The Antarctic Ice Sheet is the single largest mass of ice on Earth. It covers an area 5.4 million square miles (14 million km2), and its mass contains 11.58 million cubic miles (30 million km3) of ice. If global warming were to cause this entire sheet of ice to melt, it would raise sea levels about 215 feet (66 m).

According to a study conducted by Isabella Velicogna and John Wahr, research scientists at the University of Colorado at Boulder's Cooperative Institute for Research in Environmental Sciences, and published in Science in March 2006, each year the Antarctic Ice Sheet is losing up to 36 cubic miles (150 km3) of its mass—a trend that scientists are

The Antarctic Ice Sheet is the single largest mass of ice on Earth. Scientists from many countries visit Antarctica every year to study the continent's ice cores in an effort to understand the Earth's past and in hopes of better understanding the Earth's climate today and in the future. (NOAA)

attributing to global warming. The pair used data from NASA's Gravity Recovery and Climate Experiment (GRACE) twin satellites to conduct their analysis. As Velicogna has noted, "The ice sheet is losing mass at a significant rate. It's a good indicator of how the climate is changing. It tells us we have to pay attention."

Richard Alley, a glaciologist from Pennsylvania State University, has commented: "It looks like the ice sheets are ahead of schedule in terms of melting. That's a wake-up call. We better figure out what's going on." If this trend continues, the global sea level could rise significantly over the next several hundred years. At present, there is already so much water melting from the ice sheet and flowing into the ocean, that it has caught the attention of scientists from many countries around the world. As a comparison, each year as much meltwater flows off Antarctica into the ocean as the entire population of the United States uses in just three months. This enormous amount of water is causing the global sea level to rise 0.02 inch (0.05 cm) a year.

Some have compared Antarctica to a "slumbering giant that is being awakened." For thousands of years since the Ice Age, the ice sheet remained stable, having thinned only 985-1,150 feet (300-350 m) in the past 13,000 years. The retreat of the ice sheet during this period of time was caused by rising sea levels, not by changes in precipitation or temperature. Today's global warming poses a new situation where dramatic rises in sea level could occur and the world's ice sheets could be in jeopardy.

When the ice streams of Antarctica reach the coast and push out across the ocean, they have to flow over rocky, rough terrain. When they do this, they attach themselves to the irregular rocks along the edge of the landmass, which tend to behave like a hinge. Once the ice is "anchored," the ice continues to grow outward onto the open water. This process is what creates the huge ice shelves that are associated with Antarctica.

Currently, ice shelves cover over half of Antarctica's coastline. In fact, the ice shelves there are so significant that they equal an area about one-tenth the size of the Antarctic continent. Some of the best-known shelves are the Ross Ice Shelf and the Larsen A and B Ice Shelves. The largest is the Ross Ice Shelf in West Antarctica. It covers an area larger than the size of California and is fed by more than seven ice streams.

The ice shelves behave like a floating dock; they float up and down with the tides. As they do, they grind and grate against the rocks, slowly cracking the ice apart, forming crevasses. Because of this constant destructive process, pieces of ice shelves are constantly breaking off—calving—into icebergs. Most calving takes place during the brief period of seasonal warming. Because ice shelves are already floating in the water, they do not raise sea level when they break off. As they melt, however, sea level not only begins to rise, but also ice flows faster to the ocean that was originally held back on the land, kept in place by the ice shelf blocking the way.

Generally, the icebergs are not extremely large, and usually, the total mass of the calvings is roughly equal to the amount of new snow that falls on the continent during the year. Over the past several thousand years, this process has kept the mass balance in equilibrium.

Unfortunately, for the past few years, this has not been the case. Due to global warming, the ice shelves on the Antarctic Peninsula have been

Ice shelves are attached to the continent's shore but float on the water. They can be extremely thick. When they calve icebergs, they do not contribute to sea-level rise as they are already floating in the water. (NOAA)

rapidly melting. Several of the major shelves have experienced significant collapse. The Wordie Ice Shelf, for example, collapsed in the late 1980s. The Prince Gustav and Larsen A Ice Shelves collapsed in 1995. At the end of the 1990s, scientists at the National Snow and Ice Data Center (NSIDC), NOAA, and the University of Colorado all predicted that the Larsen B, Wilkins, and George VI Ice Shelves were at the "point of no return" and that the Larsen B Ice Shelf would be the next to disintegrate. They determined that the Larsen B had lost its structural strength during the summer of 1998, when an iceberg 75 square miles (200 km2) in size broke off of it. The Larsen B is huge: Its surface area is 4,800 square miles (12,000 km2). Reaching a thickness of 722 feet (220 m), it has probably existed since the last ice age 12,000 years ago. It was predicted that when it disintegrated, it would dump more ice into the Southern Ocean than all of the icebergs calved between 1950 and 2000.

