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Encyclopedia > Sea level

Mean sea level (MSL) is the average (mean) height of the sea, with reference to a suitable reference surface. Defining the reference level [1], however, involves complex measurement, and accurately determining MSL can prove difficult. Image File history File links Information_icon. ... Global mean surface temperatures 1850 to 2006 Mean surface temperature anomalies during the period 1995 to 2004 with respect to the average temperatures from 1940 to 1980 Global warming is the observed increase in the average temperature of the Earths atmosphere and oceans in recent decades and the projected... Sea level measurements from 23 long tide gauge records in geologically stable environments show a rise of around 8 inches per century (2 mm/year). ... Sea as seen from jetty in Frankston, Australia Look up maritime in Wiktionary, the free dictionary. ...

Contents

Measurement

Sea level measurements from 23 long tide gauge records in geologically stable environments show a rise of around 20 centimeters (8 inches) during the 20th century (2 millimeters/year).
Sea level measurements from 23 long tide gauge records in geologically stable environments show a rise of around 20 centimeters (8 inches) during the 20th century (2 millimeters/year).

To an operator of a tide gauge, MSL means the "still water level"—the level of the sea with motions such as wind waves averaged out—averaged over a period of time such that changes in sea level, e.g., due to the tides, also get averaged out. One measures the values of MSL in respect to the land. Hence a change in 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. Description Temperature changes during a similar period and possibly responsible for causing the observed sea level rise. ... Description Temperature changes during a similar period and possibly responsible for causing the observed sea level rise. ... This tidal gauge is ready to be installed underwater in a marina. ... (19th century - 20th century - 21st century - more centuries) Decades: 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s As a means of recording the passage of time, the 20th century was that century which lasted from 1901–2000 in the sense of the Gregorian calendar (1900–1999... This tidal gauge is ready to be installed underwater in a marina. ... This article is about waves in the most general scientific sense. ... It has been suggested that Earth tides be merged into this article or section. ...


Difficulties in utilization

To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or datum, called the geoid. In a state of rest or absence of external forces, the mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's gravitational field. In reality, due to currents, air pressure variations, temperature and salinity variations, etc., this does not occur, 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 (stationary) sea surface topography. It varies globally in a range of ±2 m. It has been suggested that this article or section be merged with Geodetic system. ... The GOCE project will measure high-accuracy gravity gradients and provide an accurate geoid model based on the Earths gravity field. ... Gravity is a force of attraction that acts between bodies that have mass. ...


Traditionally, one had to process sea-level measurements to take into account the effect of the 228-month Metonic cycle and the 223-month eclipse cycle on the tides. Mean sea level does not remain constant over the surface of the entire earth. For instance, mean sea level at the Pacific end of the Panama Canal stands 20 cm higher than at the Atlantic end. The Metonic cycle or Enneadecaeteris in astronomy and calendar studies is a particular approximate common multiple of the year (specifically, the seasonal tropical year) and the synodic month. ... Time-lapse series of photos of the lunar eclipse of October 2004 as seen from Northern California. ... The Pacific Ocean (from the Latin name Mare Pacificum, peaceful sea, bestowed upon it by the Portuguese explorer Ferdinand Magellan) is the largest body of water on Earth – at 165. ... Two Panamas running the Miraflores Locks The Panama Canal (Spanish: ) is a major ship canal that traverses the Isthmus of Panama in Central America, connecting the Atlantic Ocean and Pacific Oceans. ... “Atlantic” redirects here. ...


Despite the difficulties, aviators flying under instrument flight rules (IFR) must have accurate and reliable measurements of their altitudes above (or below - see Schiphol Airport) mean sea level, and the altitude of the airports where they intend to land. That problem can compound when landing on an aircraft carrier in a gravitational anomaly. In aviation mean sea level is increasingly being defined according to the reference ellipsoid defined by the World Geodetic System. Compared to a geoid, an ellipsoid is simpler to model mathematically and therefore lends itself to use with the Global Positioning System. For other uses, see Aviator (disambiguation). ... It has been suggested that Air traffic control#Instrument Flight Rules (IFR) be merged into this article or section. ... Schiphol (IATA: AMS, ICAO: EHAM) (municipality Haarlemmermeer) is the Netherlands main airport. ... The term above mean sea level (AMSL) refers to the elevation (on the ground) or altitude (in the air) of any object, relative to the average sea level. ... Four aircraft carriers, Principe de Asturias, USS Wasp, USS Forrestal and HMS Invincible (front-to-back), showing the difference in size between a supercarrier, light V/STOL carriers, and an amphibious assault carrier. ... In geodesy, a reference ellipsoid is a mathematically-defined surface that approximates the geoid, the true figure of the Earth or other planetary body. ... The World Geodetic System defines a fixed global reference frame for the Earth, for use in geodesy and navigation. ... The Global Positioning System (GPS), is currently the only fully-functional Global Navigation Satellite System (GNSS). ...


