The Science Bit … Part 3 … The Larsen B Ice Shelf Disintegration
The Science Bit … Part 3 … The Larsen B Ice Shelf Disintegration
By: Carol Pudsey
Date: March 13, 2002
Source: Pudsey, Carol. "The Science Bit … Part 3 … The Larsen B Ice Shelf Disintegration." RRS James Clark Ross Diary. Available online at British Antarctic Survey 〈http://www.antarctica.ac.uk/Living_and_Working/Diaries/RRS_James_Clark_Ross/antarctic2001_2002/jrupdate11_26.html〉 (accessed February 10, 2006).
About the Author: Carol Pudsey was the principal scientist aboard the RRS James Clark Ross, which was in the region of the Larsen B Ice Sheet when it began to disintegrate. She received her Ph.D. in geology in 1977. Pudsey worked with the British Antarctic survey between 1987 and 2005 where she participated in twelve geoscience research cruises, five of which she acted as principal scientist. Pudsey's research interests involve understanding the history and formation of sediments and ice cores.
Antarctic ice shelves are horizontal sheets of ice that extend away from the land and float on top of the ocean. Ice shelves ring the entire continent, with extremely large shelves occupying the large gulfs in western portion of Antarctica. The total volume of ice in the ice shelves off of Antarctica is 173,369 cubic miles (722,635 cubic km), which accounts for 2.4 percent of the total ice on the continent. Ice shelves comprise eleven percent of the surface area of the continent.
Ice shelves form when glaciers and ice sheets moving across the land discharge into the ocean. The ice floats on top of the ocean and the shelf spreads out over the water by the force of gravity. Friction of the ice against walls, islands, and sea floor raises, retards, and stabilizes the movement of the ice flow.
The balance of ice added and ice removed controls the size of ice shelves. They increase in size when glaciers and ice sheets contribute more ice and when there is more snowfall. They decrease in size when icebergs calve off the edge into the ocean and when ice melts into the ocean below.
The Larsen Ice Shelf is a series of small shelves on the east coast of the Antarctic Peninsula in the western part of the continent from about seventy-one to sixty-four degrees South latitude. The shelf is divided into sections: the northernmost named Larson A. The much larger Larson B abuts Larson A to the south, followed by Larson C.
In the 1940s, Larson A began retreating. In January 1995, a dramatic disintegration of the ice shelf occurred, resulting in 772.2 square miles (2000 square km) of ice breaking into small icebergs during a storm. This disintegration was attributed to the climactic warming around Antarctica in the second half of the twentieth century.
Between January 31 and March 7, 2002, the Larson B ice shelf disintegrated. Within 45 days, satellites captured the dramatic breakup of 1255 square miles (3,250 square km) of ice shelf. The British research ship, James Clark Ross, happened to be in the area of the Larson Ice Shelf during its destruction. The principal scientist on the ship, Pudsey, recorded her observations in the ship's diary on March 13, 2002.
We had been planning to survey and core in the area of the Larsen-B ice shelf where some large icebergs have calved in recent years. Early in the cruise we heard that there had been another calving event, about 8 km of icefront amounting to 600 sq. km, between 31/1/02 and 17/2/02. In the memorable words of our colleague David Vaughan at BAS Cambridge, "And if it's still warm and you're near the icefront keep your eyes open and note anything that goes bump in the night. There is no reason to think that it's all over for this season."
On March 5th we received a satellite image revealing the complete disintegration of the northern half of Larsen-B. From this and the ship's Dartcom HRPT satellite images [High Resolution Picture Transmission satellites receive digital images from National Oceanic and Atmospheric Administration environmental satellites], the mass of disintegration debris seemed to be expanding eastwards and southwards, as the Larsen-A debris was observed to do in 1995. On March 7th the ship made a considered approach in between the southern edge of the debris and a row of four very large tabular bergs extending NE of Jason Peninsula [the southern extent of the Larson B ice shelf]. These four bergs were drifting northwards towards the edge of the debris, and in the limited time available a systematic swath survey was not possible. We were able to observe and photograph the debris and take one core before escaping to the south at nightfall. The large tabular bergs which we knew to be freshly broken were an amazing sight, beautiful clean ice cliffs with a vertical or overhanging profile. The main mass of debris looked like the aftermath of a huge explosion—certainly brought home the fact that disintegration is a different process from iceberg calving. It was exciting to be there, though a little frustrating given that we couldn't see very far from the top of the ship.
The breakup of the Larsen B ice shelf was the largest single ice shelf retreat documented. The surface area lost was nearly twice the size of Rhode Island. Using information gathered from the sediments below the ice shelf, it had likely been in place for at least 400 years, and was likely formed nearly 12,000 years ago during the last Ice Age. Because ice shelves float on top of the ocean, the melting of the ice shelf has no effect on the total sea level.
Although the reason for the ice shelf disintegration is still under debate, it is clear that the local climate played an important role. The local temperature around Antarctica increased 4.5 °F (2.5 °C) between the 1952 and 2002. This is five times greater than the global temperature increase.
Increases in temperature hasten the melting of the ice shelves in two major ways. First sea surface temperatures increase, which increases melting of the ice sheet from below. Second, melt ponds are formed on the surface of the ice shelves. The water from the ponds seeps into cracks in the ice. The pressure of the water is greater than the pressure of the air and the water forces the cracks through the entire thickness of the shelf. The buildup of large cracks throughout the shelf compromises its stability.
Satellite imagery of the ice shelves contributed greatly to the understanding of the Larson B break up. NASA's (National Aeronautical and Space Administration's) MODIS sensor (a satellite imager) was able to document the changes that took place on the ice shelf. Analysis of the satellite images showed that the ice shelf disintegrated in the same regions that were covered by melt ponds. In addition, the melt ponds became smaller just before the breakup, suggesting that the cracks formed by the water reached the bottom of the shelf, allowing the melt water to flow into the ocean below.
In the aftermath of the Larson B collapse, scientists measured the flow rates of glaciers that used to flow onto the ice shelf. They found that the glaciers flowed into the ocean at speeds up to 250 percent greater then when the ice shelf was in place. The ice shelf had acted like a brake. These results have major implications for the ice sheets on continent. Although the disintegration of ice shelves do not contribute to a rise in sea level because they are already floating, adding ice from land-bound glaciers into the ocean would contribute to an increased sea level.
Computer modeling combined with the satellite imagery of the shelves have resulted in a theory that links summertime temperatures to shelf instability. These efforts demonstrate that the next shelf south of Larson B is close to its stability limit. The summertime temperatures of one of the largest ice shelves, the Ross Ice Shelf, are only slightly cooler than necessary to make it stable. Scientists predict that if the warming trend in Antarctica continues, these shelves may begin to recede in future decades.
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