Mean sea level (MSL) is a measure of the average height of the ocean's surface (such as the halfway point between the mean high tide and the mean low tide); used as a standard in reckoning land elevation.[1]
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Measurement
Sea level measurements from 23 long tide gauge Sensors continuously record the height of the water level with respect to a height reference surface close to the geoid. Water enters the device by the bottom pipe , and electronic sensors measure its height and record it to a tiny computer records in geologically stable environments show a rise of around 200 millimetres The millimetre is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length (8 inches) during the 20th century (2 mm/year).To an operator of a tide gauge Sensors continuously record the height of the water level with respect to a height reference surface close to the geoid. Water enters the device by the bottom pipe , and electronic sensors measure its height and record it to a tiny computer, MSL means the "still water level"—the level of the sea with motions such as wind waves In mathematics and science, a wave is a disturbance that travels through space and time, usually by transference of energy. Waves are described by a wave function that can take on many forms depending on the type of wave. A mechanical wave is a wave that propagates through a medium due to restoring forces produced upon its deformation. For example, averaged out—averaged over a period of time such that changes in sea level, e.g., due to the tides Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth. The tides occur with a period of approximately 12 hours and 25 minutes, and with an amplitude that is influenced by the alignment of the sun and moon and the shape of the near-shore, 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 The United Kingdom of Great Britain and Northern Ireland[note 7] is a sovereign state located off the northwestern coast of continental Europe. It is an island country, spanning an archipelago including Great Britain, the northeastern part of the island of Ireland, and many small islands. Northern Ireland is the only part of the UK with a land, mean sea level has been measured at Newlyn Newlyn is a town and fishing port in southwest Cornwall, England, United Kingdom in Cornwall Cornwall is a ceremonial county and unitary authority of England, United Kingdom, forming the tip of the south-western peninsula of Great Britain. It is bordered to the north and west by the Atlantic Ocean, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Taken with the Isles of Scilly, Cornwall and Liverpool Liverpool is a city and metropolitan borough of Merseyside, England, along the eastern side of the Mersey Estuary. It was founded as a borough in 1207 and was granted city status in 1880. Liverpool is the fourth largest city in the United Kingdom (third largest in England) and has a population of 435,500, and lies at the centre of the wider for decades, by tide gauges to provide Ordnance Datum In the British Isles, an Ordnance Datum or OD is a vertical datum used by an ordnance survey as the basis for deriving altitudes on maps. A spot height may be expressed as AOD for "above ordnance datum". Usually mean sea level is used for the datum. In particular: for the zero metres The metre , symbol m, is the base unit of length in the International System of Units (SI). Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole, its definition has been periodically refined to reflect growing knowledge of metrology. Since 1983, it is defined as the distance travelled by light in a height on UK maps.
Satellite altimeters have been making precise measurements of sea level since the launch of TOPEX/Poseidon Launched in 1992, TOPEX/Poseidon was a joint satellite mission between NASA, the U.S. space agency, and CNES, the French space agency, to map ocean surface topography. The first major oceanographic research vessel to sail into space, TOPEX/Poseidon helped revolutionize oceanography by proving the value of satellite ocean observations. The in 1992. A joint mission of NASA The National Aeronautics and Space Administration is an Executive Branch agency of the United States government, responsible for the nation's civilian space program and aeronautics and aerospace research. Since February 2006 NASA's self-described mission statement is to "pioneer the future in space exploration, scientific discovery and and CNES The Centre National d'Études Spatiales is the French government space agency (administratively, a "public administration with industrial and commercial purpose"). Its headquarters are located in central Paris and it's placed under the supervision of the French Ministries of Defence and Research. It operates out of the Guiana Space, TOPEX/Poseidon was followed by Jason-1 Jason-1 is a satellite oceanography mission to monitor global ocean circulation, study the ties between the ocean and the atmosphere, improve global climate forecasts and predictions, and monitor events such as El Niño and ocean eddies in 2001 and the Ocean Surface Topography Mission The Ocean Surface Topography Mission on the Jason-2 satellite is an international Earth observation satellite mission that continues the sea surface height measurements begun in 1992 by the joint NASA/CNES TOPEX/Poseidon mission and followed by the NASA/CNES Jason-1 mission launched in 2001 on the Jason-2 satellite in 2008.
