Climate change and ice shelves
Contents
Ice Shelves
An ice shelf is a thick slab of ice, attached to a coastline and extending out over the ocean as a seaward extension of the grounded ice sheet. Ice shelves range in thickness from about 50 to 600 meters, and some shelves persist for thousands of years. They fringe the continent of Antarctica, and occupy a few fjords and bays along the and Ellesmere Island coasts. (An ice shelf occupying a fjord is sometimes called an ice tongue.) At their seaward edge, ice shelves periodically calve icebergs, some the size of a small U.S. state or European country.
Ward Hunt Ice Shelf: This ice shelf, which fragmented in 2002, is an example of a shelf made from compressed sea ice. This Canadian RADARSAT image shows the shelf in August 2002, when a crack made its way down the length of the shelf. Image courtesy of the Alaska Satellite Facility, Geophysical Institute, University of Alaska Fairbanks. (Source: NSIDC) Formation and calving: the natural processes
Most ice shelves are fed by inland glaciers. Together, an ice shelf and the glaciers feeding it can form a stable system, with the forces of outflow and back pressure balanced. Warmer temperatures can destabilize this system by increasing glacier flow speed and—more dramatically—by disintegrating the ice shelf. Without the shelf to slow its speed, the glacier accelerates. After the 2002 Larsen B Ice Shelf disintegration, nearby glaciers in the Antarctic Peninsula (Antarctica) accelerated up to eight times their original speed over the next 18 months. Similar losses of ice tongues in have caused speed-ups of two to three times the flow rate in just one year.
While calving or disintegrating ice shelves don't raise ocean level, the resulting glacier acceleration does, and it poses a direct threat to coastal communities. More than 100 million people currently live within 1 meter of mean sea level. contains enough ice to raise sea level by seven meters, and Antarctica holds enough ice to raise sea level by 57 meters. While these ice sheets are unlikely to disappear anytime soon, even partial loss of the grounded ice could present a significant problem. In the early decades of any climate warming era, ice shelves and ice tongues are likely to play a prominent role in changing the rate of ice flow off the major ice sheets.
Antarctica's major ice shelf areas: These ice shelf areas can easily be seen in NSIDC's Mosaic of Antarctica. Image courtesy (Scambos et al. 2007). (Source: NSIDC) Ice Shelf Observations
Ice shelves fall into three categories: (1) ice shelves fed by glaciers, (2) ice shelves created by sea ice, and (3) composite ice shelves. Most of the world's ice shelves, including the largest, are fed by glaciers and are located in and Antarctica.
One example of an ice shelf composed of compacted, thickened sea ice is the Ward Hunt Ice Shelf off the coast of Ellesmere Island in northern Canada. In the Northern Hemisphere summer of 2002, this shelf fractured and calved several large pieces into the Arctic Ocean. As a result, more than 3 billion cubic meters of fresh water drained into the Arctic Ocean. Compared to many of the world's other ice shelves, however, the Ward Hunt Ice Shelf was actually quite small.
Loose tooth, 2001: These Multi-angle Imaging SpectroRadiometer (MISR) images show the progression of a Because, in any climate regime, they are typically exposed to both warmer air above and warmer ocean below, ice shelves and ice tongues respond more quickly than ice sheets or glaciers to rising temperatures. Antarctica has 15 major ice shelf areas, and 10 of the largest appear in this map: Ross, Ronne-Filchner, Amery, Larsen C, Riiser-Larsen, Fimbul, Shackleton, George VI, West, and Wilkins. The three largest are the Ross, the Ronne-Filchner, and the Amery.
Two kinds of events occurring on ice shelves have attracted the attention of scientists. One kind is iceberg calving, a natural event. The other kind is disintegration, a newly identified phenomenon suggestive of possible climate change.
Loose tooth, 1963: The CORONA mission captured this image of an earlier loose tooth on the Amery Ice Shelf, in 1963. Image courtesy of Helen A. Fricker, Scripps Institution of Oceanography. (Source: NSIDC) Calving of huge, tabular icebergs is unique to Antarctica, and the process can take a decade or longer. Calving can take the form of a large crack along the ice shelf edge. In the case of the Amery Ice Shelf, the calving area resembles a loose tooth. On a stable ice shelf, calving is a near-cyclical, repetitive process producing large icebergs every few decades. The icebergs drift generally westward around the continent, and as long as they remain in the cold, near-coastline water, they can survive decades or more. However, they eventually are caught up in north-drifting currents where they melt and break apart.
