IV. STATE OF THE ANTARCTIC ENVIRONMENT: SYNOPSIS OF SOME RECENT FINDINGS

59. The sections that follow highlight some recent findings for a selection of topics of current interest with regard to the state of the Antarctic environment. This selection is not intended to be comprehensive, but rather illustrative of prominent topics. These sections were reviewed by a range of specialists in the relevant fields.

A. Science and support activities

60. Scientific investigation is the predominant human activity in Antarctica. The number of science and support personnel working in Antarctica each season provides a crude estimate of the level of this activity. States Parties to the Antarctic Treaty report on the numbers involved in an "Annual exchange of information" required under Antarctic Treaty Consultative Meeting agreements. 19/

61. While these data provide a basic context for the level of scientific activities currently occurring in Antarctica, it is important to note that they generally do not allow a quantification of the total scientific presence in terms of person-days in the region.

62. Since regular activities began, the number of people participating in Antarctic scientific programmes grew steadily up to 1989/90 (figure II). 20/ Associated with an increased number of people participating in Antarctic activities was an increase in both the number of countries represented and the number of operating stations. The United States has the largest Antarctic science programme, comprising approximately 35 per cent of the science and support population in the 1994/95 season.

63. A preliminary assessment of the levels of science and support personnel since 1990 suggests that there has been a reduction in personnel by perhaps around one third. However, data compilation for this section was hampered by variable quality and timeliness in the reporting of activities and in the distribution of this information by States working in Antarctica. Further analysis, perhaps linked to investigation of national investment levels, would be required to draw conclusions from the apparent trend.


Figure II. Levels of science and support activity, 1941/42 to 1989/90

Source: Data from J. C. N. Beltramino, The Structure and Dynamics of Antarctic Population (New York, Vantage Press, 1993).

B. Antarctic tourism

64. Antarctica has been a tourist destination for the past 40 years, with more than 60,000 tourists estimated to have visited the area over that time. 21/ Antarctic commercial tourism has undergone a period of accelerated growth in the last decade, both in the number of passengers on ships and, more recently, in overflying aircraft (figure III).


Figure III. Levels of Antarctic tourism activity, 1980/81-1995/96

Sources:
(1) Adapted from D. J. Enzenbacher, "Tourists in Antarctica: numbers and trends", Polar Record, vol. 28, No. 164 (1993).

(2) United Kingdom of Great Britain and Northern Ireland, "Recent developments in Antarctic tourism", Information Paper 13, XIX ATCM, Seoul, 8-19 May 1995.

(3) International Association of Antarctic Tour Operators, "Preliminary overview of Antarctic tourism", Information Paper 96, XX ATCM, Utrecht, the Netherlands, 29 April-10 May 1996.

65. "Seaborne" tourists are those visiting Antarctica on commercial cruise vessels and the figures include yachts where data are known. "Air-landing" tourists are those journeying to and from Antarctica by air with a landing, while "Air-overflight" tourists are those who visit Antarctica as a specific destination by air but do not land. Overflights of Antarctica on commercial airline routes are not included in this review because this does not constitute travel to Antarctica as a specific tourist destination. The number of ships shown in figure III does not include yachts.

66. There is growing awareness of the importance of environmental issues arising from Antarctic tourism. 22/ However, research on the effects of Antarctic tourism is still in its infancy and there is still uncertainty over the level of impact arising from tourist visits to Antarctica. 23/ Antarctic Treaty recommendation XVIII-1 provides operational and environmental guidance to tour operators in Antarctica.

67. The International Association of Antarctica Tour Operators (IAATO) was founded in August 1991 and current membership comprises the majority of companies operating Antarctic tours. During the 1993/94 season, just over 83 per cent of cruise ship passengers travelled aboard ships operated by IAATO members. 24/

Recent developments and trends

68. The number of tourists visiting Antarctica in the 1995/96 season is the highest yet recorded, with increases recorded in both seaborne and air-overflight tourism (figure III). Air-landing tourism caters for a specific adventure-tourist market, and is currently very expensive: the numbers are low and appear static.

