Sea level

Source: Wikipedia, the free encyclopedia.
For other uses, see Sea level (disambiguation).

This marker indicating sea level is situated between Jerusalem and the Dead Sea.

Mean sea level (MSL, often shortened to sea level) is an

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 a long-term average of tide gauge readings at a particular reference location.[1]

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.[2] When temperatures rise, mountain glaciers and polar ice sheets 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 managed retreat.[3]

The term above sea level generally refers to the height 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.

gravity anomalies, defines the geoid of the Earth, which approximates the local mean sea level for locations in the open ocean. The geoid includes a significant depression 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).[5]

Measurement

Sea level measurements from 23 long tide 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.

Metonic 19-year lunar cycle to determine the mean sea level at an official tide gauge.[7]

Still-water level or still-water sea level (SWL) is the level of the sea with motions such as

wind waves averaged out.[8]
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 area, typically using large sets of tide gauges and/or satellite measurements.[7]

One often measures the values of MSL with respect to the land; hence a change in relative 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, 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 (5.7 in) above chart datum[10] and 1.304 m (4 ft 3.3 in) 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.
European Vertical Reference System is calibrated 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

Ocean Surface Topography Mission
on the Jason-2 satellite in 2008.

Height above mean sea level

Main article: 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

EGM96
(part of WGS84). Details vary in different countries.

When referring to

Death Valley, California
, the elevation AMSL is negative.

Difficulties in use

plumb line
  • Continent
  • 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

    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) ocean surface topography. It varies globally in a typical range of ±1 m (3 ft).[12]

    Dry land

    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.[13]
    • "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.[14]
    • "steric" refers to global changes in sea level due to thermal expansion and salinity variations.[15]
    • "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, disregarding changes in the volume of water in the oceans.

    The melting of glaciers at the end of ice ages results in isostatic 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 from the weight of cooling volcanos. The subsidence of land due to the withdrawal of groundwater is another isostatic cause of relative sea level rise.

    On planets that lack a liquid ocean,

    zero-level elevation
    , serves equivalently as a reference for the height of planetary features.

    Change

    See also: Past sea level and sea level rise

    Local and eustatic

    See also: Eustatic sea level
    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 tides 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 sea level changes
    (millimetres per year).

    Some land movements occur because of isostatic adjustment 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 (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 oceanic basins.[16] 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.[17]

    Short-term and periodic changes

    The Last Glacial Period caused a much lower global sea level.
    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:

    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 (Chandler wobble) 14-month P 5 mm
    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 <0.1m
    Ocean surface topography (changes in water density and currents) Days to weeks Up to 1 m
    southern oscillation
    		 6 mo every 5–10 yr Up to 0.6 m
    Seasonal variations
    Seasonal water balance among oceans (Atlantic, Pacific, Indian) 6 months  
    Seasonal variations in slope of water surface 6 months  
    River runoff/floods 2 months 1 m
    Seasonal water density changes (temperature and salinity) 6 months 0.2 m
    Seiches
    Seiches (standing waves) Minutes to hours Up to 2 m
    Earthquakes
    Tsunamis (catastrophic long-period waves) Hours 0.1–10+ m
    Abrupt change in land level Minutes Up to 10 m

    Recent changes

    Further information: Ocean heat content and Effects of climate change on oceans
    This section is an excerpt from Sea level rise.[edit]

    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.

    ice sheets and glaciers accounted for 44% of sea level rise, with another 42% resulting from thermal expansion of water.[21]
    : 1576 

    Sea level rise lags behind changes in the Earth's temperature by many decades, and sea level rise will therefore continue to accelerate between now and 2050 in response to warming that has already happened.[22] What happens after that depends on human greenhouse gas emissions. If there are very deep cuts in emissions, sea level rise would slow between 2050 and 2100. It could then reach by 2100 slightly over 30 cm (1 ft) from now and approximately 60 cm (2 ft) from the 19th century. With high emissions it would instead accelerate further, and could rise by 1.0 m (3+1⁄3 ft) or even 1.6 m (5+1⁄3 ft) by 2100.[20][18]: 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).[20]: 21 

    Rising seas affect every coastal and island population on Earth.

    mangroves. Crop yields may reduce because of increasing salt levels in irrigation water. Damage to ports disrupts sea trade.[24][25] 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.[26]

    Local factors like

    Pacific islands. Sea level rise will make many of them uninhabitable later this century.[31]

    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 like seawalls[32] are hard approaches. There are also 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.[33] Poorer nations may also struggle to implement the same approaches to adapt to sea level rise as richer states.

    Aviation

    Main article:
    Altitude in aviation

    Pilots can estimate height above sea level with an

    international standard atmosphere (ISA) pressure at MSL which is 1013.25 hPa or 29.92 inHg.[34]

    See also

    References

    1. ^ What is "Mean Sea Level"? Liverpool, UK: National Oceanography Centre. Retrieved 29 January 2024.
    2. ^ USGCRP (2017). "Climate Science Special Report. Chapter 12: Sea Level Rise. Key finding 1". science2017.globalchange.gov: 1–470. Archived from the original on 8 December 2019. Retrieved 27 December 2018.
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    9. ^ "Ordnance Survey Benchmark locator". Archived from the original on 27 December 2021. Retrieved 21 December 2021.
    10. ^ "Tide: Notes", Hong Kong Observatory. Archived 27 September 2022 at the Wayback Machine.
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    12. ^ "Sea Level 101: What Determines the Level of the Sea?". NASA. 3 June 2020. Retrieved 17 April 2024.
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    17. ^ "Global Warming Effects on Sea Level". www.climatehotmap.org. Archived from the original on 20 November 2016. Retrieved 2 December 2016.
    18. ^ a b c Fox-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. Retrieved 18 October 2022.
    19. ^ "WMO annual report highlights continuous advance of climate change". World Meteorological Organization. 21 April 2023. Archived from the original on 17 December 2023. Retrieved 18 December 2023. Press Release Number: 21042023.
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      . 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.
    22. ISBN 978-0-309-15176-4. Archived
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    25. ^ Holder, Josh; Kommenda, Niko; Watts, Jonathan (3 November 2017). "The three-degree world: cities that will be drowned by global warming". The Guardian. Archived from the original on 3 January 2020. Retrieved 28 December 2018.
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    32. ^ "IPCC's New Estimates for Increased Sea-Level Rise". Yale University Press. 2013. Archived from the original on 28 March 2020. Retrieved 1 September 2015.
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    34. ^ US Federal Aviation Administration, Code of Federal Regulations Sec. 91.121 Archived 26 April 2009 at the Wayback Machine
    The Wikibook Historical Geology has a page on the topic of: Sea level variations
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