric height}}
* {{annotated link|Earth:Raised beach|aka=Marine terrace}}
* {{annotated link|Vertical datum}}
* {{annotated link|World Geodetic System}}

== References ==
{{Reflist}}

== External links ==

*[https://research.csiro.au/slrwavescoast/sea-level/ Sea Level Rise:Understanding the past – Improving projections for the future]
*[https://web.archive.org/web/20100310181956/http://www.pol.ac.uk/psmsl/ Permanent Service for Mean Sea Level]
*[https://web.archive.org/web/20090708014832/http://www.agu.org/revgeophys/dougla01/dougla01.html Global sea level change: Determination and interpretation]
*[https://web.archive.org/web/20090710124721/http://yosemite.epa.gov/oar/globalwarming.nsf/content/ResourceCenterPublicationsSeaLevelRiseIndex.html Environment Protection Agency Sea level rise reports]
*[https://web.archive.org/web/20110721060035/http://www.homepage.montana.edu/~geol445/hyperglac/sealevel2/index.htm Properties of isostasy and eustasy]
*[http://sealevel.jpl.nasa.gov/ Measuring Sea Level from Space]
*[https://web.archive.org/web/20151016104450/http://www.scivee.tv/node/8324 Rising Tide Video: Scripps Institution of Oceanography]
*[http://tidesandcurrents.noaa.gov/sltrends/sltrends.shtml Sea Levels Online: National Ocean Service (CO-OPS)]
*[http://www.sonel.org/ Système d'Observation du Niveau des Eaux Littorales (SONEL)]
*[https://web.archive.org/web/20120603123140/http://ice.tsu.ru/index.php?option=com_content&view=category&layout=blog&id=1&Itemid=138 Sea level rise – How much and how fast will sea level rise over the coming centuries?]
{{physical oceanography|expanded=other}}

[[Category:Geodesy]]
[[Category:Physical oceanography]]
[[Category:Oceanographical terminology]]
[[Category:Vertical datums]]

{{Sourceattribution|Sea level}}