69. Antarctic tourist overflights became popular in the 1970s, but ceased after the crash of a DC-10 aircraft on Mt. Erebus (Ross Island) in 1979, killing all 257 passengers and crew. 25/ In the 1994/95 season, Antarctic overflights, departing from Melbourne, Australia, were reintroduced. These were continued in 1995/96 and are again planned for the 1996/97 season. 26/ These flights account for all air-overflight data in figure III. In terms of their potential environmental impact and management, overflights are very different from seaborne and air-landing tourism, owing to their relatively short duration and absence of landings.

70. In the 1995/96 season, a total of 10 tour operators are known to have conducted organized seaborne tours; this compares with 14 operators in 1994/95, the highest number recorded in a season to date. Associated with an increase in the number of tourists is an increase in the number of vessels (figure III), with at least 12 in use each season since 1991/92. 27/ The number of passengers per ship in the 1995/96 season ranged from 13 to 452, the average being 81.5. 28/

71. Most seaborne voyages are concentrated along the Antarctic peninsula during the four-month austral summer, with few landings made in the Weddell Sea and Ross Sea regions. 29/, 30/, 31/ The number of distinct sites at which tourists landed has expanded from 36 in 1989/90 to over 150 in the 1994/95 season, 32/ with probably a comparable number in 1995/96. 33/

72. The types of activities being undertaken have broadened from earlier times, with tourists now engaging in skiing, climbing, camping, sea kayaking, and other activities. Several ships now carry helicopters that can transport tourists to sites previously inaccessible, 34/ and the number of yachts appears to be increasing, with 17 venturing south of 60° S in 1991/92. 35/

73. It is at present difficult to collate complete, consistent statistics on Antarctic tourism. While historically most data have been reported to the United States, which has compiled these data in annual reports to the Antarctic Treaty Consultative Meeting, several operators are now working out of other countries and do not necessarily report to this one point. Argentina 36/ and Australia 37/ also gather and report tour data. Data on yachts are particularly difficult to locate and compile.

74. The Antarctic Treaty Consultative Meeting held at Utrecht, the Netherlands, in May 1996 agreed a prototype standard format for reporting Antarctic tour data, to be trialled in the 1996/97 season with a view to universal adoption at the 1997 Meeting. The format contains fields for data on the principal aspects of tourist activities (for example, operating companies, routes taken, site landings, activities undertaken, tour and landing durations, and impact observed), which, when agreed, should provide the basis for the development of a comprehensive, consistent and accessible international database on Antarctic tourism.

C. Fishing

75. Concern over an unregulated fishery for Antarctic krill, along with past exploitation of fur seals and whales, resulted in the adoption of the Convention on the Conservation of Antarctic Marine Living Resources. The Convention entered into force in 1982 and stringent conservation measures to halt the further decline of fish stocks were implemented by 1989. 38/ Prior to the coming into force of the Convention, many Antarctic fish stocks were overexploited. The conservation measures under the Convention currently include the setting of total allowable catches for targeted species, imposing limits on by-catch of non-target species, measures to prevent incidental mortality of seabirds, and requirements for having scientific observers on some harvesting vessels. 39/

(a) Fishery development

76. Current fishing is focused on Antarctic krill and finfish species. Harvesting of Antarctic finfish species began in 1969/70, with reported annual catches of the Notothenids (Notothenia rossii and N. squamifrons) and the icefish (Champsocephalus gunnari) often exceeding 100,000 tons and peaking at 400,000 tons in 1969/70. More than 3 million tons of finfish were reported to have been caught prior to 1995/96. 40/ However, finfish fisheries have been very low since 1992 (table 2).

