sea level

sea level. Variations in sea level over short time scales are familiar effects of tides, winds and waves, but globally mean sea levels are affected by fluctuations in the amount of water in the oceans, the shape of the oceanic basins and the warmth of the interior of the oceans. Ocean basins are constantly, albeit slowly, changing shape as a result of tectonic movements. Over geological time scales sea levels have fluctuated by several hundred metres.

During the Miocene Era the Strait of Gibraltar opened and closed several times. Each time the strait closed, the water that evaporated from the Mediterranean basin fell as rain and filled up the rest of the oceans, raising sea level by about 70 metres (230 ft). Then when the strait reopened there was a rapid fall as sea water cascaded back into the Mediterranean. Climate change has also played a major role in sea level fluctuations. Following the end of the last ice age sea level has risen by nearly 100 metres (325 ft) as the ice sheets melted, and the interior of the ocean is still continuing to warm up; the thermal expansion of water is resulting in a small annual rise of one millimetre, a tiny but significant rise for those designing coastal protection and flood control for the next century.

Currently the polar ice caps contain enough water to raise global sea levels by 60 metres (195 ft). A modest amount of global warming that leads to some melting of polar ice will result in catastrophic rises in sea level. The precise amount of rise is difficult to estimate because as ice is removed from Antarctica and Greenland there will be local adjustments in the earth's crust. However, increased sea levels in the Bengal basin are threatening the Indian Sundarbans, the world's largest delta, and its mangrove forests. Levels have risen by an average of 3.14 centimetres (1.2 in.) annually since the 1980s, submerging four islands, leaving many families homeless, and threatening local fisheries and the food chain. If this continues, thousands of square kilometres of the Sundarbans will be submerged, destroying whole communities. In 2003 the government of the Pacific state of Tuvalu, where the sea level has risen nearly three times as fast as the world average, was considering the possibility of having to evacuate its 9,300 residents.

In the north-west of Britain, the land is still rising as a result of isostatic adjustments of the earth's crust after the last glaciation. In the south-east the land is sinking by more than a millimetre a year, adding to local sea-level rises. Hence how long will the Thames Barrier remain effective in protecting London from flooding? Globally, sea level is being observed by remote sensing.

Levels oscillate about the surface of equal gravity, the so-called geoid. Sea levels over deep trenches, where gravity is anomalously high, can be depressed by 40 metres (130 ft). Variations around the geoid result from the influences of currents and eddies as well as the underlying geological structure of the seabed. See also chart datum; geological oceanography.

M. V. Angel