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From Wikipedia, the free encyclopedia

This marker indicating sea level is situated between Jerusalem and the Dead Sea.
This marker indicating sea level is situated between Jerusalem and the Dead Sea.

Mean sea level (MSL) (often shortened to sea level) is an average level of the surface of one or more of Earth's oceans from which heights such as elevation may be measured. MSL is a type of vertical datum – a standardised geodetic datum – that is used, for example, as 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 the midpoint between a mean low and mean high tide at a particular location.[1]

Sea levels can be affected by many factors and are known to have varied greatly over geological time scales. However 20th century and current millennium sea level rise is caused by global warming,[2] and careful measurement of variations in MSL can offer insights into ongoing climate change.[3]

The term above sea level generally refers to above mean sea level (AMSL).

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Transcription

This RealLifeLore video is sponsored by the upcoming film, An Inconvenient Sequel. You've probably heard news, at some point in your life, that the Earth is warming, and as result is that the Arctic ice is melting. There is no debate that Earth is warming up. The planet's average temperature as increased 1.1 °C since 1880. And 16 out of the 17 of the hottest years, ever on record, have been since 2001. NASA estimates there is 95% probability that this current warming is a direct result of human activities since the mid 20th century And 97/100 active publishing climate scientists agree that human activity is extremely likely to blame As a result of this, ice around the world is melting and raising the planet's sea level The oceans already rose 6 cm during the course of the 19th century. But they rose by 19 cm during the 20th century, over 3 times faster than they rose in the previous century. NOAA estimates that global sea level could rise by up to 2.5 meters by the year 2100. Which would have devastating consequences. If this happens entire island countries like the Maldives, Marshall Islands Kiribati, and Tuvalu would be submerged beneath the ocean and cease to exist Their entire population has to be transplanted somewhere else And this will create a situation in which hundreds of thousands of state-less people would find a new home 150 million total climate refugees from around the world may exist by 2050, and by 2100 that number could rise to 2 billion people 18% of the global population at that time. But in an even worse case scenario than this, however what would happen if all the ice on earth melted? If all the ice on Antarctica, Greenland, and the world glaciers melted in to the ocean it would rise global sea level by an incredible 68.3 meters Such a catastrophe would take 5,000 years of the current rate of the ice melting to happen, but it would severely alter the geography of our planet, in North America, Vancouver, Seattle, Portland, and most of San Diego, and Los Angeles would be underwater. San Francisco would be an island and San Jose would be destroyed by the waves. The San Francisco Bay would greatly expand in size, to completely swallow Sacremento. The Gulf of California would expand so far North, that Palm Springs would become a coastal city. While on the Atlantic Coast, the ocean would expand to swallow the cities of Corpus Christi, Houston, New Orleans, the entire state of Florida, as well as the entire state of Delaware, almost all of Long Island, and the cities of New York, Philadelphia, Baltimore, Washington D.C., and Boston. South America would also be severely flooded with two new inland seas existing on the continent. And the cities of Buenos Aires, Montevideo, and Rio de Janeiro, would all be underwater. Europe would be one of the most devastated areas of the world. The entire country of the Netherlands would be underwater. And so would nearly all of Denmark and half of Belgium. The British Isles will become more of an archipelago. And London would be TOTALLY buried beneath the sea. And Northern Germany would also be entirely underwater including the city of Berlin. And Venice in Italy would be long since be claimed by the Adriatic Sea. Istanbul would also be underwater. The Crimean Peninsula would become the Crimean Island. And the Black and Caspian Seas would be connected with one another. Which would mean that Kazakhstan, Uzbekistan, Turkmenistan, and Azerbaijan, would no longer be landlocked countries. Africa would be perhaps the least severely effected continent. But Egypt would be severely damaged by having both Alexandria and Cairo going underwater The Persian Gulf, meanwhile, would swallow the entire the entire countries of Qatar, and Bahrain, and the city of Baghdad. India and Pakistan would be badly damaged. But the entire country of Bangladesh would cease to exist in this catastrophe. Hundreds of millions of people would be underwater in China, including Beijing and Shanghai. In addition to the cities of Seoul, Pyongyang, and Tokyo. Australia would be home to a new inland sea. And finally, Antarctica would become more of an archipelago. But unlike the Antarctica of today, this Antarctica would likely be habitable and easily colonize-able by future humans. So what can we do to prevent this nightmare future from happening? Reducing humanity's usage of fossil fuels is perhaps the best way of mitigating our warming climate.These following countries produce At least 75% of their power from renewable energy resources, like solar, wind, hydro-power. Global investments in renewable energy rose by 57% between 2008 and 2015. The European Union alone more than doubled it's renewable energy production between 2000 and 2013. And recently 195 different countries signed the Paris Climate Accord in which they all agreed to cooperate in combating global warming. While the stakes have never been higher, we are close to seeing a true energy revolution. If you want to learn more about climate change, or want to take action yourself, then I strongly encourage you to go watch the upcoming film, "An Inconvenient Sequel". Watching the film is taking action, and it goes into far more depth than I ever could in a short YouTube video. The film will be released in New York and Los Angles on July 28th and will be released nation-wide on August 4th. Please go to the description below for more detail. Be inconvenient. *Narrator in trailer talking about global warming* If you enjoyed what you just watched, then I hope you'll subscribe to my channel for future videos by clicking here (windmill icon) You can visit my Patreon if you'd like to support my channel directly and receive some cool rewards And I hope to see you next Friday for another RealLifeLore video then

Contents

Measurement

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).
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).

