 #2 (permalink) Mon May 22, 2006 19:15 pm Meaning of "sea level" |
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Hi,
It may help.
Sea Level : the average level of the sea's surface in relation to the land.
Mean Sea Level (MSL) is the average (mean) height of the sea, with reference to a suitable reference surface.
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.
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. Mean sea level at the Pacific end of the Panama Canal stands 20 cm higher than at the Atlantic end.
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.
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 in northern hemisphere glaciers. T
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.
By courtesy of Wikipedia and Collins English Dictionary.
regards
Jan |
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