Guest post by Mauri Pelto
It is popularly understood that glaciologists consider West Antarctica the biggest source of uncertainty in sea level projections. The base of the 3000-m thick West Antarctic Ice Sheet (WAIS) – unlike the much larger East Antarctic Ice Sheet – lies below sea level, and it has been recognized for a long time that this means it has the potential to change very rapidly. Most of the grounded West Antarctic ice sheet drains into the floating Ross and Ronne-Filchner ice shelves, but a significant fraction also drains into the much smaller Pine Island Glacier. Glaciologists are paying very close attention to Pine Island Glacier (“PIG” on map, right) and nearby Thwaites Glacier. In the following guest post, Mauri Pelto explains why.
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 excellent example is the determination by Molina and Rowland (1974) that CFC’s will lead to losses in stratospheric ozone. The full truth of their understanding of the process was not revealed until the Antarctic ozone hole was reported in 1985 by Farman et al.
A different example, from the same time period, was the 1978 publication by 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. This is a much faster means of losing mass than melting in place. Mercer further commented that the loss of ice shelves on the Antarctic Peninsula, as has since been observed, 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 reach the sea. These are the only two 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 really huge ice shelves Ross Ice Shelf and the Ronne-Filchner Ice Shelf in which most other large WAIS outlet glaciers terminate. 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. Note in the image below that the situation is even less stable than Hughes speculated. The current grounding line is at a higher elevation than the bed of the glacier for the next 200 km inland of this grounding line. (Note, inland is to the left in the figure, below.) 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 (from Shepherd et al., 2001)
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?
In 2006 and 2007, instruments were placed directly on PIG for the first time by the British Antarctic Survey. 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 increase from 2006 to 2007 was 6.4% at 55 km from the terminus and 4.1% at 171 km inland. The extent of the fast flowing portion of PIG is seen in the figure below. 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.
Velocity map of Pine Island and Thwaites Glaciers. Rignot, 2008
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. Then 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.
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 are doing. A number of projects from the British Antarctic Survey, NASA and NSF will continue to expand the research in the area. In January 2008 Robert Bindschadler (NASA) landed on the floating ice shelf of PIG. They found the situation hazardous for plane landing but did leave behind several instruments. NSF has decided to fund establishment of a helicopter camp to safely study the ice-ocean interaction during the 2010-11 summer field season in Antarctica. In 2009 a team of British and American scientists deployed an autonomous robot submarine on six missions beneath the PIG ice shelf using sonar scanners to map the seabed and the ice shelf bottom. This fall NASA’s Operation Ice Bridge has focused much of its energy on the Pine Island Glacier. Seelye Martin of the University of Washington notes that “Pine Island Glacier is a major focus for our mission. We have four flights planned for this glacier. One of our hopes with these flights is to understand the detailed topography under the floating ice tongue. That topography controls the rate of melting there.”
Basal topography of Pine Island Glacier region (from Vaughan et al, 2006).
References
Bindschadler, R.A., History of lower Pine Island Glacier, West Antarctica, from Landsat imagery, Journal of Glaciology, 48 (163), 536-544, 2002.
Farman, J., B. G. Gardiner and J. D. Shanklin, (1985). Large losses of ozone in Antarctica reveal seasonal ClOx/NOx interaction, Nature, 315, 207-210.
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.
Molina, M.J. and F. S. Rowland (1974). Stratospheric sink for chlorofluoromethanes: Chlorine atom-catalysed destruction of ozone, Nature, 249, 810-812.
Rignot E (2008). “Changes in West Antarctic ice stream dynamics observed with ALOS PALSAR data”. Geophys. Res. Lett. 35: L12505. doi:10.1029/2008GL033365.
Rignot, E.J. (1998). Fast recession of a West Antarctic Glacier, Science, 281, 549-551.
Rignot, E.J., D.G. Vaughan, M. Schmeltz, T. Dupont, and D.R. MacAyeal (2002). Acceleration of Pine Island and Thwaites Glacier, West Antarctica, Annals of Glaciology, 34, 189-194.
