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Sea levels - about as level as the proverbial playing field


 
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Professor Greg Skilbeck plumbs the depths of sea level with some surprising conclusions.     
Sea level is a construct that most of us think is relatively simple. Fill a bath with water and anyone can see the water level as horizontal - something that will rise (or fall) evenly with more (or less) water added. Oceanography is more complicated, with several factors contributing to sea level, not least being the stability of the ocean floor and the fluidity of the upper solid Earth. Absolute sea level is particularly difficult because, apart from the centre of the earth, there are no completely stationery reference points for ocean measurements of sea level.

Sea level points are dependent on where they are situated and given that the upper 300 kilometres of the solid part of the earth is slowly moving (measured over tens of thousands of years), sea level is variable.


The ice on the poles during an Ice Age pushes down onto this upper 300 kilometres, forcing this material to flow towards the equator and, by the effect of gravity, making sea levels there rise. Conversely, when the ice melts, that driving force breaks down, causing sea levels at the equator to fall.

The extremes of sea levels both occur in regions not far from the equator (and also from each other). Using agreed fixed points, scientists have calculated a huge 180 metres difference in sea levels, with the lowest being a point just south of India and the highest being a point near the edge of New Guinea.

In addition, many oceanic islands and atolls are also subject to another factor in relative sea level – subsidence. They are in fact perched atop oceanic volcanoes which are weighty masses that locally press down on the upper 300 km of the earth, resulting in a very slow, natural subsidence of the islands. In these cases, tropical islands rely on the growth of the coral to match the natural sinking, thereby keeping an atoll’s head above water. Islands composed of sandy, unconsolidated sediments can be built or consumed by natural tidal currents, meaning some will cease to exist and new ones can appear.

Global warming brings a variety of additional factors that can contribute to changes in the rate of rise in sea level, including the expansion of the oceans with increase in their temperature as well as the increased volume of water from the melting of the polar ice. Very recently, new research has shown that contrary to previous assumptions, both the eastern and western Antarctic ice are melting in line with melting measured elsewhere, and the predicted 50 cm rise in sea levels this century have now been revised to double this level.


 

 
Above: Antarctic iceberg graveyard. Image by Katherina Petrou.

Text: V.B. February 2009

    

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