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40-578: It reaches a height of 3,346 feet above sea level . Named after Peter Kemp , who, in the brig Magnet , is reported to have sighted land in 1833. In 1930, BANZARE under Sir Douglas Mawson in the Discovery delineated the coastline from the junction with Enderby Land to that with Mac. Robertson Land. In 1936 the coast was recharted by RRS William Scoresby . 67°30′00″S 57°30′00″E  /  67.5000°S 57.5000°E  / -67.5000; 57.5000 This Kemp Land location article

80-509: A chart datum in cartography and marine navigation , or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and, consequently, aircraft flight levels . A common and relatively straightforward mean sea-level standard is instead a long-term average of tide gauge readings at a particular reference location. Sea levels can be affected by many factors and are known to have varied greatly over geological time scales . Current sea level rise

120-491: A datum . For example, hourly measurements may be averaged over a full Metonic 19-year lunar cycle to determine the mean sea level at an official tide gauge . Still-water level or still-water sea level (SWL) is the level of the sea with motions such as wind waves averaged out. Then MSL implies the SWL further averaged over a period of time such that changes due to, e.g., the tides , also have zero mean. Global MSL refers to

160-491: A topographic map variations in elevation are shown by contour lines . A mountain's highest point or summit is typically illustrated with the AMSL height in metres, feet or both. In unusual cases where a land location is below sea level, such as Death Valley, California , the elevation AMSL is negative. It is often necessary to compare the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called

200-440: A "mean sea level" is difficult because of the many factors that affect sea level. Instantaneous sea level varies substantially on several scales of time and space. This is because the sea is in constant motion, affected by the tides, wind , atmospheric pressure, local gravitational differences, temperature, salinity , and so forth. The mean sea level at a particular location may be calculated over an extended time period and used as

240-409: 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,

280-432: A few metres, in timeframes ranging from minutes to months: Between 1901 and 2018, the average sea level rose by 15–25 cm (6–10 in), with an increase of 2.3 mm (0.091 in) per year since the 1970s. This was faster than the sea level had ever risen over at least the past 3,000 years. The rate accelerated to 4.62 mm (0.182 in)/yr for the decade 2013–2022. Climate change due to human activities

320-463: A sharp reduction in greenhouse gas emissions, this may increase to hundreds of millions in the latter decades of the century. Local factors like tidal range or land subsidence will greatly affect the severity of impacts. For instance, sea level rise in the United States is likely to be two to three times greater than the global average by the end of the century. Yet, of the 20 countries with

360-483: A spatial average over the entire ocean area, typically using large sets of tide gauges and/or satellite measurements. One often measures the values of MSL with respect to the land; hence a change in relative MSL or ( relative sea level ) 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, or both. In the UK, the ordnance datum (the 0 metres height on UK maps)

400-419: Is a stub . You can help Misplaced Pages by expanding it . Sea level Mean sea level ( MSL , often shortened to sea level ) is an average surface level of one or more among Earth 's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datum  – a standardised geodetic datum  – that is used, for example, as

440-473: Is a period of almost exactly 19 years after which the lunar phases recur at the same time of the year. The recurrence is not perfect, and by precise observation the Metonic cycle defined as 235 synodic months is just 2 hours, 4 minutes and 58 seconds longer than 19 tropical years . Meton of Athens , in the 5th century BC, judged the cycle to be a whole number of days, 6,940. Using these whole numbers facilitates

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480-585: Is calibrated to the Amsterdam Peil elevation, which dates back to the 1690s. 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. Height above mean sea level ( AMSL )

520-427: Is due to change in either the volume of water in the world's oceans or the volume of the oceanic basins . Two major mechanisms are currently causing eustatic sea level rise. First, shrinking land ice, such as mountain glaciers and polar ice sheets, is releasing water into the oceans. Second, as ocean temperatures rise, the warmer water expands. Many factors can produce short-term changes in sea level, typically within

560-451: Is mainly caused by human-induced climate change . When temperatures rise, mountain glaciers and polar ice sheets melt, increasing the amount of water in the oceans, while the existing seawater also expands with heat. Because most of human settlement and infrastructure was built in response to a more-normalized sea level with limited expected change, populations affected by sea level rise will need to invest in climate adaptation to mitigate

600-442: Is the intercalation of a thirteenth lunar month in a calendar year from time to time. The traditional lunar year of 12 synodic months is about 354 days, approximately eleven days short of the solar year. Thus, every 2 to 3 years there is a discrepancy of 22 to 33 days, or a full synodic month. For example, if the winter solstice and the new moon coincide, it takes 19 tropical years for the coincidence to recur. The mathematical logic

