There is plenty of evidence for large changes in sea level during the Earth's history. For instance, during the warm period before the onset of the last ice age, about 120 000 years ago, the global average temperature was a little warmer than today (Figure 4.6). Average sea level then was about 5 or 6 m higher than it is today. When ice cover was at its maximum towards the end of the ice age, some 18 000 years ago, sea level was over 100 m lower than today, sufficient, for instance, for Britain to be joined to the continent of Europe.
The main cause of the large sea level changes was the melting or growth of the large ice-sheets that cover the polar regions. The low sea level before 18 000 years ago was due to the amount of water locked up in the large extension of the polar ice-sheets. In the northern hemisphere these extended in Europe as far south as southern England and in North America to south of the Great Lakes. Also the 5 or 6 m higher sea level during the last warm interglacial period resulted from a reduction in the Antarctic and Greenland ice-sheets. But changes over shorter periods are largely governed by other factors that combine to produce a significant effect on the average sea level.
During the twentieth century observations show that the average sea level rose by about 20 cm.4 The largest contribution to this rise is from thermal expansion of ocean water; as the oceans warm the water expands and the sea level rises (see box below). Other significant contributions come from the melting of glaciers and as a result of long-term adjustments that are still occurring because of the removal of the major ice-sheets after the end of the last ice age. Contributions from the ice caps of Greenland and Antarctica are relatively small. A further small contribution to sea level change arises from changes in terrestrial storage of water, for instance from the growth of reservoirs or irrigation.
Since around 1990 much improved observations of changes of sea level with global coverage have become possible through satellite-borne altimeters that can measure with great accuracy the height of the sea surface at any location. In Figure 7.1 are shown the largest contributions to sea level rise for the periods 1961-2003 and 1993-2003 as estimated from climate models and indicates that these contributions when summed show good agreement with observations, providing some confidence in the modelling methods. The figure also illustrates the substantial increase in the rate of sea level rise, especially due to thermal expansion, experienced during the most recent decade, 1993-2003.
The same methods and models used to estimate twentieth-century sea level trends have been applied to provide estimates of the sea level rise during the twenty-first century. An example for SRES scenario A1B is shown in Figure 7.2 with an uncertainty range (5% to 95% probability) for the decade 2090-99
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