Several terms are used to describe the changing relationships between sea level and dry land. When the term "relative" is used, it connotes change that is not attributed to any specific cause. The term "eustatic" refers to global changes in the sea level due to water mass added (or removed from) the oceans (e.g. melting of ice sheets). The term "steric" refers to global changes in sea level due to thermal expansion and salanity variations. The term "isostatic" refers to changes in the level of the land masses due to thermal buoyancy or tectonic effects and implies no real change in the volume of water in the oceans. The melting of glaciers at the end of ice ages is an example of eustatic sea level rise. 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. Variations in CO2, temperature and dust from the Vostok ice core over the last 400 000 years For the animated movie, see Ice Age (movie). ...


On other 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", serves equivalently as a reference for the height of planetary features. Planetary science, also known as planetology and closely related to planetary astronomy, is the science of planets, or planetary systems, and the solar system. ...


Changes through geologic time

Comparison of two sea level reconstructions during the last 500 Myr. The scale of change during the last glacial/interglacial transition is indicated with a black bar. Note that over most of geologic history long-term average sea level has been significantly higher than today.
Comparison of two sea level reconstructions during the last 500 Myr. The scale of change during the last glacial/interglacial transition is indicated with a black bar. Note that over most of geologic history long-term average sea level has been significantly higher than today.
Sea level change since the end of the last glacial episode. Changes displayed in meters.
Sea level change since the end of the last glacial episode. Changes displayed in meters.

Sea level has changed over geologic time. As the graph shows, sea level today is very near the lowest level ever attained (the lowest level occurred at the Permo-Triassic boundary about 250 million years ago). For this reason, sea level is more prone to rise than fall today, and small changes in climate can have noticeable effects during human lifetimes. Image File history File links Phanerozoic_Sea_Level. ... Image File history File links Phanerozoic_Sea_Level. ... Image File history File links Post-Glacial_Sea_Level. ... Image File history File links Post-Glacial_Sea_Level. ...


During the most recent ice age (at its maximum about 20,000 years ago) the world's sea level was about 130 m lower than today, due to the large amount of sea water that had evaporated and been deposited as snow and ice in northern hemisphere glaciers. The majority of the glaciers had melted by about 10,000 years ago, but minor glacial melting has continued (with occasional reversals) throughout recorded human history. More detail about the changes in sea level for the past 140,000 years can be seen by accessing this chart. Variations in CO2, temperature and dust from the Vostok ice core over the last 400 000 years For the animated movie, see Ice Age (movie). ...


Hundreds of similar glacial cycles have occurred throughout the Earth's history. Geologists who study the positions of coastal sediment deposits through time have noted dozens of similar basinward shifts of shorelines associated with a later recovery. This results in sedimentary cycles which in some cases can be correlated around the world with great confidence. This relatively new branch of geological science linking eustatic sea level to sedimentary deposits is called sequence stratigraphy. The planet Earth, photographed in the year 1972. ... This article may be too technical for most readers to understand. ...


See also

The term above mean sea level (AMSL) refers to the elevation (on the ground) or altitude (in the air) of any object, relative to the average sea level. ... Sea level measurements from 23 long tide gauge records in geologically stable environments show a rise of around 8 inches per century (2 mm/year). ... The World Geodetic System defines a fixed global reference frame for the Earth, for use in geodesy and navigation. ...

External links


  Results from FactBites:
 
USGS FS 002-00: Sea Level and Climate (1003 words)
Climate-related sea-level changes of the last century are very minor compared with the large changes in sea level that occur as climate oscillates between the cold and warm intervals that are part of the Earth's natural cycle of long-term climate change.
During cold-climate intervals, known as glacial epochs or ice ages, sea level falls because of a shift in the global hydrologic cycle: water is evaporated from the oceans and stored on the continents as large ice sheets and expanded ice caps, ice fields, and mountain glaciers.
Sea levels during several previous interglacials were about 3 to as much as 20 meters higher than current sea level.
  More results at FactBites »

 
 

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