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Moulins, Calving Fronts and Greenland Outlet Glacier Acceleration

Filed under: — group @ 18 April 2008 - (Español)

Guest Commentary by Mauri Pelto

The net loss in volume and hence sea level contribution of the Greenland Ice Sheet (GIS) has doubled in recent years from 90 to 220 cubic kilometers/year has been noted recently (Rignot and Kanagaratnam, 2007). The main cause of this increase is the acceleration of several large outlet glaciers. There has also been an alarming increase in the number of photographs of meltwater draining into a moulin somewhere on the GIS, often near Swiss Camp (35 km inland from the calving front). The story goes—warmer temperatures, more surface melting, more meltwater draining through moulins to glacier base, lubricating glacier bed, reducing friction, increasing velocity, and finally raising sea level. Examining this issue two years RealClimate suggested this was likely the correct story. A number of recent results suggest that we need to take another look at this story.


The Acceleration:

Jakobshavn Glacier, West Greenland, retreated 30 km from 1850-1964, followed by a stationary front for 35 years. Jakobshavn has the highest mass flux of any glacier draining an estimated 6% of the GIS. The glacier terminus region also had a consistent velocity of 19 meters/day (maximum of 26 m in glacier center), from season to season and year to year, the glacier seemed to be in balance, as I noted in a 1989paper. This is the fastest glacier in the world, no steroids needed. After 1997 it began to accelerate and thin rapidly, reaching an average velocity of 34 m/day in the terminus region. The glacier thinned at a rate of up to 15 m/year and retreated 5 km in six years. Jakobshavn has since slowed to near its pre-1997 speed, the terminus retreat is still occurring, but likewise is.

Helheim Glacier, East Greenland had a stable terminus from the 1970’s-2000. In 2001-2005 the glacier retreated 7 km and accelerated from 20 m/day to 33 m/day, while thinning up to 130 meters in the terminus region. Kangerdlugssuaq Glacier, East Greenland had a stable terminus history from 1960-2002. The glacier velocity was 13 m/day in the 1990’s. In 2004-2005 it accelerated to 36 m/day and thinned by up to 100 m in the lower reach of the glacier. Helheim and Kangerdlugssuaq combined drain 8 % of GIS. Hence, they are more than canaries in the coal mine. In 2006, the velocity of Helheim and Kangerdlugssuaq decreased to near the 2000 level, the terminus of Helheim advanced a bit (Howat et al., 2007).

The first mechanism for explaining the change in velocity is the “Zwally effect”, which relies on meltwater reaching the glacier base and reducing the friction through a higher basal water pressure. A moulin is the conduit for the additional meltwater to reach the glacier base. This idea, proposed by Jay Zwally, was observed to be the cause of a brief seasonal acceleration of up to 20 % on the Jakobshavns Glacier in 1998 and 1999 at Swiss Camp (Zwally et al., 2002). The acceleration lasted two-three months and was less than 10% in 1996 and 1997 for example. They offered a conclusion that the “coupling between surface melting and ice-sheet flow provides a mechanism for rapid, large-scale, dynamic responses of ice sheets to climate warming”. The acceleration of the three glaciers had not occurred at the time of this study and they were not concluding or implying that the meltwater increase was the cause of the aforementioned acceleration. However, many others have made this assertion and are investigating (Stearns and Hamilton, 2007). Examination of recent rapid supra-glacial lake drainage documented short term velocity changes due to such events, but they had little significance to the annual flow of the large glaciers outlet glaciers (Das et.al, 2008).

The second mechanism is a “Jakobshavn effect”, coined by Terry Hughes, (1986), where a force small imbalance of forces caused by some perturbation can cause a substantial non-linear response. In this case an imbalance of forces at the calving front propagates up-glacier. Thinning causes the glacier to be more buoyant, even becoming afloat at the calving front, and is responsive to tidal changes. The reduced friction due to greater buoyancy allows for an increase in velocity. This is akin to letting off the emergency brake a bit. The reduced resistive force at the calving front is then propagated up glacier via longitudinal extension in what R. Thomas calls a backforce reduction (Thomas, 2003 and 2004). For ice streaming sections of large outlet glaciers (in Antarctica as well) there is always water at the base of the glacier that helps lubricate the flow. This water is, however, generally from basal processes, not surface melting.