Floating ice tongues downstream from large outlet glaciers are generally more broken up by crevasses. Calving of the ice tongues releases armadas of smaller, steep-sided icebergs that drift south sometimes reaching North Atlantic shipping lanes. Calving of the large glacier, Jacobshavn, on the east coast of is responsible for the majority of icebergs reaching Atlantic shipping and fishing areas off of Newfoundland and most likely shed the iceberg responsible for the sinking of the Titanic in 1912. These denizens of the ocean are now tracked by the {C}National Ice Center in the United States, along with other organizations.
Jacobshavn Glacier retreat: The rapidly retreating Jakobshavn Glacier in western Greenland drains the central ice sheet. This image shows the glacier in 2001, flowing from upper right to lower left. Terminus locations before 2001 were determined by surveys and more recent contours were derived from Landsat data. The more recent retreat lines indicated in this image are longer than the earlier ones, and the increasing area of retreat suggests the possibility of increasing glacier acceleration as more ice flows into the fjord. Ice flowing into the fjord, however, would still have to pass through the same bottleneck of rock. NASA image by Cindy Starr, based on data from Ole Bennike and Anker Weidick (Geological Survey of Denmark and Greenland) and Landsat data. (Source: NSIDC) In recent years, calving of the largest ice tongues in Greenland (in particular, Jacobshavn, Helheim, and Kangerdlugssuaq) has accelerated probably due to warmer air and/or ocean temperatures. As the ice tongues have retreated, the reduced backpressure against the glacier has allowed these glaciers to accelerate significantly.
Large tabular iceberg calvings are natural events that occur under even stable climatic conditions, so are not a meaniful indicator of warming or changing climate. Over the past several decades, however, meteorological records have revealed atmospheric warming on the Antarctic Peninsula (Antarctica), and the northernmost ice shelves on the peninsula have retreated dramatically. In fact, since 1974, seven ice shelves have retreated by a total of approximately 13,500 square kilometers.
Disintegration of the Larsen B Ice Shelf: The event began on January 31, 2002. Several weeks later, the ice shelf had completely shattered. MODIS image courtesy of Ted Scambos and Terry Haran, National Snow and Ice Data Center, University of Colorado, Boulder. (Source: NSIDC) The most pronounced ice shelf retreat has occurred on the {C}Larsen Ice Shelf, located on the eastern side of the Antarctic Peninsula's northern tip. The shelf is divided into three regions from north to south: A, B, and C.
In January 1995, two events on the Larsen attracted public attention: the calving of a 70- by 25-kilometer iceberg from the Larsen B; and the disintegration of the remainder of the Larsen A, which began retreating in the 1980s. Although the iceberg attracted more attention, the disintegration may have been more closely related to climate change. The breakup pattern in the Larsen A, in which 2,000 square kilometers disintegrated into small icebergs, was at that time an unprecedented observation.
Extent of Larsen Ice Shelf retreat: Colored lines mark the ice shelf's extent in 1947, 1961, 1993, and 2002. MODIS image courtesy of Ted Scambos, National Snow and Ice Data Center, University of Colorado, Boulder. (Source: NSIDC) In the year 2002, satellites recorded a larger disintegration than what occurred in 1995 (see Larsen B Ice Shelf Collapses in Antarctica). Between 31 January and 5 March 2002, approximately 3,250 square kilometers of the Larsen B shattered, releasing 720 billion tons of ice into the Weddell Sea. It was the largest single disintegration event in 30 years of ice shelf monitoring. Preliminary studies of sediment cores suggest that it may have been this ice shelf's first collapse in 12,000 years.
Building on earlier research, Ted Scambos of NSIDC, Christina Hulbe of Portland State University, and Mark Fahnestock of the University of New Hampshire have developed a theory of how ice shelves disintegrate (see melt pond theory). Sufficiently warm summer temperatures and an impermeable surface that prevents water from being absorbed lead to melt ponds on the shelf. This meltwater can later fill small surface cracks. Depending on the amount of water and the depth of a crack, the water can deepen the crack and eventually wedge through the ice shelf.
