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Pine Island Glacier hypothesis to emergent event

May 13, 2014 11:23 am0 comments

This Post is an update to one I wrote in 2009 for Realclimate. It is warranted given the increasing observations in papers by Rignot et al (2014), Favier et al (2014), and Mouginot et al (2014) that the Pine Island Glacier exiting into the Amundsen Sea in West Antarctica has entered a period of unstable retreat. Roald Amundsen won the race to the South Pole in 1911, the outlet glaciers in the sea named for him are winning the race back to the sea.

Dr. Mauri Pelto

Dr. Mauri Pelto

In science there are instances when a specific mechanism is understood and a hypothesis posed based on an understanding of the processes involved, prior to the initiation or observation of the those processes. An example from glaciology that we have watched develop was from the late John Mercer, Ohio State U., who argued that a major deglaciation of the West Antarctic Ice Sheet (WAIS) may be in progress within 50 years. This conclusion was based on the fact that the WAIS margin was ringed with stabilizing ice shelves, and that much of the ice sheet is grounded below sea level. The loss of ice shelves — Mercer proposed — would allow the ice sheet to thin, grounding lines to retreat and the ice sheet to disintegrate via calving much than by melting in place. Mercer further commented that the loss of ice shelves on the Antarctic Peninsula, as has now been observed at Wilkins, Gustav and Larsen A and B, would be an indicator that this process of ice sheet loss due to global warming was underway.

Mercer’s ideas led Terry Hughes (1981) (my doctoral advisor at U. of Maine) to propose that the WAIS had a “weak underbelly” in Pine Island Bay. This bay in the Amundsen Sea is where the Pine Island Glacier (PIG) and Thwaites Glacier (TG) reach the sea. These are the two most significant outlet glaciers draining the north side of the WAIS. Together they drain 20% of the WAIS. Hughes called this area the “weak underbelly” because these glaciers lack the huge ice shelves, such as Ross Ice Shelf and the Ronne-Filchner Ice Shelf in which most other large WAIS outlet glaciers terminate. The response of these glaciers he noted would lead to instability and collapse of WAIS. Both glaciers have a relatively rapid flow from the WAIS interior to the calving margin. Further the low surface slopes and smooth flow patterns of PIG suggested to Hughes that there was no indication of a landward rise in the elevation of the glacier bed; such a rise would help stabilize the glacier. Without a rise in the bed, glacier thinning and retreat could result in continual grounding line retreat. The grounding line is where the bottom of the glacier comes in contact with the ground below the ice sheet, in this case the sea bottom. The grounding line is an anchoring point for the outlet glaciers. The length of the glacier that is grounded is both slowed and stabilized by resulting basal friction. Beyond the grounding line toward the margin, the floating ice shelf is susceptible to rapid calving retreat and as the grounding line retreats, so would the calving front. The floating portion is also susceptible to basal melt, much more powerful in this region than surface melt. Hence, in the winter of 1985 I found myself plotting the bathymetry in font of Pine Island Glacier for Hughes and Tom Kellogg.

It is true that PIG calved a large iceberg nearly 700 square kilometers, during the fall of 2013, this is not key to our story, though it did receive lots of attention. Instead the story is what is happening at the base of this glacier that is leading to profound changes in its behavior. The story is as glaciologists predicted there would be thinning, grounding line retreat, terminus retreat, acceleration and increasing mass loss. PIG and TG have enough ice volume to raise sea level by 1 meter.

NASA MODIS from November 2013 indicating break off of iceberg from Pine Island Glacier.

NASA MODIS from November 2013 indicating break off of iceberg from Pine Island Glacier.


Note in the first image below that the situation is even less stable than Hughes speculated. The grounding line is at a higher elevation than the bed of the glacier for the next 200 km inland of this grounding line. The deeper the basin, the thicker the ice must be to maintain grounding. This makes it tough to slow grounding line retreat once it begins in a deepening basin.



Basal topography profile of Pine Island Glacier, inland is to the left. (from Shepherd et al., 2001)


In this next image from Vaughan et al (2006) the scope of the trench, blue penetrating under the ice sheet, is evident.


Basal topography of Pine Island Glacier region (from Vaughan et al, 2006)


The weak underbelly idea was forgotten for some time. While I was attending a conference on rapid glacier flow in Vancouver BC in 1986, data were presented that showed no acceleration of Pine Island Glacier. This was further noted for the entire 1970′s to early 1990′s period by Lucchita and others (1995). Then, in 1998, Rignot (1998) used satellite imagery to identify that the grounding line of Pine Island Glacier had retreated 5 km from 1992 to 1996. In the same year, Wingham and others (1998) observed a 10 cm per year thinning in the drainage basins for Thwaites and PIG during the 1990′s. Shepherd and others (2001) noted thinning in the fast flow areas of the glacier of 1.6 m/year between 1992 and 1999. This led them to conclude that the observed inland thinning and acceleration of PIG was a response to enhanced glacier bed lubrication. Not from surface melting of course as there is next to none on this glacier. Rignot and others (2002) noted that the glacier had accelerated 18% over a 150 km long section of the glacier in the fast flow area between 1992 and 2000. Change was afoot: after 50 years of apparent stability, the glacier calving front was retreating, and the grounding line was retreating indicating reduced bedrock anchoring. The reduction in basal friction would then lead to faster flow and more thinning. Was this just a short-term increase?

