This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (October 2011)

This marker indicating the sea level is placed on the path from Jerusalem to the Dead Sea.

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] MSL also plays an extremely important role in aviation, where standard sea level pressure is used as the measurement datum of altitude at flight levels.

Measurement[link]

Sea level measurements from 23 long tide gauge records in geologically stable environments show a rise of around 200 millimetres (7.9 in) during the 20th century (2 mm/year).^{[citation needed]}

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, the Ordnance Datum (the 0 metres height on UK maps) is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Prior to 1921 the datum was MSL at the Victoria Dock, Liverpool.

In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collapsed data about the sea level. It is used for a part of continental Europe and main part of Africa as official sea level.

Satellite altimeters have been making precise measurements of sea level since the launch of TOPEX/Poseidon in 1992. A joint mission of NASA and CNES, TOPEX/Poseidon was followed by Jason-1 in 2001 and the Ocean Surface Topography Mission on the Jason-2 satellite in 2008.

Difficulties in utilization[link]

1. Ocean. 2. Reference ellipsoid.
3. Local plumb line. 4. Continent. 5. Geoid

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) ocean 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 is not constant over the surface of the Earth. For instance, mean sea level at the Pacific end of the Panama Canal stands 20 cm (7.9 in) higher than at the Atlantic end.

Sea level and dry land[link]

Sea level sign (2/3 of the way up the cliff face) above Badwater Basin, Death Valley National Park, USA

Several terms are used to describe the changing relationships between sea level and dry land. When the term "relative" is used, it means change relative to a fixed point in the sediment pile. The term "eustatic" refers to global changes in sea level relative to a fixed point, such as the centre of the earth, for example as a result of melting ice-caps. 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 relative to a fixed point in the earth, possibly due to thermal buoyancy or tectonic effects; it implies no change in the volume of water in the oceans. The melting of glaciers at the end of ice ages is one 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.

Sea level change[link]

Local and eustatic sea level[link]

Water cycles between ocean, atmosphere, and glaciers.

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 and tides 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 adjustment of the mantle to the melting of ice sheets 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. Changes in ground-based ice volume also affect local and regional sea levels by the readjustment of the geoid and true polar wander. Atmospheric pressure, ocean currents and local ocean temperature changes can affect LMSL as well.

Eustatic change (as opposed to local change) results in an alteration to the global sea levels due to changes in either the volume of water in the world oceans or net changes in the volume of the ocean basins.^[2]

Short term and periodic changes[link]

There are many factors which can produce short-term (a few minutes to 14 months) changes in sea level.

Long term changes[link]

Sea-level changes and relative temperatures

Various factors affect the volume or mass of the ocean, leading to long-term changes in eustatic sea level. The primary influence is that of temperature on seawater density and the amounts of 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 also 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[link]

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 billions 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 lower in salinity, 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.^[3] The collapse of the grounded interior reservoir of the West Antarctic Ice Sheet would raise sea level by 5–6 m.^[4]
• 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^[5] 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[link]

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.^[6]

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.

[edit] 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:^[7]

• 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[link]

For at least the last 100 years, sea level has been rising at an average rate of about 1.8 mm per year.^[8] The majority of this rise can be attributed to the increase in temperature of the sea and a slight resulting thermal expansion of the upper 500m of sea water. Additional contributions, as much as one-fourth of the total, come from water sources on land such as melting snow and glaciers and extraction of groundwater for irrigation and other agricultural and human needs. (see global warming).^[9]

Aviation[link]

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.^[10]

Flight level[link]

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 they 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[link]

Notes[link]

External links[link]

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)
• Système d'Observation du Niveau des Eaux Littorales (SONEL)
• Sea level rise - How much and how fast will sea level rise over the coming centuries?