77. Krill (targeted species Euphausia superba) harvesting began in 1972/73 and over 5 million tons have been taken to date. 41/ The current krill catch is around 90,000 tons a year which, although less than in the peak years of the early 1980s, is still the largest catch in Antarctic waters. An experimental fishery for stone crabs (Lithodidae) at South Georgia and Shag Rocks was undertaken by a United States fishing boat in July 1992 (table 2). 42/

Table 2. Total reported fish catches in the region covered by the Commission for the Conservation of Antarctic Marine Living Resources from 1991/92 to 1995/96

Total reported catch (tons)
Species1991/921992/931993/941994/95
Antarctic krill302 9618877683 962118 715
Patagonian toothfish12 4975 7885 6488 889
Mackerel icefish650283 974
Greyrockcod0000
Lanternfish51 91501140
Antarctic crabs029900
Source: CCAMLR, Statistical Bulletin 1996 (1986-1995) (Hobart, Australia, CCAMLR, 1996).

(b) Recent trends, threats and initiatives

78. Ecosystem monitoring. CCAMLR maintains an active Working Group on Ecosystem Monitoring and Management. The working group has constructed a framework which will allow information collected from established monitoring programmes to be integrated into management advice. 43/

79. Krill fishery. Krill (Euphausia superba) are the key food for most Antarctic marine birds and mammals, and krill research efforts are central to CCAMLR management. 44/ In recent years, there has been a decline in catches of Antarctic krill (table 2), primarily due to economic factors and driven by a reduction in the Russian Federation and Ukrainian fishing effort for this species. 45/ The current catch is less than 10 per cent of the total allowable catch, which itself is set at 10 per cent of the estimated krill biomass.

80. Seabird mortality. Incidental mortality of seabirds during longline fishing operations has been widely reported as being a significant problem. 46/, 47/, 48/ Mitigation measures adopted by CCAMLR include the setting of longlines only at night, using streamer lines to deter birds from baited hooks, and a prohibition on dumping trash during longline operations. CCAMLR 49/, 50/ has reported a reduction in the by-catch of albatross as a result. However, incidental mortality of white-chinned petrels occurs with night settings 51/ and it is still unknown to what extent albatross populations may be recovering.

81. Marine debris. Management measures have been implemented by CCAMLR to reduce incidental mortality and the impact of marine debris on biota. The use of plastic packaging bands to secure bait boxes on fishing vessels is prohibited and monitoring programmes have detected reductions in debris over the last year. 52/

82. Trawl impact. Most finfishing in the Southern Ocean has been conducted with bottom trawls. 53/ Bottom trawl gear is dragged along the seabed, re-suspending sediments and disturbing the benthos. 54/ Although precise effects on the rich benthic fauna of the Southern Ocean are unknown, it is speculated that trawling could have serious, long-lasting impact owing to the low resilience of the slow-growing communities to disturbance.

83. Illegal fishing. There have been reports of illegal fishing of D. eleginoides. The illegal take is believed to equal or exceed the total allowable catch set by CCAMLR, seriously threatening sustainable management of this fishery. 55/ It is not known what effect this level of exploitation is having on fish populations. CCAMLR has introduced a revised scheme of international inspection in an attempt to combat this problem.

84. Squid fishery. The ommastrephid squid (Martialia hyadesi) has been recognized by the United Kingdom as having potential for exploitation within the CCAMLR area. 56/ In 1989, a catch of approximately 8,000 tons was taken by the United Kingdom, but no further fishery has taken place.

D. Long-range pollutants

85. Antarctica is the least populated and industrialized continent, with human activities minimal and highly localized. Research has been conducted on the presence and transport of pollutants in Antarctic marine and terrestrial ecosystems. Antarctic case studies can be used to provide a baseline against which to assess both current and future levels of global pollution. 57/

(a) Origin and deposition of long-range pollutants

86. Long-range pollutants in Antarctica originate predominantly from the industrialized areas of the world. 58/ Many such pollutants are transported to Antarctica in the upper atmosphere as vapour, 59/ while others are transported to the Antarctic by ocean currents. Air reaching Antarctica from outside has to pass through the zone of cyclonic storms that surrounds the continent. This acts as a filter, removing some of the particles and reactive gases from the air and depositing them in the Southern Ocean.

