Chapter 11 of the 2001 IPPC report focussed on changes in global and regional sea level in the 20th and 21st century while giving a background to movements of sea level as a response to the waxing and waning of ice sheets during the Quaternary ice age and anticipating changes a thousand years into the future. At the Last Glacial Maximum, approximately 20,000 years ago, ice spread out extensively from the polar regions and covered much of the UK. As water was taken up by the growth of the ice sea level was ~120 m below present at the edge of the continental shelf. As the ice retreated rapidly between 15,000 and 6,000 years before present sea levels rose rapidly at a rate of approximately 10 mm per year based on geological evidence. Since 6,000 the rate of rise was slower at approximately 0.1- 0.2 mm per year over the last 3,000 years. Over these more recent millenia sea level has varied less than ±0.3- 0.5m around the present level.

Based on historical tidal measurements http://www.nbi.ac.uk/psmsl/ global average sea level rise has ranged between 1.0 and 2.0 m in the 20th century with the report concluding that there is no evidence for any significant acceleration in this period.

Rising sea levels are caused by thermal expansion of seawater as temperatures increase and increased inflow of freshwater from melting ice caps and glaciers.Melting of ice on the surface of sea water does not contribute as it was originally formed from seawater. If all glaciers, the Greenland Ice sheet, the West Antarctic Ice sheet or all ice sheets melted they would lead respectively to increased sea levels of ~0.5 m, ~6 m, ~8 m or >80 m. It is to be expected that the recent rapid rise in global sea temperatures will have led to an increase in sea level rise from thermal expansion and there is clear evidence of major retreat of glaciers

Glacial Retreat of Waxeggkees Glacier in upper Zillertal above Mayrhofen, Tirol, Austria
1912	2003

and West Antarctic Ice sheets. Evidence from Antarctica as a whole is still unclear, but the West Antarctic Ice sheet is particularly vulnerable to changing sea levels as it is grounded on rock below sea level. If it started to break up it could suddenly collapse leading to a rapid increase in sea level. The consensus view in the IPCC report was that this was unlikely to happen in the 21st century.

Recent research based on 177 tide gauge stations with a global coverage gives a mean sea level rise over the last 55 years of 1.7 ± 0.2 mm per year. Evidence based on satellite altimetry suggests that at least for coastal stations sea level rise over the decade 1993-2002 was significantly above the global average. If this is representative of global conditions, as some models suggest, sea level rise may be accelerating. Further new evidence shows that the rate of ice loss from glaciers and ice sheets has more than doubled since 1988 suggesting that the IPCC calculation of a rise of 0.11 to 0.77m in the 21st century is an underestimate. Calculations made on the new evidence indicate that glacier melt could increase sea levels by 2m this century. The IPCC estimated that other processes such as ocean warming would cause an additional 0.1 to 0.4m sea-level rise by the year 2100. Combined together the new estimate is a substantial increase that would have grave consequences for coastal regions and countries with low topography such as Bangladesh.