Scott J.B.T., Gudmundsson G.H., Smith A.M., Bingham R.G., Pritchard H.D., Vaughan D.G. (2009). “Increased rate of acceleration on Pine Island Glacier strongly coupled to changes in gravitational driving stress”. The Cryosphere 3: 125-131. http://www.the-cryosphere.net/3/125/2009/tc-3-125-2009.html.
Shepherd A., Wingham D.J., Mansley J.A.D., Corr H.F.J. (2001). “Inland thinning of Pine Island Glacier, West Antarctica”. Science 291: 862-864. doi:10.1126/science.291.5505.862.
Vaughan D.G., Corr H.F.J., Ferraccioli F., Frearson N., O’Hare A., Mach D., Holt J.W., Blankenship, D.D., Morse, D.L., Young, D.A. (2006). “New boundary conditions for the West Antarctic ice sheet: Subglacial topography beneath Pine Island Glacier”. Geophysical Research Letters 33: L09501. doi:10.1029/2005GL025588.
Wingham D.J., Wallis D.W., Shepherd A. (2009). “The spatial and temporal evolution of Pine Island Glacier thinning, 1995 – 2006”. Geophysical Research Letters 36. doi:10.1029/2009GL039126.
#46, Jim Fassa–
Start with the “Science Links” on the RC sidebar, to the right of your page.
The very first one, “AIP: The Discovery of Global Warming,” is universally recommended. It’s got a ton of information, organized historically, but with hyperlinking that lets you go deeper, or pursue an interesting point, and is well-written and well-researched.
There are also links to the IPCC reports, and the Summary for Policy Makers is succinct and clear. That might be another one to try early on.
#41- The bed beneath the main trunk of PIG in the basin upstream of the grounding line is thought to be water saturated till. This provides a different hydrology than a conduit system. It is more distributed. In terms of greater accumulation in the upper basin driving greater movement. This could happen on GIS but has not, the changes in thickness at the summit have been minor compacted to ice sheet thickness and such a change would take time. Why is this not the scenario here? Again look where the thinning and acceleration are most pronounced, at the terminus. Where did the process first become apparent, near the terminus. This suggests a downstream not an upstream mechanism driving this change. In addition the upglacier area would have to thicken appreciably before any acceleration occurs. This would be visible in the altimetry record.
Followed up on the Columbia Glacier references. Posting the links I found in case other amateurs are not already familiar with that situation.
http://ak.water.usgs.gov/glaciology/columbia/index.htm
in particular
http://ak.water.usgs.gov/glaciology/columbia/reports/fs%20091-96/FS-091-96%20Columbia%20Glacier%20Alaska.pdf
Figure 9. Changes in the surface elevation and bed profile of Columbia Glacier.
Looks like a really good example, albeit on a smaller scale than the PIG.
Hit the Submit button too fast, the relevant quotation is,
“Research at Columbia and other
Alaska glaciers can be applied to the Antarctic.
where most of the world’s ice exists.
The West Antarctic Ice Sheet is also a tidewater
glacier, but more than a thousand
times the size of Columbia Glacier. A similar
instability may exist there, and scientists
say a rapid retreat could cause a rise of
sea level of several meters in a few centuries.”
Question: while the heat input from local volcanism is currently negligible (am I understanding this right?) would that change if there was some major activity?
Also: would that major increase in volcanic activity (if there were one) be something that, under ordinary circumstances would be negligible, but because of the human-induced warming have a greater effect?
Another way to put it: minus AGW, the sub-ice-sheet volcanoes would not matter. With it, do they matter now?
My impression, given what has been posted here, is possibly no. But intuitively I was thinking that if you had a set of eruptions — basically if we were all really, really unlucky — then you could have a more serious problem.
And how much of the ice sheet has to go before Florida is underwater? At what point do we get the “tipping” where it becomes catastrophic? That is, should we be building very big dikes around New York right now?
Sorry if this seems obvious, I just want t make sure I am understanding what I am reading.
[Response: There’s no question that volcanic eruptions under the ice could make a difference, but they’d have to be in the right place, and they aren’t. And keep in mind that volcanoes erupt for a while and then stop. In contrast, the warm water that is under the floating ice (ice shelf) is a persistent source of heat under just the right place.