Physical oceanography – Waves and Currents Waves Airy wave theory Ballantine Scale Benjamin–Feir instability Boussinesq approximation Breaking wave Clapotis Cnoidal wave Cross sea Dispersion Infragravity waves Edge wave Equatorial waves Fetch Freak wave Gravity wave Internal wave Kelvin wave Luke's variational principle Mild-slope equation Radiation stress Rogue wave Rossby wave Rossby-gravity waves Sea state Seiche Significant wave height Sneaker wave Soliton Stokes boundary layer Stokes drift Surf wave Swells Tsunami Undertow Ursell number Wave base Wave–current interaction Wave height Wave power Wave setup Wave shoaling Wave radar Wave turbulence Waves and shallow water Shallow water equations Wind wave Wind wave model More... Circulation Atmospheric circulation Baroclinity Boundary current Coriolis effect Downwelling Eddy Ekman layer Ekman spiral Ekman transport El Niño-Southern Oscillation Geostrophic current Gulf Stream Halothermal circulation Humboldt Current Hydrothermal circulation Global circulation model Langmuir circulation Longshore drift Loop Current Maelstrom Ocean current Ocean dynamics Ocean gyre Rip current Thermohaline circulation Shutdown of thermohaline circulation Subsurface currents Sverdrup balance Whirlpool Upwelling GLODAP MOM POM WOCE More... Tides Amphidromic point Earth tide Head of tide Internal tide Lunitidal interval Perigean spring tide Rule of twelfths Slack water Spring/neap tide Tidal bore Tidal force Tidal power Tidal race Tidal range Tidal resonance Tide Tide gauge Tideline More...   Physical oceanography – Other Landforms Abyssal fan Abyssal plain Atoll Bathymetric chart Cold seep Continental margin Continental rise Continental shelf Contourite Hydrography Guyot Oceanic basin Oceanic plateau Oceanic trench Passive margin Seabed Seamount Submarine canyon Coastal geography More... Plate tectonics Black smoker Convergent boundary Divergent boundary Fracture zone Hydrothermal vent Marine geology Mid-ocean ridge Mohorovičić discontinuity Vine–Matthews–Morley hypothesis Oceanic crust Outer trench swell Ridge push Seafloor spreading Slab pull Slab suction Slab window Subduction Transform fault Volcanic arc More... Ocean zones Benthic Deep ocean water Deep sea Littoral Mesopelagic Oceanic zone Pelagic Photic Surf zone Sea level Current sea level rise Future sea level Sea-level curve DART GLOSS NOOS OSTM WGS Acoustics Ocean acoustic tomography Sofar bomb SOFAR channel Underwater acoustics Hydroacoustics Other Alvin Argo Benthic lander Color of water Marginal sea Mooring Ocean Ocean energy Ocean exploration Ocean observations Ocean pollution Ocean reanalysis Ocean surface topography Ocean thermal energy conversion Oceanography Pelagic sediments Sea surface microlayer Sea surface temperature Seawater Science On a Sphere Thermocline Underwater glider Water column World Ocean Atlas NODC More...

Anote Tong
President of Kiribati
Incumbent
Assumed office 10 July 2003
Vice President	Teima Onorio
Preceded by	Tion Otang (Acting)
Personal details
Born	(1952-06-11) 11 June 1952 (age 60) Tabuaeran, Gilbert and Ellice Islands (now Kiribati)
Political party	Pillars of Truth
Spouse(s)	Meme Tong
Alma mater	University of Canterbury London School of Economics

Tong's Presidential residence

Anote Tong (Chinese: 汤安诺; pinyin: Tāng Ānnuò; born 11 June 1952) is an I-Kiribati politician with Chinese heritage who is the President of Kiribati. He won the election in July 2003 with a slim plurality of votes cast (47.4%) against his brother, Dr. Harry Tong (43.5%) and the private lawyer Banuera Berina (9.1%).^[1] The elections were contested by the opposition, due to allegations of electoral fraud but the High Court of Tarawa had confirmed that there was no fraud. He was easily re-elected on 17 October 2007, for a second term (64%). In 2012, Tong was reelected for a third term, although with a significantly smaller percentage than in the previous two elections.^[2]

Biography[link]

Personal life[link]

Born in Tabuaeran, he is the son of a Chinese migrant who settled in the Gilberts after World War II and of Nei Keke, from the island of Maiana in Kiribati,^[3] he went to St Bede's College for his secondary school education, graduated from Canterbury University with a degree in Science, and then gained a Masters in Economics degree from the London School of Economics.^[4]

He is married to an I-Kiribati woman, First Lady Nei Meme, and has seven children. Tong is originally from the island of Maiana, located in central Kiribati.^[5]

Political career[link]

During the campaign, he promised to review the lease of a spy and satellite tracking base used by the People's Republic of China and "to take appropriate actions at the right time." On 7 November, he established relations with the Republic of China on Taiwan, which led the People's Republic of China to sever relations and vacate its satellite base nearly a month later.

