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Larsen Ice Shelf

Larsen Ice Shelves A, B, C, and D
Location of the Antarctic Peninsula within Antarctica

The Larsen Ice Shelf is a long 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. In 2005, it covered approximately 78500 km2 of the earth's sea with exceptionally thick ice. 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.[1] 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.[2] Further south, Larsen D and the much smaller Larsen E, F and G are also named.[3]

The breakup of the ice shelf since the mid-1990s has been widely reported,[4] with the collapse of Larsen B in 2002 being particularly dramatic. A large section of the Larsen C shelf was reported to have broken away in July 2017.[5]

Ice shelf

Processes around an Antarctic 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 of roughly 1,000,000 square kilometres (390,000 sq mi) (twice the size of Texas or France). 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. Clams were observed clustered about the vents.[6]

The former Larsen A region, which was the farthest 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.[7] 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, likely due to the removal of a buttressing effect of the ice shelf.[8] 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.[9]

Breakup

An image of the collapsing Larsen B Ice Shelf and a comparison of this to the U.S. state of Rhode Island.

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 degrees C (0.9 degrees F) per decade since the late 1940s.[10] According to a paper published in Journal of Climate in 2006, the peninsula at Faraday station warmed by 2.94 degrees C (5.3 degrees F) 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.[11]

Larsen A

The Larsen A ice shelf disintegrated in January 1995.[2]

Larsen B

The collapse of Larsen B, showing the diminishing extent of the shelf from 1998 to 2002.

From 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, an area comparable to the US state of Rhode Island.[12] In 2015, a study concluded that the remaining Larsen B ice-shelf will disintegrate by 2020, based on observations of faster flow and rapid thinning of glaciers in the area.[13]

Larsen B was stable for at least 10,000 years, essentially the entire Holocene period since the last glacial period.[7] By contrast, Larsen A was absent for a significant part of that period, reforming about 4,000 years ago.

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".[14] It broke in a time of three weeks or less, with a factor in this fast break-up being the powerful effects of 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.[15][16] Other likely factors in the break-up were the higher ocean temperatures and the decline of the ice of the peninsula.[17]

Larsen C

2016 rift in Larsen C, wide view
Four figures showing 1) how the buoyancy of an ice shelf supports the descending glacier, slowing its motion, 2) how warmer temperatures reduce the mass of the ice shelf and provide more meltwater to lubricate the glacier, causing it to move faster, 3) how a missing ice shelf leads to more rapid glacier motion and rapid calving into the sea, and 4) how this leads to a thinner glacier with a steeper surface which moves even faster
Glacier–ice shelf interactions.

As of July 2017, Larsen C is the fourth largest ice shelf in Antarctica, with an area of about 44,200 km2 (17,100 sq mi).[18]

Satellite radar altimeter measurements show that between 1992 and 2001 the Larsen Ice Shelf lowered by up to 0.27 ± 0.11 meters per year.[19] In 2004, a report concluded that although the remaining Larsen C region appeared to be relatively stable,[20] continued warming could lead to its breakup within the following decade.[21]

The breakaway process had begun by mid-2016.[22][23] On 10 November 2016 scientists photographed the growing rift running along the Larsen C ice shelf,[24] 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.[25] This was predicted to cause the calving of between nine and twelve percent of the ice shelf, 6,000 km2 (2,300 sq mi), an area greater than the US state of Delaware,[18] or twice the size of Luxembourg.[26] The calved fragment was predicted to be 350 m (1,150 ft) thick and to have an area of about 5,000 km2 (1,900 sq mi).[18] The resulting iceberg was predicted to be among the largest icebergs ever recorded, unless it would break into multiple pieces.[25]

On 1 May 2017 members of MIDAS reported that satellite images showed a new crack, around 15 km (9 mi) long, branching off the main crack approximately 10 km (6 mi) behind the previous tip, heading toward the ice front.[27] Scientists with Swansea University in the UK say the crack lengthened 18 km (11 mi) from 25 May to 31 May, and that less than 13 km (8 mi) of ice is all that prevents the birth of an enormous iceberg. "The rift tip appears also to have turned significantly towards the ice front, indicating that the time of calving is probably very close," Adrian Luckman and Martin O'Leary wrote on Wednesday in a blog post for the Impact of Melt on Ice Shelf Dynamics and Stability project, or MIDAS. "There appears to be very little to prevent the iceberg from breaking away completely." The larger swath of the Larsen C ice shelf that sat behind the calved iceberg "will be less stable than it was prior to the rift" and may rapidly disintegrate in the same manner as Larsen B did in 2002.[28]

