Sea Level:
Frequently Asked Questions and Answers

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1. What is "Mean Sea Level"?

Mean Sea Level (MSL) has a least two different meanings:

1. For a tide gauge operator, MSL means the 'still water level'. The 'still water level' (without waves) is averaged over some period of time, such as a month or a year. This MSL is measured relative to fixed marks on the land known as 'benchmarks'.

Because mean sea level is measured relative to land, MSL can change either due to a change in the volume of the water or due to a movement of the land. For this reason measurements from a tide gauge are sometimes called 'relative sea level'.

The process of calculating MSL from tide gauge data is described in the Intergovernmental Oceanographic Commission manuals which are available from the training manuals section of this website. All MSL data in the PSMSL are processed in this way.

2. The second meaning of MSL is used by geodesists – scientists who measure the shape of the Earth. For a geodesist, MSL is the local height of the global Mean Sea Surface (MSS) above a level surface, known as the 'geoid'.

A level surface is one on which a ball would not roll when placed on it. However, level surfaces are not necessarily flat. The Earth is nearly round and so a good approximation is that the level surface of the Earth is a sphere.

However, the Earth is not in fact perfectly round but is slightly flattened at the poles. This is due to the rotation (spin) of the Earth that produces a slight bulge at the equator. We call this shape an 'oblate spheroid' or an 'ellipsoid'. This ellipse has a smallest radius of 6356.752 km at the poles and a largest radius of 6378.137 km at the equator.

Even this ellipsoid is not completely level. Concentrations of mass in the Earth's interior, as well as topography on the Earth's surface, such as mountains and seamounts, have a gravitational attraction which distorts this level surface (the geoid) compared to the ellipsoid. The difference between the Mean Sea Surface and the ellipsoid closest to it can be 100m in either direction, depending on location.

As a result, the MSS observed from space, with the reference ellipsoid subtracted from it, is very complicated. Over shorter distances there are undulations in the sea floor which are visible due to the gravitational attraction of seamounts and underwater mountains.

If the oceans did not move the MSS and the geoid would have the same shape. However, winds, and the heating and cooling of the atmosphere, drive currents. These currents lead to differences between the MSS and the geoid of 1-2 m, even after averaging out time dependent motions such as tides. The differences in the MSS generated by the currents means that the Atlantic is 1m lower on the north side of the Gulf Stream than further south.

2. What is meant by "altitude above sea level" and how is the zero reference determined? Is it a constant with respect to space and time?

Heights above sea level, such as mountain peak heights, have traditionally been defined in terms of a measurement of 'mean sea level' at one or more locations. The value of mean sea level, once determined at the location, was then carried around the country by levelling, using methods similar to those used by surveyors in the road or construction industry.

For example, in the U.K. the height above sea level is defined 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. This definition replaced an earlier Ordnance Datum Liverpool based on sea level in that port in 1844. So, a height of a Scottish mountain means height above the sea level at Newlyn many years ago. Similarly, Normaal Amsterdam Peil in the Netherlands is approximate mean sea level at Amsterdam and represented by a marker in the city hall. French heights are relative to mean sea level at Marseille at a particular epoch.

ODN was carried around the country by levelling. One can think of the levelling results by imagining a network of thin, and in some places very deep, canals across the country. The water level at each location, which determines the 'zero' level at that point, is such that the water will not flow in any direction. In the case of the United Kingdom, ODN determines the 'zero' reference level for the network.

In many countries with two coastlines, such as the U.S. or India, there are often two or more datums because the distance from the sea to the mountains can be great and errors creep into the measurements. In addition, sea level along coasts can be different in different places due to dynamic ocean effects, i.e. mean sea level is not a 'level surface' (see FAQ #1).

3. Is sea level of the Atlantic and Pacific oceans the same? If so, why are locks required in Panama?

Sea level is about 20 cm higher on the Pacific side than the Atlantic due to the water being less dense on the Pacific side, on average, 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 is determined by geodetic levelling from one side to the other. This levelling follows a 'level' surface which will be parallel to the geoid (see FAQ #1). 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 did not contain 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 the Pacific to the Atlantic. An analogy, though imperfect because there are many other factors, is a comparison between Panama and the Drake Passage off the south tip of Chile, which has a west-east flow. (The flow in the Drake Passage is primarily wind-driven, 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. 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 two sides of Panama, so, if there was a sea level canal, there would be major tidal currents through it.

4. Why is the sea level higher off the coast of Bermuda than New York?

A strong northward current, the Gulf Stream, flows between Bermuda and New York. This current is deflected to the right by the Coriolis effect (a result of the rotation of the Earth), towards Bermuda. Thus, sea levels at Bermuda are typically 1 metre higher than at Charleston, South Carolina, with respect to the geoid, see FAQ #1.

5. Is there any evidence that global sea level has been rising in the 20th century and currently is rising?

Tide gauge records from around the world show that on average global sea level has been rising over the past few hundred years. The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (IPCC, 2007) concluded that global sea level had risen during the 20th century by approximately 1.7 ± 0.5 mm/yr, increasing to over 3.1 ± 0.7 mm/year in the 1990s. The 1993-2003 value was derived from satellite observations, but also has been confirmed by tide gauge measurements.

