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Controversy concerning the significance of the present rate of global sea level rise, and how it may relate to the greenhouse effect and anthropogenic global warming hypotheses (AGW collectively) is very much in the public sphere due to it having "gone politicized". As the news media often does, highly complicated matters are over-simplified to give viewers and readers easy-to-digest meals that do not require much thinking—this is where the problem inlies in understanding of sea level.
Mean sea level (MSL) is the average (mean) height of the sea, with reference to a suitable reference surface. Defining the reference level (Permanent Service for Mean Sea Level), however, involves complex measurement, and accurately determining MSL can prove difficult. Finding the MSL change involves comparing the local height of the mean sea surface with a "level" reference surface or datum, called the geoid. In a state of rest with absence of external forces (totally stagnant water), the mean sea level would be the same at every point on the Earth. The geoid would only deviate from the perfect sphere in this theoretical model with local differences in MSL from local deviations in the Earth's gravitational field. In reality, due to currents, air pressure variations, temperature variations, salinity variations, etc., this does not occur, and prevents certain verifiable long term averages from being calculated. The location-dependent, but persistent in time, separation between mean sea level and the geoid is referred to as "stationary sea surface topography," which varies globally by ±2 meters, further offsetting the MSL.
Traditionally, one must have 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 (both having to do with the moon's effect on sea level). MSL never remains constant over the surface of the entire earth. For instance, MSL at the Pacific end of the Panama Canal stands approximately 20 centimeters (0.6 ft) higher than at the Atlantic end.
Despite the difficulties, aviators flying under instrument flight rules (IFR) must have accurate and reliable measurements of their altitudes above (or below, for airports such as in the Netherlands) local MSL, and the altitude of the airports where they intend to land.
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 melting of glaciers at the end of ice ages is an example of eustatic sea level rise. The term "isostatic" refers to changes in the land level, of land masses due to thermal buoyancy or tectonic effects and implies no real change in the amount of water in the oceans, although isostatic changes change the MSL because it is relative to the land. 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 often experience relative sea level rise as a result of isostatic cooling of the rock which causes the land to sink.
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