In June 2017 the speed of the imminent Larsen C iceberg accelerated, with the eastern end moving at 10 metres (33 ft) per day away from the main shelf.[29] As discussed by the Project MIDAS researchers on their site: “In another sign that the iceberg calving is imminent, the soon-to-be-iceberg part of Larsen C ice shelf has tripled in speed to more than 10 meters per day between 24 and 27 June 2017. The iceberg remains attached to the ice shelf, but its outer end is moving at the highest speed ever recorded on this ice shelf.”[30]

On 7 July the Project MIDAS blog report stated: "The latest data from 6th July reveal that, in a release of built-up stresses, the rift branched several times. Using data from ESA's Sentinel-1 satellites, we can see that there are multiple rift tips now within 5 km (3.10 miles) of the ice edge. We expect that these rifts will lead to the formation of several smaller icebergs."[31]

Imagery from NASA's Aqua MODIS showing the complete break of the ice shelf as of 12 July 2017
Radar imagery from ESA's Sentinel-1B taken on 12 July 2017, showing the complete break

On 12 July 2017, Project MIDAS announced that a large, 5,800-square-kilometre (2,200 sq mi) portion of Larsen C had broken from the mainland at some point between 10 and 12 July.[5][32] The iceberg, designated A-68, weighs more than a trillion tons[33][34] and is more than 200 m (700 ft) thick.[35][36]

Project MIDAS updated their blog information on 19 July 2017 regarding Larsen C by revealing that a possible new rift appeared to be extending northwards from the point where A-68 had broken off in mid-July. The project researchers felt this questionable new rift might turn towards the shelf edge, compounding the risk that it would "continue on to Bawden ice rise" which is considered "a crucial point of stabilization for Larsen C Ice Shelf.”[37]

A68's departure from Antarctica did not immediately affect global sea levels. However, a number of glaciers discharge onto the shelf from the land behind it, and they may now flow faster due to reduced support from the ice shelf. 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 (4 in).[38]

Larsen D

The Larsen D Ice Shelf is between Smith Peninsula in the south and Gipps Ice Rise. It is considered to be generally stable. Over roughly the past fifty years it has advanced (expanded) whereas comparable George VI, Bach, Stange, and Larsen C ice shelves have retreated (to a much greater net extent). The most recent survey of Larsen D measured it at 22,600 km2. There is fast ice along the entire front. This makes it difficult to interpret the ice front because the semi-permanent sea ice varies in thickness and may be nearly indistinguishable from shelf ice.[39]

