from Climate Alert Volume 8, No. 3 May-June 1995

Scientists Express Concern About How Stable the East and West Antarctic Ice Sheets May Be

If they partially collapse, how high will seas rise? What will be the effect on climate change?

A Special Report by Nancy C. Wilson

The calving of an immense iceberg in West Antarctica and potential changes in the East Antarctic Ice Sheet have focused attention on both sides of the ice-bound continent this year. A huge tabular iceberg broke off from the Larsen ice shelf in West Antarctica in January, and shortly afterward a 40-mile long crack opened in the adjoining shelf area. From the air, a large section of the ice shelf looked as if it was cracking into rubble. On the other side of the continent, speculation about how global change might affect the stability of the East Antarctic Ice Sheet led the National Science Foundation to sponsor an international meeting at Woods Hole Oceanographic Institution on what we know about the warmest period in recent geologic eras, the Pliocene, when temperatures may have been 5.5 ° to 11° C (10° to 20° F) higher than at present.

"The enormous volume of ice within the Antarctic Ice Sheet - about 30 million cubic kilometers (13.5 million square kilometers or 5.3 million square miles) - contains about 91 percent of the total volume of glacier ice on Earth. If it melted, it would raise the sea level about 65 meters (213 feet)," says Richard S. Williams, Jr., United States Geological Survey geologist. "Significant changes in the volume and dynamics of the ice sheet, therefore, would cause global environmental changes. In turn, changes in the temperature of the atmosphere and oceans can affect the volume and dynamics of the Antarctic Ice Sheet." Williams is senior editor of an ongoing project, the production of an 11-volume USGS Professional Paper, Satellite Image Atlas of Glaciers of the World, which will provide a baseline inventory of the areas of glacier ice on the planet, derived from images acquired by the Landsat series of satellites during the decade 1972-82. When completed, it will be possible to compare future satellite images with this data base to measure changes. Williams is also editor of the "Antarctica" volume of the Atlas; publication of the last volume of the professional paper is scheduled for late 1998.

The polar regions are particularly sensitive to global climate change, and some global circulation models (GCMs) predict that warming induced by an increase in atmospheric CO2 will affect the poles first and most strongly. Because Antarctica appears to play a key role in climate patterns around the world, Williams has emphasized the need to understand the effect of changes in the continent on the rest of the planet. Improving comprehension of the interrelationships - the processes governing Antarctica and those influencing the other continents and oceans - will help us assess indicators of global environmental change such as regional and global variation in glacier volume and area.

The average air temperature over the Antarctic Peninsula, a finger of land in the west jutting out toward South America, has risen 2.5° C in the last 50 years, much more rapidly than the rest of the world. The highly variable climate of the region is sensitive to the amount of winter sea ice, frozen sea water that forms around the Peninsula in the fall and which mostly melts in summer. Average annual temperatures have now increased to -3° C, plant life is exploding, vegetation is reported to have increased 25-fold. The iceberg which broke off from the Larsen ice shelf on the east side of the Peninsula in January is 37 by 77 kilometers and 183 meters thick, the size of Rhode Island or Luxembourg. Without intact ice shelves to cool them, winds blowing over Antarctica will be warmer than usual this year and carry more moisture.

Ice shelves and outlet glaciers, which move seaward on a decadal time scale, are floating extensions of a grounded ice sheet. They are composed of fresh water ice that originally fell as snow and are largely considered geographically permanent features. When an ice shelf or an outlet glacier extends far enough seaward to be affected by tides and currents, a tabular or irregular piece breaks off and floats away. While the calving process is usually part of a natural cycle, the calving of a large iceberg may be a sign of a more serious disintegration. It could take time before the implications of a change in ice-shelf area are fully understood, according to the British Antarctic Survey (BAS). In theory, the breaking up of ice shelves could be a first step toward diminution of the West Antarctic Ice Sheet. If the rate of calving increases substantially, this could be a signal of global warming. The striking warming of the Peninsula that has taken place since 1940 may be part of a natural fluctuation; it is also possible that human-induced global warming could be speeding up the process.

The chart below shows some of the many calving events in the last 25 years. In 1986, three huge icebergs, larger than this year's, broke loose into Antarctic seas. Two were grounded nearby in shallow water and are still there. The third drifted north, reaching the tropical latitudes before melting completely, scientists believe.

AREA AND ESTIMATED VOLUME OF SELECTED CALVING EVENTS IN ANTARCTICA

LOCATION	DATE	AREA (km2)	ESTIMATED VOLUME (km3
Trolltunga	Pre-1969	5,000	1,100
Wordie Ice Shelf	1974-1979	250	37.5
Larsen Ice Shelf	1986	11,225	2,250
Filchner Ice Shelf	1986	10,700	3,210
Thwaites Glacier Tongue	1986	1,600	?
Ross Ice Shelf	1987	5,508	1,650
Shirase Glacier	1973-1988	600	?
Larsen Ice Shelf	1995	2,849	521

Source: R.S.Williams, Jr., & J.G.Ferrigno, USGS

The West Antarctic Ice Sheet rests on a submerged volcanic archipelago about the size of the Philippine Islands, with great ice streams flowing rapidly across the two major ice shelves (Ross and Ronne-Filchner) toward the sea. If the above sea level, grounded part of the West Antarctic Ice Sheet were to melt or float, sea level would rise about 6 meters (nearly 20 feet) around the world, with devastating effects, particularly on the Earth's coasts and low-lying islands. (See Climate Alert, vol. 8 #2, an issue completely devoted to the possible consequences of sea level rise around the world).

