The Info List - Sea Level

--- Advertisement ---

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 elevations may be measured. MSL is a type of vertical datum – a standardised geodetic reference point – 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. The careful measurement of variations in MSL can offer insights into ongoing climate change, and sea level rise has been widely quoted as evidence of ongoing global warming.[2] The term above sea level generally refers to above mean sea level (AMSL).


1 Measurement

1.1 Height above mean sea level

1.1.1 Difficulties in use

2 Dry land 3 Change

3.1 Local and eustatic 3.2 Short term and periodic changes 3.3 Recent changes

4 Aviation 5 See also 6 References 7 External links


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.[3] 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. 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
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. 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. 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[4] 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
Ocean Surface Topography Mission
on the Jason-2 satellite in 2008. Height above mean sea level[edit] Main article: Elevation 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
World Geodetic System
84 is increasingly used to define heights; however, differences up to 100 metres (328 feet) 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[edit]

Ocean Reference ellipsoid Local plumb line Continent 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 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 (stationary) 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
Metonic cycle
and the 223-month eclipse cycle on the tides.[5] Dry land[edit]

This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (August 2015) (Learn how and when to remove this template message)

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", serves equivalently as a reference for the height of planetary features. Change[edit] See also: Past sea level, Current sea-level rise, Future sea level, and Sea
level rise Local and eustatic[edit]

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.[6] Short term and periodic changes[edit]

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.[7]

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

and precipitation (may also follow long-term pattern) Days to weeks  

Ocean surface topography
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 (standing waves) Minutes to hours Up to 2 m


Tsunamis (generate catastrophic long-period waves) Hours Up to 10 m

Abrupt change in land level Minutes Up to 10 m

Recent changes[edit] Main article: Current sea level rise Further information: Global warming, Ocean
heat content, and Effects of global warming on oceans For at least the last 100 years, sea level has been rising at an average rate of about 1.8 mm (0.1 in) per year.[8] 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.[9] Aviation[edit] Main article: Altitude in 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.[10] See also[edit]

Above ground level Amsterdam Ordnance Datum Before Present Chart datum Extreme points of Earth Geopotential height Height above average terrain List of places on land with elevations below sea level Marine terrace Meltwater pulse 1A Metres above the Adriatic Normaal Amsterdams Peil Normal height Normalhöhennull Normalnull North West Shelf Operational Oceanographic System Ordnance datum
Ordnance datum
(UK) Orthometric height Vertical datum World Geodetic System


^ What is " Mean
Level"? (Proudman Oceanographic Laboratory). ^ Solomon et al., Technical Summary, Section 3.4 Consistency Among Observations in IPCC AR4 WG1 2007; Hegerl et al., Executive summary, Section 1.3: Consistency of changes in physical and biological systems with warming in IPCC AR4 SYR 2007. ^ US National Research Council, Bulletin of the National Research Council 1932 page 270 ^ "Evaluating models of sea state bias in satellite altimetry". Journal of Geophysical Research. NASA. 99 (C6): 12581. 1994. Bibcode:1994JGR....9912581G. doi:10.1029/94JC00478. Roman Glazman Greysukh, A. M., Zlotnicki, V. ^ "Stonehenge pt 3". www.celticnz.co.nz. Retrieved 2017-02-18.  ^ "Eustatic sea level". Oilfield Glossary. Schlumberger Limited. Retrieved 10 June 2011.  ^ "Global Warming Effects on Sea
Level". www.climatehotmap.org. Retrieved 2016-12-02.  ^ 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.  ^ 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.  ^ US Federal Aviation Administration, Code of Federal Regulations Sec. 91.121

External links[edit]

The Wikibook Historical Geology has a page on the topic of: Sea
level variations

Wikimedia Commons has media related to Sea

Level Rise:Understanding the past – Improving projections for the future Permanent Service for Mean
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?

v t e

Physical oceanography


Airy wave theory Ballantine scale Benjamin–Feir instability Boussinesq approximation Breaking wave Clapotis Cnoidal wave Cross sea Dispersion Edge wave Equatorial waves Fetch Gravity wave Green's law Infragravity wave Internal wave Iribarren number Kelvin wave Kinematic wave Longshore drift Luke's variational principle Mild-slope equation Radiation stress Rogue wave Rossby wave Rossby-gravity waves Sea
state Seiche Significant wave height Soliton Stokes boundary layer Stokes drift Stokes wave Swell Trochoidal wave Tsunami


Undertow Ursell number Wave action Wave base Wave height Wave power Wave radar Wave setup Wave shoaling Wave turbulence Wave–current interaction Waves and shallow water

one-dimensional Saint-Venant equations shallow water equations

Wind wave



Atmospheric circulation Baroclinity Boundary current Coriolis force Coriolis–Stokes force Craik–Leibovich vortex force Downwelling Eddy Ekman layer Ekman spiral Ekman transport El Niño–Southern Oscillation General circulation model Geostrophic current Global Ocean
Data Analysis Project Gulf Stream Halothermal circulation Humboldt Current Hydrothermal circulation Langmuir circulation Longshore drift Loop Current Modular Ocean
Model Ocean
dynamics Ocean
gyre Princeton ocean model Rip current Subsurface currents Sverdrup balance Thermohaline circulation


Upwelling Whirlpool World Ocean
Circulation Experiment


Amphidromic point Earth tide Head of tide Internal tide Lunitidal interval Perigean spring tide Rip tide Rule of twelfths Slack water Tidal bore Tidal force Tidal power Tidal race Tidal range Tidal resonance Tide
gauge Tideline


Abyssal fan Abyssal plain Atoll Bathymetric chart Coastal geography Cold seep Continental margin Continental rise Continental shelf Contourite Guyot Hydrography Oceanic basin Oceanic plateau Oceanic trench Passive margin Seabed Seamount Submarine canyon Submarine volcano

Plate tectonics

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


Benthic Deep ocean water Deep sea Littoral Mesopelagic Oceanic Pelagic Photic Surf Swash


Deep-ocean Assessment and Reporting of Tsunamis Future sea level Global Sea
Level Observing System North West Shelf Operational Oceanographic System Sea-level curve Sea
level rise World Geodetic System


Deep scattering layer Hydroacoustics Ocean
acoustic tomography Sofar bomb SOFAR channel Underwater acoustics


Jason-1 Jason-2 ( Ocean
Surface Topography
Mission) Jason-3


Argo Benthic lander Color of water DSV Alvin Marginal sea Marine energy Marine pollution Mooring National Oceanographic Data Center Ocean Ocean
exploration Ocean
observations Ocean
reanalysis Ocean
surface topography Ocean
thermal energy conversion Oceanography Pelagic sediment Sea
surface microlayer Sea
surface temperature Seawater Science On a Sphere Thermocline Underwater glider Water column World Ocean