6. What are the causes of current global sea level rise?

The global sea level changes either due to ocean volume or ocean mass changes. Ocean volume change is associated with thermal expansion of the ocean; as ocean water warms up, it expands, increasing the volume of global ocean producing a sea level rise. Change in the mass of the ocean is mainly due to melting of glaciers and ice sheets with some contribution from water stored in continental reservoirs or groundwater extraction. It is debated, however, which of the two causes: expansion of ocean waters, or freshwater input from the continents, dominated the 20th century global sea level rise. Nevertheless, observations over the last couple of decades suggest increasing contributions from both thermal expansion of the oceans and the melting of glaciers and ice sheets.

7. What are the causes of local sea level change?

While global average sea level change gives a good measure of the total variation of heat and mass in the ocean, no location on earth will observe this average value. In some places, sea level is rising by more than the global average while in others it is falling. There are many causes to for these local differences in sea level. There are decadal changes in wind patterns, which drive currents. Heating and cooling of the oceans is uneven, which drives local difference in volume change. As ice sheets melt, the surface of the Earth deforms (see FAQ #11). Other very local effects such as earthquakes, ground water extraction, and subsidence of the land will cause an apparent change of sea level at that location. Further explanation can found on the geophysical signals page. All of these causes of sea level change must be considered when trying to understand an individual observation. To explore the local variations in sea level trends, as well as the how those trends change with the time period used to generate the estimate, please visit our trends page.

8. How much is global sea level projected to increase by 2100?

The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) suggested 18-59 cm sea level rise by 2100 with an additional 10-20 cm on the upper limit associated with rapid dynamical changes in ice flow of the ice sheets [Meehl et al., 2007]. This range reflects different emission scenarios as well as uncertainties in the climate models and is the subject of ongoing research and debate.

9. How do GPS (Global Positioning System) measurements help us to understand sea level change?

Tide gauges measure the sea level with respect to the nearby land. In some places, there can be highly-localised vertical motion of the land, such as displacement due to earthquakes or subsidence due to ground water pumping. When studying regional or global sea level trends, people often want to remove these localised effects. In addition, there can also be regionally-coherent signals in land motion, such as motion of the continental plates and displacement caused by the continued response of the Earth due to the collapse of the large ice sheets at the end of the last ice age. GPS measurements made near the tide gauge are used to remove this land contribution to the sea level observations.

Another reason for removing GPS-derived estimates of vertical land motion is to be able to compare the tide gauge observations with altimetry. Altimetry is different from tide gauges in that it measures the sea level in a globally-defined system, such as the measurements being referenced to the centre of mass of the Earth system. GPS results can be used to both remove the land motion from the tide gauge records and place the results into the same reference system as the altimetry results, typically the International Terrestrial Reference Frame (ITRF).

It should be noted, though, that there are many circumstance where one should not remove the land motion from the tide gauge records. For the inundation of a city, for example, the result is the same if the sea level rise is caused by either the sea surface going up or the land subsiding. Thus, the relevant sea level rate is derived from tide gauge records. In addition, the changes indicated by the tide gauge records do not depend upon a reference frame, and thus are not subject to possible systematic errors that might be present in the ITRF.

For more information see the GPS section under PSMSL's geophysical signals page or the geodesy section of PSMSL's reading lists.

10. How do tide gauges which measure the pressure of water (and hence the water height) take account of atmospheric pressure?

The pressure exerted on a gauge mounted in the water is the result of the mass of the column of water and air directly above it. In order to calculate the water height from the pressure due to the water, we must first remove the portion of the pressure that comes from the air column.

The atmospheric component is accounted for using a second, land-based pressure measurement nearby. The difference between the two sensors gives the pressure solely due to the water. This pressure is directly proportional to the water height, providing we assume the density of the water does not change. For more detailed information on tide gauge measurement techniques, see the IOC Manuals and Guides No. 14.

11. How much does sea level rise if all of the world's ice melts?

There are two different types of ice to consider: sea ice and ice grounded on land. First, if all the floating sea ice in the world melted, there would be nearly no change in sea level, because floating ice displaces its own weight of water. However, if land ice melts, the melt water will raise sea level. All the world's glaciers and small ice caps, outside of Greenland and Antarctica, contain approximately 0.4 m of sea level equivalent. The large Greenland and Antarctic ice sheets contain approximately 7 and 57 m respectively. Consequently, if all of the world’s ice melted in a very much warmer world, sea level would be approximately 65 m higher.

Note, though, that when land ice melts the resulting sea level rise will not be uniform. As the ice melts, the land beneath the ice sheet rebounds and the gravitation pull associated with the ice sheet decreases. These both contribute to a sea level fall near the ice sheets. Conversely, far from the ice sheet, the sea level rise would be greater than the global average. There are also dynamic oceanographic effects associated with the cold, fresh melt water entering the oceans. These dynamic sea level changes are more difficult to model, but are also important to understand future sea level change.

12. How much would rising sea levels depress the Earth's crust with the additional weight of water?

The answer to this problem varies from place to place around the world's coasts, depending upon geometry of the ocean near a particular point. The computation needed to answer this question involves calculating depressions of distributed loads of a certain size. For example, start by assuming a water disc with a height of one metre. As the radius of the disc changes, the vertical displacement in the centre changes:

  1 km radius    -0.3 mm 
 10 km radius    -2 mm
100 km radius   -11 mm
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 two metres in the southern North Sea depresses the crust by 20 to 30 mm in near coastal areas.

For more FAQs related to tides, see the Tides: questions and answers page of the National Tidal and Sea Level Facility, and the "Education" pages of the NOAA/NOS Centre for Operational Oceanographic Products and Services.