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 A geodetic datum is a reference from which measurements are made. In surveying and geodesy, a datum is a set of reference points on the earth's surface against which position measurements are made, and (often) an associated model of the shape of the earth (reference ellipsoid) to define a geographic coordinate system. Horizontal datums are used, called the geoid The geoid is that equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest, and extended through the continents . According to C.F. Gauss, who first described it, it is the "mathematical figure of the Earth," a smooth but highly irregular surface that. 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 Gravitation, or gravity, is one of the four fundamental interactions of nature , in which objects with mass attract one another. In everyday life, gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped. Gravitation causes dispersed matter to coalesce, thus accounting for 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) ocean surface topography The ocean surface has highs and lows, similar to the hills and valleys of Earth's land surface depicted on a topographic map. These variations, called "ocean surface topography" or "dynamic sea surface topography" are mapped using measurements of sea surface height relative to Earth's geoid. Earth's geoid is a calculated. 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 The Metonic cycle or Enneadecaeteris in astronomy and calendar studies is a particular approximate common multiple of the tropical year and the synodic month. The Greek astronomer Meton of Athens observed that a period of 19 tropical years is almost exactly equal to 235 synodic months, and rounded to full days counts 6940 days. The difference and the 223-month eclipse cycle Eclipses may occur repeatedly, separated by certain intervals of time: these intervals are called eclipse cycles. The series of eclipses separated by a repeat of one of these intervals is called an eclipse series 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 The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east end of the Panama Canal The Panama Canal is a 77 km ship canal in Panama that joins the Atlantic Ocean and the Pacific Ocean and is a key conduit for international maritime trade. Annual traffic has risen from about 1,000 ships in the canal's early days to 14,702 vessels in 2008, displacing a total 309.6 million Panama Canal/Universal Measurement System (PC/UMS) tons stands 20 cm (8 in) higher than at the Atlantic The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about 106,400,000 square kilometres , it covers approximately twenty percent of the Earth's surface and about twenty-six percent of its water surface area. The first part of its name refers to the Atlas of Greek mythology, making the Atlantic the " end.
Sea level and dry land
Sea water sign (2/3 of the way up the cliff face) above Badwater Basin Badwater Basin is an endorheic basin in Death Valley , Inyo County, California, noted as the lowest point in North America, with an elevation of 282 ft (86 m) below sea level. Mount Whitney, the highest point in the contiguous 48 states, is only 76 miles to the west, Death Valley National Park It is the hottest and driest of the national parks in the United States. The second-lowest point in the Western Hemisphere is in Badwater Basin, which is 282 feet below sea level. The park is home to many species of plants and animals that have adapted to this harsh desert environment. Some examples include creosote bush, Bighorn Sheep, Coyote,, USASeveral 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 Water is a chemical substance with the chemical formula H2O. Its molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state, water vapor or steam mass added to (or removed from) the oceans An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface (~3.61 X 1014 m2) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas (e.g. melting of ice sheets An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² , thus also known as continental glacier. The only current ice sheets are in Antarctica and Greenland; during the last glacial period at Last Glacial Maximum (LGM) the Laurentide ice sheet covered much of Canada and North America, the Weichselian). The term "steric" refers to global changes in sea level due to thermal expansion Thermal expansion is the tendency of matter to change in volume in response to a change in temperature. When a substance is heated, its particles begin moving and become active thus maintaining a greater average separation. Materials which contract with increasing temperature are rare; this effect is limited in size, and only occurs within limited and salinity Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates. Salinity in Australian English and North American English may also refer to the salt content of soil variations. The term "isostatic" refers to changes in the level of the land masses due to thermal buoyancy or tectonic Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth (or other planets) and particularly with the forces and movements that have operated in a region to create these structures effects and implies no real change in the volume of water in the oceans. The melting of glaciers A glacier is a perennial mass of ice which moves over land. A glacier forms in locations where the mass accumulation of snow and ice exceeds ablation over many years. The word glacier comes from French via the Vulgar Latin glacia, and ultimately from Latin glacies meaning ice. The corresponding area of study is called glaciology at the end of ice ages An "ice age" or, more precisely, "glacial age" is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in an expansion of continental ice sheets, polar ice sheets and alpine glaciers. An ice age is a natural system. Within a long-term ice age, individual pulses is an example of eustatic sea level rise. The subsidence of land due to the withdrawal of groundwater Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of lithologic formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called is an isostatic cause of relative sea level rise. Paleoclimatologists Paleoclimatology is the study of climate change taken on the scale of the entire history of Earth. It uses records from ice sheets, tree rings, sediment, and rocks to determine the past state of the climate system on Earth 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 Planetary science is the scientific study of planets, moons, and planetary systems, in particular those of the Solar System. It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation, interrelations and history. It is a strongly interdisciplinary field, originally growing from 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.
