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 the midpoint between a mean low and mean high tide at a particular 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 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.[3] This variation from a perfect sphere is the geoid of the Earth. It causes a significant depression in the Indian Ocean, about 1,200 km (746 mi) southwest of India, where the surface reaches a depth of 106 m (348 ft) below the global mean sea level.[4]

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.

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.[citation needed
]

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

wind waves averaged out.[6]
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.[citation needed]

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

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.[8] 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

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 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

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.[citation needed
]

In the rare case that a location is below sea level, the elevation AMSL is negative. For one such case, see Amsterdam Airport Schiphol.[citation needed]

Difficulties in use

plumb line
  • Continent
  • Geoid

    • 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

    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.

    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.[10]
    • "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.[11]
    • "steric" refers to global changes in sea level due to thermal expansion and salinity variations.[12]
    • "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; it implies no change in the volume of water in the oceans.[dubious – discuss]

    The melting of glaciers at the end of ice ages results in eustatic post-glacial rebound. 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 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 (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.[13]

    Short-term and periodic changes

    Global sea level during the Last Glacial Period
    Melting glaciers are causing a change in sea level

    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.[14]

    Periodic sea level changes
    Diurnal and semidiurnal astronomical tides 12–24 h P 0.2–10+ m
    Long-period tides    
    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
    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
    southern oscillation
    		 6 mo every 5–10 yr Up to 0.6 m
    Seasonal variations
    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
    Seiches (standing waves) Minutes to hours Up to 2 m
    Earthquakes
    Tsunamis (generate catastrophic long-period waves) Hours Up to 10 m
    Abrupt change in land level Minutes Up to 10 m

    Recent changes

    This section is an excerpt from Sea level rise.[edit]

    Between 1901 and 2018, the average global sea level rose by 15–25 cm (6–10 in), or an average of 1–2 mm per year.

    temperate glaciers accounted for 21%, with Greenland accounting for 15% and Antarctica 8%.[17]: 1576  Sea level rise lags changes in the Earth's temperature. So sea level rise will continue to accelerate between now and 2050 in response to warming that is already happening.[18] What happens after that will depend on what happens with human greenhouse gas emissions. Sea level rise may slow down between 2050 and 2100 if there are deep cuts in emissions. It could then reach a little over 30 cm (1 ft) from now by 2100. With high emissions it may accelerate. It could rise by 1 m (3+1⁄2 ft) or even 2 m (6+1⁄2 ft) by then.[19][20] In the long run, sea level rise would amount to 2–3 m (7–10 ft) over the next 2000 years if warming amounts to 1.5 °C (2.7 °F). It would be 19–22 metres (62–72 ft) if warming peaks at 5 °C (9.0 °F).[19]
    : 21 

    Rising seas ultimately impact every coastal and island population on Earth.

    Crop production falls because of salinization of irrigation water and damage to ports disrupts sea trade.[23][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]
    Areas not directly exposed to rising sea levels could be affected by large scale migrations and economic disruption.

    At the same time, local factors like

    Pacific islands—many of those would be rendered uninhabitable by sea level rise later this century.[31]

    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[32] 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.[33] 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.[34] 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.[35] 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.[36]
    Further information: Ocean heat content and Effects of climate change on oceans

    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.[37]

    See also

    References

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    External links

    The Wikibook Historical Geology has a page on the topic of: Sea level variations
    Wikimedia Commons has media related to Sea level.
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