If the Zwally effect is the key than since meltwater is a seasonal input, velocity would have a seasonal signal. If the Jakobshavn effect is the key the velocity will propagate up-glacier, the terminus velocity will be impacted by tides, and there will be no seasonal cycle.

On Jakobshavn the acceleration began at the calving front and spread up-glacier 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 become afloat, detached from their bed (Hamilton et al, 2006). This had been the case at Jakobshavn for the last 50 years, but not for Helheim or Kangerdlussuaq. In each case the major outlet glaciers accelerated by at least 50%, much larger than the impact noted due to summer meltwater increase. On Jakobshavn the acceleration was not restricted to the summer, persisting through the winter when surface meltwater is absent.

As a result of the above Luckman et al. ( 2006) concluded:

“The most plausible sequence of events is that the thinning eventually reached a threshold, ungrounded the glacier tongues and subsequently allowed acceleration, retreat and further thinning. It is reasonable to believe that the 1998 Jakobshavn speed-up, also following a long period of stability, was triggered by the same processes of thinning but occurred earlier and after a shorter period of thinning because the tongue was already afloat.”

Examination of the acceleration of other glaciers such as the Petermann Glacier indicate a much smaller acceleration than that observed on three glaciers we have focused, and indeed it is in the summer and of a magnitude that the Zwally effect could explain (Rignot, 2005). Other large outlet glaciers such as the Rinks and Daugaard-Jensen have been stable since 1960 (Stearns et al, 2005). Many other lesser outlet glaciers have accelerated substantially.

That each of the three glaciers has a reduced velocity in 2006 and 2007 despite some exceptional melt conditions in 2007 further suggests that meltwater is not the dominant driver of the acceleration of the main outlet glaciers. Temporarily, there appears to be a force imbalance at the glacier fronts. This will reduce the annual contribution to rising sea level from glacier dynamic changes. The bad news is that the degree of acceleration that can occur via the Jakobshavn effect is greater in these cases than that from the Zwally effect. The Zwally effect is nonetheless real and also implies a direct sea level impact of greater melt.

The Jakobshavn is of particular importance as it has a bed below sea level for at least 80 km inland from the terminus. In this reach there are no significant pinning points, or abrupt changes in slope or width (Clarke and Echelmeyer, 1996) that would help stabilize the glacier during retreat. It is the only outlet glacier of GIS to lack these, and can then (via backforce reductions) tap into the heart of GIS. We know that surface melting is a slow process for raising sea level. but as Greenland’s major outlet glaciers have recently shown, rapid acceleration can quickly deliver large volume of ice to the ocean. The pace of change is not glacial.


Clarke, T.S. & Echelmeyer, K. 1996: Seismic-reflection evidence for a deep subglacial trough beneath Jakobshavns Isbræ, West Greenland. Journal of Glaciology 42(141), 219–232.

Hughes, T. (1986), The Jakobshavn effect. Geophysical Research Letters, 13, 46-48.
Pelto, M.S., Hughes, T.J. & Brecher, H.H. 1989: Equilibrium state of
Jakobshavns Isbræ, West Greenland. Annals of Glaciology 12, 127–131.,

Thomas, R. H. Abdalati W, Frederick E, Krabill WB, Manizade S, Steffen K, (2003) Investigation of surface melting and dynamic thinning on Jakobshavn Isbrae, Greenland. Journal of Glaciology 49, 231-239.

Thomas RH (2004), Force-perturbation analysis of recent thinning and acceleration of Jakobshavn Isbrae, Greenland, Journal of Glaciology 50 (168): 57-66.


221 Responses to “Moulins, Calving Fronts and Greenland Outlet Glacier Acceleration”

  1. 151
    Nick Barnes says:

    kenlevenson@147: I was very struck by the 3D images at membrane.com, but note that the vertical scale and choice of colour make it impossible to resolve heights within 100m of sea-level, where it really matters. For a much clearer view, see the paper by Bamber et al at NSIDC:
    http://nsidc.org/data/docs/daac/nsidc0092_greenland_ice_thickness/parca_paper1.pdf
    As you can see from that, the central basin is fairly shallow and nowhere connects with the sea below sea-level.