As for tipping points, I personally think this term is greatly overused, and is inappropriate in most place (Arctic sea ice for example). But the tidewater glacier situation is one place where it is entirely appropriate, and where a tipping point can be well defined, and in two different ways. On the one hand, continued melting from below — or increased melting from above (this isn’t imminent, but will happen at some point in the next century or two) — will eventually lead to a tipping point in time where rapid retreat begins, regardless of what ocean or air temperatures do from that point on. On the other hand, the point at which the grounding line retreats from its current high ground is a clearly-defined physical location — hence another tipping point.–eric]
Jesse:
http://www.google.com/search?q=how+much+of+the+ice+sheet+has+to+go+before+Florida+is+underwater%3F
> At what point do we get the “tipping”
There’s no single point. Think “slippery slope.”
> should we be building very big dikes around New York right now?
No.
[Response: I’m not sure that we should be building dikes around New York, but in point of NYC has plan to address sustainability, and the threat from sea level rise is part of the things being considered seriously. Some might call this a response to lefty brainwashing, but I think it remarkably forward thinking.–eric]
Chris G good point with respect to the Columbia Glacier. This is a much different type of glacier. However, the thinking in terms of why it was going to retreat significantly parallels that of PIG. It terminated on a shallow shoal, really mostly an island. Behind this was a deep water basin. If it retreated into this basin the glacier would begin to calve fueling a calving retreat. The shallow point at the terminus was a pinning point of stability. Similarly if PIG grounding line retreats into the deeper basin, it is hard to imagine a stable position during a resultant calving retreat, in that deep basin stretch. The terminus of PIG is not the key it is the grounding line location.
Chas Webster #36: Thank you so much for keeping us up to date on George Will. We now know that, given the evidence that each decade is warmer than the last, the obvious conclusion is that we’re cooling. This may well continue for decades. And (about equally important, measured by the space devoted to it) no worries about climate, ‘cuz sometimes people like to get too worked up about sharks.
I’m feeling much better now.
Thank you Eric for the pointer to New York’s plan — much smarter than “building very big dikes … right now” — reducing fossil fuel use; increasing efficiency; revising flood plain maps, insurance and building codes. Smart.
#52 Mauri, thank you for your patient explanations.
From Shepherd et al., 2001
“The glacier develops a driving stress in excess of 100 kPa in surmounting the bedrock trough, and the associated upstream thickening appears to determine the location of the present grounding line.”
Since the trough is impeding the outward flow of the glacier, doesn’t this suggest that if the grounding line retreats into the trough, the glacier mass will no longer be impeded, and the driving stress which would previously build up would now be able to move the ice forward, and so tend to offset the retreat of the grounding line?
Terran: You are correct a reduction in driving stress would occur, if the grounding line retreats off of the current higher terrain. That is what causes the acceleration, less driving stress holding the ice back. This acceleration is what we are talking about, less friction, less buttressing-less braking. This acceleration in our experience leads to more calving, more retreat and more acceleration. It does not in general lead to a readvance to a point of stability.
Take a look at the image at http://pigiceshelf.nasa.gov/img/landsat_pig.jpg
Notice how the surface smooths out beyond the green line-grounding line-on the floating section, where there is no be interaction to generate surface roughness. Also notice at the top of this image the grounding line swings closer to the terminus. Note the substantial rifting that is apparent at the grounding lines closest point to the ice front. These rifts are zones of weakness as has been noted on Wilkins and Larsen Ice Shelf. If these rifts are enhanced that is a warning sign. Rignot (2002) indicated that they were increasing in this area. This is a 2001 image and the large rift beyond this that spread across the glacier in 35 days or so, led to an iceberg calving event.
To Jim Fassa — take a look at my article, published last summer. A copy of it is on my web site at
http://www.burgy.50megs.com/climate.htm
re: 50 of Kevin McKinney
The graphs in the pdf you referenced show many almost “instant” collapses of the WAIS across 5 million years, but the graphs aren’t fine grained enough to say if “instant” is ~10,000 years or less. Still, if “instant” is even 1000 years, the year to year increase in melting for low-lying coastal regions will be dramatic.
Re #46
Jim you should look at the links in the ‘start here’ section of the website (see menu at the top).
