The global sea level has already risen by around between 10 to 25 centimetres during the last 100 years, at the rate of 1 to 2 millimetres per year. Measuring past and current changes in sea level, however, is extremely difficult. There are many potential sources of error and systematic bias, such as the uneven geographical distribution of measuring sites and the effect of the land itself as it rises and subsides.
It is likely that most of this rise in sea level has been due to the increase in global temperature over the last 100 years. Global warming should, on average, cause the oceans to warm and expand thus increasing sea level. Climate models indicate that about 25% of the rise in sea level this century has been due to the thermal expansion of seawater. A second major cause of rising sea level is the melting of land-based ice caps. Presently, it is uncertain to what extent the melting of the Greenland and Antarctic ice caps has contributed to global sea level rise during the 20th century.
Forecasts of a rising sea level are based on climate model results, which indicate that the Earth's average surface temperature may increase by between 1.4 and 5.8oC during the 21st century. Global warming is expected to cause a further rise of between 9 and 88 centimetres by the year 2100, with a best estimate of 50 centimetres, if emissions of greenhouse gases remain uncontrolled. This expected rate of change (an average of 5 cm per decade) is significantly faster than that experienced over the last 100 years.
Forecasting sea level rise, however, involves many uncertainties. While most scientists believe that man-made greenhouse gas emissions are changing the climate, they are less sure about the details, and particularly the speed, of this change. Global warming is the main potential impact of greenhouse gas emissions, but other aspects of the climate besides temperature may also change. For example, some studies suggest that changes in precipitation will increase snow accumulation in Antarctica, which may help to moderate the net sea level rise. Another complication is that the sea level would not rise by the same amount all over the globe due to the effects of the Earth’s rotation, local coastline variations, changes in major ocean currents, regional land subsidence and emergence, and differences in tidal patterns and sea water density.
Nevertheless, some areas of Antarctica have warmed by 2.5oC during the past 50 years, a rate of warming 5 times faster than for the Earth as a whole. Whilst scientists believe this to reflect mostly regional changes in climate, the recent summertime disintegration of the Larsen Ice Shelf has renewed speculation that climatic changes in the polar regions have the potential to cause severe impacts via a rise in global sea level over the next 100 to 200 years.