  2. 152
    Steve Bloom says:

    Re #146: That’s fair enough based on the present melt rate, but extrapolating it to much higher rates seems a little shaky. Recall that RA was probably the main target of Hansen’s “reticence” comment a while back and was more responsible than anyone for the conservative SLR estimate in the AR4.

  3. 153

    Re: #149

    Dear Hank,

    I assume you already know this stuff, but this link might provide some googling ideas, vis-à-vis GRACE measurements:

    http://www.geoinformatics.com/asp/default.asp?t=article&newsid=3257

  4. 154
    David B. Benson says:

    d18O is thought to be a proxy for ice volume.
    Using the measurements in

    ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/greenland/summit/gisp2/isotopes/d18o20y.txt

    from 30 yap (1980 CE) back to about 10300 ypb, we
    can obtain some idea regarding possible futures for
    ice volume. First I scanned the orginal data for
    the Younger Dryas (which is before the Holocene)
    and the 8.2 ypb event (which is in the Holocene).
    In both intervals indeed the d18O level was lower
    in comparison to dates both earlier and later.

    I averaged the data in 100 year intervals and placed
    each interval into one of 7 bins, bin00 (lowest d18O,
    most ice) to bin06 (highest d18O, least ice). The
    last 100 year interval, corresponding to 1881 CE to
    1980 CE, is in bin03, the median bin for the Holocene.
    As for potential futures, consider the up-transitions
    from bin to bin in the Holocene, in 100-year jumps from
    bin03.

    up 1 bin:
    from ~1670 to ~1570
    from ~3770 to ~3670
    from ~6470 to ~6370
    from ~7170 to ~7070
    from ~9970 to ~9870

    up 2 bins:
    from ~2570 to ~2470
    from ~3370 to ~3270
    from ~4270 to ~4170
    from ~5270 to ~5170
    from ~9770 to ~9670

    up 3 bins:
    from ~9370 to ~9270

    where the ‘~’ reminds us these are units of years
    before present (ybp). Now up 3 bins takes us to
    a climatic local maximum for d18O (in Central
    Greenland at least), although around ~6870 may
    well be the climatic optimum for high d18O there.
    The point is that sea stands rose to about 3 meters
    higher than now during the mid-Holocene. Of course
    nothing here says that all of this rise occurred
    in a mere 100 years.

    One the other hand, such a large transition has
    only occured the once in the past, out of the 23
    different centuries that d18O was in bin03 in the
    103 centuries studied; that’s 4% of the transitions.

    Disclaimer: I am an amateur with regard to the
    subject at hand, especially with respect to
    determining ice volume, and so sea stand, from d18O.

  5. 155
    mg says:

    149. the Bamber paper linked in 151 has an interesting balance velocity plot which can be compared with the location and frequency of glacial earthquakes on Greenland (eg Ekstrom et al 2006). presumably the icequakes may play an important role in development of the percolation threshold as moulin networks interconnect prior to sheet collapse, or at least through the transitional architectures. not sure what the order parameters might be given the degree of nonergodicity and nonmarkovian nature of the system. presumably techniques such as Hurst and rescaled range analyses may provide some insight but the whole system is so far far-from-equilibrium. heard that IBM are setting up a supercomputer facility here in UK to model these things – an interesting development!

  6. 156
    kenlevenson says:

    149. Hank Roberts – good questions, can anyone answer them?

    150. JHC – Since it doesn’t look like we’ll be pumping sulfur into the stratosphere perhaps this is the geo-engineering feat needed! ;)

    151. Nick Barnes – I don’t think the bottom needs to connect to the ocean, and this idea makes it scarier to me. It looks like an elevated tub where sides are made of mountain ranges and big “holes” where ice must be currently damming the outflows. What happens when these ice dams are penetrated, crack, and open? Images of sudden catastrophic earthen dam collapses come to mind. Based on the Bamber paper the northern half of the western coast seems, to my amateur eyes, where this sort of “break out” would most likely occur. Pure “chicken little” conjecture, maybe – but is it possible?

