 | ||
The Larsen Ice Shelf is a long, fringing ice shelf in the northwest part of the Weddell Sea, extending along the east coast of the Antarctic Peninsula from Cape Longing to the area just southward of Hearst Island. It is 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. In finer detail, the Larsen Ice Shelf is a series of shelves that occupy (or occupied) distinct embayments along the coast. From north to south, the segments are called Larsen A (the smallest), Larsen B, and Larsen C (the largest) by researchers who work in the area. Further south, Larsen D and the much smaller Larsen E, F and G are also named.
Contents
The breakup of the ice shelf since the mid 1990s has been widely reported, with the collapse of Larsen B in 2002 being particularly dramatic.
Ice shelf
The collapse of Larsen B has revealed a thriving chemotrophic ecosystem 800 m (half a mile) below the sea. The discovery was accidental. U.S. Antarctic Program scientists were in the north-western Weddell Sea investigating the sediment record in a deep glacial trough twice the size of Texas. Methane and hydrogen sulfide associated with cold seeps is suspected as the source of the chemical energy powering the ecosystem. The area had been protected by the overlying ice sheet from debris and sediment which was seen to be building up on the white microbial mats after the breakup of the ice sheet. The clams were observed clustered about the vents.
Studies show that the former Larsen A region, which was the furthest north and was just outside the Antarctic Circle, had previously broken up in the middle of the present interglacial and reformed only about 4,000 years ago. The former Larsen B, by contrast, had been stable for at least 10,000 years. The ice of the shelf is renewed on a much shorter time-scale and 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. Data collected in 2007 by an international team of investigators through satellite-based radar measurements suggests that the overall ice-sheet mass balance in Antarctica is increasingly negative.
Breakup
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 were linked by The Independent newspaper in 2005 to ongoing climate warming in the Antarctic Peninsula, about 0.5 °C per decade since the late 1940s. According to a paper published in Journal of Climate in 2006, the peninsula at Faraday station warmed by 2.94 °C from 1951 to 2004, much faster than Antarctica as a whole and faster than the global trend; this localized warming is caused by anthropogenic global warming, through a strengthening of the winds circling the Antarctic. Once the disintegration of all three of Larsen A, B and C has completed, the enormous Larsen Ice Shelf viewed in 1893 by Carl Anton Larsen and his crew aboard the Jason would largely be gone - less than a century and a half after its discovery.
Larsen A
The Larsen A ice shelf disintegrated in January 1995.
Larsen B
During 31 January 2002 to March 2002 the Larsen B sector partially collapsed and parts broke up, 3,250 km2 (1,250 sq mi) of ice 220 m (720 ft) thick, covering an area comparable to the US state of Rhode Island. In 2015 a study concluded that the remaining Larsen B ice-shelf will disintegrate by the end of the decade, based on observations of faster flow and rapid thinning of glaciers in the area.
Larsen B was stable for at least 10,000 years, essentially the entire Holocene period since the last glacial period, according to Queen's University researchers. 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." What especially surprised glaciologists was the speed of the breakup, which was a mere three weeks (or less). Factors they had not anticipated were 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. 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 Center:
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.
Andrew Fleming (remote sensing manager at the British Antarctic Survey) said to Reuters: "The Larsen B shattered like car safety glass into thousands and thousands of pieces. It disappeared in the space of about a week."
Larsen C
Larsen C is the fourth largest ice shelf in Antarctica, with an area of about 50,000 km2 (19,000 sq mi).
In 2004, a report concluded that although the remaining Larsen C region appeared to be relatively stable, continued warming could lead to its breakup within the next decade.
News reports in summer of 2016 suggested that this process has begun. On November 10, 2016 scientists photographed the growing rift running along the Larsen C ice shelf, showing it running about 110 kilometres (68 mi) long with a width of more than 91 m (299 ft), and a depth of 500 m (1,600 ft). By December 2016, the rift had extended another 21 km (13 mi) to the point where only 20 km (12 mi) of unbroken ice remained and calving was considered to be a certainty in 2017. This will cause the collapse of between nine and twelve percent of the ice shelf, 6,000 km2 (2,300 sq mi), an area greater than the size of the US state of Delaware. After calving, the broken fragment will be 350 m (1,150 ft) thick and have an area of about 5,000 km2 (1,900 sq mi). If it calves without breaking into small fragments, it will be among the largest icebergs ever recorded. For comparison, the predicted iceberg's area is one quarter the size of Wales, or the area of Trinidad and Tobago.
Sea-level is not affected directly by the fragment breaking away from Antarctica, as the fragment is already floating on the ocean. The breakup will however leave the remainder of the shelf more vulnerable to future collapse. If all the ice that the Larsen C shelf currently holds back were to enter the sea, it is estimated that global waters would rise by 10 cm (3.9 in).