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Bergs ahoy

image: Research headerEarly in his career, Douglas MacAyeal observed the movements of Antarctica's icebergs standing in subzero weather with his eye pressed against a cold surveyor's instrument called a theodolite. It was the only way for the geophysical sciences professor to measure gradual shifts in the Ross Ice Shelf, the continent's largest ice shelf, which extends from the Eastern Antarctic mainland into the southern Ross Sea.

Luckily for MacAyeal, advances in satellite and computer technology have now made it possible to track ice shelf activity from afar. On March 17, MacAyeal made the most significant discovery of his career while nestled in the cozy confines of his Chicago office.

Working with colleagues at the University of Wisconsin's Antarctic Meteorological Research Center, MacAyeal spotted something on satellite images that others had somehow missed: an iceberg the size of Connecticut had broken off the Ross Ice Shelf approximately 200 miles east of McMurdo Station, the National Science Foundation's Antarctic research station. The iceberg--named B15 because it is the 15th iceberg formed in what scientists refer to as Antarctica's "B region"--is the largest iceberg to break off, or "calve," since 1956, and stretches 22 miles across and 183 miles long. Breakoffs of this size occur only once every 50 or 100 years. Estimates put the 1956 berg at 207 miles long by 62 miles wide.

MacAyeal had been studying satellite images to track shifts in the Ross Ice Shelf since 1996. In particular, he had his eye on a major crack on the shelf's New Zealand side, accurately anticipating that the crack would lead to the calving of a new iceberg. Now B15, perhaps the biggest object ever to sail the seas, offers scientists a rare research opportunity.

Glaciologists have long believed the mechanical breakup of ice, and subsequent shoveling of that ice out to sea, influences global climate change because it reduces the size of the ice shelf. But until now, scientists haven't been able to determine what causes the ice to breakup, whether tides, currents, or ocean warming. "This particular berg is exciting and important because it's the first time we have the chance to study the process and begin our efforts to understand that process in the context of global change," says MacAyeal. "This event can teach us about possible future events that signal global change and the response to greenhouse warming."

MacAyeal, whose research is supported in part by the National Science Foundation, made his first of six trips to Antarctica as an undergraduate at Brown University. A term paper he wrote on the theory of ice ages led to a job as a field assistant in Antarctica measuring the ice flow of the Ross Ice Shelf. "I fell in love with the work and have been dealing with or associated with ice dynamics ever since," says MacAyeal, who went on to earn a Ph.D. in geophysical sciences from Princeton University.

In 1987 MacAyeal won the James Macelwane Medal for outstanding young investigator from the American Geophysical Union, and in 1998 he received the Richardson Medal of service from the International Glaciological Society. He now considers himself a glaciologist with interests in ice dynamics and paleoclimatology, or what caused the ice ages, and his recent discovery ties both interests neatly together.

Earlier MacAyeal built a computer model of Antarctica that simulates the tides, currents, and ice movements. MacAyeal now can use that model to simulate B15's path and predict how the ocean's tides will affect the berg. Already, the iceberg is ten miles out to sea and has caused, by repeatedly banging back into the Ross Ice Shelf, two other large icebergs to calve, B17 and B18, which MacAyeal predicted.

MacAyeal hopes to travel to Antarctica in the fall to outfit the iceberg with Global Position System units, which will track the iceberg's position. They should yield data on the tides and currents of Antarctica that will further help scientists understand global climate changes by tracking the berg's path and eventual melting.

MacAyeal says he expects to be studying B15 for the rest of his career, as the iceberg could be floating the seas, in smaller and smaller pieces, for the next 20 or 30 years. He also plans to pursue a related interest of his in a climate just as imposing: ice caps on Mars. --Molly Tschida

  JUNE 2000

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