640-411: Is the elevation (on the ground) or altitude (in the air) of an object, relative to a reference datum for mean sea level (MSL). It is also used in aviation, where some heights are recorded and reported with respect to mean sea level (contrast with flight level ), and in the atmospheric sciences , and in land surveying . An alternative is to base height measurements on a reference ellipsoid approximating

680-723: Is the main cause. Between 1993 and 2018, melting ice sheets and glaciers accounted for 44% of sea level rise , with another 42% resulting from thermal expansion of water . Sea level rise lags behind changes in the Earth 's temperature by many decades, and sea level rise will therefore continue to accelerate between now and 2050 in response to warming that has already happened. What happens after that depends on human greenhouse gas emissions . If there are very deep cuts in emissions, sea level rise would slow between 2050 and 2100. It could then reach by 2100 slightly over 30 cm (1 ft) from now and approximately 60 cm (2 ft) from

720-741: Is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Before 1921, the vertical datum was MSL at the Victoria Dock, Liverpool . Since the times of the Russian Empire , in Russia and its other former parts, now independent states, the sea level is measured from the zero level of Kronstadt Sea-Gauge. In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of 0.146 m (5.7 in) above chart datum and 1.304 m (4 ft 3.3 in) below

760-503: The Antikythera mechanism which offers unexpected evidence for the popularity of the calendar based on it. The (19-year) Metonic cycle is a lunisolar cycle, as is the (76-year) Callippic cycle . An important example of an application of the Metonic cycle in the Julian calendar is the 19-year lunar cycle insofar as provided with a Metonic structure. Meton introduced the 19 year cycle to

800-616: The Attic calendar in 432 BC. In the following century, Callippus developed the Callippic cycle of four 19-year periods for a 76-year cycle with a mean year of exactly 365.25 days. Around AD 260 the Alexandrian computist Anatolius , who became bishop of Laodicea in AD 268, was the first to devise a method for determining the date of Easter Sunday. However, it was some later, somewhat different, version of

840-568: The Jewish people . It is similar to, but slightly different in usage from, the Greek Metonic cycle (being based on a month of 29 + 13753 ⁄ 25920 days, giving a cycle of 6939 + 3575 ⁄ 5184  ≈ 6939.69 days ), and likely derived from or alongside the much earlier Babylonian calendar. It is possible that the Polynesian kilo-hoku (astronomers) discovered the Metonic cycle in

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880-643: The geoid . In the absence of external forces, the local mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's gravitational field which, in itself, does not conform to a simple sphere or ellipsoid and exhibits gravity anomalies such as those measured by NASA's GRACE satellites . In reality, the geoid surface is not directly observed, even as a long-term average, due to ocean currents, air pressure variations, temperature and salinity variations, etc. The location-dependent but time-persistent separation between local mean sea level and

920-973: The 19th century. With high emissions it would instead accelerate further, and could rise by 1.0 m ( 3 + 1 ⁄ 3  ft) or even 1.6 m ( 5 + 1 ⁄ 3  ft) by 2100. In the long run, sea level rise would amount to 2–3 m (7–10 ft) over the next 2000 years if warming stays to its current 1.5 °C (2.7 °F) over the pre-industrial past. It would be 19–22 metres (62–72 ft) if warming peaks at 5 °C (9.0 °F). Rising seas affect every coastal and island population on Earth. This can be through flooding, higher storm surges , king tides , and tsunamis . There are many knock-on effects. They lead to loss of coastal ecosystems like mangroves . Crop yields may reduce because of increasing salt levels in irrigation water. Damage to ports disrupts sea trade. The sea level rise projected by 2050 will expose places currently inhabited by tens of millions of people to annual flooding. Without

960-481: The Metonic 19-year lunar cycle which, as the basic structure of Dionysius Exiguus ' and also of Bede 's Easter table, would ultimately prevail throughout Christendom , at least until in the year 1582, when the Gregorian calendar was introduced. The Coligny calendar is a Celtic lunisolar calendar using the Metonic cycle. The bronze plaque on which it was found dates from c. AD 200, but the internal evidence points to

1000-580: The average sea level. In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collated data about the sea level. It is used for a part of continental Europe and the main part of Africa as the official sea level. Spain uses the reference to measure heights below or above sea level at Alicante , while the European Vertical Reference System

1040-460: The calendar itself being several centuries older, created in the Iron Age. The Runic calendar is a perpetual calendar based on the 19-year-long Metonic cycle. It is also known as a Rune staff or Runic Almanac. This calendar does not rely on knowledge of the duration of the tropical year or of the occurrence of leap years. It is set at the beginning of each year by observing the first full moon after