Global warming could create an even more catastrophic scenario in East Antarctica, where a much larger ice sheet (the East Antarctic Ice Sheet), up to 4.8 kilometers (nearly 3 miles) thick, rests on a buried mountainous continent.

Scientists disagree on whether the East Antarctic Ice Sheet melted during the Pliocene - three to four million years ago. The implications are important for Global Circulation Models (GCMs) and for our ability to predict possible consequences of global environmental change: how much sea level might rise, what differences to anticipate in the world's climate if there were open water next to Antarctica.

Mark Kurz of WHOI convened a Pliocene Antarctic Glaciation Workshop in April 1995 to bring together the opposing views about the scientific evidence of conditions, particularly in the Sirius Group rocks, for or against ice sheet changes during the Pliocene. In an argument that has been going on for many years, the "stabilists," basing their position on geomorphology (a science dealing with relief features of the earth's surface), imply that there was no melting. They present evidence of enduring desert pavements in the Transantarctic Mountains, produced in a very cold, dry environment, that have been dated back about 10 million years and are very similar to conditions today. The "dynamicists" argue that evidence from fossils implies there was melting. Their evidence centers on whether diatoms and fossil wood, indicating a much warmer climate, coexist in the same deposit. The tiny marine diatom fossils give the age of the deposit because this species has been dated as belonging to the Pliocene era. If the diatoms are used to date a deposit, you have to exclude the possibility they were blown in from another area. (Some diatoms have been found in places where they could only have been blown in.)

Both sides of the controversy agreed at the meeting that it is important to drill a core in the Sirius Group rocks to determine whether the diatoms exist at great depth (and therefore are unlikely to have been windblown) or only at the very top or near cracks.They also agreed that some key locations, several near the American base at McMurdo, should be sampled, laying out a future research agenda.

Another question raised was how the Sirius Group came to be at such a high elevation. The diatom fossils found in the Sirius Formation are far above the level of any contemporary glaciers.Were they deposited near sea level and then rapidly uplifted or deposited in a shallow sea and then scraped off by an ice sheet and deposited elsewhere later?

Controversy at the workshop over global sea level changes may be resolved by new satellite altimetry from TOPEX-POSEIDON.

Despite the importance of the Antarctic Ice Sheet - and its tremendous mass - we are not yet certain whether it is growing or shrinking, Williams states. Thirty years of satellite data and intensive ground and airborne surveys are still not enough to yield firm conclusions, but they can show trends. (And we must keep in mind, Williams warns, that change may not come as a straight-line function; sudden drastic change in the cryosphere component of the Earth system happened before and could occur again.)

Data accumulated to date show that several ice shelves around the Antarctic Peninsula are shrinking; parts of the Larsen Ice Shelf are in an active state of disintegration. The BAS has reported a dense plume of ice fragments extending several hundred kilometers seaward. The Wordie Ice Shelf on the other side of the peninsula no longer exists, Williams reports.

Although many geological processes take centuries or even millennia to produce obvious changes, glaciers cause noticeable changes in a shorter time, responding to warming or cooling by growing or shrinking, causing sea level to rise or fall. If the ice locked up in Antarctica and Greenland together were to melt, world sea level would rise 75 meters (plus or minus 5 meters) or as much as 260 feet, Williams estimates.

While Antarctica and Greenland contain the largest glaciers, smaller ones respond much more quickly to climate change. Most smaller glaciers have been in recession for the last 100 years. (Some readvanced for a few years, and then in the 80s went into retreat again.) According to a study made 10 years ago by Mark Meier of the Institute for Arctic and Alpine Research (INSTAAR) of the University of Colorado, the mass losses from small glaciers are the most probable source of about one-third of the 10 - 20 cm sea level rise of the past century.

"Melting of the Greenland and Antarctic Ice Sheets does not appear to have been a factor in sea level rise over the past century, although the present state of mass balance of both ice sheets is unknown," according to a 1985 report by the National Research Council. (Changes in the mass balance result in an advance or retreat of a glacier's margin.)

Launching of Landsat 7 in 1997 will facilitate comparison of historic Landsat images with future information and help us monitor glacier changes. Unfortunately, systematic, repetitive data on long-term changes in glaciers on a global basis have not been gathered. And although Landsat data were originally made available to scientists and others at the cost of reproduction, the commercialization of the Landsat program has meant that data are no longer systematically and repetitively gathered, and the data are priced beyond the project budgets of most academic and governmental environmental change research scientists, according to Williams. Some early Landsat data have already been lost because of lack of funding for permanent storage.