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Antarctic Ice ShelfThe first direct evidence linking human activity to the collapse of Antarctic ice shelves is published this week in the Journal of Climate. Scientists from the British Antarctic Survey (BAS), the Centre for Polar Observation and Modelling, University College London, and the Katholieke Universiteit Leuven, (Belgium) reveal that stronger westerly winds in the northern Antarctic Peninsula, driven principally by human-induced climate change, are responsible for the marked regional summer warming that led to the retreat and collapse of the northern Larsen Ice Shelf.

The Larsen ice shelf at the northern end of the Antarctic Peninsula experienced a dramatic collapse between January 31 and March 7, 2002. First, melt ponds appeared on the ice shelf during these summer months (seen in blue on the shelf), then a minor collapse of about 800 square kilometers occurred. Finally, a 2600 square kilometer collapse took place, leaving thousands of sliver icebergs and berg fragments where the shelf formerly lay. Brownish streaks within the floating chunks mark areas where rocks and morainal debris are exposed from the former underside and interior of the shelf.
Global warming and the ozone hole have changed Antarctic weather patterns such that strengthened westerly winds force warm air eastward over the natural barrier created by the Antarctic Peninsula’s 2 km-high mountain chain. On days when this happens in summer temperatures in the north-east Peninsula warm by around 5 degrees C, creating the conditions that allowed the drainage of melt-water into crevasses on the Larsen Ice Shelf, a key process that led to its break-up in 2002.
Lead author Dr Gareth Marshall from the British Antarctic Survey said, “This is the first time that anyone has been able to demonstrate a physical process directly linking the break-up of the Larsen Ice Shelf to human activity. Climate change does not impact our planet evenly - it changes weather patterns in a complex way that takes detailed research and computer modelling techniques to unravel. What we’ve observed at one of the planet’s more remote regions is a regional amplifying mechanism that led to the dramatic climate change we see over the Antarctic Peninsula.”
Reference: Marshall GJ, Orr A, van Lipzig NPM, King JC. 2006. The impact of a changing Southern Hemisphere Annular Mode on Antarctic Peninsula summer temperatures. Journal of Climate 19: 5388*5404.
The collapse of the 3250 km2 Larsen B Ice Shelf took place in 2002. During the past 40 years the average summer temperatures in this region of the north-east Peninsula has been 2.2°C. The western Antarctic Peninsula has showed the biggest increase in temperatures (primarily in winter) observed anywhere on Earth over the past half-century.
Ice sheet - is the huge mass of ice, up to 4 km thick, that covers Antarctica’s bedrock. It flows from the centre of the continent towards the coast where it feeds ice shelves.
Ice shelf - is the floating extension of the grounded ice sheet. It is composed of freshwater ice that originally fell as snow, either in situ or inland and brought to the ice shelf by glaciers. As they are already floating any disintegration (like Larsen B) will have no impact on sea level. Sea level will rise only if the ice held back by the ice shelf flows more quickly into the sea.
Loss of ice shelves near the Antarctic Peninsula is one of the most obvious signs of climate change. Elsewhere in Antarctica ice shelves are shrinking, which most scientists believe is because of a recent increase in the rate at which the ocean melts the ice.
British Antarctic Survey is a world leader in research into global issues in an Antarctic context. It is the UK’s national operator and is a component of the Natural Environment Research Council. It has an annual budget of around £40 million, runs nine research programmes and operates five research stations, two Royal Research Ships and five aircraft in and around Antarctica. More information about the work of the Survey can be found on our website: http://www.antarctica.ac.uk

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