There are two usual answers to this question depending on your background:
 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.
 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 more-or-less 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 . [return to top]
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 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 (i.e. levelling, which is equivalent to imagining a set of ficticious thin canals with no flow permeating the country which allow the ODN level, the 'zero' reference level of the height system, to be transferred from one place to another). So, a height of a Scottish mountain, for example, means height above ODN, which is the 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 the town hall/opera building) (see http://nl.wikipedia.org/wiki/Normaal_Amsterdams_Peil and http://nl.wikipedia.org/wiki/Afbeelding:Nap_reference2.jpg for more information). French heights are relative to mean sea level at Marseille at a particular epoch.
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.[return to top]
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).
Note also that the tides have opposite phase on the 2 sides of Panama, so, if there was a sea level canal, there would be major tidal currents through it.[return to top]
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).[return to top]
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 suggested Reading List (1): Books on Tides and Sea Levels in the PSMSL training web pages. The reports themselves should be obtainable from any decent library.[return to top]
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.[return to top]
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 scientific reviews of the subject, consult the Intergovernmental Panel on Climate Change (IPCC) Working Group II reports listed in the PSMSL training web pages. Articles on climate changes (including sea level changes) and their impacts on the developing countries can be found in The TIEMPO Climate Cyberlibrary.[return to top]
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.[return to top]
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. [return to top]
The deepest part of the oceans is the Marianas 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.[return to top]
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).
Tide gauge pressure sensors are either 'absolute' systems (ie. they measure the total pressure under water which of course includes the pressure load of the atmosphere), or 'differential' systems which measure the water-head pressure alone by subtracting the atmospheric pressure from the total pressure within the pressure transducer. An alternative form of 'differential' sensor can be constructed with the use of an 'absolute' sensor together with a separate barometer. Either way, it is important to realise that 'sea level' is not the same as 'sub-surface total pressure'.For more detailed information on tide gauge measurement techniques, see the 'IOC Manuals and Guides No.14' (especially Volume 3 which is the most recent). [return to top]
As a rule of thumb (no more), 1°C of global air temperature rise amounts to about 10 cm of thermal expansion of sea level, but the exact amount depends on the ocean model used to estimate this. Further reading on how much water is stored where can be gained form the IPCC Third Assessment Report chapter on Sea Level (Cambridge Univ Press).[return to top]
To a first approximation, if all the floating sea ice in the world melted, there would be no change in sea level at all, as the floating ice will have displaced its own weight of water. However, if land ice melts, that will raise sea level. All the world's glaciers and small ice caps contain approximately 0.5 m of sea level equivalent between them, while the great Greenland and Antarctic ice sheets contain approximately 7 and 61 m respectively. Consequently, if all the wolrd's ice melted in a very much warmer world, sea level would be approximately 70 m higher.
However, when land ice melts the distribution of the mass of water around the global ocean is by no means uniform. A large melting would result in a modification in the Earth's gravity field which would result in the sea level change being higher in some places than in others. [return to top]
The answer to this problem is quite complex. The computation involves computing depressions of distributed loads of a certain size. For example, for a 1 metre load of water over discs of different radii, the vertical displacements at the centre are:-
Thus, the depression just due to the weight of water in the local area that is flooded is probably only a few tenths of a mm. For the crustal displacement due to an increase in sea level over a whole sea area (such as the North Sea), a larger mass is involved. For example, our work on storm surge loading shows that a storm surge of about 2 metres in the southern North Sea depresses the crust by 20 to 30 mm in near coastal areas. [return to top]