87. The transport of atmospheric pollutants between continents is indicated by similarities in the contaminant patterns in Antarctica to those observed in the remainder of the southern hemisphere. 60/ Examples of such pollutants include chlorofluorocarbons (CFCs), responsible for Antarctic ozone depletion (see sect. E below), trace gases such as carbon dioxide and methane, radioactive debris from past atmospheric nuclear bomb tests and accidents, heavy metals and hydrocarbons. 61/, 62/, 63/ Low concentrations of these pollutants (in the ng (nanogram) kg-1 range) have sometimes hampered accurate analysis of contamination, and spatial and temporal variability has made it difficult to establish mean values or identify change. 64/

88. Once over Antarctica, pollutants may be deposited within snowflakes, or by direct deposition to the snow surface. The ice preserves a historical record of the atmosphere, with ice-core studies revealing global changes in trace gases, and some pollutants such as lead. Deposition processes are not well understood, and assumptions that the concentrations seen in snow can be related simply to the concentrations in the air mass are questionable. 65/ The ratio of atmospheric pollution reaching the Antarctic continent to that deposited in the Southern Ocean appears unknown.

89. A range of pollutants indicating intercontinental transport have been identified in Antarctica. Examples of the general trends observed are seen in heavy metals and hydrocarbons.

(b) Heavy metals

90. Heavy metals have received the most attention in studies of pollutants in polar snow and ice. 66/ Studies of heavy metals in Southern Ocean waters and biota are scarce, and suffer from high variability and analytical difficulties. 67/ Lead (Pb) is an example of a heavy metal widely dispersed in the Antarctic environment. It is distributed primarily as a result of its use as tetra-alkyl Pb as an additive to petroleum. Pre-industrial levels were typically 0.3-0.5 ng kg-1, derived from crustal dust and possible volcanic input. 68/ Between 1920 and 1950, lead concentrations varied around a mean of 2.5 ng kg-1, with a clear increase to 6 ng kg-1 between 1950 and 1980. 69/ This represents a twelvefold to twentyfold increase in concentration. Subsequent decreases can be linked to increasing usage of lead-reduced fuels. 70/ Isotopic ratios suggest an anthropogenic lead content in Antarctic sea water.

(c) Hydrocarbons

91. Levels of hydrocarbon pollution from anthropogenic activities are very low and localized in Antarctica compared with other areas of the world. Along with low levels of natural biogenic hydrocarbons, this makes the Antarctic an ideal stage on which to measure baselines and assess global hydrocarbon pollution. 71/, 72/ However, it is important to differentiate between global and local sources of pollution, with local outputs having the potential to jeopardize detection of global pollution. 73/, 74/ An example of local hydrocarbon pollution was the release of 600,000 litres of Diesel Fuel Arctic into Arthur Harbour, Antarctic Peninsula in January 1989, from the sinking of the Argentine ship Bahia Paraiso. 75/

92. Chlorinated hydrocarbons have been recorded in Antarctic biota, snow, ice and air. 76/, 77/, 78/ These substances have no known natural source. 79/ Chlorinated hydrocarbon residues are also reported in species of moss and lichen from varying locations. 80/

E. Ozone depletion

93. The discovery of substantial ozone depletion over Antarctica was unexpected and necessitated a major revision to theories of stratospheric chemistry. Although scientists had predicted the possibility of ozone depletion, 81/ the discovery of the ozone "hole" over Antarctica by Farman and others 82/ had not been anticipated. The processes leading to ozone depletion over the polar regions are now broadly understood. Chemical reactions on clouds in the stratosphere convert chlorine and bromine from inactive reservoir species into forms which catalytically destroy ozone in the presence of sunlight. 83/ Ozone depletion persists until warming of the polar stratosphere removes the stratospheric clouds and causes the breakdown of the polar vortex in early summer.

