The text below was scraped using OCR and then corrected. There may still be errors though – if you find any, let me know and I will fix them.
Thick ice sheets perched above sea level cover more than 10% of the earth’s area, which is about equal to the total area devoted to farming. The Antarctic and Greenland icecaps contain about 99% of this ice. According to George Denton, complete melting of the Greenland ice sheet would raise sea level 6.5 meters. the West Antarctic sheet 5.5 meters. and the East Antarctic Ice Sheet 54 meters, for a total of 66 meters (217 feet).
Vulnerability to destruction appears to be the reverse order of formation with the East Antarctic Ice Sheet having formed first and now extending from the Transantarctic Mountains across a low bedrock plain to the sea. The Greenland Ice Sheet apparently formed much later. and now covers a smooth low plain fringed by mountains, so that most of the ice flows to the ocean through about 20 large outlet glaciers. The ephemeral West Antarctic Ice Sheet formed last between the Transantarctic Mountains and the West Antarctic Archipelago so that its ice is partially grounded below sea level to depths as low as 2500 meters
Mercer has hypothesized that the West Antarctic ice may have been wholly destroyed in the last interglacial age, and Fairbridge reports that sea level was 6 meters higher than present at the peak of that warm period, approximately 125,000 years ago. The ice sheet which replaced it is believed to have been partly destroyed during the Altithermal Age, when sea level is reported to have been about 3 meters higher than present,
On a worldwide basis, a 3 meter rise would dislocate all existing port facilities, inundate low-lying coastal structures. cause the disappearance of most of the world’s beaches, and disrupt the habitat of fauna now existing in coastal wetlands. In the U.S. a 3 meter rise – if not blocked by dikes – would submerge the fertile delta farmlands in the Great Valley of California, and would submerge virtually the entire land surface at Galveston, Port Arthur New Orleans, Naples, the Florida Keys, Miami, Charleston, Norfolk, Portsmooth (VA), and Atlantic City. A 6 meter rise would submerge virtually all of Sacramento, Biloxi, Gulfport, and Mobile, and would inundate Washington DC from the Lincoln Memorial to the base of Capitol Hill. In other parts of the world, low-lying river deltas which are heavily farmed and densely populated are particularly vulnerable to a rise in sea level. These include the deltas of the Rhine, Nile, Ganges, Mekong, Yangtse, and Huang
A band of very strong westernly winds has prevented sea ice from building a protective fringe around most of the East Antarctic Ice Sheet, whereas in West Antarctica, the large Ross and Filchner-Ronne Ice Shelves have formed in relatively protected bays. Ice shelves there rise and fall with the tide to produce large ‘strand cracks’ where they adjoin grounded glaciers
The Ross and Filchner-Ronne Ice Shelves are thickest where they are fed by these glaciers, so that they presently act as buttresses to the large icestreams feeding into them. Weertmen has found that these large ice streams have a lubricating bottom-water layer, so that the ice streams are moving at a ‘small-scale surge’ velocity of about 0.6 kilometers per year. What has concerned Mercer and other knowledgeable glaciologists that any environmental change that diminishes or destroys the buttressing ice shelves would also diminish or destroy the ice grounded below sea level. As Hughes describes it, the West Antarctic marine ice sheet is inherently unstable and can be rapidly carved away by calving bays which migrate up surging icestreams. so that a relatively minor climatic fluctuation along the ice shelf calving barrier can unleash dynamic processes independent of climate that can cause calving bays to remorselessly carve out the living heart of a marine ice sheet.” Thus while very large continental ice sheets such as the former Laurentide Ice Sheet. may waste away slowly over millenia by in situ melting and peripheral retreat, the West Antarctic Ice Sheet may be destroyed catastrophically.
Weertman has suggested that large scale surge may cause the West Antarctic Ice Sheet to discharge one third to one half its volume into the oceans in possibly 100 years. CO2-induced warming might therefore result in sea level rising as fast as 0.2 to 0.3 meter per decade (8 to 12 inches) if catastrophic destruction results. For comparison, Fairbridge’s data show rises of 0.2 meter per decade to have been common in the past and 0.3 meter per decade to be the most rapid calculated.
How much CO2 buildup can the earth tolerate before the iceshelf buttresses are destroyed? Glaciologists have ventured some educated guesses which generally fall within the next 50 years if the CO2 buildup continues at present.