  7. 157
    Mauri Pelto says:

    There would not be substantial voids in the ice. There is some. Thus, GRACE tells the story. The Joughin and Das papers was cited in this article, and was one reason behind the timing of this post, as it indicates the limits on the Zwally Effect. There are sufficient outlets to the ocean for substantial drawdown via calving. This post does not try to address how far before outlet glacier calving would be limited. But calving is a more rapid means to reduce melt and thus, the prospect of more rapid sea level rise is heightened by the Jakobshavn effect in the near term. Eventually maybe melting will be the key, but in the near term it is calving that is the dynamic response mechanism that we have observed is the key to rapid retreat of tidewater glaciers around the world. Melting in place of adjacent non-calving glaciers has been much slower. this is evident on the glaciers adjacent to the Jakobshavn that are not calving.

  8. 158
    Nick Barnes says:

    kenlevenson@156: my point is that this bedrock data shows that the ocean cannot undermine the GIS — the Jakobshavn effect (which this post is about) — except at the coasts and a little way upstream on some of the outlet glaciers. This particular mechanism was giving me particular concern about Greenland, as it was one way in which a huge amount of heat might be delivered to the basement ice under the GIS, leading to a catastrophic collapse. It seems that in fact this is not possible. We shouldn’t be complacent about the GIS, but if considering the risk of catastrophic collapse, we can restrict ourselves to other mechanisms which might deliver that heat to the basement. Principally, drainage of surface meltwater and rain (the Zwally effect).

  9. 159
    Hank Roberts says:

    Mauri, I’m curious if we could tell a void from a water-filled space in the ice, do you know? Do you think it’s possible to rule out an outburst flood of meltwater from underneath the Greenland ice, by knowing how much liquid water may be accumulating as the ice cracks and lakes drain?

    I’m thinking of how decades ago the big meltwater outbursts were pictured as lakes of water sitting on top of ice and behind barriers, but the more recent descriptions I’ve seen do talk about the meltwater accumulating underneath rather than above the ice.

    I know about some of the fluid movement mapped under the Antarctic ice but haven’t seen much about that from Greenland, except this most recent article in Science about seeing the lake water make the ice move as it drained down.

  10. 160
    Nick Barnes says:

    In contrast, much of the WAIS is grounded deep below sea-level, which is certainly connected to the Southern Ocean not just by shallow water but by very deep areas.

  11. 161
    David B. Benson says:

    Re #154: “Over longer time periods, this ratio [d18O] indicates the average temperature of the regions between the evaporation site and the coring site.”

    From:

    http://www.gisp2.sr.unh.edu/MoreInfo/Ice_Cores_Past.html

    So it appears to be of no immediate predictive value. Apologies for wasting your time.

  12. 162
    mg says:

    artic sea ice cover has been shown to be in a state of self-organised citicality

    http://www.cosis.net/abstracts/EGU04/00344/EGU04-A-00344-2.pdf

    might it not be true also for the ice sheet and its features?

  13. 163
    JCH says:

    Does this sentence from the article need some attention:

    If the Zwally effect is the key than since meltwater is a seasonal input, velocity would have a seasonal signal.

    On the 3D rendition, if you tilt your screen, and look carefully at the colors, I think you’ll agree that it’s a more accurate representation of an ice-free Greenland than it might have seemed at first blush. My description of it showing a ring of islands is incorrect.

  14. 164
    kenlevenson says:

    Mauri and Nick,
    Thank you for your clarifying responses and patience. It’s starting to make more sense to me. Although my questions still go in the direction of Hank’s #159. Unfounded fear or no?

  15. 165
    David B. Benson says:

    kenlevenson (164) — In the mid-Holocene, temperatures were much warmer than even now in Greenland. The sea stand was about 3 meters higher than now. So the Greenland ice sheet was still mostly there.

    Similarly for the Eem (Eemian interglacial) with sea stands estimated to be about 4–6 meters higher than now.

  16. 166

    Guys, isn’t the question really whether or not at some point in the future the GIS could become so riddled all the way to the bottom that it would collapse here and there under its own weight, and how to determine if GIS is becoming more and more prone to collapse? Or, at some point, will the glacier fronts just keep melting away and pull back until they no longer calve?