Precise determination of a "mean sea level" is difficult to achieve because of the many factors that affect sea level.[4] Instantaneous sea level varies quite a lot 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 easiest way this may be calculated is by selecting a location and calculating the mean sea level at that point and use it as a datum. For example, a period of 19 years of hourly level observations may be averaged and used to determine the mean sea level at some measurement point.

Still-water level or still-water sea level (SWL) is the level of the sea with motions such as wind waves averaged out.[5] 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 a spatial average over the entire ocean.

One often measures the values of MSL in respect to the land; hence a change in relative 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 vertical datum was MSL at the Victoria Dock, Liverpool. Since the times of the Russian Empire, in Russia and other former its 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 1.230m below the average sea level. 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. As for Spain, the reference to measure heights below or above sea level is placed in Alicante. Elsewhere in Europe vertical elevation references (European Vertical Reference System) are made to the Amsterdam Peil elevation, which dates back to the 1690s.

Satellite altimeters have been making precise measurements of sea level[6] 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

Height above mean sea level (AMSL) is the elevation (on the ground) or altitude (in the air) of an object, relative to the average sea level datum. It is also used in aviation, where some heights are recorded and reported with respect to mean sea level (MSL) (contrast with flight level), and in the atmospheric sciences, and land surveying. An alternative is to base height measurements on an ellipsoid of the entire Earth, which is what systems such as GPS do. In aviation, the ellipsoid known as World Geodetic System 84 is increasingly used to define heights; however, differences up to 100 metres (328 feet)[citation needed] exist between this ellipsoid height and mean tidal height. The alternative is to use a geoid-based vertical datum such as NAVD88.

When referring to geographic features such as mountains on a topographic map, variations in elevation are shown by contour lines. The elevation of a mountain denotes the highest point or summit and is typically illustrated as a small circle on a topographic map with the AMSL height shown in metres, feet or both.

In the rare case that a location is below sea level, the elevation AMSL is negative. For one such case, see Amsterdam Airport Schiphol.

Difficulties in use

To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or geodetic 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 (mean) ocean surface topography. It varies globally in a range of ± 2 m.

Historically, adjustments were made to sea-level measurements to take into account the effects of the 235 lunar month Metonic cycle and the 223-month eclipse cycle on the tides.[7]

Dry land

Sea level sign seen on cliff (circled in red) at Badwater Basin, Death Valley National Park
Sea level sign seen on cliff (circled in red) at Badwater Basin, Death Valley National Park

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" or zero-level elevation, serves equivalently as a reference for the height of planetary features.

Change

Local and eustatic

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's oceans or net changes in the volume of the ocean basins.[8]

Short term and periodic changes

Melting glaciers can cause a change in sea level
Melting glaciers can cause a change in sea level

There are many factors which can produce short-term (a few minutes to 14 months) changes in sea level. Two major mechanisms are causing sea level to 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.[9]

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

Recent changes

For at least the last 100 years, sea level has been rising at an average rate of about 1.8 mm (0.07 in) per year.[10] Most of this rise can be attributed to the increase in temperature of the sea and the resulting slight thermal expansion of the upper 500 metres (1,640 feet) of sea water. Additional contributions, as much as one-quarter 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 uses.[11]

Aviation

Pilots can estimate height above sea level 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, the altimeter is set to the international standard atmosphere (ISA) pressure at MSL which is 1013.25 hPa or 29.92 inHg.[12]

See also

References

  1. ^ What is "Mean Sea Level"? (Proudman Oceanographic Laboratory).
  2. ^ USGCRP (2017). "Climate Science Special Report. Chapter 12: Sea Level Rise. Key finding 2". science2017.globalchange.gov. Retrieved 2018-12-27.
  3. ^ "The strange science of melting ice sheets: three things you didn't know". The Guardian. 12 September 2018.
  4. ^ US National Research Council, Bulletin of the National Research Council 1932 page 270
  5. ^ [1]
  6. ^ Glazman, Roman E; Greysukh, Alexander; Zlotnicki, Victor (1994). "Evaluating models of sea state bias in satellite altimetry". Journal of Geophysical Research. 99 (C6): 12581. Bibcode:1994JGR....9912581G. doi:10.1029/94JC00478.Roman Glazman Greysukh, A. M., Zlotnicki, V.
  7. ^ "Stonehenge pt 3". www.celticnz.co.nz. Retrieved 2017-02-18.
  8. ^ "Eustatic sea level". Oilfield Glossary. Schlumberger Limited. Retrieved 10 June 2011.
  9. ^ "Global Warming Effects on Sea Level". www.climatehotmap.org. Retrieved 2016-12-02.
  10. ^ Bruce C. Douglas (1997). "Global Sea Rise: A Redetermination". Surveys in Geophysics. 18 (2/3): 279–292. Bibcode:1997SGeo...18..279D. doi:10.1023/A:1006544227856.
  11. ^ Bindoff, N.L.; Willebrand, J.; Artale, V.; Cazenave, A.; Gregory, J.; Gulev, S.; Hanawa, K.; Le Quéré, C.; Levitus, S.; Nojiri, Y.; Shum, C.K.; Talley, L.D.; Unnikrishnan, A. (2007). "Observations: Oceanic Climate Change and Sea Level" (PDF). In Solomon, S.; Qin, D.; Manning, M.; Chen, Z.; Marquis, M.; Averyt, K.B.; Tignor, M.; Miller, H.L. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press.
  12. ^ US Federal Aviation Administration, Code of Federal Regulations Sec. 91.121

External links

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