Gallery

See also

Notes and references

  1. ^ U.S. Geological Survey Geographic Names Information System: Larsen Ice Shelf
  2. ^ a b Fox, Douglas. "Witness to an Antarctic Meltdown". 307 (1): 54–61. doi:10.1038/scientificamerican0712-54. 
  3. ^ Rignot, E; Jacobs, S; Mouginot, J; Scheuchl, B (13 June 2013). "Ice Shelf Melting Around Antarctica" (PDF). Science. 341 (6143): 266–270. doi:10.1126/science.1235798. Retrieved 21 January 2017. 
  4. ^ Chris Wickham (9 May 2012). "Warm water threatens vast Antarctic ice shelf (+video)". The Christian Science Monitor / Reuters. Retrieved 20 January 2017. 
  5. ^ a b "Iceberg four times the size of London breaks off from Antarctica ice shelf". The Daily Telegraph. 12 July 2017. 
  6. ^ Domack, Eugene; Ishman, Scott; Leventer, Amy; Sylva, Sean; Willmott, Veronica; Huber, Bruce (19 July 2005). "A Chemotrophic Ecosystem Found Beneath Antarctic Ice Shelf" (PDF). Eos, Transactions American Geophysical Union. American Geophysical Union. 86 (29). Bibcode:2005EOSTr..86..269D. doi:10.1029/2005EO290001. Retrieved 20 October 2016. 
  7. ^ a b "Ice Shelf disintegration threatens environment, Queen's study" (Press release). Kingston, Ontario: Queens University. 3 August 2005 – via American Association for the Advancement of Science's Eurekalert. 
  8. ^ Rignot, E.; Casassa, G.; Gogineni, P.; Krabill, W.; Rivera, A.; Thomas, R. (2004). "Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B ice shelf" (PDF). Geophysical Research Letters. 31 (18): L18401. Bibcode:2004GeoRL..3118401R. doi:10.1029/2004GL020697. Retrieved 22 October 2016. 
  9. ^ Perlman, David (2008) "Antarctic Glaciers Melting More Quickly" San Francisco Chronicle (26 January) p. A2, online
  10. ^ Connor, Steve (2005) "Ice shelf collapse was biggest for 10,000 years since Ice Age" The Independent, London (4 August), online
  11. ^ Marshall et al., "The Impact of a Changing Southern Hemisphere Annular Mode on Antarctic Peninsula Summer Temperatures", Journal of Climate, vol. 19, pp. 5388–5404, October 2006.
  12. ^ Hulbe, Christina (2002). "Larsen Ice Shelf 2002, warmest summer on record leads to disintegration". Portland State University. 
  13. ^ "NASA Study Shows Antarctica's Larsen B Ice Shelf Nearing Its Final Act" (Press release). NASA. 14 May 2015. 
  14. ^ Pearce, Fred (2006). The Last Generation: How Nature Will Take Her Revenge for Climate Change. Eden Project Books. p. 92. ISBN 1-903919-87-8. 
  15. ^ "Larsen B Ice Shelf Collapses in Antarctica". National Snow and Ice Data Center. 18 March 2002. Retrieved 2017-07-12. 
  16. ^ "Antarctic Ice Shelf Collapse Triggered By Warmer Summers". Office of News Services, University of Colorado at Boulder. 16 January 2001. Retrieved 2017-07-12. 
  17. ^ "Experts challenge ice shelf claim". Two scientists have claimed that climate change was not the only cause of the collapse of a 500bn tonne ice shelf in Antarctica six years ago. BBC News. 7 February 2008. Retrieved 21 October 2016. 
  18. ^ a b c "Growing Crack in Antarctica's Larsen C Ice Shelf Spotted by NASA's MISR". 
  19. ^ Shepherd, Andrew; Wingham, Duncan; Payne, Tony; Skvarca, Pedro (2003-10-31). "Larsen Ice Shelf Has Progressively Thinned". Science. 302 (5646): 856–859. ISSN 0036-8075. PMID 14593176. doi:10.1126/science.1089768. 
  20. ^ Riedl C, Rott H, Rack W (2004) "Recent Variations of Larsen Ice Shelf, Antarctic Peninsula, Observed by Envisat" Proceedings of the 2004 Envisat & ERS Symposium, Salzburg, Austria, online
  21. ^ Rignot, Eric (2007) "Mass Balance and Ice Dynamics of Antarctic Peninsula Glaciers for IPY2007-2008" Proposal #359, International Polar Year Expression of Intent.
  22. ^ Adrian Luckman; Daniela Jansen; Martin O'Leary; the MIDAS team (18 August 2016). "A growing rift on Larsen C". projectmidas.org. Retrieved 21 October 2016. 
  23. ^ Zee Media Bureau (23 August 2016). "A massive rift is threatening Antarctic Larsen C ice shelf to collapse". zeenews.india.com. Retrieved 21 October 2016. Media report on Project MIDAS 
  24. ^ Loff, Sarah, ed. (13 December 2016). "Rift in Antarctica's Larsen C Ice Shelf". John Sonntag (image credit). NASA. Retrieved 5 January 2017. 
  25. ^ a b McGrath, Matt (5 January 2017). "Huge Antarctic iceberg poised to break away". Science & Environment. BBC. Retrieved 5 January 2017 – via BBC.com. 
  26. ^ Nicola Davis (12 July 2017). "Iceberg twice size of Luxembourg breaks off Antarctic ice shelf". Guardian. Retrieved 13 July 2017. 
  27. ^ "Huge Antarctic ice shelf crack now has second branch". 
  28. ^ "A giant crack in Antarctic ice is 'days or weeks' from breaking off a Delaware-size iceberg". Business Insider. Retrieved 2 June 2017. 
  29. ^ O'Leary, Martin; Luckman, Adrian; Project MIDAS. "Larsen C iceberg accelerates ahead of calving". Project MIDAS. Retrieved 1 July 2017. 
  30. ^ "Larsen C Ice Shelf Rift Approaching Its End, Outer Edge Moving Away From Ice Shelf At Speed Of 33 Feet Per Day". CleanTechnica. Retrieved 2017-07-01. 
  31. ^ "Larsen C rift branches as it comes within 5 km of calving". Project MIDAS. Retrieved 7 July 2017. 
  32. ^ "Giant iceberg splits from Antarctic". BBC. 12 July 2017. 
  33. ^ "Massive iceberg breaks away from Antarctica". CNN. 12 July 2017. 
  34. ^ "Larsen C calves trillion ton iceberg". Project MIDAS. 12 July 2017. Retrieved 12 July 2017. 
  35. ^ Amos, Jonathan (12 July 2017). "Giant iceberg splits from Antarctic". BBC. Retrieved 12 July 2017. 
  36. ^ MIDAS, Project. "Larsen C calves trillion ton iceberg". Project MIDAS. Retrieved 2017-07-12. 
  37. ^ MIDAS, Project. "Larsen C responds to the calving of A68". Project MIDAS. Retrieved 2017-07-20. 
  38. ^ "Huge Antarctic iceberg poised to break away". BBC News. 6 January 2017. Retrieved 6 January 2017. 
  39. ^ Overview of areal changes of the ice shelves on the Antarctic Peninsula over the past 50 years. The Cryosphere Discussions. 3 pp579-630. URL: [www.the-cryosphere-discuss.net]

External links