The Larsen Ice Shelf is a long, fringing ice shelf in the northwest part of the Weddell Sea, extending along the east coast of Antarctic Peninsula from Cape Longing to the area just southward of Hearst Island. Named for Captain Carl Anton Larsen, the master of the Norwegian whaling vessel Jason, who sailed along the ice front as far as 68°10' South during December 1893.[1]
In finer detail, the Larsen Ice Shelf is a series of three shelves that occupy (or occupied) distinct embayments along the coast. From north to south, the three segments are called Larsen A (the smallest), Larsen B, and Larsen C (the largest) by researchers who work in the area. The Larsen A ice shelf disintegrated in January 1995. The Larsen B ice shelf disintegrated in February 2002. The Larsen C ice shelf appears to be stable for the time being, though scientists predict that, if localized warming continues at its current rate, the shelf could disintegrate at some point within the foreseeable future.[2]
The Larsen disintegration events were unusual by past standards. Typically, ice shelves lose mass by iceberg calving and by melting at their upper and lower surfaces. The disintegration events are linked to the ongoing climate warming in the Antarctic Peninsula, about 0.5 °C per decade since the late 1940s, which is a consequence of localized warming of the Antarctic peninsula.[3] This localized warming is caused by anthropogenic global warming, according to some scientists through strengthening of the Antarctic annular winds.[4]
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During 31 January 2002–7 March 2002 the Larsen B sector collapsed and broke up, 3,250 km² of ice 220 m thick disintegrated, meaning an ice shelf covering an area comparable in size to the US state of Rhode Island collapsed in a single season.[5] Larsen B was stable for up to 12,000 years, essentially the entire Holocene period since the last glacial period, according to Queen's University researchers.[6] By contrast, Larsen A "was absent for a significant part of that period and reformed beginning about 4,000 years ago," according to the study.
Despite its great age, the Larsen B was clearly in trouble at the time of the collapse. With warm currents eating away the underside of the shelf, it had become a "hotspot of global warming."[7] What especially surprised glaciologists was the speed of the breakup, which was a mere three weeks (or less). A factor they had not anticipated was the powerful effects of liquid water; ponds of meltwater formed on the surface during the near 24 hours of daylight in the summertime, then the water flowed down into cracks and, acting like a multitude of wedges, levered the shelf apart, almost in one fell swoop.[8][9] Global increase in air temperature was not the only factor contributing to the break according to Ted Scambos, of the University of Colorado's national snow and ice data centre.
It's likely that melting from higher ocean temperatures, or even a gradual decline in the ice mass of the peninsula over the centuries, was pushing the Larsen to the brink— Ted Scambos[10]
Although the remaining Larsen C region, which is the furthest south, appears to be relatively stable for now,[11] continued warming could lead to its breakup within the next decade.[12] If disintegration should occur with this last major sector, which is larger in size than the US states of New Hampshire and Vermont combined — then the enormous Larsen Ice Shelf viewed in 1893 by Carl Anton Larsen and his crew aboard the Jason will largely be gone in just over a century after its discovery.
The collapse of Larsen B has revealed a thriving ecosystem 800 m (half a mile) below the sea. "Despite near freezing and sunless conditions, a community of clams and a thin layer of bacterial mats are flourishing in undersea sediments. [...] The discovery was accidental. U.S. Antarctic Program scientists were in the northwestern Weddell Sea investigating the sediment record in a deep glacial trough twice the size of [the US state of] Texas."[13]
Studies show that in the middle of the present interglacial the former Larsen A region, which was the furthest north and outside the Antarctic Circle, had previously broken up and reformed only about 4,000 years ago, although the former Larsen B had been stable for at least 10,000 years.[6] The maximal ice age on the current shelf dates from only two hundred years ago. The speed of Crane Glacier increased threefold after the collapse of the Larsen B and this is likely to be due to the removal of a buttressing effect of the ice shelf.[14] Recent data collected by an international team of investigators through satellite-based radar measurements suggests that the overall ice-sheet mass balance in Antarctica is increasingly negative.[15]