  17. 167
    David B. Benson says:

    “Moreover, work on Greenland ice cores suggests
    an inverse relationship between temperature and
    accumulation rates during the last 7 ka., and the ice sheet is
    therefore especially prone to volume reduction during
    periods of warm climate, such as the middle Holocene
    (CUFFEYand CLOW, 1997). FUNDER’s (1989) summary
    of Greenland glacial records supports this view, noting that
    middle Holocene ice margins were considerably inland
    from present-day, and re-advanced after ca. 4 ka.”

    From:
    Journal of Coastal Research SI 36 65-80 (ICS 2002 Proceedings) Northern Ireland ISSN 0749-0208
    Middle Holocene Sea-Level and Evolution of The Gulf of Mexico Coast (USA)
    Michael D. Blum, Amy E. Carter, Tracy Zayac, and Ron Goble

    http://www.science.ulst.ac.uk/ics2002/blum_md%20et%20al.pdf

  18. 168
    JCH says:

    This paper looks to be somewhat on topic:

    http://www.nature.com/nature/journal/v404/n6778/full/404591a0.html

    Is the mid-Holocene so simply invoked?

  19. 169
    mg says:

    168 JCH thanks for that link. If the west antarctica ice sheet were to partially disintegrate first (since many commentators seem to think that is the most vulnerable) giving a rise, say, of a couple of metres, what might the impact be on the oceanic / moulin-riddled-GIS interfacial dynamics? Might the sequence of events be the signature of an MWP (giving a WAIS contribution of say 2 metres and a GIS contribution of – as per the link – about 5 metres? The linked paper suggests that GIS was much steeper – does that tie in with the glacial earthquakes now being observed compromising the ice sheet edges?

  20. 170
    Hank Roberts says:

    JCH, interesting Nature link, that looks helpful:
    http://www.nature.com/nature/journal/v404/n6778/images/404591ac.2.jpg

  21. 171
    Mauri Pelto says:

    Lakes form at the bottom of a glacier or on the surface. Because ice crystals deform under pressure, and pressure is substantial within a glacier or ice sheet it is not possible to have substantial void volumes. Ice under pressure would deform and flow into this void. This happens to much of the seasonal hydrology system each winter. Without water flow to keep tunnels open, they close, then in spring maximum water pressures often occur befor the conduit system redevelops. Once opened the flowing meltwater can maintain these narrow conduits. However, the meltwater does not have enough heat to melt much. At the base of the glaciers even in the summer next to these streams, you will see new ice coating the bedrock in places. The moulin ice riddling is science fiction. No ice sheet or glacier collapses due to riddling by moulins. I still see a persistent misconception about the ability of meltwater to melt glacier ice and riddle the glacier with holes. I work on glaciers with lots of melt and they are not weakened by all the meltwater drainage. The meltwater is not a very capable melter of ice. Ice is unlike rock which does not deform under the pressure and temperatures observed on glaciers.

  22. 172
    Hank Roberts says:

    That’s helpful, Dr. Pelto. I think this is one of those trivial questions. I asked a system operator once what he meant when he said I’d asked a trivial question. He thought for a moment and then said “it’s not worth my time to explain, and you’d never figure it out for yourself.”

    I very much appreciate your taking the time to address it clearly. I think we amateurs don’t have much of a sense of what’s going on deep under thick ice, and so imagine by analogy to the very different experience we have of shallow thin ice melting.

    Seeing all the recent reports of water moving under the icecaps, it’s hard to have a sense of what’s going on without hearing from someone like you who’s actually looked into it.

  23. 173

    Re: #171

    Dear Dr. Pelto,

    Thank you for your explanation. I think I have finally been disabused of my misapprehension of the ability of moulins to increase to the point that they endanger the structural integrity of the GIS.

  24. 174
    Aaron Lewis says:

    Ice flows under pressure. There are no voids in thick ice. However, hydro-fracture can be a relatively rapid process. Alternatively, volumes of water can sink through ice in a process very much like the classic physics demonstration where a wire is passed through a block of ice using weights and gravity. This happens when water on top of ice is more than just over 6 meters deep.

    The heat advected into the ice by the stream of water in a moulin may be small in comparison to the heat required for substantial melting. However, advection results in conditions very different from those when conduction is assumed to be the primary mechanism for heat transfer to the base of the ice and our thinking must be adjusted accordingly.

