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Polar sea ice extent

Studies reported that the Arctic sea ice extent had shrunk by 7.4 per cent over the past 25 years, with record-low coverage in September 2002 (Johannessen and others 2003). An analysis of 30 years of satellite data suggests that the loss of Arctic sea ice is also accelerating (Cavalieri and others 2003).
There are projections that much of the sea ice, until now thought to be permanent, will melt during the summer by the end of this century if the current trend in global warming continues. This will have major direct impacts on indigenous people and Arctic marine wildlife such as polar bears and seals, and will also open the region to increased development pressure as access by sea to valuable natural resources becomes easier (Figure 1). The global impacts may also be significant as absorption of solar radiation increases, and could lead to changes in the world ocean circulation (UCL 2003, NASA 2003, Laxon and others 2003).

Figure 1: Projection of Arctic sea ice extent by 2030 and possible ice-free transpolar shipping routes

Source: New Scientist 2002

In the Antarctic, satellite data indicate that the overall extent of sea ice has gradually increased since 1977, following a significant decrease in the early part of the 1970s (Cavalieri and others 2003). This reversal demonstrates a greater variability in the trend for Antarctic sea ice extent in comparison to a more consistent trend of decline in the Arctic.

However, on the Antarctic peninsula, and in the Amundsen Sea sector of west Antarctica, several ice shelves that were thousands of years old have disintegrated within the last few decades – a sign of rapid change in the region (Shepard and others 2003).

Argentine scientists have documented surges in tributary glaciers following these break-ups (de Angelia and Skvarca 2003). Their study – with new results from theoretical models – suggests that ice shelves may play an important role in the stability of some inland ice streams, and their loss could result in the accelerated collapse of glaciers.

Particular attention is being paid to the behaviour of critical ice flows in the West Antarctic ice sheet, which could raise global sea level by five metres, were it to collapse completely (Casassa 2003).
The break-up of the Ward Hunt sea ice-shelf in northern Canada (Box 1), the Arctic’s largest, was also reported in 2003 (Mueller and others 2003). The Ward Hunt has existed for some 3 000 years. When it was mapped by explorers in the 1870s it was 1 000 km long and 60 km wide. By the late 1990s, its dimensions had decreased to some 70 km by 20 km, and now it’s disintegration is almost complete.

This sequence may be attributed to natural climate variability. However, just as the recent collapse of ice shelves in west Antarctica has been interpreted as evidence of accelerated climate change in that region, the break-up of the Ward Hunt may suggest the same in the high Arctic (Mueller and others 2003).

Box 1: Ward Hunt Ice-shelf

Source: NASA and Earth Observatory

The Arctic’s largest ice-shelf is breaking up. The Ward Hunt Ice-shelf is a remnant of the compacted snow and ancient sea ice that extended along the northern shores of Ellesmere Island in Northern Canada until the early 20th century. Rising temperatures have reduced the original shelf into a number of smaller shelves, the largest of which was the Ward Hunt Ice-shelf on the northwest fringe of the island. Between 2000–2002, the Ward Hunt Ice-shelf began to crack and eventually broke in two. This Standard Beam Mode RADARSAT-1 image, which was acquired September 27, 2003, clearly shows a large crack dividing the ice-shelf in half. The crack runs from the Arctic Sea to the right of Ward Hunt Island and the bright white ice grounded there and back to the rougher, mountainous region.

Source: Earth Observatory 2003
Break-up of the Ward Hunt Ice-shelf.


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