If you have any further questions for the PSMSL please could you include
your home country (not always obvious from the email address),
as this should help us to give answers with a more local bias.
There are two usual answers to this question depending on your background:
[1] MSL to an operator of a tide gauge means the 'still water level' (i.e.
the level of the sea with high frequency motions such as wind waves
averaged out) averaged over a period of time such as a month or a year such
that periodic changes in sea level (e.g. due to the tides) are also
averaged out. The values of MSL are measured with respect to the level of
marks on land (called 'benchmarks'). Consequently, 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 is located. For information on how to
compute MSL from tide gauge data, see the IOC/GLOSS Manuals in the
PSMSL training web pages . All
'MSL data' in the PSMSL database are derived from tide gauge data by this
means.
[2] MSL to a geodesist (a person who studies the shape of the Earth) usually
means the local height of the global Mean Sea Surface (MSS) above a
'level' reference surface, or datum, called the geoid.
So the next question is "What is a level surface"? Consider that
a ball placed on a
slope would roll downhill, but a ball placed on a level surface would
stay still. Another way to think of this is that you do not have to do
any work to move along a 'level' surface. However, a level surface is
not flat. Since the Earth is round, a first
approximation is that level surfaces are spheres. In fact, the effect
of the Earth's rotation, and of the resulting bulge at the equator,
means that a better approximation is an oblate ellipsoid: an ellipse
with a smallest radius of 6356.752 km at the poles, and a largest
radius of 6378.137 km at the equator, rotated around the Earth's rotation
axis.
This ellipsoid is still not precisely a level surface. Concentrations
of mass in different parts of the earth's interior, and topography
(mountains, seamounts etc) all result in a gravitational attraction
which deforms the level surfaces. The level surface closest to the MSS,
known as the geoid, departs from an ellipsoid by about 100 m
in each direction, depending on position on the earth. For this
reason, a map of the MSS measured from space, with the reference
ellipsoid subtracted off, shows a very complicated shape reflecting to
some extent (but at shorter wavelengths only) the undulations in the
shape of the ocean floor (with each seamount below the ocean surface
producing a gravitational attraction towards it, resulting in a small
bulge in the sea surface above it).
If the oceans did not move, and were not affected by winds and air
pressure, then MSS and geoid would be the same surfaces.
However, there are steady currents in the ocean, driven by winds and
atmospheric heating and cooling, which give rise to differences in sea
level around the world. Therefore, the MSS is not a 'level' surface, and it
departs from the geoid (which is) by about 1-2 m, even after averaging
out the effects of tides and other time-dependent motions. For
example, the Atlantic Ocean north of the Gulf Stream is about 1 m
lower than further south (see 4), and the Atlantic as a whole is about 40 cm
lower than the Pacific. There is even a sea level difference of about
20 cm across the Panama Canal (see 3).
It can be seen that MSL (the local height of the MSS above the geoid)
is a complicated quantity, which depends not only on the
volume of water in the oceans, and the shape of the ocean basins, but
also on the earth's gravitational field and rotation rate (which
determine the shape of the geoid), and on patterns of currents within
the oceans. The importance of the differences between these different
surfaces depends on the scientific application. But it will also be realised
that, because of ocean, climate and geological changes, MSL is not
constant in time, its 'mean value' being determined only for a
particular epoch. For more information on MSL and geodesy, see the
various books and reports mentioned in the
PSMSL training web pages .
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Heights above sea level, like mountains or whatever, have traditionally
been defined (and still are in most cases) in terms of a measurement of
'mean sea level' at one or more locations. That mean sea level, once
determined at the location, is then carried around the country by means of
surveyors' spirit levelling apparatus (i.e. better versions of the levelling
apparatus you must have seen surveyors use in the road or construction industry).
So, for example, here in the U.K. we define heights above sea level in
terms of 'Ordnance Datum Newlyn' (ODN) which is a level defined by
the mean level of the sea at
Newlyn in Cornwall in S.W.England in the period
May 1915 to April 1921 (which replaced an earlier Ordnance Datum Liverpool
based on sea level in that port in 1844). That level
has been carried around the country by surveyors' methods. So, a height of
a Scottish mountain, for example, means height above the sea level at
Newlyn many years ago. Similarly, NAP (Normaal Amsterdam Peil) in
the Netherlands is approximate mean sea level at Amsterdam (and
represented by a mark in a shopping centre). French heights are relative to
mean sea
level at Marseille.
In many countries, like the US or India with 2 coastlines there are
often two or more datums as the distance from the sea to the mountains
(or whatever) can be great and errors creep into the measurements. Also
for oceanographic reasons sea level can really be different in different
places i.e. mean sea level is not a 'level surface' (see 1).
There are many complications. The sea level itself can change (sea level is
believed to have increased by between 10 and 20 cm during the last 100 years) and
land levels can change due to geological reasons.
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Sea level is about 20 cm higher on the Pacific side than the Atlantic due to
the water being less dense on average on the Pacific side and due to the
prevailing weather and ocean conditions. Such sea level differences are
common across many short sections of land dividing ocean basins.
The 20 cm difference business is determined by geodetic levelling from one
side to the other. A datum called Panama Canal Datum is used. When you use
spirit levelling you follow a 'level' surface (to our perceptions, see 1)
which will be parallel to the geoid (which is geometrically a 'lumpy' surface).
The geoid is the surface of constant gravitational potential (plus
a 'centrifugal potential' term) which on average
coincides with the sea surface i.e. a 'level' surface in everyday language.
The 20 cm difference at Panama is not unique. There are similar 'jumps'
elsewhere e.g. Skagerrak, Indonesian straits.