Robin has noted that ice shelves are absent from coasts of the Antarctic Peninsula where sea temperature rises above 1.5 degrees C during the warmest month. Mercer notes that the average midsummer air temperatures at the fronts of the Ross and Filchner.Ronne Ice Shelves are now about -4 degrees to -5 degrees C, so that with the anticipated amplification of a CO2,-induced warming at polar latitudes, the threshold of ice shelf destruction may have been surpassed substantially before atmospheric CO2, has doubled. This conclusion is consistent with that part of the paleotemperature record at Camp Century which corresponds to the Altithermal high stand. While the peak warmth noted at Camp Century a half century ago exceeded the Altithermal temperatures shown by ice core, we note that the Southern Hemisphere generally experienced less warming a half century ago, possibly due to the flywheel effect of the greater amount of ocean waters in that hemisphere.
A more conservative school of thought concerning the possibility of a CO2- induced sea level rise is represented in the National Research Council‘s Energy and Climate (1977), prepared by a panel chaired by Roger Revelle. This report suggests that high latitude warming might well bring increased snowfall to Antarctica and perhaps to parts of Greenland also. A similar hypothesis was advanced by G.C. Simpson in 1929. except that Simpson
theorized that if temperatures continued to rise, summer melting would increase and a greater proportion of precipitation would be in the form of rain The NRC panel thought it likely that the temperature in the Antarctic would still remain below the freezing point, so that “melting at or near the surface of the ice probably would not occur.”
The geological record seems to indicate that the effects of global warming are far more likely to cause ice sheet shrinkage than ice sheet growth. The major retreats and advances of the former Northern European Ice Sheet between 10.000 and 14,000 years ago appear to correlate directly with temperature fluctuations at Camp Century, although the accuracy of darings may not be great enough to demonstrate this conclusively.
Any added ice buildup due to increased precipitation will cause an ice sheet to establish a new profile of equilibrium due to the plastic flow of ice. The rate is highly dependent on temperature. Figures from Paterson‘s The Physics of Glaciers (1969) show the response time of glacial ice at -50 degrees C to be about 2500 years, at -27 degrees C about 250 years, and at- 6 degrees C about 25 years. The slowest response time (2500 years) would
correspond to ice at the surface of the high East Antarctic Plateau, but owing to the constant influx of geothermal heat at the base, the characteristic response time for the ice sheet as a whole is not known. The response time of the bulk of the Greenland ice is generally between 25 and 250 years. For short response times, adding greater and greater amounts of snow on top is like piling wet sand on wet sand rather than getting higher, it spreads out at the base.
A unique feature of a very large ice sheet is that it is generally immune from traveling barometric depressions which bring precipitation as snow. Because of the dryness of air over very cold ice and reflection of most of the solar radiation, barometric pressure tends to remain high to create a nearly permanent anticyclone. Air descends in the central portion and flows outward in every direction. Thus the geographic center of Antarctica receives only about 3 centimeters of precipitation annually in the form of hoar frost. Winds drift the frost crystals to give a deceptive appearance of snow piling up on man’s installations.
Because the Greenland Ice Sheet is relatively small, it may diminish rapidly in response to a CO2-induced warming similarly to the former Northern European Ice Sheet. Thus it may not be far behind the West Antarctic Ice Sheet in terms of vulnerability, especially in view of the fact that it is situated at relatively low latitudes with a branch of the Gulf current flowing along the west coast. The southern and western flanks appear to be particularly vulnerable to warming. On the central plateau at elevations of 2000 to 3000 meters, the mean maximum temperatures in June presently reach 4 degrees to -8.5 degrees C. Radiation, which is the most important source of heat during the ablation season, increases greatly from the sky and clouds as the atmospheric temperature increases above 0 degrees C.
Whereas half of the ablation in Greenland occurs as meltwater runoff and half as iceberg calving, most of the ablation in the East Antarctic occurs by iceberg calving. It has been surmised that an increase in air temperature there of 15 degrees C may be required to create a zone of ablation wide enough to diminish the area of that vast ice sheet.
A meeting of knowledgeable glaciologists will soon be held under the auspices of AAAS and DoE at the University of Maine at Orono to analyze further the vulnerability of the West Antarctic Ice Sheet to CO2-induced warming, and to recommend further research. Heading the meeting will be Charles R. Bentley of the Geophysical and Polar Research Center, University of Wisconsin.
Annotation – see this by Hughes, 1981
“Possible collapse of the West Antarctic ice sheet by surges of Thwaites and Pine Island Glaciers into the Pine Island Bay polynya of the Amundsen Sea was a subject addressed in papers by Lingle and Clark (1979) and Thomas ([979), and in abstracts by Denton and others ( 1979) and Hughes ( 1979), that were published in Vol. 24, No. 90 of the Journal of Glaciology. This concept was first developed in 1975 by George H. Denton and me as part of our CLIMAP responsibilities to reconstruct the maximum Antarctic ice sheet and then to disintegrate the marine West Antarctic portion. Lingle and Clark (1979) have acknowledged us and CLIMAP in this regard, and we are grateful to them…”