What links the retreat of Jakobshavn Isbrae, Wilkins Ice Shelf and the Petermann Glacier?

Petermann Glacier is a much different glacier than the others mentioned above. Its velocity of 2-3 m/day (Higgins, 1990) is much lower than 10-30 m/day observed on the other marine terminating outlet glaciers. It is located on the northwest corner of Greenland and certainly experiences less melting and less snowfall. The lower 80 km (in length) and 1300 km2 (in area) of the glacier is afloat. This makes it (by area) the largest floating glacier in the Northern Hemisphere. The ice front is not impressive,unlike the faster outlet glaciers. The calving front protrudes a mere 5-10 m above sea level, reflecting the fact that the ice at the front is only 60-70 m thick. Further up-glacier, the ice at the grounding line is 600-700 m thick. The combination of velocity and thickness yield the volume of material calved each year. Petermann Glacier calves 0.6 km3 (Higgins, 1990), whereas Jakobshavns yields close to 40 km3. The thinning between the grounding line and the calving front is mainly via melting as the snowline is at 900 m. The low slope leads to very low velocities, giving the low-lying floating section plenty of time to melt, and surface melt ponds are common.

The Petermann Glacier lost a substantial area, 29 km2 due to calving this summer (Box 2008c), and a crack well back of the calving front indicates another 150 km2 is in danger. The volume of the ice lost is much less than that from the loss of a comparable area by Jakobshavn because the ice is an order of magnitude thinner. Once again the key to this glacier’s second major ice loss this decade after limited retreat in the last century, is thinning of the floating tongue, which weakens the glacier. The loss of this ice should then lead to acceleration, developing more crevassing and glacier retreat. The crack seen in the image of Petermann Glacier (ASTER image provided by Ian Howat of Ohio State) is more of a rift, like those on Larsen Ice Shelf, than a crevasse. This transverse rift is further connected to longitudinal-marginal rifts. Illustrating the poor connection of the Petermann Glacier to its margin and lack of a stabilizing force this margin has, even 15 km behind the calving front. This is not the only rift of its kind on the glacier. Also note that like on Larsen Ice Shelf the rift crosscuts surface streams.

A series of Landsat images from 2002, 2006 and 2007 illustrate the shift in the terminus and in the position of key rifts A, B and C. The distance back from the terminus has diminished for A and B from 2002 to 2007. In 2006 to 2007 the shift in the position of C is also evident.

As in the case on Jakobshavns, Helheim and others the key is the pre-conditioning phase of thinning, that leads to more calving, that leads to more acceleration, and that generates retreat. In a recent paper in press in the Journal of Glaciology Ian Howat and others examined changes in terminus position, surface elevation and flow on 32 glaciers along the southeast coast of Greenland from 200-2006. Their key conclusion was that the

… ratio of retreat to the along-flow stress-coupling length is proportional to the relative increase in speed, consistent with typical ice flow and sliding laws. This affirms that speedup results from loss of resistive stress at the front during retreat, which leads to along-flow stress transfer. Many retreats began with an increase in thinning rates near the front in the summer of 2003, a year of record high coastal-air and sea-surface temperatures.

This indicates again the importance of pre-conditioned thinning via melting.

Wilkins Ice Shelf (WIS) refused to hibernate this winter. A previous post noted that the recent collapse of Wordie Ice Shelf, Mueller Ice Shelf, Jones Ice Shelf, Larsen-A and Larsen-B Ice Shelf on the Antarctic Peninsula has made us aware of how dynamic ice shelf systems are.

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