Sea level change
Local and eustatic sea level
Water cycles between ocean An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface (~3.61 X 1014 m2) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas, atmosphere The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night. Dry air contains roughly (by volume) 78% nitrogen, 21%, and glaciers A glacier is a perennial mass of ice which moves over land. A glacier forms in locations where the mass accumulation of snow and ice exceeds ablation over many years. The word glacier comes from French via the Vulgar Latin glacia, and ultimately from Latin glacies meaning ice. The corresponding area of study is called glaciology.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 In fluid dynamics, wind waves or, more precisely, wind-generated waves are surface waves that occur on the free surface of oceans, seas, lakes, rivers, and canals or even on small puddles and ponds. They usually result from the wind blowing over a vast enough stretch of fluid surface. Some waves in the oceans can travel thousands of miles before and tides Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth. The tides occur with a period of approximately 12 hours and 25 minutes, and with an amplitude that is influenced by the alignment of the sun and moon and the shape of the near-shore 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 Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic heights can exist at the Earth's surface adjustment of the mantle The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiated by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is a rocky shell about 2,890 km thick that to the melting of ice sheets An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² , thus also known as continental glacier. The only current ice sheets are in Antarctica and Greenland; during the last glacial period at Last Glacial Maximum (LGM) the Laurentide ice sheet covered much of Canada and North America, the Weichselian 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 Post-glacial rebound is the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period, through a process known as isostatic depression. It affects northern Europe (especially Scotland, Fennoscandia and northern Denmark), Siberia, Canada, and the Great Lakes of Canada and the United States. Atmospheric pressure Atmospheric pressure is the force per unit area exerted against a surface by the weight of air above that surface in the Earth's atmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low pressure areas have less atmospheric mass above, ocean currents An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis force, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun. Depth contours, shoreline configurations and interaction with other currents and local ocean temperature Historically, two equivalent concepts of temperature have developed, the thermodynamic description and a microscopic explanation based on statistical physics. Since thermodynamics deals entirely with macroscopic measurements, the thermodynamic definition of temperature, first stated by Lord Kelvin, is stated entirely in empirical, measurable changes also can affect LMSL.
Eustatic change (as opposed to local change) results in an alteration to the global sea levels, such as changes in the volume of water in the world oceans or changes in the volume of an ocean basin.
Short term and periodic changes
There are many factors which can produce short-term (a few minutes to 14 months) changes in sea level.
| 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 |
| Evaporation and 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 |
| El Niño/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 |
Medium term changes
Sea-level changes and relative temperaturesVarious factors affect the volume or mass of the ocean, leading to long-term changes in eustatic sea level. The two primary influences are temperature (because the volume of water depends on temperature), and the mass of water locked up on land and sea as fresh water in rivers, lakes, glaciers, polar ice caps, and sea ice. Over much longer geological timescales, changes in the shape of the oceanic basins and in land/sea distribution will affect sea level.
Observational and modelling studies of mass loss from glaciers and ice caps indicate a contribution to sea-level rise of 0.2 to 0.4 mm/yr averaged over the 20th century.
