Sea level

The sea level is the average height of the surface of the Mer, compared to an adequate reference level.

Measure

It is difficult to carry out a direct measurement of the mean level of the sea. The Altimétrie satellitale makes it possible nevertheless to bring back the altitude of the sea to a terrestrial reference frame (Geoid or geodetic Système). One can also measure the variation of the mean level according to time. This variation is used as indication in particular on the Climate warming. However, it is not possible to either take a direct measurement of the variations of the mean level . Indeed of many disturbances affect measurements with in particular in the order of importance, the Marée, the effects of the Atmospheric pressure, the Houle,…

These disturbances are known as high frequencies, because their signature is fast in time: a few seconds for the Vague S and a few hours (days) for the Tide S.

In order to obtain an estimate of the variation of the mean level in time, it is necessary to withdraw these disturbances. The variations of the mean level are very slow thus low frequency. Thus, it is enough to apply to the time serieses of measurements (recorded with a Marégraphe) mathematical a function known as low-pass Filtre . This function has as a characteristic to preserve only the low frequencies of a signal. Thus, the disturbances are eliminated from the signal. It is necessary however to apply this filter to have a recording of great quality and long life (1 minimum year).

Reference

The measurement a height of the sea level immediately poses the problem of a point of reference, i.e. a point that one supposes fixes and which is used as origin for measurements.

The precise localization of this point is related to the definition of a geodetic reference frame, a whole of points whose coordinates are known. Several systems of this type coexist; in France, the national geographical Institut uses inter alia a geodetic network covering the territory of the country, whose origin of altitude is determined by a Marégraphe located at Marseilles: to define the sea level in another place, visible since the dry land, can then be done by levelling.

In open sea, a modern definition calls upon a Geoid of reference, a surface covering the sphere in such way that the terrestrial Gravité is to him always Perpendiculaire in any point. In the absence of external forces, the sea level would coincide with this geoid, since it would be about an equipotential surface of the terrestrial field of gravity. Actually, the differences of pressure, temperature, salinity and the currents marine make that it is not the case, even on a long-term average: on a sphere scale, the sea level is thus not constant and the variations reach ±2 m compared to the geoid of reference. The level of the Pacific Ocean with an end of the Panama Canal is for example 20 cm higher than that of the Atlantic Ocean with the other end.

The geoid of reference is a complex surface. To simplify the problem, one often has recourse to a Ellipsoïde of reference (WGS 84), easier to model. The resulting sea level varies on the other hand much more, being able to move away from a hundred meters compared to the ellipsoid of reference by the fact of gravitational anomalies.

Variations

The sea level was not always the same one during the ages. Its oscillations are due to multiple factors, in particular with changes of Climat, themselves determined by various causes.

The paleoclimatologists employ several terms to define the changes between the sea level and the dry land:

  • a eustatic variation implies a modification of the volume of water in the oceans, generally following a climate change. The cast iron of glaciers at the glacial end of one era in is an example.
  • an isostatic variation indicates that the sea level changed without the volume of water in the oceans not being modified. For example, following the cast iron of the glaciers which recovered it during the last glaciation, the Scandinavia became lighter and rises few mm per annum, thus making lower locally the level of the the Baltic.
  • a relative variation implies an absence of known specific cause.

It is possible to determine a chronology of the variations of the sea level by studying the deposits of sediments on the coasts tectoniquement stable, such as for example the east coast of North America.

During the glacial last era (of which the maximum go backgoes back approximately 20.000 years), the sea level was about 100 m lower than currently. In spite of that, the sea level seems to be nowadays with the one of the levels low since several hundreds of million years.

Recent chronology

The plank following tent to summarize the variations of the sea level on the 30.000 last years and its implications on the geography (mainly European).

Forecast

The forecasts give a rise in the sea level from 11 to 77 centimetres at the end in.

If all the ice which is on the continent the Antarctic and with the Greenland founded, the sea level would rise of 70 meters.

See too

Internal bonds

External bonds

  • Chronology of the variations
  • '' Modelling shoreline evolution associated with Earth glaciation '', proposing charts of the evolutions of the sea level for the 20.000 last years.
  • Forecasts

Simple: Sea level Zh-classical: 海平面

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