Glacier-ice shelf interactions: In a stable glacier-ice shelf system, the glacier's downhill movement is offset by the buoyant force of the water on the front of the shelf. Warmer temperatures destabilize this system by lubricating the glacier's base and creating melt ponds that eventually carve through the shelf. Once the ice shelf retreats to the grounding line, the buoyant force that used to offset glacier flow becomes negligible, and the glacier picks up speed on its way to the sea. Image by Ted Scambos and Michon Scott, National Snow and Ice Data Center, University of Colorado, Boulder. (Source: NSIDC) The formation of melt ponds depends most upon summer temperatures. Although a single warm summer cannot lead to collapse, a series of warm summers transforms permeable snow into impermeable ice, allowing melt ponds to form during subsequent warm summers. A glacier can also respond to summer warming. Even when the temperature of interior glacial ice remains below freezing, meltwater can percolate through the glacier to its base and decrease friction between the glacial ice and the underlying rock. This is a seasonal phenomenon, and with a stable ice shelf in place, glacier acceleration ends with the warm summer temperatures. If the ice shelf shatters, however, the picture changes.
A critical feature of an ice shelf is the "grounding line," the point where the underside of the ice shelf detaches from land and floats on the ocean water. If an ice shelf retreats to the grounding line, the shelf's shape changes. More ice protrudes above the water line, and the ocean water exerts little buoyant pressure on the ice. As a result, the flow of the glacier meets very little resistance. In the 18 months following the Larsen Ice Shelf disintegration, glaciers feeding that ice shelf accelerated between three- to eight-fold. Similar mechanisms are at work in the Jakobshavn Ice Stream in.
Iceberg A54 calving: Left: Aerial photo of the A54 iceberg calving from the remainder of the Larsen Ice shelf. Photo courtesy of Ted Scambos, National Snow and Ice Data Center, University of Colorado, Boulder. Right: NASA Moderate Resolution Imaging Spectroradiometer (MODIS) image courtesy of NASA Earth Observatory. (Source: NSIDC) The images show a tabular iceberg calving from an ice shelf. This iceberg happens to be calving from the remnant piece of the Larsen B ice shelf at the southwestern corner of the embayment. At the time these images were acquired, the Larsen B sported melt ponds. Although still intact, the Larsen C had snow firn nearly in the same state as that on Larsen B, namely dense enough to support extensive ponding.
If all the glaciers feeding the Larsen B Ice Shelf were to flow into the ocean, they would raise ocean level by only a few millimeters. Greenland's glaciers and those feeding the Ross Ice Shelf, however, would have a more significant effect.
The Ross Ice Shelf is the main outlet for several major glaciers from the West Antarctic Ice Sheet. This single ice sheet contains enough above-sea-level ice to raise global sea level by five meters. At present, the Ross Ice Shelf's mean annual temperature is well below freezing. Although summer temperatures in the warmest part of this shelf are currently just a few degrees too cool for the formation of melt ponds, there is no evidence of a strong warming trend on the Ross Ice Shelf at this time.
Further reading
- Hughes, T. 1983. On the disintegration of ice shelves: The role of fracture. Journal of Glaciology 29(101): 98-117.
- Jeffries, M.O. 2002. Ellesmere Island ice shelves and ice islands. U.S. Geological Survey Professional Paper 1386-J J147-J163.
- Joughin, I., W. Abdalati, and M. Fahnestock. 2004. Large fluctuations in speed on Greenland's Jakobshavn Isbræ glacier. Nature 432: 608-610.
- Rignot, E., G. Casassa, P. Gogineni, W. Krabill, A. Rivera, and R. Thomas. 2004. Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B ice shelf. Geophysical Research Letters doi:10.1029/2004GL020697.
- Scambos, T., C. Hulbe, and M. Fahnestock. 2003. Climate-induced ice shelf disintegration in the Antarctic Peninsula. In Antarctic Peninsula climate variability: historical and paleoenvironmental perspectives, Antarctic Research Series 79: 79-92.
- Scambos, T.A., J.A. Bohlander, C.A. Shuman, and P. Skvarca. 2004. Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica. Geophysical Research Letters doi:10.1029/2004GL020670.
- Vaughan, D.G., and C.S.M. Doake. 1996. Recent atmospheric warming and retreat of ice shelves on the Antarctic Peninsula. Nature 379: 328-331.
- Weertman, J. 1973. Can a water-filled crevasse reach the bottom surface of a glaier? In International Association of Scientific Hydrology (Symposium at Cambridge 1969: Hydrology of Glaciers) 95: 139-145.
- Zwally, H.J., W. Abdalati, T. Herring, K. Larson, J. Saba, and K. Steffen. 2002. Surface melt-induced acceleration of Greenland ice-sheet flow. Science 297: 218-222.
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