Glaciers and ice caps
Each year about 8 mm (0.3 inch) of water from the entire surface of the oceans falls into the Antarctica and Greenland ice sheets as snowfall. If no ice returned to the oceans, sea level would drop 8 mm every year. To a first approximation, the same amount of water appeared to return to the ocean in icebergs and from ice melting at the edges. Scientists previously had estimated which is greater, ice going in or coming out, called the mass balance, important because it causes changes in global sea level. High-precision gravimetry from satellites in low-noise flight has since determined Greenland is losing millions of tons per year, in accordance with loss estimates from ground measurement.
Ice shelves float on the surface of the sea and, if they melt, to first order they do not change sea level. Likewise, the melting of the northern polar ice cap which is composed of floating pack ice would not significantly contribute to rising sea levels. Because they are fresh, however, their melting would cause a very small increase in sea levels, so small that it is generally neglected.
- Scientists previously lacked knowledge of changes in terrestrial storage of water. Surveying of water retention by soil absorption and by reservoirs outright ("impoundment") at just under the volume of Lake Superior agreed with a dam-building peak in the 1930s-1970s timespan. Such impoundment masked tens of millimetres of sea level rise in that span. ( Impact of Artificial Reservoir Water Impoundment on Global Sea Level. http://www.sciencemag.org/cgi/content/full/320/5873/212?rss=1. B. F. Chao,* Y. H. Wu, Y. S. Li).
- If small glaciers and polar ice caps on the margins of Greenland and the Antarctic Peninsula melt, the projected rise in sea level will be around 0.5 m. Melting of the Greenland ice sheet would produce 7.2 m of sea-level rise, and melting of the Antarctic ice sheet would produce 61.1 m of sea level rise.[2] The collapse of the grounded interior reservoir of the West Antarctic Ice Sheet would raise sea level by 5–6 m.[3]
- The snowline altitude is the altitude of the lowest elevation interval in which minimum annual snow cover exceeds 50%. This ranges from about 5,500 metres above sea-level at the equator down to sea level at about 70° N&S latitude, depending on regional temperature amelioration effects. Permafrost then appears at sea level and extends deeper below sea level polewards.
- As most of the Greenland and Antarctic ice sheets lie above the snowline and/or base of the permafrost zone, they cannot melt in a timeframe much less than several millennia; therefore it is likely that they will not, through melting, contribute significantly to sea level rise in the coming century. They can, however, do so through acceleration in flow and enhanced iceberg calving.
- Climate changes during the 20th century are estimated from modelling studies to have led to contributions of between –0.2 and 0.0 mm/yr from Antarctica (the results of increasing precipitation) and 0.0 to 0.1 mm/yr from Greenland (from changes in both precipitation and runoff).
- Estimates suggest that Greenland and Antarctica have contributed 0.0 to 0.5 mm/yr over the 20th century as a result of long-term adjustment to the end of the last ice age.
The current rise in sea level observed from tide gauges, of about 1.8 mm/yr, is within the estimate range from the combination of factors above[4] but active research continues in this field. The terrestrial storage term, thought to be highly uncertain, is no longer positive, and shown to be quite large.
Geological influences
Comparison of two sea level reconstructions during the last 500 Ma. 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.At times during Earth's long history, the configuration of the continents and seafloor have changed due to plate tectonics. This affects global sea level by determining the depths of the ocean basins and how glacial-interglacial cycles distribute ice across the Earth.
The depth of the ocean basins is a function of the age of oceanic lithosphere: as lithosphere becomes older, it becomes denser and sinks. Therefore, a configuration with many small oceanic plates that rapidly recycle lithosphere will produce shallower ocean basins and (all other things being equal) higher sea levels. A configuration with fewer plates and more cold, dense oceanic lithosphere, on the other hand, will result in deeper ocean basins and lower sea levels.
When there were large amounts of continental crust near the poles, the rock record shows unusually low sea levels during ice ages, because there was lots of polar land mass upon which snow and ice could accumulate. During times when the land masses clustered around the equator, ice ages had much less effect on sea level.
