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The breakout in March 2008 of the Wilkins Ice Shelf on the Antarctic Peninsula is the latest drama in a region that has experienced unprecedented warming over the last 50 years. In the past 30 years seven floating ice shelves retreated, with very little of their area now remaining. The changes give us clues about the impact of climate change across Antarctica in the coming centuries.
Ice shelves are the floating extensions of a grounded ice sheet. Although a few small ice shelves exist in the Arctic, most occupy bays around the coast of Antarctica. They were once thought to be permanent features of the Antarctic landscape. The largest ice shelf, the Ronne-Filchner, covers an area slightly smaller than Spain. Over many decades ice shelves find their natural size when the amount of snow falling on the surface, and the amount ice delivered by glaciers, balances the rate of ice loss through melting and iceberg calving. A change in any of these factors will cause an ice shelf to change its size to find a new equilibrium.
The long mountainous landmass known as the Antarctic Peninsula has always been warmer than the interior of the Antarctic continent. Each summer produces significant amounts of meltwater on the Antarctic Peninsula’s ice shelves. Each shelf can tolerate only so much meltwater before they weaken and begin to retreat — scientists call this the ‘limit of viability’.
As the climate on the Antarctic Peninsula has warmed — by 3°C over the last 50 years — the limit of viability for ice shelves has moved southwards. Ice shelves that used to be stable are now retreating.
Since the 1950s, a total of 25,000 km 2 of ice shelf has been lost from around the Antarctic Peninsula. In volume, this is the equivalent of the UK domestic water requirement for around 1,000 years.
For some ice shelves, loss of area has occurred progressively over several decades. For others there have been dramatic episodes of collapse. Some progress in determining the exact mechanisms responsible is now being made.
The loss of ice from ice shelves has very little direct impact on sea level, but the acceleration of glaciers draining ice from the grounded ice sheet has been reported as a consequence of ice-shelf retreat in several places. Overall, this and other effects are leading to the northern Antarctic Peninsula to contribute 0.16mm per year to global sea-level rise.
In 1991, the Wordie Ice Shelf was the first ice shelf to be shown to be retreating. However, it was in 1996, after the retreat of the Prince Gustav and the dramatic break-up of the Larsen-A ice shelves, that BAS first presented evidence that these events were caused by climate change.
Sea-bed sediment cores indicate that before their recent loss, some ice shelves were present continuously for at least 10,000 years. This suggests that the current ice-shelf retreat, and climate warming, are unprecedented on this timescale and are not solely part of a natural cycle of change.
Currently, the Wilkins Ice Shelf is pinned by only a narrow peninsula of ice between two islands and is unlikely to survive more than a few more summers. Once lost, a further section may follow swiftly. Larsen C, which is currently stable, may begin to retreat. If climate change continues on the Antarctic Peninsula, it is likely that more ice shelves will be lost in the coming century.
© NERC-BAS 2007