Terrence J. Hughes

In June, the International Glaciological Society conducted an International Symposium on Fast Glacier Flow in Yakutat, Alaska. Of the many scientific presentations, one that stands out was given by H. Jay Zwally of NASA, who, with colleagues, found that diurnal changes in summer melting rates in the ablation zone just north of Jakobshavns Isbrae, the fastest ice stream draining the Greenland Ice Sheet, correlated with an immediate increase in ice velocity. Their findings indicate that at least some surface meltwater rapidly found its way to the bed and increased the rate of basal sliding. This correlation has profound consequences about the response of ice sheets, past and present, to climate forcing. The idea that the Greenland Ice Sheet was responding slowly is no longer credible, opening the possibility that ice sheets have a rapid response to slow climate warming, beyond a threshold that allows quantities of surface meltwater to reach the bed.

This prospect was dramatically reinforced later in 2002, when NASA satellite imagery showed that the floating terminus of Jakobshanvs Isbrae was breaking apart. Also, William Krabill, along with other NASA scientists, found the breakup correlated with rapid increases in ice velocity and surface lowering in the Jakobshavns ice drainage system over distances up to 100 kilometers upstream from the calving front.

Earlier in 2002, Larsen B Ice Shelf on the east side of the Antarctic Peninsula disintegrated completely: 3,370 square kilometers within a few days in February and March. At the fall workshop of the West Antarctic Ice Sheet Initiative (WAIS), held in Virginia, Douglas MacAyeal of the University of Chicago gave a laboratory demonstration of how the catastrophic disintegration took place. He and colleagues Ted Scambos, Christina Hulbe and Mark Fahnestock also produced a theoretical model of disintegration, in which ice slabs separated by crevasses tipped over and wedged apart neighboring slabs until their vertical alignment became horizontal, as observed in satellite imagery. Larsen A Ice Shelf had undergone a similar rapid disintegration in January of 1995. Larsen C Ice Shelf, the furthest south portion, should be next. At the same time, satellite images of ice streams supplying the Larsen Ice Shelf have been showing widespread evidence of rapid velocity increase and surface lowering, having been freed from buttressing by the ice shelf, as reported by Hernan De Angelis and Pedro Skvarca of the Instituto Antartico in Argentina (Science, March 7, 2003).

Eric Rignot of CalTech's Jet Propulsion Laboratory and Stanley Jacobs of Columbia University's Lamont-Doherty Earth Observatory linked these changes along the Antarctic Peninsula to rapid bottom melting below all the ice shelves surrounding Antarctica, wherever measurements were made (Science, June 14, 2002). This linkage raises the possibility that a catastrophic breakup of these ice shelves — similar to the breakup of Larsen B Ice Shelf — might also take place. Concurrently, a rapid upstream increase of ice velocity and surface lowering in the ice streams that supply the ice shelves could occur, once ice-shelf buttressing is reduced or removed entirely.

Substantial surface lowering is already taking place in West Antarctica even without removal of buttressing ice shelves. H. Jay Zwally and 15 of his NASA and other colleagues reported the capability for high-precision laser measurements of ice elevations in Greenland and Antarctica from the Ice, Cloud, and Land Elevation Satellite (ICESat).

They presented do you know where they presented these results? results from earlier elevations mapped by the European Remote Sensing (ERS) satellites showing surface lowering rates of 30 centimeters per year at the calving fronts of Pine Island Glacier and Thwaites Glacier, with ice-lowering rates decreasing upstream but affecting the whole ice drainage basin for these two ice streams. This drainage basin is one-third of the remaining grounded part of the West Antarctic Ice Sheet, a mass of ice that, if melted, would add 6 meters to sea level (H. Jay Zwally et al., Journal of Geodynamics, vol. 34, p. 405-445, 2002).

Honoring glaciologists

During the June International Symposium on Fast Glacier Flow in Yakutat, Alaska, the International Glaciological Society honored several with its highest award, the Seligman Crystal: Garry Clarke (University of British Columbia), past president of the Society, and Geoffrey Boulton (University of Edinburgh), the first glacial geologist to receive the Crystal. Also present were five former awardees Barclay Kamb (USA, 1977), Johannes Weertman (USA, 1983), Mark Meier (USA, 1985), Charles Raymond (1999, USA) and Hans Rothlisberger (Switzerland, 1992).

In 1970, British glaciologists named the West Antarctic ice streams that enter the Ross Ice Shelf, giving them the titles A through E (adding Ice Stream F in 1972).
As a fitting close to a decade of glaciological field research on these ice streams, and as part of the National Science Foundation's (NSF) West Antarctic Ice Sheet Initiative (WAIS), the U.S. Advisory Committee on Antarctic Names (ACAN) (upon the urging of Julie Palais at NSF) recommended renaming the ice streams. The new names honor John Mercer (Ice Stream A), Kees van der Veen (tributary B 1 of Whillans Ice Stream, formerly Ice Stream B), Barclay Kamb (Ice Stream C), Robert Bindschadler (Ice Stream D), Douglas MacAyeal (Ice Stream E), and Keith Echelmeyer (Ice Stream F). The intervening ice ridges were renamed in honor of Howard Conway (Ridge AB), Hermann Engelhardt (Ridge BC), Charles Raymond (Ridge CD), Sion Shabtaie (Ridge DE), and William Harrison (Ridge EF).

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Hughes is a professor of geological sciences and Quaternary studies in the Department of Geological Sciences and Climate Change Institute at the University of Maine in Orono, Maine.

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