Another image, after the event Mauri points to above:
http://earthobservatory.nasa.gov/IOTD/view.php?id=8384
With respect to Mauri #57, Terran #60, and Mauri #61:
First, thanks!
On the grounding line, I would imagine it is an indicator of where the balance of forces lies (with some offset for the viscosity of ice) rather than being a key or event driver, if that is what Mauri meant.
Terran, as I imagine it, there are huge driving forces on a column of ice a thousand meters high. These forces would be high enough to deform the lower ice faster than it could melt wherever the forces were not in equilibrium. So, as the leading edge melts, this deformation and filling would be first observed as a thinning of the sheet and, in particular, the trunk or main ice stream channel, as well as an acceleration of the stream. (Which is what we are seeing now.) I don’t see the GL moving until after any previously built up driving force (height of unsupported ice column) has already been released (deformed to a lower height). I imagine calving as occurring when the loss (ie. from melting) from underneath is faster than the ice above, under less pressure, is willing to deform. Maybe that isn’t how an expert would put it, but it seems to be a working model for this layman.
Thinking on the rate of collapse, and going back to the Columbia Glacier as an example. That glacier has been retreating about 0.8 km per year, under similar topography conditions as the PIG. The PIG is a little higher and a little deeper; so, larger forces are involved and maybe the ice flows/deforms faster. Let’s just give it a round number of 1 km per year of retreat. It’s about 200 km before the topography starts rising again. So, if you want an intuitive guess as to how long it will take, let’s say about 200 years to clear the main channel and a bit longer for the rest. Kind of mundane for a Hollywood movie. Of course, the calving front will be a spectacular 30 km across the main trunk and, where Columbia has earth on each side, the PIG has ice on both sides of the channel; so, it will be more complicated than the Columbia collapse.
I’m not sure if they were the same model as DeConto’s, but at Professor Tim Naish’s inaugural lecture at Victoria University of Wellington, New Zealand, he showed the results of a detailed melt model of the WAIS, and if I understood correctly it took something like 200-300 years for most of the sheet to melt. This timescale of melt was also confirmed by the ANDRILL record (Naish headed at least one of these expeditions) and another paleo record showing that the sea level rose rapidly around a corresponding time. There’s no question that the sheet could melt in under 500 years – it’s done it before, and that was without massive CO₂ forcing…
[Response: Careful here. It isn’t directly about ‘melt’. This part of Antarctica will remain well below freezing for some time to come. It’s about increased flow of the ice into the ocean, and from *there* it melts (and of the icebergs raise sea level whether they melt or not). Note that 300 years is a very firm minimum time frame. Deconto and Pollard’s results actually suggest anywhere from 300 to 6000 years. That’s a sea level rise of about 1 m/century to as little as 5 cm/century. Either is ‘serious’ I suppose but the range is pretty big. We simply don’t know yet which end member is the best estimate of reality.–eric]
Still all very interesting and I think that I understand the basics but the main point with me is : what is the number?
We have the IPCC report on SLR which no scientist believed (me neither and I am not a scientist) when it was published and has now unofficially been updated by the experts to 1m by the end of the 21C.
We now need to fill in the missing bits like can the increased speed of galciers themselves lead to a number higher than 1m by the end of the century or do we need a big chunk of the sheet to fall off to achieve anything like 3m or maybe much more by the end of the following century.
As I understand it GIS is stuck by virtue of its topography (am I correct?) and limited to 2m max.
So where does this lead with the WAIS?
Does the increasing speed of glaciers and subsequent calving (which I understand can be significant when the grounding line (?) is so deep) provide sufficient floating ice to affect SLR significantly?
Or do we need a fracture somewhere?
The paleological record shows maybe big changes. They surely cant come just from the speed of glaciers.
If I were a practising scientist I would lend a hand but I am not.
[Response: That 2 m max number is actually Tad Pfeffer’s estimate for *EVERYTHING* including Antarctica and Greenland in the next 100 years. So much less than 2 m from Greenland alone. However, 2 m is a lot! And Pfeffer’s calculations don’t go beyond 100 years. In 200 years, it may be a very different story (i.e. upwards of 3 m or more, easily).–eric]
that should read paleontological (I think).
