Sea level 62529 221230855 2008-06-23T16:59:32Z AndreasJS 469010 /* See also */ {{onesource|date=December 2006}} {{for|considerations of sea level change, in particular rise associated with possible [[global warming]]|sea level rise}} {{otheruses}} ==Mean sea level (MSL)== 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. ===MSL 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) and flight level (FL)=== 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 {{convert|18000|ft|m}}, the flight level is one-eight-zero. == Measurement == [[Image:Recent Sea Level Rise.png|thumb|right|250px|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 [[wave]]s averaged out—averaged over a period of time such that changes in sea level, e.g., due to the [[tide]]s, 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 Newlyn|Ordnance Datum]] for the ''zero metres'' height on UK maps. == 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 (geodesy)|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 [[gravity|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 Ocean|Pacific]] end of the [[Panama Canal]] stands 20 cm higher than at the [[Atlantic Ocean|Atlantic]] end. Despite the difficulties, [[aviator]]s flying under [[instrument flight rules]] (IFR) must have accurate and reliable measurements of their altitudes above (or below - see [[Schiphol Airport]]) [[above mean sea level|mean sea level]], and the altitude of the [[airport]]s 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]]. ==Sea level and dry land== 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 [[ocean]]s (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 [[tectonics|tectonic]] effects and implies no real change in the volume of water in the oceans. The melting of [[glacier]]s at the end of [[ice age]]s 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. [[Paleoclimatologist]]s 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, [[planetologist]]s 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 == [[Image:Phanerozoic_Sea_Level.png|thumb|250px|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.]] [[Image:Post-Glacial Sea Level.png|thumb|right|250px|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 [[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 [http://www.grida.no/climate/ipcc_tar/wg1/fig11-4.htm this chart]. Hundreds of similar glacial cycles have occurred throughout the [[History of Earth|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 [[sediment]]ary 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]]. ==See also== *[[Above mean sea level]] *[[List of places on land with elevations below sea level]] *[[North West Shelf Operational Oceanographic System]] *[[Sea level rise]] *[[World Geodetic System]] * [[Orthometric height]] * [[Normal height]] * [[Geopotential height]] == External links == *[http://www.cmar.csiro.au/sealevel Sea Level Rise:Understanding the past - Improving projections for the future] *[http://www.pol.ac.uk/psmsl/ Permanent Service for Mean Sea Level] *[http://www.agu.org/revgeophys/dougla01/dougla01.html Global sea level change: Determination and interpretation] *[http://yosemite.epa.gov/oar/globalwarming.nsf/content/ResourceCenterPublicationsSeaLevelRiseIndex.html Environment Protection Agency Sea level rise reports] *[http://www.homepage.montana.edu/~geol445/hyperglac/sealevel2/index.htm Properties of isostasy and eustasy] [[Category:Oceanography]] [[Category:Cartography]] [[Category:Climate change]] [[ar:سطح البحر]] [[ast:Nivel de la mar]] [[bn:সমুদ্র সমতল]] [[map-bms:Nang dhuwur laut]] [[ca:Nivell mitjà del mar]] [[de:Meeresspiegel]] [[el:Μέση Στάθμη Θάλασσας]] [[es:nivel del mar]] [[eo:Marnivelo]] [[eu:Itsas maila]] [[fr:Niveau de la mer]] [[fur:Nivel dal mâr]] [[gl:Nivel medio do mar]] [[zh-classical:海平面]] [[id:Permukaan laut]] [[is:Sjávarmál]] [[it:Livello del mare]] [[he:גובה פני הים]] [[ka:ზღვის დონე]] [[lv:Jūras līmenis]] [[ml:സമുദ്രനിരപ്പ്]] [[nl:Zeeniveau]] [[ja:海面]] [[nn:Havnivå]] [[oc:Nivèl de la mar]] [[pl:Poziom morza]] [[pt:Nível médio das águas do mar]] [[ru:Уровень моря]] [[simple:Sea level]] [[fi:Merenpinnan taso]] [[sv:Havsyta]] [[ta:சராசரி கடல் மட்டம்]] [[te:సముద్రమట్టం]] [[th:ระดับน้ำทะเล]] [[vi:Mực nước biển]] [[zh:海平面]]