Mean
sea level (MSL) is the average (mean) height
of the ocean's surface (especially that halfway between mean high
and low tide); used as a standard in reckoning land
elevation.
Measurement
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
, mean sea level has been measured at Newlyn
in Cornwall
and Liverpool
on Merseyside for
decades, by tide gauges to provide Ordnance Datum for the zero
metres height on UK maps.
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
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
does
not remain constant over the surface of the entire
earth.
For
instance, mean sea level at the Pacific
end of the
Panama
Canal
stands 20 cm (8 in) higher than at the Atlantic
end.
Sea level and dry land
Several 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 mass added to (or removed from) the
oceans (e.g. melting of
ice sheets). 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 masses
due to thermal buoyancy or
tectonic
effects and implies no real change in the volume of water in the
oceans. The melting of
glaciers at the end
of
ice ages is an 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
Local and eustatic sea level
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.
Atmospheric pressure,
ocean currents and local ocean
temperature 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
temperatures
Various 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. It can however be
argued that if ice shelves melt it is a precursor to the melting of
ice sheets on Greenland and Antarctica .
- 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. ( 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. The collapse of the grounded interior reservoir
of the West Antarctic Ice
Sheet would raise sea level by 5-6 m.
- 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 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
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.
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 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). For this reason, sea level
is more prone to rise than fall today, and small changes in
climate can have noticeable effects during
human lifetimes.
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:
- 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
For at least the last 100 years, sea level has been rising at an
average rate of about 1.8 mm per year. Scientists believe that the
majority of this rise can be attributed to human-induced
global warming.
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.
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 . 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
Notes
- What is "Mean Sea Level"?
Proudman Oceanographic
Laboratory
- Geologic Contral on Fast Ice Flow - West Antarctic
Ice Sheet. by Michael Studinger, Lamont-Doherty Earth
Observatory
- US Federal Aviation
Administration, Code of Federal Regulations Sec. 91.121
References
- What is "Mean Sea Level"?
Proudman Oceanographic
Laboratory
- Geologic Contral on Fast Ice Flow - West Antarctic
Ice Sheet. by Michael Studinger, Lamont-Doherty Earth
Observatory
- US Federal Aviation
Administration, Code of Federal Regulations Sec. 91.121
External links