metres above sea level near the centre of East Antarctica, and 2,538 m below sea level in West Antarctica. Maximum measured thickness is up to 4,776 m (Drewry, 1983).
The Antarctic Ice Sheet is by far the largest body of ice on earth. It contains 29 million km3 of ice which is about 90% of the world's supply of ice and about 70%of all fresh water. Wind blown and fallen snow accumulates on the surface at a mean rate of about 150 kg m2 yr1 and is compacted with increasing depth of burial. True glacial ice is formed at depths between 40 m near the coast and 160 m inland. This ice moves under gravity generally downwards and outwards from the interior of Antarctica towards the coast, increasing from close to zero as ice divides to about 4 km yr1 in the fastest outlet glaciers. Movement is concentrated in ice streams (Bentley, 1987) which flow at speeds typically about 1,000 m yr1 into valley or outlet glaciers onshore, or ice shelves or tongues of glacial ice protruding offshore. Further details of the ice sheet are given in many publications including Gow (1965), Drewry (1983,1986), Robin and Swithinbank (1987) and Robin (1988).
Because of its size, composition and geographic position, the ice sheet is an important feature of the earth. It affects global climate and sea level, as well as patterns of weather and ocean currents in the Southern Hemisphere. It is believed to respond slowly to change and may be a stabilizing component of the dynamic atmosphere-ocean-cryosphere system which dominates the global environment. At depth, it contains a detailed and probably unparallelled record of past climate, snowfall, air temperature, atmospheric gases and solids such as dust and volcanic products deposited over the last 200,000 years or so. It has been shown, for example, that present concentrations of C02 in the atmosphere are greater than they have been over the last 160,000 years, including the last interglacial period (Barnola et al., 1987). Measurement of impurities in the snow and ice including methane, lead and radioactive elements provides a guide and baseline to global pollution. Ice plays and has played an important role on our planet and Antarctica contains the only marine and terrestrial ice sheets of continental size enabling the study, dating, modelling and even prediction of a wide range of processes that occurred before, during and after world glaciations. Antarctica is also the largest collector and preserver of extraterrestrial matter (e.g., meteorites and dust) on earth. A proposal to bury high-level radioactive waste in the ice sheet (Zeller et al., 1974) was dropped when it was discovered that the base of the ice sheet is melting in places (Robin and Swithinbank, 1987).
The mechanisms by which the Antarctic ice sheet interacts with the rest of the globe, global processes and the biosphere are known in a general way but not always in detail (e.g., Deacon, 1984; Oerlemans and van der Veen, 1984). Ice sheets exist as a consequence of climate, geographic position and topography. Antarctica itself strongly influences climate through powerful feedback processes. The growth and decay of Antarctic ice perturb and are perturbed by the global energy and water budgets. The high albedo (a ratio of reflected and incident radiation) of snow and ice interact with various climate-determining factors to influence the response of the climate system. The accumulation of ice has completely changed the colour and relief of Antarctica burying whole mountain ranges. Being white and covered with snow, the ice sheet reflects back most incoming solar radiation and radiates long-wave radiation as well. A vast, powerful heat sink has therefore been created which helps drive both the poleward transport of energy and mass from the solar-heated equatorial regions and the circulation of atmosphere and oceans. Cyclonic storms and frontal waves are continually being formed in the circumpolar interface between cold Antarctic and warm, moist subtropical air masses. These storms orbit Antarctica from west to east and with the predominantly westerly wind they create north of 60°S, help drive the Antarctic Circumpolar Current which flows eastward around the continent (Deacon, 1984). Weather patterns and ocean currents far to the north are also affected. Seasonal changes driven by earth-sun relationships impact on these processes and directly influence biota, for example through animal and bird migrations.
Global sea levels are possibly the most spectacular linkage between Antarctica, the atmosphere and the oceans. The Antarctic Ice Sheet contains sufficient ice that, if melted, would raise sea level by about 73 m (Robin, 1986). A rapid or imminent rise of this order is most unlikely because of the slow response time of most of the ice sheet to atmospheric or climatic changes and the large amount of warming (up to 15°C; Robin, 1986) required. However, recent evidence suggests that growth and collapse of substantial parts of the ice sheet occur on time scales of a million years rather than ten million years as previously thought (Webb et al., 1984). On the other hand, the West Antarctic Ice Sheet is in close contact with the ocean via large ice shelves and could be susceptible to changes in it as well as climatic changes. The ice sheet has fluctuated in thickness and diameter on time scales of thousands to tens of thousands of years, especially between glacial and interglacial periods, but there is disagreement about the precise nature and scale of changes to it (Stuiver et al., 1981; Robin, 1986). Possible fluctuations over time periods of hundreds of years could produce catastrophic change in sea level of up to 6 m. This ice sheet is situated mainly in the Pacific Sector of the Antarctic region.