(a) Development

94. From 1978 to 1987, the ozone "hole" grew, both in depth (total ozone loss in a column) and in area (figure IV). This growth was not linear but seemed to fluctuate with a two-year period influenced by equatorial winds. 84/ Ozone depletion was significantly less in 1988 but in the period 1989-1991 was as large as in 1987. The Antarctic ozone "hole" continued to grow during the early 1990s, although the extremely large "holes" of 1992 and 1993 were due in part to sulfate aerosols from the Mt. Pinatubo eruption which increased the effectiveness of chlorine- and bromine-catalysed ozone destruction. 85/ A record low ozone recording (85 Dobson units) was measured in the spring of 1993. In 1995, the ozone decline started earlier than in any previous year, while the rate of decline was the most rapid on record. 86/


Figure IV. Antarctic spring ozone minima within the ozone "hole" from 1979 to 1994. The solid filled graph shows the average area of the ozone "hole" (ozone values lower than or equal to 220 Dobson units)

Source: Adapted from J. R. Herman and others, "Meteor-3/TOMS observations of the 1994 ozone hole", Geophysical Research Letters, vol. 22, No. 3 (1995).

Vertical soundings over the South Pole in September and October 1995 showed nearly complete destruction of ozone at altitudes between 15 and 20 km. Total ozone values over Antarctica in September and October 1995 were extremely low, with the minimum values only slightly higher than the record low values observed in 1993.

(b) Implications

95. Increased surface solar ultraviolet-B radiation (UV-B), attributable to the depletion of stratospheric ozone, poses a threat to Antarctic ecosystems. UV-B is detrimental not only for primary terrestrial colonizers such as cyanobacteria and algae, but also for lichens and mosses, higher plants and invertebrates. 87/ Terrestrial colonizers may have protective repair mechanisms, but the long-term effects are not well understood. 88/

96. Evidence has been found of direct effects from an increase in the UV-B radiation in Antarctic waters by comparing phytoplankton productivity both within and outside the "hole" area. One study indicated a 6-12 per cent reduction in phytoplankton productivity in the marginal ice zone. 89/ Karentz and others 90/ concluded that Antarctic marine phytoplankton might have protective repair mechanisms and produce pigment when required. In terms of ecological consequences, McMinn and others 91/ concluded that the displacement of UV-sensitive species by UV-tolerant ones might be more important than a decline in overall productivity. The long-term effects of increased UV on ecosystems are difficult to predict and very little is known at present. 92/

97. Recent scientific findings

  1. The atmospheric growth rates of several major ozone-depleting substances have slowed, demonstrating the expected impact of the Montreal Protocol (1987) and its Amendments and Adjustments.

  2. The conclusion that chlorine and bromine compounds, coupled with surface chemistry on naturally occurring particles in the polar stratosphere, are the cause of polar ozone depletion has been further strengthened. SCAR has noted that other compounds (such as methyl-bromide) in addition to CFCs are now known to destroy ozone.

  3. The 1993 record ozone "hole" was due primarily to a colder than normal stratosphere, a stable polar vortex and volcanic aerosols originating from Mt. Pinatubo.

  4. The maximum area covered by the ozone "hole" is nearing its limiting value set by the low temperature region within the polar vortex.

  5. Peak global ozone losses are expected to occur during the late 1990s.

  6. The link between a decrease in stratospheric ozone and the increase in surface UV radiation has been further strengthened.

  7. Stratospheric ozone losses cause a global mean negative radiative forcing.

F. Sea ice

98. The presence and variability of sea ice around Antarctica constitute one of the most salient characteristics in the southern hemisphere. The dramatic variation in sea-ice coverage, from 4 million km2 in late summer to almost 20 million km2 in late winter, more than doubles the effective ice-covered area of the Antarctic continent. 93/ This large seasonal fluctuation affects the exchange of energy, mass and momentum between the ocean and atmosphere and, together with the seasonal fluctuation of Arctic sea ice, plays an important role in global climate.