Over most of geologic time, long-term sea level has been higher than today (see graph above). Only at the Permian-Triassic boundary ~250 million years ago was long-term sea level lower than today. Long term changes in sea level are the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term.[5]
During the glacial/interglacial cycles over the past few million years, sea level has varied by somewhat more than a hundred metres. This is primarily due to the growth and decay of ice sheets (mostly in the northern hemisphere) with water evaporated from the sea.
The Mediterranean Basin's gradual growth as the Neotethys basin, begun in the Jurassic, did not suddenly affect ocean levels. While the Mediterranean was forming during the past 100 million years, the average ocean level was generally 200 metres above current levels. However, the largest known example of marine flooding was when the Atlantic breached the Strait of Gibraltar at the end of the Messinian Salinity Crisis about 5.2 million years ago. This restored Mediterranean sea levels at the sudden end of the period when that basin had dried up, apparently due to geologic forces in the area of the Strait.
| Long-term causes | Range of effect | Vertical effect |
|---|---|---|
| Change in volume of ocean basins | ||
| Plate tectonics and seafloor spreading (plate divergence/convergence) and change in seafloor elevation (mid-ocean volcanism) | Eustatic | 0.01 mm/yr |
| Marine sedimentation | Eustatic | < 0.01 mm/yr |
| Change in mass of ocean water | ||
| Melting or accumulation of continental ice | Eustatic | 10 mm/yr |
| • Climate changes during the 20th century | ||
| •• Antarctica (the results of increasing precipitation) | Eustatic | -0.2 to 0.0 mm/yr |
| •• Greenland (from changes in both precipitation and runoff) | Eustatic | 0.0 to 0.1 mm/yr |
| • Long-term adjustment to the end of the last ice age | ||
| •• Greenland and Antarctica contribution over 20th century | Eustatic | 0.0 to 0.5 mm/yr |
| Release of water from earth's interior | Eustatic | |
| Release or accumulation of continental hydrologic reservoirs | Eustatic | |
| Uplift or subsidence of Earth's surface (Isostasy) | ||
| Thermal-isostasy (temperature/density changes in earth's interior) | Local effect | |
| Glacio-isostasy (loading or unloading of ice) | Local effect | 10 mm/yr |
| Hydro-isostasy (loading or unloading of water) | Local effect | |
| Volcano-isostasy (magmatic extrusions) | Local effect | |
| Sediment-isostasy (deposition and erosion of sediments) | Local effect | < 4 mm/yr |
| Tectonic uplift/subsidence | ||
| Vertical and horizontal motions of crust (in response to fault motions) | Local effect | 1–3 mm/yr |
| Sediment compaction | ||
| Sediment compression into denser matrix (particularly significant in and near river deltas) | Local effect | |
| Loss of interstitial fluids (withdrawal of groundwater or oil) | Local effect | ≤ 55 mm/yr |
| Earthquake-induced vibration | Local effect | |
| Departure from geoid | ||
| Shifts in hydrosphere, aesthenosphere, core-mantle interface | Local effect | |
| Shifts in earth's rotation, axis of spin, and precession of equinox | Eustatic | |
| External gravitational changes | Eustatic | |
| Evaporation and precipitation (if due to a long-term pattern) | Local effect | |
Changes through geologic time
Comparison of two sea level reconstructions during the last 500 Ma. 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 metres.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).
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, mostly in the Laurentide ice sheet. The majority of this had melted by about 10,000 years ago.
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 most up-to-date chronology of sea level change during the Phanerozoic shows the following long term trends:[6]
- Gradually rising sea level through the Cambrian
- Relatively stable sea level in the Ordovician, with a large drop associated with the end-Ordovician glaciation
- Relative stability at the lower level during the Silurian
- A gradual fall through the Devonian, continuing through the Mississippian to long-term low at the Mississippian/Pennsylvanian boundary
- A gradual rise until the start of the Permian, followed by a gentle decrease lasting until the Mesozoic.
