When the area of an ice cap exceeds 50,000 km2, it is considered an ice sheet. The two ice sheets that currently exist on the Earth, however, are significantly larger than this minimal area: Greenland ~1.7 million km2, and Antarctica ~12.3 million km2. At the time of the last glacial maximum, approximately 21,000 years ago, ice sheets covered nearly three times this area, including large sections of North America and Europe. Sea level at that time was 120-135 meters (394-443 feet) lower than it is today, revealing land bridges that are currently underwater, such as the Bering Strait, linking Alaska and eastern Siberia.
Ice sheets, like their smaller ice cap and glacier counterparts, move under their own weight. The dynamics of the ice within an ice sheet, as well as its overall evolution, are very complex. Ice may move slowly, as it deforms under the influence of gravity. It may speed up as meltwater reaches the bedrock on which the ice rests, slow down, or come to a complete stop, in response to the underlying roughness or slope. Ice can also be transported out of the ice sheet by fast-flowing ice streams, which reach the ocean and form tidewater or outlet glaciers. The ice may extend out into the water, forming ice shelves, particularly in Antarctica. These shelves are subjected to ocean tides, which eventually cause pieces of ice to break off along the margins, yielding icebergs.
NASA monitors the extent and thickness of these ice sheets, using a comprehensive suite of surface, aircraft and satellite observations. In addition, these data are being used in the development of increasingly sophisticated models, with the goal of better predicting the future evolution of the ice sheets, and their impact on climate and sea level.
The Greenland Ice Sheet
The Jakobshavn Glacier in western Greenland drains the central ice sheet, and it is retreating inland faster than any other. This image shows the glacier in 2001. The glacier flows from upper right to lower left. The fjord beyond the glacier terminus is packed with seasonal ice and icebergs. Terminus locations before 2001 were determined by surveys; more recent contours were derived from Landsat data. Without measurements of ice thickness, however, the picture of ice loss is incomplete. (NASA image by Cindy Starr, based on data from Ole Bennike and Anker Weidick of the Geological Survey of Denmark and Greenland and Landsat data.)
The Greenland Ice Sheet is the largest mass of ice in the northern hemisphere. Covering approximately 80 percent of Greenland, it has a mean thickness of nearly 1,800 meters (1.1 miles), but in places, reaches nearly twice that. Two domes, one in the south and one in the north, are connected by a north-south ridge, which divides drainage basins that flow westward from those that flow eastward. Beneath the southern part of the ice sheet, and along its eastern margin, the bedrock surface is very rugged and currently stands 500 to 1,000 meters (0.3 to 0.6 miles) above sea level. In the north and west, however, large areas of the underlying bedrock are smooth, and lie below sea level.
The last decade saw a significant increase in the net rate of ice loss from Greenland, from approximately 7 Gt per year to 171 Gt per year [1 gigatonne (Gt) = 1012kg]. This loss stems, in part, from warmer temperatures in the Arctic, which have increased the rate of melting at the ice sheet surface. This meltwater is thought to make its way to the underlying bedrock, increasing lubrication at the surface, and making it easier for ice to flow through outlet glaciers to the ocean.
Furthermore, warming ocean temperatures have caused thinning and, in some cases, breakup of ice tongues. These floating ice masses, similar to ice shelves, buttress outlet glaciers, slowing the drainage of ice from the ice sheet. When they break up, the ice increases in speed, as it no longer encounters resistance on reaching the ocean, until a new equilibrium is established. A dramatic example of this occurred at the Jakobshavn Isbrae in western Greenland. Beginning in 1996, a gradual retreat, that had been ongoing for several decades, began to accelerate, as the ice tongue thinned and broke up. This continued until 2004, but the retreat of the ice front continues.
The ice island that calved off the Petermann Glacier in northwestern Greenland on August 5, 2010, was continuing its slow migration down the fjord 11 days later. The Advanced Land Imager (ALI) on NASA's Earth Observing-1 (EO-1) satellite captured this natural-color image on August 16, 2010.
Even more recently, in August 2010, northwest Greenland's Petermann Glacier shed the largest iceberg observed in nearly 50 years. Nearly four times the size of the island of Manhattan, it broke off from the glacier's ice tongue that extends into the waters of the surrounding fjord.
The Antarctic Ice Sheet
The Antarctic Ice Sheet actually consists of the East Antarctic Ice Sheet (EAIS) and the West Antarctic Ice Sheet (WAIS), which are separated by the Transantarctic Mountains. In contrast to Greenland, nearly the entire land surface of Antarctica is covered by ice. The EAIS contains the vast majority of that ice, and is more than 5 times the size of the WAIS. The mean thickness in the EAIS is more than 2600 meters (1.6 miles), while in the WAIS it is only about 1800 meters (1.1 miles). Much of the bedrock surface beneath the EAIS currently lies below sea level. If that ice were removed, however, the bedrock would rebound, eventually to an elevation above sea level, as the weight of the ice was lifted from it. As a result, the EAIS is considered a continental ice sheet. The WAIS, on the other hand, is considered a marine ice sheet, as it resides on bedrock, below sea level, that was a sea floor at some point in its history.
The overall state of the Antarctic Ice Sheet is difficult to assess. Although the EAIS is generally considered to be stable, the marine nature of the WAIS has made it the focus of concern for many years. Currently, the Pine Island and Thwaites glaciers, in the Amundsen Sea Embyament, are viewed as being especially vulnerable, given their interactions with warm ocean currents. Satellite monitoring has revealed this area to have the largest loss of ice across the entire continent. Simulations show that weakening of this area could lead to the loss of the entire WAIS, which would raise sea level by nearly 3.3 meters (11 feet).
An additional region of concern is the Antarctic Peninsula, where warming air and ocean temperatures have led to the dramatic breakup of numerous ice shelves. As with Jakobshavn Isbrae, in Greenland, this has led to increased ice flow velocities, as the buttressing effect of the ice shelves disappears. One of the more recent and spectacular breakups occurred in early 2008, with the disintegration of the Wilkins Ice Shelf.
(Credit: National Snow and Ice Data Center, Colorado)
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