(a) Development of Antarctic sea ice

99. Sea ice forms when small randomly oriented ice crystals aggregate into a thin sheet, which then grows downward by the freezing of water onto its base. If there is significant wave action, the ice agglomerates into nearly circular discs or "pancakes" which adhere to each other and raft together forming a continuous consolidated cover up to tens of centimetres thick. 94/ Four factors are dominant in the development of sea ice: oceanic heat flux, or convection currents; atmospheric temperature; wave action; and oceanographic currents. 95/ Of particular importance in the development of Antarctic sea ice (as compared with Arctic sea ice) are the rapid expansion of the ice cover and the formation of "snow-ice" after the ice surface has been flooded by sea water. 96/

(b) Spatial distribution

100. In the austral summer, sea ice is confined primarily to the western Weddell Sea, the southern Bellingshausen and Amundsen Seas and the south-east Ross Sea,

with a narrow fringe of ice usually evident around much of the continent. 97/ The winter maximum ice edge extends furthest north in the eastern Weddell Sea, and furthest south in the western Bellingshausen Sea (figure V). Although there is consistency in the seasonal cycle, the magnitudes of the ice extents vary from year to year. Recent research suggests that these fluctuations may be related to the El Nino/Southern Oscillation (ENSO) climate periodicities. 98/

101. Open water occurs within the sea ice as leads and polynyas, where there is intense ocean-atmosphere interaction and significant ice growth and thickening. Leads occur throughout the sea ice and are a consequence of differential ice motion, driven primarily by winds. Polynyas are recurrent zones of open water or low ice concentration observed in the same areas within the sea ice. Some of the largest polynyas occur in the Weddell and Ross Seas, where they result from a combination of oceanographic and atmospheric forcing. Jacobs and Comiso 99/ found that the Ross Sea polynyas were influenced by both upwelling of relatively warm saline water along the continental slope and strong katabatic winds driving off the continent.


Figure V. Seasonal variability of Antarctic sea ice (shaded region) for the period 1973-1992: (a) mean minimum ice coverage, (b) mean maximum ice coverage

Source: Adapted from I. Simmonds and T. H. Jacka, "Relationships between the international variability of Antarctic sea ice and the Southern Oscillation", Journal of Climate, vol. 8 (1995).

(c) Thickness

102. The Antarctic sea-ice cover is considerably thinner than Arctic sea ice of a similar age. 100/, 101/, 102/, 103/ First-year ice of east Antarctica is typically 0.4-0.6 m thick, 104/ while multi-year ice in the western Weddell Sea - the largest area of perennial ice in Antarctica - is typically 1-3 m thick. 105/ The late winter-ice cover throughout the Pacific sector of the Southern Ocean appears to be thicker (mean 0.9 m) than that in much of the Weddell Sea and the east Antarctic sea ice. 106/

103. Recent scientific findings

  1. An analysis of global ice cover between 1978 and 1994 has detected no statistically significant change in Antarctic sea ice, 107/ although interannual variability is high and the time series of synoptic data is relatively short. In the same period, Arctic sea ice appears to have decreased by 5.5 per cent.

  2. Significant reductions in the summer sea-ice cover have been identified in the Bellingshausen/Amundsen Seas in the late 1980s/ early 1990s that are consistent with a warming climate west of the Antarctic peninsula. 108/

  3. Parkinson 109/ calculated a "sea-ice season" between 1979 and 1986 which showed increases in the length of the season in the Ross Sea but decreases in the Weddell and Bellingshausen seas.

  4. Antarctic sea-ice coverage shows periodicities that agree well with variations in the ENSO climate periodicities. 110/

G. Ice sheets

104. Over 87 per cent of the Earth's fresh water exists in a frozen state with more than 90 per cent of that ice situated on the Antarctic continent. 111/ The Antarctic ice sheet and floating ice-shelf extensions are a significant element in the global climate system, with high reflection of sunlight combined with its altitude being important influences. The relative size of the ice sheet has a direct effect on global sea level and it is estimated that between 62 and 70 m of sea level equivalent are locked up in this mass of ice. 112/

(a) Mass balance of the Antarctic ice sheet

105. The ice-sheet mass balance is the most relevant and least well-known parameter needed to model the effects of global warming on the Antarctic ice sheet. 113/ The total ice-sheet mass balance is the sum of the net mass balance over the upper surface of the ice sheet (that is, precipitation less melting, sublimation, evaporation and deflation), melt losses at the lower boundary and iceberg calving at the margin. Over the past several thousand years, melting and calving of ice beyond the grounding line of the ice sheet have maintained an approximate balance between accumulation and attrition. 114/ Estimates of the current mass balance are crude, with uncertainties of between 20 and 50 per cent in all budget terms. 115/ The most up-to-date evaluation 116/ suggests that the Antarctic ice sheet is losing mass to the oceans; however, further investigation, particularly into the attrition terms, is required before a reliable statement can be made.

