The International Panel on Climate Change (IPCC) expert group has reviewed estimates of future sea-level change, including the most recent studies. There are problems in making a comparison because these studies are based on different time periods and on different assumptions concerning greenhouse gas emissions and other factors. Generally, however, sea-level rise is projected to be in the range 10-30 cm by about 2030.

A major uncertainty in the estimation of future sea-level change concerns the ratio of the contributions of thermal expansion and glacier melt. Of the earlier studies by Polar Research Board, 1985, some de-emphasized the role of thermal expansion. Gornitz et al. (1982) assumed a future thermal expansion to alpine glacier melt ratio of 1:1, although their study ascribes the major part of sea-level rise observed over the last century to thermal expansion. Revelle (1983) estimates that thermal expansion would contribute about 30 cm of the 72 cm rise associated with a global warming of 3-4o C by 2085. Only 20 cm of the total rise is attributed to melting of the Greenland ice sheet and mountain glaciers. An additional rise of 17 cm, based on trend extrapolation, was included. Revelle assigned an uncertainty of + 25% to the total rise of 72 cm. Both Gornitz et al. and Revelle studies assumed that the contribution from Antarctica was zero. The Hoffman et al. (1983) estimates were derived from model studies which project both global warming and thermal expansion rates. It was assumed that the glacial contribution would be between one and two times the thermal expansion contribution.

The first study to consider in detail the potential contribution of glaciers to sea-level rise was the work of the Polar Research Board (1985). The estimated contribution of alpine glacier and Greenland ice-sheet melt was put at 10-30 cm, with a global warming of 1.5-4.5 oC. Greater uncertainty surrounded the Antarctic contributions which was eventually estimated at between -10 cm and +100 cm. Sea-level fall was considered a possibility since more moisture might be carried polewards in a warmer world to accumulate as snow and ice at high latitudes. The most likely contribution of Antarctic melt was estimated at a few decimeters and a total sea-level rise of 10-160 cm was preferred.

The low, best, and high estimates are obtained using a climate sensitivity to a doubling of CO2 of 1.5oC, 2.5oC, and 4.5oC, respectively. The estimated contributions of mountain glaciers came from a simple global glacier melt model. This model required the specification of three global parameters: initial ice volume in 1861 (when the world’s glaciers were last estimated to have been in equilibrium), glacier response time, and glacier sensitivity to temperature. The estimates for the Greenland and Antarctic ice sheets are based only on observations of the changes in mass balance over the last 100 years and do not account for possible changes in dynamic behaviour such as calving and surging. It is considered that these latter effects operate over time scales greater than 100 years. It is assumed that the Greenland ice sheet is reducing in size and thus contributes positively to sea-level rise. Melting is, however, partially offset by increased snowfall over the higher parts of the ice sheet due to the enhanced hydrological cycle. This latter effect is considered to dominate in the case of the Antarctic ice sheet, hence its contribution to sea-level rise is negative.

Beyond 2100, the greatest uncertainty in estimates of sea-level rise relates to the possible rapid disintegration of the West Antarctic ice sheet if warming continues. In the longer-term future, sea level can be expected to fall in the onset of the next glaciation. In present-day coastal areas, falling sea level is initially likely to be accompanied by a reduction in the frequency and strength of extreme events such as storm surges. The major climatic effects of the very large falls in sea level during glacial episodes related to the associated increase in continentality at specific location.