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Professor discovers process behind glacier disintegration

Glaciologist and Portland State geology professor Christina Hulbe was part of an international team of scientists who have reported on the dramatic disintegration of a 2,000 year old floating ice shelf in the eastern part of Antarctica, called Larsen B. Their work concerning the effect of melt-ponds on the break-up of glacial ice shelves has brought them widespread attention, beginning with a paper published in the Journal of Glaciology in December of 2000.

That paper was the result of years of work following the disintegration of Larsen Ice Shelf A in 1995, a similar event to that of Larsen B, and for Hulbe posed a serious problem. “In my work I focus on the movement of ice, and this raises the sort of questions you start to want to ask but cannot design in a field experiment,” Hulbe explains. “It’s difficult to understand exactly how things got there with fieldwork, and so one needs a physical model of the system, a computer simulation.”

“When Larsen A disintegrated, it was completely shocking. This was not the normal way that one thinks of ice shelves,” Hulbe recalls, “and with the physics that I understood, I could not understand why that happened.” Working with Ted Scambos of the National Snow and Ice Data Center in Boulder, Colorado, and Mark Fahnestock of the University of Maryland at College Park, she then began to use satellite remote sensing togather images, finding large amounts of melt-ponds showing up as dark spots on the surface, which was unusual. What this indicated, according to Hulbe, was that there was lots of melting on the ice shelf and that it had been going on for awhile.

Beyond this, however, what interested Hulbe and her colleagues was what this melt water was actually doing. She was most interested in finding out if there was indeed a correlation between the melt ponds and “catastrophic collapse” of ice shelves. What they found to be true was that the crevasses, or cracks, which occur normally on the glacier were becoming filled with water. Since water has a greater density than ice, the finite depths of the glacier were now being opened up even more, like a wedge, and the stresses at the tip of the crevasse were becoming greater.

While this type of possibility has always been recognized as physically true, for Hulbe and her colleagues it was a question of “assessing whether itwas a viable cause for these events, does the real physical system allow these physics to work.” Hulbe found that “numerical models were well-suited to answer these questions,” and so her team created models of the Larsen ice shelf systems, in an inverse study, where “the question was, what are the minimum set of criteria under which results would be attainable?”

Hulbe discovered that the results she was looking for, disintegration of an ice shelf, was “pretty easily attainable, with a crevasse depth of only 6 to 15 meters.” Working largely on the observations of Larsen A, a link was made to the climate in that they found summertime temperatures to be the climate indicator for ice shelf viability, not the annual mean temperature, as had been previously thought.

What this does, according to Hulbe, is “refine the understanding of processes in general. Now we can make predictions as well, depending on the summertime temperature. So if there are more warm summers, there may be more events like this.”

Though the amount of ice disintegrated from Larsen A and B is trivial in comparison to the total ice volume of Antarctica, consisting of less than one-thousandths of a percent, and it does not affect sea water level, it still covered as much surface area as the state of Rhode Island, and would bury Portland under 2 kilometers of ice. This was also uncharacteristic behavior that had not been observed before, and was coming after the warmest summer on record.

While there has been a 2 degrees Celsius warming on the planet since 1940, Hulbe acknowledged that on a larger geological time scale, the planet has been cooling since its hottest peak about 6,000 years ago. In these larger shifts in time, there is a variety of oscillation and it is difficult to say whether this current warming is natural or anthropogenic, or caused by humans, though Hulbe did warn that there is increased sentiment that it is due to the latter, since there is increased carbon dioxide in the atmosphere due to human activity, which causes solar radiation to be

trapped and increase warming.