1080-470: The construction of a lunisolar calendar . A tropical year (about 365.24 days) is longer than 12 lunar months (about 354.36 days) and shorter than 13 of them (about 383.90 days). In a Metonic calendar (a type of lunisolar calendar ), there are twelve years of 12 lunar months and seven years of 13 lunar months. In the Babylonian and Hebrew lunisolar calendars , the years 3, 6, 8, 11, 14, 17, and 19 are

1120-484: The entire Earth, which is what systems such as GPS do. In aviation, the reference ellipsoid known as WGS84 is increasingly used to define heights; however, differences up to 100 metres (328 feet) exist between this ellipsoid height and local mean sea level. Another alternative is to use a geoid -based vertical datum such as NAVD88 and the global EGM96 (part of WGS84). Details vary in different countries. When referring to geographic features such as mountains, on

1160-482: The geoid is referred to as (mean) ocean surface topography . It varies globally in a typical range of ±1 m (3 ft). Several terms are used to describe the changing relationships between sea level and dry land. The melting of glaciers at the end of ice ages results in isostatic post-glacial rebound , when land rises after the weight of ice is removed. Conversely, older volcanic islands experience relative sea level rise, due to isostatic subsidence from

1200-624: The greatest exposure to sea level rise, twelve are in Asia , including Indonesia , Bangladesh and the Philippines. The resilience and adaptive capacity of ecosystems and countries also varies, which will result in more or less pronounced impacts. The greatest impact on human populations in the near term will occur in the low-lying Caribbean and Pacific islands . Sea level rise will make many of them uninhabitable later this century. Pilots can estimate height above sea level with an altimeter set to

1240-551: The height of planetary features. 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 long enough that fluctuations caused by waves and tides are smoothed out, typically a year or more. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can occur at rates similar to sea level changes (millimetres per year). Some land movements occur because of isostatic adjustment to

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1280-453: The long (13-month) years of the Metonic cycle. This cycle forms the basis of the Greek and Hebrew calendars. A 19-year cycle is used for the computation of the date of Easter each year. The Babylonians applied the 19-year cycle from the late sixth century BC. According to Livy , the second king of Rome, Numa Pompilius (reigned 715–673 BC), inserted intercalary months in such a way that "in

1320-481: 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 sea level change (global as opposed to local change)

1360-541: 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, the altimeter is set to the international standard atmosphere (ISA) pressure at MSL which is 1013.25 hPa or 29.92 inHg. Metonic 19-year lunar cycle The Metonic cycle or enneadecaeteris (from Ancient Greek : ἐννεακαιδεκαετηρίς , from ἐννεακαίδεκα, "nineteen")

1400-534: The poles and 6,371.001 km (3,958.756 mi) on average. This flattened spheroid , combined with local gravity anomalies , defines the geoid of the Earth, which approximates the local mean sea level for locations in the open ocean. The geoid includes a significant depression in the Indian Ocean , whose surface dips as much as 106 m (348 ft) below the global mean sea level (excluding minor effects such as tides and currents). Precise determination of

1440-415: The same way Meton had, by trying to make the month fit the year. Sea level calculations also depend on the Metonic cycle. https://tidesandcurrents.noaa.gov/publications/Understanding_Sea_Level_Change.pdf The Metonic cycle is the most accurate cycle of time (in a timespan of less than 100 years) for synchronizing the tropical year and the lunar month ( synodic month ), when the method of synchronizing

1480-466: The twentieth year the days should fall in with the same position of the sun from which they had started". As "the twentieth year" takes place nineteen years after "the first year", this seems to indicate that the Metonic cycle was applied to Numa's calendar. Diodorus Siculus reports that Apollo is said to have visited the Hyperboreans once every 19 years. The Metonic cycle has been implemented in

1520-411: The weight of cooling volcanos. The subsidence of land due to the withdrawal of groundwater is another isostatic cause of relative sea level rise. On 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" or zero-level elevation , serves equivalently as a reference for

1560-530: The winter solstice. The oldest one known, and the only one from the Middle Ages, is the Nyköping staff , which is believed to date from the 13th century. The Bahá'í calendar , established during the middle of the 19th century, is also based on cycles of 19 solar years. A Small Maḥzor ( Hebrew מחזור, pronounced [maχˈzor] , meaning "cycle") is a 19-year cycle in the lunisolar calendar system used by

1600-419: The worst effects or, when populations are at extreme risk, a process of managed retreat . The term above sea level generally refers to the height above mean sea level (AMSL). The term APSL means above present sea level, comparing sea levels in the past with the level today. Earth's radius at sea level is 6,378.137 km (3,963.191 mi) at the equator. It is 6,356.752 km (3,949.903 mi) at

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