    With conduction as the primary heat transfer mechanism, the colder (and stronger) ice will always be toward the bottom of the or the ice sheet. In this case, there is no possibility of the ice sheet undergoing rapid collapse. This is the conventional wisdom.

    However, if advection by melt water is the primary heat transfer mechanism, then the basal ice may be as warm as the upper layers of ice, and be too warm (and too weak) to support the weight of the ice above it (without extensive ice buttresses around it.) In this case, there is a weak foundation that is subject to progressive collapse, if the surrounding buttress weakens. Remember, ice flows under pressure. As the foundation ice warms, it will put increasing stress on the buttress. As the buttress warms, it will become weaker and the system will collapse. Thus, the real question is “How much heat is being advected?”, and “How much heat is being conducted into the GIS?” My intuition is that advection will increase much faster than conduction. I would be much happier if I did not trust my physics. Unfortunately the very first lesson that I learned at the university was to trust the “the physics” rather than any particular textbook or author.

    Finally, as water or ice falls under the force of gravity, potential energy is converted to kinetic energy. When the falling object strikes another object, the kinetic energy is converted to heat. If that heat is transferred to ice, that ice becomes warmer and weaker. The heat may not be enough to melt the ice, but the ice will be weakened. (On the other hand, measuring such melt would be difficult.) That water at the base of a glacier is at 0C says only that it has transferred the heat that it gained in falling to the base of the glacier, to other, colder ice. The ice, that gained that heat, is now weaker. In fact, that weaker ice may have become so weak that it flowed together to fill the moulin that acted as the transfer conduit. Thereby, it may be hard to identify just where the heat from the falling water now resides. Nevertheless, somewhere in the glacier, there is a volume of weaker ice. Heat does not just disappear – even in a glacier.

  25. 175
    Nick Barnes says:

    Aaron Lewis @ 174: on potential energy. Worst case scenario: a meltwater pond on a very thick ice sheet which falls 3km through a moulin to the basement. Each kg of water converts 30kJ of potential energy to heat in the basement (in fact the heat will be distributed through the depth of the ice sheet, as friction slows the falling water). That same kg of water also carries 334 kJ of latent heat. So even in this worst-case scenario the potential energy is negligible.

  26. 176
    Phillip Shaw says:

    Re: #171

    Dr. Pelto,

    I greatly appreciate your time and patience in helping us laypeople better understand ice sheet dynamics. But I confess I’m confused by your statements that meltwater and moulins don’t significantly weaken glaciers or ice sheets.

    After the Larsen B ice shelf broke up a number of popular press articles pointed to the meltwater lakes and moulins as having been a major factor in precipitating or magnifying the disintegration of the ice. Because the meltwater was only present for several months each year it would have had to act quickly and powerfully to weaken the ice shelf. Did the writers simply misrepresent the science, or do meltwater and moulins act differently on a floating ice shelf than on a grounded ice sheet?

  27. 177
    David B. Benson says:

    Phillip Shaw (176) wrote “… do meltwater and moulins act differently on a floating ice shelf than on a grounded ice sheet?” The meltwater at the bottom of a floating ice shelf just freshens the sea water at the outlet. The meltwater at the bottom of a glacier digs a tunnel downhill.

    Disclaimer: I’m an amateur.

  28. 178
    JCH says:

    Would there be enough pressure in a floating ice shelf to plasticize the ice to fill the voids?

  29. 179
    Aaron Lewis says:

    Re 175
    The energy from one little melt pond is negligible. The melt from weeks of warm, moist onshore winds from the North Atlantic or the Arctic Ocean every melt season for season after season is not negligable.

    Advection can move more heat into the ice during the melt season than conduction can remove during the winter. Thus, the heat accumulation can be cummulative year after year and decade after decade.

    Look at the weather patterns on Greenland’s East Coast last May and June. Then, promise me that they will not occur again.