Examining the specific area around the grounding line in more detail shows how close to the basin the Grounding line is. This article is an examination of observation of rapid changes of a significant WAIS glacier, and the implications for this glacier. Further to note that proposed mechanics were well assessed 30 years ago, and the scientific community is launching a significant effort to monitor and quantify this glacier. It is the results of this that could lead to the big picture answers many seek. http://glacierchange.wordpress.com/
Thanks for the understandable-to-a-layperson post.
I have a question. Would the rising sea level have any impact on this glacier & ice shelf? I’m thinking that ice floats (esp in salt water, I suppose), and since this glacier bed is below sea level, and if sea water were to get into it (or even at front edge points where it meets the sea), a rising sea level might put even more upward pressure on the glacier.
I know glaciers are heavy, so that might not be the case, but the sky is big and CO2 molecules small, and yet we have the natural GH effect and AGW….
[Response: Perfectly reasonable layperson question. Yes, of course ice floats! It doesn’t matter that they are “heavy”. What matters is that they are less dense than liquid water. Indeed, one can measure the up and down of the ice shelves and even the grounded ice upstream with the rising and falling tides on a daily basis. So there is no question that rising sea level can ‘unground’ these glaciers. The question though, is how fast. That’s a much harder problem to answer — hence all the research going into this.-eric]
Sea level rise of the rates we are seeing now, have a greater influence on larger, thinner ice shelf systems. The PIG is not dominated by its ice shelf, hence the potential for rapid ice stream response. However, the grounding line of ice of the thickness of PIG is not as responsive as say the thin ice at the margin of Petermann Glacier in Greenland or Wilkins Ice Shelf.
re #49 terran
Today’s Science has a paper that is relevant to your discussion of accelerating Greenland ice mass loss in relation to its components:
M. van den Broeke et al. (2009) Partitioning Recent Greenland Mass Loss Science 326, 984-986 (November 13, 2009)
http://www.sciencemag.org/cgi/content/abstract/326/5955/984
abstract: Mass budget calculations, validated with satellite gravity observations [from the Gravity Recovery and Climate Experiment (GRACE) satellites], enable us to quantify the individual components of recent Greenland mass loss. The total 2000–2008 mass loss of ~1500 gigatons, equivalent to 0.46 millimeters per year of global sea level rise, is equally split between surface processes (runoff and precipitation) and ice dynamics. Without the moderating effects of increased snowfall and refreezing, post-1996 Greenland ice sheet mass losses would have been 100% higher. Since 2006, high summer melt rates have increased Greenland ice sheet mass loss to 273 gigatons per year (0.75 millimeters per year of equivalent sea level rise). The seasonal cycle in surface mass balance fully accounts for detrended GRACE mass variations, confirming insignificant subannual variation in ice sheet discharge.
Not only tides, but also waves from distant storms, may move the floating ice enough to crack pieces loose; this research used summertime solar-powered instruments so reports the correlation with distant N. Hemisphere winter storms; they comment that S.H. winter storms would too but were not recorded (no solar power for their instruments).
http://www.earth.northwestern.edu/people/emile/PDF/EAO186.pdf
GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L17502, doi:10.1029/2006GL027235, 2006
Re #74, Hank–
Solar power in high latitudes. . . it’s a sometime thing. (H/T to the Gershwin brothers.)
Mauri, Eric, Chris G, and all, thank you for an enlightening conversation (from my perspective, at any rate :).
If I may add one more speculative question : are the portions of glacial sheets formed during periods of high ice flux less stable, and more prone to calving, than those formed during slow flux?
Terran: An examination of a map of glacier velocity for either the Pine Island presented in this post or of the ice streams feeding the Ross Ice Shelf indicate that most of the ice sheet region is not a fast flow region.
http://www.spri.cam.ac.uk/research/projects/icestreamstoppage/
Compare the entire basin area to the magenta fast flow regions in either. Most of the deformation in an ice stream is in the bottom portion and thus most of the thickness of the ice stream is just carried along. Take a look at the internal layering of PIG in Figure 3 of the following paper. Notice the continuity of most of the layering, some of which is quite old. This illustrates two things, that PIG flow regime has not changed much for quite awhile and two that all the action from topography and changing basal conditions is happening lower in the ice stream.