- of or pertaining to eustasy
Mean sea level (MSL) is the average (mean) height of the sea, with reference to a suitable reference surface. Defining the reference level http://www.pol.ac.uk/psmsl/puscience/index.html#1, however, involves complex measurement, and accurately determining MSL can prove difficult.
Mean Sea Level as used by military Flight Surgeons and Aerospace Units: Using pressure to measure altitude results in two other types of altitude. True or MSL (mean sea level) is the next best measurement to absolute - and in some ways better. MSL tells you how far you are above an imaginary line at sea level. If you then know the elevation of terrain, the next step is to determine how far you are above ground. It also tells you how thin the air is, which determines your physiological response to that altitude. True Altitude (MSL) has been adjusted for local high or low pressure conditions. FL or Flight Level is another related term that is measured in hundreds of feet. At a standard pressure that correlates to , the flight level is one-eight-zero.
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.
In the UK, mean sea level has been measured at Newlyn in Cornwall and Liverpool on Merseyside for decades, by tide gauges to provide Ordnance Datum for the zero metres height on UK maps.
Difficulties in utilizationTo 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.
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.
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 gravity 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.
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 salinity 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.
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.
Changes through geologic time
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 Permian-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.
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. Most "recently" (within last 10,000 years) a "normal" interglacial increase of Earth temperature was interrupted at a level of a few degrees below peak, and this probably correlates with slowdown in the sea level increase at present. There is no clear data, was this Earth temperature decrease a result of human activities or just a new step in continuing "Earth freezing". More detail about the changes in sea level for the past 140,000 years can be seen by accessing this chart.
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.
eustatic in Arabic: سطح البحر
eustatic in Asturian: Nivel de la mar
eustatic in Bengali: সমুদ্র সমতল
eustatic in Banyumasan: Nang dhuwur laut
eustatic in Catalan: Nivell mitjà del mar
eustatic in German: Meeresspiegel
eustatic in Modern Greek (1453-): Μέση Στάθμη Θάλασσας
eustatic in Spanish: Nivel del mar
eustatic in Esperanto: Marnivelo
eustatic in Basque: Itsas maila
eustatic in French: Niveau de la mer
eustatic in Friulian: Nivel dal mâr
eustatic in Galician: Nivel medio do mar
eustatic in Classical Chinese: 海平面
eustatic in Indonesian: Permukaan laut
eustatic in Icelandic: Sjávarmál
eustatic in Italian: Livello del mare
eustatic in Hebrew: גובה פני הים
eustatic in Georgian: ზღვის დონე
eustatic in Latvian: Jūras līmenis
eustatic in Malayalam: സമുദ്രനിരപ്പ്
eustatic in Dutch: Zeeniveau
eustatic in Japanese: 海面
eustatic in Norwegian Nynorsk: Havnivå
eustatic in Occitan (post 1500): Nivèl de la mar
eustatic in Polish: Poziom morza
eustatic in Portuguese: Nível médio das águas do mar
eustatic in Russian: Уровень моря
eustatic in Simple English: Sea level
eustatic in Finnish: Merenpinnan taso
eustatic in Swedish: Havsyta
eustatic in Tamil: சராசரி கடல் மட்டம்
eustatic in Telugu: సముద్రమట్టం
eustatic in Thai: ระดับน้ำทะเล
eustatic in Vietnamese: Mực nước biển
eustatic in Chinese: 海平面