  30. 180
    mg says:

    this is what Jim Hansen wrote in a lengthy paper here
    http://www.columbia.edu/~jeh1/2006/CaseForVermont_20060907.pdf
    which at paragraph 49 has:

    “49. The primary question, however, is whether the rate of melt has the potential to accelerate in a rapid, nonlinear fashion. Some hint of that possibility is contained in recent earthquake data for Greenland. Seismometers around the world have detected an increasing number of earthquakes on Greenland between 1993 and 2005, as shown by the green bargraph in Figure 22. The location of these earthquakes is near the outlets of major ice streams on Greenland, as shown by the red circles in Figure 22. The earthquakes, which have magnitudes between 4.6 and 5.1, are an indication that large pieces of the ice sheet lurch forward and then grind to a halt from friction with the ground.”

    the Figure 22 (in the linked pdf) referred to can be compared with the figure linked in 170.

    what is the precursor / trigger for the “rapid, nonlinear fashion” ? is it to do with collective dynamics across the ice sheet / oceanic combine ?

  31. 181
    Mauri Pelto says:

    Good point P. Shaw and JCH. Yes, surface water on an ice shelf behaves much differently, and I have seen no mention of moulins for Larsen B or Wlikins Ice Shelf. The surface water filling crevasses helps weaken the ice shelf. But yes the ice shelves in this case are fairly thin and ice deformation is slower, but it still was not the internal drainage system that seemed to breakup the ice sheet. If you look at the disintegration it produces good sized tabular icebergs, which indicates crevasse control. Again this happened from the calving front, and represents a rapid calving retreat not an internal collapse. Thus, it the surface water played a role, but it was an inbalance in forces at the calving front that allowed the retreat to propogate. Nick, it has never been demonstrated that advection occurs in ice sheets. It may but the signal is too weak to be identified yet. And even warm ice is not weak. Note many glaciers are temperate, and the bottom of all rapidly moving glaciers are at the pressure melting point, warm, they do not collapse at that point. Nick, trust your physics but that means you must understand the properties of ice too.

  32. 182
    Hank Roberts says:

    While puzzling over this earlier I left the bits I found in this thread over at William’s blog Stoat:
    http://scienceblogs.com/stoat/2007/02/why_do_science_in_antarctica.php#c859866

    As I puzzled I found mention of what seemed to me to be three different situations, at least (each with varying amounts of overburden ice pressing down)

    One:
    Continental icecap sitting more or less where it is, bottom below sea level, pushing down on rock or sediment, where water at the base can run uphill and get squirted out at the edges due to pressure at the middle, and the pressure/temperature at the bottom allows liquid water.

    Look at some of the radar maps of Antarctica (I’m looking for links); I recall the topography under the ice is very crisp looking at the center, you can see dendritic drainage patterns extending from the central ‘sea’ out to the edges. And out at the edge all that clearly discernible rock is no longer visible — there are layers of sediment that look like they were pushed out from under the central ice, around the perimeter.

    I don’t know if Greenland has anything similar, though I wonder.

    Two:
    A glacier above sea level — where once water entering from above declines air space is left as water drains out below and the pressure of the ice can force the openings to close, and air presumably can circulate as well cooling off the void by evaporation — “ice caves” for a while, closing up under time and pressure.

    Three:
    Floating sea ice. I know there are observations of cracks forming in ice sheets due to tidal change and to episodes of waves from distant storms. I’d wondered if there were enough rise in sea level to crack them at the point where they ‘hinge’ and become floating. There are observations that the cracks once opened don’t weld themselves shut with equal strength; they fill up with a loose mass of snow and ice and air debris that blows in.

    Then there’s more — circulating seawater opening channels under the ice, perhaps up into the ice flows (the place an unmanned submersible was lost in Antarctica might be one such, I don’t know).

    This is a quick memory dump, cites would be in the very tolerant William Connolley’s thread over at Stoat.

  33. 183
    Hank Roberts says:

    So — can we set the other stuff aside, til our hosts open a thread for those again? The International Polar Year probably has most of the likely people out in the field or frantically writing papers on it now (grin).

    But — here and now:

    We have Mauri Pelto here to talk about his area of expertise — “Moulins, Calving Fronts and Greenland Outlet Glacier Acceleration” — and it’s a big area.

    Dr. Pelto, let me try pointing back to the topic, what question _should_ one of us amateurs have thought to ask you that no one has? Anything obvious that hasn’t been paid enough attention in the news, that you find remarkable or puzzling or instructive?