This is when the scientific community which had been keeping a careful watch on the situation at PIG intensified their research efforts. This work was led by NASA and the British Antarctic Survey (BAS). In 2006 and 2007, instruments were placed directly on PIG for the first time by the BAS. Four GPS receivers monitored ice flow from 55 to 171 km inland of the calving front at the center of the glacier (Scott and others, 2009). Glacier velocities had been noted at each site in 1996; by 2007 the respective increase in velocity was 42%, 36%, 34% and 26% respectively, an approximately 2 to 3% annual increase. The extent of the fast flowing portion of PIG is seen in the Figure 3. A separate data set, radar based was used by Rignot (2008) to identify a 42% acceleration of PIG between 1996 and 2007 accompanied by most of its ice plain becoming ungrounded. Warner and Roberts (2013; Figure 4) provide an update in Figure 3 below indicating the acceleration continued with 2009-2011 having the highest velocities at both transects on PIG. The northing transect (top) is approximately 25 km inland from the calving front across the glacier. The easting transect runs up the middle of the glacier. In each the acceleration has been from close to 3000 m/year in 2001-2002 to 4000+m/year 2009-2011.

PIG_TG- velocity

Figure 3 Velocity map of Pine Island and Thwaites Glaciers, (Rignot, 2008)


pig velocity profile

Figure 4. From Warner and Roberts (2013) velocity on a transverse and longitudinal profile from 2002 to 2011.


Scott and others (2009) pointed out that the greater thinning toward the grounding line and terminus increased the surface slope and the gravitational driving stress, further promoting acceleration. Wingham and others (2009) reported that the 5400 km2 central trunk of the glacier had experienced a quadrupling in the average rate of volume loss quadrupled from 2.6 km3 a year in 1995 to 10.1 km3 a year in 2006. PIG had an annual volume flux at the front of 28 km3 a year, so this increase is a marked change. Their observations were that the region of lightly grounded ice at the glacier terminus is extending upstream, and the changes inland are consistent with the effects of a prolonged disturbance to the ice flow, such as the effects of ocean-driven melting. Further examination of the bed topography by Vaughan and others (2006) indicates that most of the bed of the drainage basin of PIG is more than 500 meters below sea level, and there is a particularly deep basin in the eastern section of the upper basin. The observed acceleration, retreat of the grounding line, thinning of the lower section of the glacier and the observed elevation of the basal topography provide no indication that this is not a weak underbelly of WAIS. Pine Island Glacier retreated 31 km at its center, with most retreat in 2005-2009 when the glacier un-grounded from its ice plain (Rignot et al 2014). This will lead to continued grounding line retreat. Pritchard et al (2012) noted the exceptional nature of the thinning of PIG and several other glaciers ending in the Amundsen Sea, averaging nearly 1 m/year across the ice tongue of PIG. This thinning will also lead to less pinning on the margins, which MacGregor et al (2012) pointed out is an important stabilizer along with the grounding line across the main stem of the glacier. The result of the thinning measured from above was seen below, Jenkins et al (2010) using an autosub found that the glacier was lifting off the buttressing sub=glacial ridge that it had been grounded upon, which also lets more warm water underneath. Depoorter et al (2014) noted that PIG loses over 100 Gigatons per year to basal melt, and with both warmer water and an expanded exposed base, this number will only increase. Favier et al (2014) in their Figure 1 (Figure 5) here provide a view of velocity and potential grounding line change. That led them to discuss the irreversible nature of the retreat. Favier et al (2014) in their modelling of irreversible retreat note that the loss in glacier mass should increase 400-500%.


pig iceshed

Figure 5. From Favier et al (2014) a look at grounding line retreat into the basin.


pig_tongue copy

Figure 6. Grounding line from above in 2013, notice the rifting that is a weakness that results in calving (Pelto, 2013).



Figure 7. Grounding line from NASA interferoetommetry. Thinning of the ice tongue due to retreat leads to the unpinning of the glacier not just in the middle but also less pinning on the margins.


pid height change

Figure 8. Change in surface elevation resulting from thinning in the Amundsen Sea region, Pritchard et al (2012).


I wrote in 2009 that the evidence does indicate that one of the basic underlying principles, proposed by Mercer and Hughes, of what can stabilize or destabilize WAIS was right on the money. The evidence reviewed does not fully confirm the weak underbelly hypothesis, but it provides enough evidence that we had best monitor the situation and expand our attempts to understand it. That is just what the glaciological and scientific community did. A number of projects from the British Antarctic Survey, NASA and NSF have continued to intensify in the area. I mention B.Bindshacler, C.Doake, D. Holland, T. Hughes, S. Jacobs, A. Jenkins, E.Rignot, A. Shepard, R. Thomas, D. Vaughan, R. to name a few who have worked collectively to answer what is happening. This would not have happened without cooperative foresight. They have landed upon, drilled through, sent autosubs underneath, targeted satellites and plane overflights, all to understand what is happening in this key location on our planet. This is not the end of the scientific research in the region, for example instruments have been lowered through boreholes drilled in PIG to measure currents and melting beneath the ice sheet. Nearby Thwaites Glacier is equally important and has its own story.


Bindschadler, R.A., History of lower Pine Island Glacier, West Antarctica, from Landsat imagery, Journal of Glaciology, 48 (163), 536-544, 2002.
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Favier, L. et al., (2014). Retreat of Pine Island Glacier controlled by marine ice-sheet instability,
Nature Climate Change 4 117-121.
Hughes T. (1981). “The weak underbelly of the West Antarctic Ice Sheet”. Journal of Glaciology 27: 518-525.
Luchitta, B., Rosanova, C., and Mullins, K. (1995). Velocities of Pine Island Glacier, West Antarctica. Annals of Glaciology, 21, 277-283.
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Editor’s note: The original featured image of the iceberg break off of Pine Island Glacier was accidentally labeled “2014” in the original version of this post. The break off occurred in November 2013 and the date of the image was corrected. Thank you Dave for the spot.  


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