What links the retreat of Jakobshavn Isbrae, Wilkins Ice Shelf and the Petermann Glacier?
Guest commentary from Mauri Pelto
Changes occurring in marine terminating outlet glaciers of the Greenland Ice Sheet and ice shelves fringing the Antarctic Peninsula have altered our sense of the possible rate of response of large ice sheet-ice shelf systems. There is a shared mechanism at work that has emerged from the detailed observations of a number of researchers, that is the key to the onset and progression of the ice retreat. This mechanism is shared despite the vastly different nature of the environments of Jakobshavns Isbrae, Wilkins Ice Shelf and the Petermann Glacier.
We reviewed in a previous post the first mechanism for explaining the change in velocity of Greenland’s large outlet glacier - the Zwally effect - and why it is not the key. This mechanism relies on meltwater reaching the glacier base via moulins and reducing the friction at the base of the glacier. This idea was observed to be the cause of a brief seasonal acceleration of 10- 20 % on the Jakobshavns Glacier in 1998 and 1999 at Swiss Camp 35 km inland from the calving front (Zwally et al., 2002). Examination of recent rapid supraglacial (i.e. on the surface) lake drainage documented short term velocity changes due to such events around 10%, but little significance to the annual flow of the large glaciers outlet glaciers (Das et.al, 2008).
The second mechanism is a dynamic thinning of the terminus zone of the marine terminating outlet glacier reducing the effective bed pressure, allowing acceleration - the Jakobshavn effect. The reduced resistive force at the calving front due to the thinner ice, now experiencing greater flotation, is then propagated "up glacier" (Hughes, 1986; Thomas, 2003 and 2004). If the Jakobshavn effect is the key the velocity increase will propagate up-glacier, there will be no seasonal cycle, and thinning and acceleration would be greatest near the terminus.
That the thinning and acceleration is greatest for marine terminating outlet glaciers has indeed been demonstrated by Sole et. al. (2008). That acceleration began at the calving front and spread upglacier 20 km in 1997 and up to 55 km inland by 2003 (Joughin et al., 2004). On Helheim the thinning and velocity propagated up-glacier from the calving front. Each of the glaciers fronts did respond to tidal variations indicating they had started floating, detached from their bed (Hamilton et al, 2006). This summer, Jason Box and others at Ohio State University observed that Jakobhavns Isbrae retreated again, losing 15 km2, and maintaining an accelerated pace from the northern branch of the ice stream as opposed to the greater retreat and acceleration of the southern branch 2001-2005 (Box, 2008). This was accompanied by the second consecutive year of substantial retreat of the glacier just north of Jakobshavn, Sermeq Avannarleq which had been quite stable for much of the last century (Box , 2008b). Sole et. al. (2008) also noted that the recent thinning and acceleration was not limited to just the now more famous Helheim, Jakobshavn and Kangderlugssuaq Glaciers, but included Rinks Isbrae, Equaluit, Cristian IV and all others they observed. Note the greater flow of the southern ice stream in 2000, compare to the northern ice stream in this image from Ian Joughin:
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.
[Much more to be read at the link below!]
Link to the realclimate post: http://www.realclimate.org/index.php/archives/2008/10/what-links-the-retreat-of-jakobshavn-isbrae-wilkins-ice-shelf-and-the-petermann-glacier/
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