Recent changes
Main article: Current sea level riseFor at least the last 100 years, sea level has been rising at an average rate of about 1.8 mm per year.[7] The majority of this rise can be attributed to the increase in temperature of the sea and the resulting thermal expansion of sea water. Additional contributions come from water sources on land such as melting snow and glaciers (see global warming).[8]
Aviation
Using pressure to measure altitude results in two other types of altitude. Distance above true or MSL (mean sea level) is the next best measurement to absolute. MSL altitude is the distance above where sea level would be if there were no land. If one knows the elevation of terrain, the distance above the ground is calculated by a simple subtraction.
An MSL altitude—called pressure altitude by pilots—is useful for predicting physiological responses in unpressurized aircraft (see hypoxia). It also correlates with engine, propeller, and wing performance, which all decrease in thinner air.
Pilots can estimate height above terrain with an altimeter set to a defined barometric pressure. Generally, the pressure used to set the altimeter is the barometric pressure that would exist at MSL in the region being flown over. This pressure is referred to as either QNH or "altimeter" and is transmitted to the pilot by radio from air traffic control (ATC) or an Automatic Terminal Information Service (ATIS). Since the terrain elevation is also referenced to MSL, the pilot can estimate height above ground by subtracting the terrain altitude from the altimeter reading. Aviation charts are divided into boxes and the maximum terrain altitude from MSL in each box is clearly indicated. Once above the transition altitude (see below), the altimeter is set to the international standard atmosphere (ISA) pressure at MSL which is 1013.2 HPa or 29.92 inHg.[9]
Flight level
MSL is useful for aircraft to avoid terrain, but at high enough altitudes, there is no terrain to avoid. Above that level, pilots are primarily interested in avoiding each other, so adjust their altimeter to standard temperature and pressure conditions (average sea level pressure and temperature) and disregard actual barometric pressure—until descending below transition level. To distinguish from MSL, such altitudes are called flight levels. Standard pilot shorthand is to express flight level as hundreds of feet, so FL 240 is 24,000 feet (7,300 m). Pilots use the international standard pressure setting of 1013.25 hPa (29.92 inHg) when referring to Flight Levels. The altitude at which aircraft are mandated to set their altimeter to flight levels is called "transition altitude". It varies from country to country. For example in the U.S. it is 18,000 feet, in many European countries it is 3,000 or 5,000 feet.
See also
- Above mean sea level
- Flood myth
- Geopotential height
- List of places on land with elevations below sea level
- Normal height
- Normaal Amsterdams Peil
- Normalhöhennull
- North West Shelf Operational Oceanographic System
- Orthometric height
- Sea level rise
- World Geodetic System
Notes
- ^ What is "Mean Sea Level"? Proudman Oceanographic Laboratory
- ^ "Some physical characteristics of ice on Earth". Climate Change 2001: The Scientific Basis. http://www.grida.no/climate/ipcc_tar/wg1/412.htm#tab113.
- ^ Geologic Contral on Fast Ice Flow - West Antarctic Ice Sheet. by Michael Studinger, Lamont-Doherty Earth Observatory
- ^ GRID-Arendal. "Climate Change 2001: The Scientific Basis". http://www.grida.no/climate/ipcc_tar/wg1/428.htm. Retrieved 2005-12-19.
- ^ Müller, R. Dietmar; et al. (2008-03-07). "Long-Term Sea-Level Fluctuations Driven by Ocean Basin Dynamics". Science 319 (5868): 1357–1362. doi:10.1126/science.1151540. PMID 18323446.
- ^ Haq, B. U.; Schutter, SR (2008). "A Chronology of Paleozoic Sea-Level Changes". Science 322 (5898): 64. doi:10.1126/science.1161648. PMID 18832639. http://www.sciencemag.org/cgi/content/full/322/5898/64.
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- ^ US Federal Aviation Administration, Code of Federal Regulations Sec. 91.121
External links
| Wikimedia Commons has media related to: Sea level |
- Sea Level Rise:Understanding the past - Improving projections for the future
- Permanent Service for Mean Sea Level
- Global sea level change: Determination and interpretation
- Environment Protection Agency Sea level rise reports
- Properties of isostasy and eustasy
- Measuring Sea Level from Space
- Rising Tide Video: Scripps Institution of Oceanography
- Sea Levels Online: National Ocean Service (CO-OPS)
Categories: Oceanography | Cartography
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