Greenpeace Antarctica Tour 1997 |
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Greenpeace International January 1997 The Antarctic continent is surrounded by floating sheets of ice called ice shelves which fringe almost half of the coastline. [1] As early as 1968 the rapid breakup of ice shelves around the Antarctic Peninsula has been considered as a "sign that dangerous warming is beginning in Antarctica". [2] Calving icebergs from these shelves ordinarily represent the major source of ice discharge from the continent. For example, more than 11,000 sq km of the Larsen Ice Shelf and 11,500 sq km of the Filchner Ice Shelf broke off into the Weddell Sea. [3] While these events were not necessarily outside the bounds of natural calving rates, the total mass discharged from these events was three to four times the annual Antarctic snow accumulation. A pattern of warming with an increase of 2.5C since 1945 is evident over the Antarctic Peninsula. [4] At the same time five northerly shelves [5] have retreated dramatically as the region has warmed. [6] Four, more southerly, shelves [7] exhibit little or no change. David Vaughan and Chris Doake from the British Antarctic survey concluded however that "if the warming trend continues, other nearby ice shelves on the Antarctic Peninsula may be at risk". [6] Vaughan and Doake argue that the behaviour of ice shelves in the Antarctic Peninsula indicates that there may be "an abrupt thermal limit on ice-shelf viability" and that this limit has been driven progressively southwards by the regional atmospheric warming. [6] They conclude that "ice shelves appear to be sensitive indicators of climate change". They also note that the spatial and temporal pattern of ice-shelf retreat is similar to that proposed by earlier predictions in 1978. [8] One of the characteristics of ice shelf retreat is their rapid final collapse. Researchers from the University of Innsbruck in Austria and the Argentine Antarctic Institute, in analysing the dramatic collapse of the North Larsen Ice Sheet in January 1995, concluded that "the break-up followed a period of steady retreat that coincided with a regional trend of atmospheric warming". [9] Vaughan and Doake describe the collapse of North Larsen Ice Shelf (Larsen-A) as "most dramatic" with "a loss of 1,300 sq km in 50 days in which the ice shelf broke up into thousands of small icebergs producing a plume 200 kilometres into the Weddell Sea". [6] In February of the same year, a giant iceberg, the size of Luxemburg (2,600 sq km) also calved of Larsen-B just south of the disintegration. Further south, Larsen-B has begun to show signs of "imminent progressive retreat" with iceberg calving and the opening of crevasses behind the ice front in 1995 [6] and again in the winter of 1996 (Vaughan, DG, pers. comm. 10 Jan 1997). Researchers point out that the calculated temperature threshold for ice shelf viability had not yet been reached over Larsen-A at the time of its collapse, implying that either the existing temperature maps of the area are inadequate or that the warming trend is stronger than anticipated. [6] Another possibility may also involve basal melting although little is known of the oceanographic characteristics adjacent to the shelf (Vaughan, DG, pers. comm. 10 Jan 1997). (Note that our research programme in the area will contribute to knowledge with respect to this question.) None of these possibilities are at all reassuring. The exact mechanisms for ice shelf retreat remain uncertain, although they are may be linked with surface warming. Simple conduction of heat from the surface of the ice would work too slowly to explain the rapidity of observed changes. However, there may be additional mechanisms that affect the rate of disintegration, such as decreased ice strength. For instance, Doake and Vaughan [10] analysed the iceberg calving of the Wordie Ice Shelf and confirmed earlier results [11] that ice fracturing was enhanced by increased quantities of melt water, which in turn are a result of persistent warming in the region. Warming at the surface may change the temperature structure of the ice shelf by altering the balance of accumulation and melting so that cracks and crevasses are not healed over by the annual accumulation of snow and ice. [6] As temperatures have risen northern Antarctic Peninsula selves
have retreated or collapsed. In coming years, further climatic
warming will threaten the more southerly ice shelves and some are
already showing signs of imminent decay. The disintegration of
the ice shelves along the Antarctic Peninsula, as way suggested
as early as 1968, is a first sign of dangerous climatic warming
in Antarctica and this warming will continue unless greenhouse
gases are cut dramatically.
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