- For considerations of sea level change, in particular rise associated with possible global warming, see sea level rise.
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Mean sea level (MSL) is the average height of the sea, with reference to a suitable reference surface. Defining the reference level , however, involves complex measurement, and accurately determining MSL can prove difficult.
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.
Difficulties in Utilization
To extend this definition far from land 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 gravity 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) sea 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 Saros cycle on the tides. Mean sea level does not remain constant over the surface of the entire earth. Mean sea level at the Pacific end of the Panama Canal stands 20 cm higher than at the Atlantic end.
Despite the difficulties, aviators using instrument flight rules must have accurate and reliable measurements of their altitudes above (or below – see Schiphol Airport) mean sea level, and the altitude of the airports where they intend to land. That problem can compound when landing on an aircraft carrier in a gravitational anomaly. In aviation mean sea level is increasingly being defined with reference to an ellipsoid defined by the World Geodetic System. Compared to a geoid, an ellipsoid is simpler to model mathematically and therefore lends itself to use with the Global Positioning System.
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 changes in the amount of water in the oceans, usually due to climatic changes. 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 amount 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 Changes Through Geologic Time
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 Permo-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 noticable effects during human lifetimes.
During the most recent ice age (at its maximum about 18,000 years ago) the world's sea level was about 320 feet lower than today, due to the large amount of sea water that had evaporated and been deposited as snow and ice in northern hemisphere glaciers. The majority of the glaciers had melted by about 10,000 years ago, but minor glacial melting has continued (with occasional reversals) throughout recorded human history. A more detailed chart of sea level for the past 140,000 years can be seen by clicking here.
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 sea level to sedimentary deposits is called sequence stratigraphy.