106. The future mass balance of the Antarctic ice sheet has been addressed in several studies. 117/, 118/, 119/ Results suggest that under a warming climate, increased precipitation over the Antarctic ice sheet will lead to greater accumulation, but when atmospheric temperature rises above 5° C of the present day temperature, the Antarctic ice sheet would begin to decline.

(b) Iceberg-calving and ice-shelf melting

107. Calving of icebergs is the largest factor in the attrition of the Antarctic ice sheet. 120/ Recent estimates based on both ship and satellite data suggest iceberg-calving to be only slightly less than the total annual accumulation. 121/ Ice-shelf melting is the other principal element in the attrition of the ice sheet, with approximately 80 per cent of all ice-shelf melting occurring at the base of the circumpolar ice shelves more than 100 km from the ice front. 122/ Recent research has shown that steady ice-shelf retreat has been occurring on the Antarctic peninsula over the past 50 years. 123/, 124/, 125/, 126/ These ice shelves appear to be sensitive indicators of climate change.

(c) Ice-sheet stability

108. The East Antarctic Ice Sheet is a very old and stable feature being grounded primarily above sea level. The "marine-based" West Antarctic Ice Sheet, on the other hand, rests on a bed well below sea level and is believed by many glaciologists to be inherently unstable and prone to collapse. Ice streams - large river-like currents of ice - may be very important to the equilibrium of "marine" ice sheets. 127/ They have the potential to increase the speed of ice-sheet collapse by transporting ice from the interior to the ice-sheet margin at speeds one to two orders of magnitude faster than the general ice flow. Understanding the nature of ice-sheet - stream - shelf transition is crucial to resolving the question of stability of the Antarctic ice sheet.

(d) Effect on sea level

109. The sea level has risen an average of 6 mm year-1 since the last glacial maximum (18,000 years BP), 128/ while the best estimate of sea level rise, evaluated for IPCC, over the past 100 years is 1.5 mm yr-1. 129/ The most probable contributions to sea level rise, over the last 100 years, are thermal expansion (0.4 mm yr-1), melting of small ice caps and glaciers (0.4 mm yr-1) and losses from the Greenland ice sheet (0.25 mm yr-1). 130/ This leaves 0.45 mm yr-1 of the IPCC 1.5 mm yr-1 "best estimate" unaccounted for. If the Antarctic ice sheet is in fact losing mass to the oceans, then only a small fraction of this deficit would need to come from grounded ice to account for the unallocated 0.45 mm yr-1 portion of sea level rise.

110. Drewry and Morris, 131/ however, stress that the present contribution of the Antarctic ice sheet to sea level rise cannot be reliably estimated by treating the continent as a single unit. Accumulation and ablation rates vary depending on several factors, including the nature of the ice sheet, the surrounding topography and ice dynamics. Of particular importance is the topography beneath the ice sheet which may introduce thresholds of stability and instability causing a stepped response to climate change. 132/

111. Recent scientific findings

  1. Long-term warming trends on the Antarctic peninsula are so large that they appear to be statistically significant. 133/, 134/

  2. Five northerly ice shelves on the Antarctic peninsula have retreated dramatically over the past 50 years, perhaps in response to atmospheric warming. 135/

  3. The recent dramatic collapse of the Larsen Ice Shelf, Antarctic peninsula, implies that after an ice shelf retreats beyond a critical limit it may collapse rapidly. 136/

  4. Catastrophic collapse of the West Antarctic Ice Sheet is not supported by recent three-dimensional numerical models. 137/