If the canal was open sea and not locks (i.e. if somehow a deep open cutting
had been made rather than the canal system over the mountains) then there
WOULD be a current flowing from Pacific to Atlantic. An analogy (although
not a perfect one because
there are many other factors) is that you could compare Panama to the Drake
Passage off the south tip of Chile
which has a west-east flow (but mostly wind-driven of course, but
Pacific-Atlantic density must play some role).
Locks are needed in the Panama Canal because the canal climbs over the hills
and makes use of mountain
lakes. Therefore, locks would be needed even if sea level was the same on
the two sides. (So, for example, there are also locks on canals here in England
which is much less mountainous than Panama).
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Any current which flows for longer than a day is, effectively, turning
in circles as the Earth rotates. In the same way that an object moving
in circles requires a force at right angles to its motion, like the Moon
orbiting the Earth, any mean current in the ocean also requires a
force (to the left, in the northern hemisphere) to balance its
steady motion. This balance, between motion and a pressure force
to the left, is known as geostrophy. It as also the reason why winds
tend to blow along isobars, rather than directly from regions of high
to low pressure, as you see every day on the weather.
The strong northward current, the Gulf Stream, which flows between
Bermuda and New York results in sea level at Bermuda being about 1 metre
higher than say Charleston (that is to
say 'higher' with respect to a surface called the 'geoid', see 1. above).
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Global-average sea level is believed to have risen by between 10-20
cm during the past century and best estimates
are that it will rise by approximately 50 cm in the next 100 years
(i.e. an acceleration of a factor of 3 in the rate). Rising sea levels
are largely a consequence of the thermal expansion of the ocean,
melting of low latitude glaciers (Alps, Rockies etc.) and many other
factors, each of which are reviewed every few years by the
Intergovernmental Panel on Climate Change (IPCC). For full references of
the IPCC reports see the
PSMSL training web pages. The reports themselves
should be obtainable from any decent library.
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No. Long term changes in sea level measured at the coast (e.g. by
tide gauges) are a consequence of 'real' changes in the level of the
ocean (e.g. due to climate change), to which must be added changes in
the level of the land.
Changes in ocean level due to climate change can be greater in some
places than others because the ocean circulation will adapt to accommodate
the new climate regime (see the IPCC reports for a review).
Most knowledge of the global pattern of vertical land movements
comes from geological data which are included in geodynamic models
of the Earth. The main geological process involved is called
Glacial Isostatic Adjustment (GIA). For example, in the UK, GIA results in sea level
rising less rapidly in Scotland than in southern England. However, there
are other geological processes, violent changes due to earthquakes being the
most dramatic.
Land level changes are now being investigated by geodetic research groups
using the Global Positioning System and Absolute
Gravity techniques. See again the
PSMSL training web pages.
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This is a complicated question which depends on where you live, the
tides and storms in your area, and the type of coastal infrastructure.
For the best reviews, consult the Intergovernmental Panel on Climate
Change (IPCC) Working Group II reports listed in the
PSMSL training web pages .
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8. I want to buy a house by the sea. I have heard about climate change
and I want to ask, is this a sensible thing to do?
It is probably as sensible as it would be without climate change,
given the ferocity of winter storms in many places
even in 'normal' times. The rise
of 50 cm projected for the next 100 years is expected to occur mostly
in the second half of the next century. Consequently, rises of level
for the next 20-30 years (your remaining lifetime) can be expected to
be similar to those for the past 30 years (of order 10 cm). You should
enquire from your local authorities what their policy is for coastal
protection in your area, taking into consideration the potential sea
level changes in the future. If coastal protection is not adequate
already, climate change may make the problem worse.
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9. Where can I find out mean sea level information for the Mediterranean?
For general background reading, try the books by Paolo Pirazzoli,
which contain a wealth of information on Mediterranean sea levels.
Absolute ('real') changes in Mediterranean sea level over the last 2000
years are very unlikely to have been more that 10-20 cm or so with changes
in land levels have been much larger resulting in apparent local changes.
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The deepest part of the oceans is the Mariana Trench in the northern
Pacific Ocean. It's depth is given as 10924m below mean sea level.
The deepest in the Atlantic is the Puerto Rico Trench (8605 m), that
in the Indian Ocean is the Java Trench (7125 m) and that in the Arctic
is the Eurasian Basin (5450 m).
Data sets of the topography of the Earth can be obtained from (amongst
other places) the
US National Geophysical Data Center.
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Tide gauges are used to measure the sea level with respect to the
underlying solid earth. In areas of tectonic motion (continental plate
activity), continues glacial isostatic adjustment or serious subsidence
due to fluid pumping, changes in sea level are contaminated by vertical
land movement signals. If GPS is used to measure the vertical land movement
at some point near the tide gauge, then this signal can be removed from the sea
level measurements. Removing the land movement signal improves the estimated
sea level change.
GPS at tide gauges is also used to refer the sea level
measurements to a global external reference system, such as the International
Terestrial Reference Frame (ITRF).
For more information see
http://www.soest.hawaii.edu/cgps_tg/CGPS@TG_v9.html or
http://www.pol.ac.uk/psmsl/training/geodesy_etc.html
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Tide gauge pressure sensors are either 'absolute' systems (ie. they measure
the total pressure under water which of course includes the atmospheric
pressure), or 'differential' systems which measure the water-head pressure
(ie. sea level with the atmospheric pressure removed).
Sometimes people use an 'absolute' sensor and have a separate barometer. Either
way, one needs sea level and not 'sub-surface total pressure'.
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