Tong was overwhelmingly re-elected to his seat in parliament in the August 2007 parliamentary election.^[6] On 17 October 2007, he was re-elected as president by a large majority. The opposition boycotted the election due to the exclusion of two opposition candidates, including Tong's brother Harry.^[7]

Tong was re-elected to a third, and final, four-year term as President in the January 2012 election.^[8] Tong won a little over 40% of the popular vote.^[8] He defeated two challengers, including his nearest rival, Tetaua Taitai, by more than 2,000 votes.^[8] Tong reappointed Teima Onorio to a third term as Vice President of Kiribati on 19 January 2012, as part of his cabinet appointments for his third term.^[9]

Speaking up on climate change[link]

President Tong has attracted international attention by warning that his country may become uninhabitable by the 2050s due to rising sea levels and salination provoked by climate change. Tong has stated on several occasions that Kiribati may cease to exist altogether, and that its entire population of 94,000 may need to be resettled as climate refugees. In June 2008, he stated that Kiribati may already have reached "the point of no return"; he added: "To plan for the day when you no longer have a country is indeed painful but I think we have to do that."^{[10][11][12][13][14][15]}

President Tong has sought possibilities for ultimately relocating the entire population of his country to other countries. His stated plan is for the people of Kiribati to "receive job training and then seek skilled jobs in other nations", so that they may become productive members of their host society, and avoid becoming merely "environmental refugees". "The plan has already begun to be implemented, with small groups of nurses going to Australia for training and other workers to New Zealand."^[16]

Specifically, Tong's government has "signed on to New Zealand’s Recognised Seasonal Employer (RSE) scheme and Australia’s Pacific Seasonal Worker Pilot Scheme (PSWPS), which provide seasonal employment opportunities in fruit-picking and horticulture industries". Only small numbers of I-Kiribati have been able to benefit so far. Tong has also reached an agreement with Australia to set up a Kiribati-Australia Nurses Initiative, whereby about eighty I-Kiribati receive nursing training in Australia, with an aim to "attain Australian nursing qualifications and industry experience". "[S]ome will go back to Kiribati to work the health system, but others will stay in Australia to send remittances home to households and community."^[17]

So far, however, no country has agreed to relocate substantial numbers of I-Kiribati. President Levy Mwanawasa of Zambia did tell President Tong that there was "plenty of room" in his country for I-Kiribati migrants, but he died suddenly in office in August 2008.^[18] In February 2012, Tong visited Fiji to seek to buy lands from the Fijian government, to resettle I-Kiribati migrants.^[19]. Speaking on Fiji One, Tong explained that the migration of his country's population would begin with a fairly small number of trained, skilled workers, while others were in the process of obtaining similar skills, to make themselves useful to the host nation: "We don't want 100,000 people from Kiribati coming to Fiji in one go. They need to find employment, not as refugees but as immigrant people with skills to offer, people who have a place in the community".^{[20] [21]}

In 2008, his government declared 150,000 square miles (390,000 km²) "of [the] Phoenix Islands marine area a fully protected marine park, making it off limits to fishing and other extractive uses". This, the Phoenix Islands Protected Area, was made a United Nations World Heritage Site. Its "rich biodiversity includ[es] an abundance of healthy corals, big sharks, groupers, tuna, giant clams and other critters that have been depleted in much of the rest of the world". Tong explained that it was intended as "a significant contribution to the world community in the hope they would also act".^[22][23]

Declaring himself "extremely disappointed" by the outcome of the 2009 United Nations Climate Change Conference, Tong has "call[ed] a meeting in November [2010] in Kiribati, inviting large countries—the big polluters—and have them meet with the victims, the most vulnerable states: ours and the Marshall Islands and the Maldives".^[24]

References[link]

External links[link]

Wikimedia Commons has media related to: Anote Tong

• NOW on PBS: 'Paradise Lost'
• ABC News: President of Kiribati: 'Our Very Lives Are At Stake'
• Address to the 63rd Session of the United Nations General Assembly, 25 September 2008

v t e Presidents of Kiribati Ieremia Tabai • Rota Onorio* • Ieremia Tabai • Teatao Teannaki • Tekiree Tamuera* • Ata Teaotai* • Teburoro Tito • Tion Otang* • Anote Tong *denotes acting