Sea level
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.[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
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.
Measurement
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 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
Satellite altimeters have been making precise measurements of sea level since the launch of
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
When referring to
Difficulties in use
It is often necessary to compare the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the
Dry land
This section needs additional citations for verification. (January 2024) |
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.[12]
- "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.[13]
- "steric" refers to global changes in sea level due to thermal expansion and salinity variations.[14]
- "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,
Change
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 long enough that fluctuations caused by
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.[15] 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.[16]
Short-term and periodic changes
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
Between 1901 and 2018, average global sea level rose by 15–25 cm (6–10 in), an average of 1–2 mm (0.039–0.079 in) per year.
Rising seas affect every coastal and island population on Earth.
Local factors like
Aviation
Pilots can estimate height above sea level with an
See also
- Above ground level– Height measured with respect to the underlying ground surface
- Amsterdam Ordnance Datum, also known as Normaal Amsterdams Peil – Vertical datum
- Before Present – Time scale used in 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
- Meltwater pulse 1A – Period of rapid post-glacial sea level rise
- Metres above the Adriatic – Vertical datum used in parts of Europe
- Normal height – Altitude above quasigeoid or mean sea level
- 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
- Raised beach, also known as Marine terrace – Emergent coastal landform
- Sea level drop – Drop relative to land rebounding from weight of ice
- Sea level equation– Rise of land masses after glacial period
- Vertical datum – Reference surface for vertical positions
- World Geodetic System – Geodetic reference system
References
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- ^ "Eustatic sea level". Oilfield Glossary. Schlumberger Limited. Archived from the original on 2 November 2011. Retrieved 10 June 2011.
- ^ "Global Warming Effects on Sea Level". www.climatehotmap.org. Archived from the original on 20 November 2016. Retrieved 2 December 2016.
- ^ IPCC, 2019: Summary for Policymakers. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D. C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N. M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, New York, US. https://doi.org/10.1017/9781009157964.001.
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- ^ a b c IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J. B. R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, New York, US, pp. 3−32, doi:10.1017/9781009157896.001.
- .
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.
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Box SYN-1: Sustained warming could lead to severe impacts
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- ^ Choi, Charles Q. (27 June 2012). "Sea Levels Rising Fast on U.S. East Coast". National Oceanic and Atmospheric Administration. Archived from the original on 4 May 2021. Retrieved 22 October 2022.
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- ^ 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: Asia. In Climate Change 2022: Impacts, Adaptation and Vulnerability [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, United Kingdom and New York, New York, US, pp. 1457–1579 |doi=10.1017/9781009325844.012.
- ^ Mycoo, M., M. Wairiu, D. Campbell, V. Duvat, Y. Golbuu, S. Maharaj, J. Nalau, P. Nunn, J. Pinnegar, and O. Warrick, 2022: Chapter 15: Small islands. In Climate Change 2022: Impacts, Adaptation and Vulnerability [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, United Kingdom and New York, New York, US, pp. 2043–2121 |doi=10.1017/9781009325844.017.
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External links
- Sea Level Rise:Understanding the past – Improving projections for the future
- Permanent Service for Mean Sea Level
- Global sea level change: Determination and interpretation
- Environment Protection Agency Sea level rise reports
- Properties of isostasy and eustasy
- Measuring Sea Level from Space
- Rising Tide Video: Scripps Institution of Oceanography
- Sea Levels Online: National Ocean Service (CO-OPS)
- Système d'Observation du Niveau des Eaux Littorales (SONEL)
- Sea level rise – How much and how fast will sea level rise over the coming centuries?