Such a catastrophic change in sea level could come about due to future climatic warming, particularly oceanic warming, induced by an increased concentration of gases in the atmosphere due to the 'greenhouse effect' (Anonymous, 1985a). Climatic warming is likely to initiate a small rise in sea level due to thermal expansion of the oceans and increased melting of glaciers outside the Antarctic (Robin, 1986). This could be of the order of 20-140 cm (Bolin et al., 1986). In theory, the higher sea level would tend to cause Antarctic glacial ice to be afloat slightly further south than it is now. In addition and possibly more important, higher sea temperatures may accelerate bottom melting and frontal decay of ice shelves, reducing the size of the shelves (Mercer, 1978; Anonymous, 1985a). While decay of floating ice does not alter sea level, it would reduce the friction between ice shelf and coast, lessening the back pressure which ice shelves exert "upstream" on the inland ice and increasing the flow of ice off the land (Thomas, 1979a). The ice sheet which most scientists believe at present is either in mass balance (that is, total accumulation equals total wastage) or slowly thickening may therefore begin to discharge faster and retreat in areas where it is presently resting on land far below sea level (Anonymous, 1985a). In theory, such a retreat could also start naturally due to a natural increase or decrease of continental ice or other glacial instabilities. Wilson (1964), Hughes (1973), Weertman (1974) and others have suggested or modelled ways in which the Antarctic ice sheet, particularly the West Antarctic Ice Sheet, may become unstable naturally. Such a retreat would represent a negative mass balance of (or a loss of mass from) the ice sheet.
A negative mass balance or retreat of a grounded ice sheet adds mass (water) to the ocean and atmosphere, causing sea level to rise. The ice in West Antarctica is grounded 500-1,200 m below sea level and the land beneath slopes down towards the ice sheet interior in many places (Fig. 4.1). These key factors may mean that a retreat of this nature once started may become irreversible leading to a disintegration of the West Antarctic Ice Sheet and a rise in sea level of a few metres.
There is no conclusive evidence that such a disintegration has ever occurred before (Robin, 1986; Robin and Swithinbank, 1987). Moreover, the likelihood, sequence or time scale of such events or counteracting processes (e.g., increased snowfall) are not certain (Robin, 1979; Anonymous, 1985a). Differences in snow accumulation rates over the Antarctic ice sheet during glacial and interglacial times may be more important to sea level because of the sheer size of the ice sheet and the amount of ice it contains. Corresponding differences in the elevation of the ice sheet surface may, therefore, account for much of the 7 m rise in sea level that is believed to have occurred during the last interglacial (Robin, 1986). In addition, it is believed that greenhouse-induced changes of melting and accumulation in the ice sheets of Greenland and Antarctica will tend to counterbalance each other. The topography and dynamics of ice sheet outlets may also stabilize discharge of the ice sheet (Mclntyre, 1985). Further study is needed of the processes of ice dynamics involved in the Pacific Sector of the Antarctic. Ice streams draining the West Antarctic Ice Sheet and flowing into the Ross Ice Shelf are being studied for the part they may play in these processes (Anonymous, 1985a; Bentley et al., 1987).
The ice sheet-atmosphere-ocean interaction has other significant aspects. It drives the annual cycle of Antarctic sea ice which is important biologically, logistically and climatically. Formation and decay of this sea ice, melting at the base of ice shelves and iceberg decay affect salinity and hence the density and vertical circulation of Antarctic sea water, and especially the production of Antarctic Bottom Water. Higher Antarctic temperatures, melting and increased production of icebergs would tend to lessen surface seawater density and therefore alter the density structure of the world's oceans and deep circulation. A decrease in sea ice extent and increases in snowfall and the albedo of sea ice, all possible consequences of higher temperatures, would also alter the energy and moisture exchange between the ocean and atmosphere as well as between the subtropical and Antarctic regions.
It is still not clear whether the Antarctic ice sheet is growing or shrinking. The vast size of Antarctica plus a scarcity of data on snow accumulation, glacier flow and ice-shelf wastage have made it difficult to make an accurate assessment of the mass balance of the ice sheet. Climate change is underscoring the need to make a reliable assessment as a 1% change in the volume of the ice sheet is equivalent to a 70 cm rise or fall in sea level. Comparison of ice dynamic model predictions with layering observed in the West Antarctic Ice Sheet suggests that a large part of it has been stable with no signficant changes in mass balance, ice velocity or shape during the past 20,000-40,000 years (Whillans, 1976). Recent calculations for the whole Antarctic ice sheet suggest that total accumulation of snow and ice of about 2 x 103 km3 yr1 (2 x 101S kg yr1) (Giovinetto and Bentley, 1985) is probably nearly balanced by the outflow and wastage. A discrepancy of between zero and plus 20% possibly implies a growing ice sheet (Budd and Smith, 1985). Proxy information from deep bore-holes in the ice sheet leads to similar conclusions (Robin and Swithinbank, 1987). However, the rate of iceberg formation may have been grossly underestimated and, if so, Antarctica may be losing, not gaining, mass (Orheim, 1985).
Research including the development of various computer models on Antarctic ice mass balance, dynamic behaviour, and decay as well as past climate and physical conditions in surrounding seas are continuing. By the year 2100, a global warming may have produced a sea level rise of an appreciable fraction of a metre (Anonymous, 1985a; Bolin et al., 1986) and probably a significant increase in Antarctic snowfall. Increased discharge of glacial ice from the West Antarctic Ice Sheet into the sea is therefore a possibility. Some workers believe that this may have already started on the coast of the Amundsen Sea (Stuiver et al., 1981; Hughes, 1983), although others indicate that nothing unusual is happening there (Crabtree and Doake, 1982; Robin, 1986).
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