Predictions rang true in January 2002. Through the use of a highly specialized satellite imaging system called the Moderate Resolution Imaging Spectroradiometer (MODIS), the shattering of a huge piece of the shelf into the ocean was monitored. It sent thousands of icebergs adrift into the Weddell Sea. The shelf did not dissolve all at once, however; the process occurred over a 35-day period, January 31-March 5, 2002. In total, about 1,255 square miles (3,250 km2) of ice disintegrated. The volume of ice that broke free measured 720 billion tons (653 billion metric tons). According to NSIDC, over the past five years, the Larsen B shelf has lost 2,201 square miles (5,700 km2) of ice, leaving it presently with only about 40 percent of its original mass. This total loss represents an area larger than the state of Delaware; the loss in 2002 alone, larger than the state of Rhode Island.

This occurrence is significant. It is the largest single event in a series of ice shelf retreats in the Antarctic Peninsula over the past three decades. Climatologists at NSIDC, which is part of the Cooperative Institute for Research in Environmental Science at the University of Colorado at Boulder, claim the retreats are due to a significant warming of the South Pole climate. By their calculations, the region has warmed 0.8°F (0.5°C) each decade since 1940. Because of this, the extent of seven of the existing ice shelves have shrunk 5,212 square miles (13,500 km2) since the mid-1970s.

A large portion of the Larsen B Ice Shelf disintegrated in 2002, as evidenced in these MODIS satellite images from January 31, 2002 (upper left); February 17, 2002 (upper right); February 23, 2002 (lower left); and March 5, 2002 (lower right). Earth scientists predict that if global warming continues, incidences like this will become more common. (NSIDC)

These scientists believe there is a process in place that promotes the disintegration. They believe that melted pond water that sits on the ice's surface in the late summer contributes to fracturing the shelf because it percolates into the ice and fills the cracks. The weight of the water forces the ice apart. Researchers at NSIDC have been able to identify this process from satellite imagery.

The meltwater theory explains the disintegration of the Larsen B shelf breakup shown in the photos on page 65. In late January 2002 (summer in Antarctica), there was indeed extensive meltwater ponding on the shelf. That summer experienced unusually warm temperatures and an extended melting season. In February, satellite imagery showed several of the melt-water ponds disappear; scientists believed that they drained through open fractures into the ice. Once the fracturing reached a certain threshold, the ice sheet began breaking apart. The area of the shelf that was destroyed coincided exactly with the area that was covered by melt ponds in late January.

Scientists from 30 countries, including the United States, Britain, Taiwan, Russia, Finland, Ukraine, Japan, Argentina, Chile, Australia, Brazil, France, Italy, and Germany, conduct research in Antarctica, looking for answers to the delicate natural balance there and the effect global warming is having on the ecosystem. They are also looking for other mechanisms that contribute to ice breakup. Three other ideas have been introduced:

1. Meltwater that seeps between the ice crystals warms the shelf, where it then significantly reduces its strength.

2. Meltwater seeps into shallow cracks, expands the cracks as it refreezes during the winter, and slowly pries the ice apart.

3. Ocean warming and subice currents drag against the underside of the ice, then pull at the ice and weaken it.

Even though the melt of ice shelves does not threaten the rise of sea level, what is of concern is that the ice shelves act as a block to many inland glaciers and keep them from freely flowing into open water. If the ice shelves are gone, there is nothing to hold back the active glaciers. If they are not restrained and start advancing into the ocean and melting, this will contribute significantly to sea-level rise.

This, in fact, happened recently. In 2004 the ice floes that had fed the Larsen B Ice Shelf began moving faster toward the sea and started to thin. Since this occurrence, climate modelers, such as those at USGS and the NSIDC, are now putting this contingency into their prediction equations in order to increase the predictive accuracy of their models.

Scientists studying the glaciers on the ice sheet there today claim there is no longer a mass balance. Instead, there is a steady net loss of ice: More icebergs are calving into the ocean from rapidly moving ice streams than ice is being added to the continent via snowfall. While they are not ruling out the fact that the loss of ice mass is due to natural variations, they strongly support the theory that the main cause is anthropogenic (human caused).

Currently, the Ross Ice Shelf is only a few degrees too cool in the summer to experience a similar sequence of events, but scientists are keeping a close eye on it. Its dimensions are enormous. It spans an area 497 miles (800 km) across, encompassing 188,032 square miles (487,000 km2). Larger in size than the state of California, it is monitored closely as seasonal temperatures in the Southern Hemisphere continue to climb.

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