Geotimes
Highlights
Glaciology
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.