  34. 184
    Phil. Felton says:

    Check out these photos showing the break up of multiyear ice, notably in the Beaufort sea, it was a solid piece last year at this time:
    A month ago:
    http://manati.orbit.nesdis.noaa.gov/ice_image21/D08090.NHEAVEH.GIF
    Today:
    http://manati.orbit.nesdis.noaa.gov/ice_image21/D08119.NHEAVEH.GIF

  35. 185
    JCH says:

    You have this situation where the GIS is accumulating snow/ice at elevations above 1500 meters, and losing elevation below. I assume this was a big part of the reason why the GIS was much steeper during the mid-Holocene as the combination would have that result.

    What understand Mauri to be saying is the GIS sort of recreates its drainage system each melt season, and that the drainage systems tubing size is somewhat self-limiting because of the water’s low temperature. In my experience with water, it sort of settles for the easiest way out, so it would not really be looking for additional glacier to vandalize. In the winter, what I take him to be saying, is the incredible pressure deforms the ice and it sort of heals itself. Is it possible warm ice is even better at doing that?

    I sort of doubt the ice sheet is healing itself much under around 1000 meters, but I don’t understand how that pressure extends out into the boundary ice. Also, it seems unlikely this deformation healing would be as efficient above some elevation – like maybe 2000 meters.

    So back to this steeper GIS situation, which it sounds like will develop as the coming decades pass by. The only things I can imagine causing non-linear melting, is a situation where significant amounts of ice that is above 1500 meters suddenly falls below 1500 meters, or if the 1500 meter “tree line” goes up quite a bit.

    Is the that 1500 meter line going to move up? I don’t understand all of this stuff, but I don’t see how it could not move up as it gets warmer. If it moves up, would further additional precipitation offset that situation? So far, that seems to be the case. Will it continue to be the case – more warming, more precipitation on Greenland? It’s pretty interesting how the GIS can defend its heights.

    Assuming it gets steeper, is a steeper ice sheet as strong? Was the missing boundary ice buttressing the domes? It seems like it would have to have been. Would the domes of a steeper GIS eventually be susceptible to a form of calving? I realize ice hitting the ground at lower elevations does not equal calving into the ocean, but it’s unlikely to accumulate ice again, and the area it represented above 1500 meters will not accumulate again, and it’s going to melt quickly enough. So it would be a sort of double-edged melting event.

    This paper answered most of my precipitation questions:

    http://www.the-cryosphere.net/1/21/2007/tc-1-21-2007.pdf

  36. 186
    Mauri Pelto says:

    I appreciate the notes of thanks for responses. But I appreciate the questions, it makes it evident that scientists in communicating with each other leave gaps in our explanations of scientific processes, that allow ample room for alternative visulatizations. I will now be able to make a more comprehensive explanation of this topic than I did one week ago. Hank in #182 your observation #1 I did not really follow. Observation #2 is true as to the closing off. the cooling via evaporation, while in the bulk of the ice sheet the ice is cold already, at the base it is not. Observation three, calving is affected by tides, Jakobshavns has a tidal flexure point you can identify. It is an area of weakness, which is why pinning points are important. Pinning points provide a support that a free floating ice tongue would lack, and where tidal flexure would be more dominant.

  37. 187
    Hank Roberts says:

    Mauri, my #1 was thinking almost entirely of Antarctica (as far as I know).

    I’ll try to clarify it briefly but, seriously, if this isn’t possibly relevant to Greenland, let’s not go off to Antarctica til the Polar Year news is in!

    So to answer re my #1, I put earlier speculation here:
    http://scienceblogs.com/stoat/2007/02/why_do_science_in_antarctica.php#comment-353350

    I then found I’d been scooped (grin) by New Scientist
    http://environment.newscientist.com/article/mg19225801.800.html
    and typed some quotes into the following post at Stoat. Below I’ll put a shorter excerpt. The NS link here only goes to a teaser, I was copying from the paper magazine from our library.

    ——excerpt———
    ‘You can have lakes sloping down the sides of mountains, you can have uphill waterfalls, it’s wacky’ [Don Blankenship, geophysicist at U. Texas] …. Blankenship fears that warming since the end of the last ice age has melted the base of the ice, and this may already be priming some parts of the ice sheet to slip. East Antarctica could be ready to open its floodgates.

    “David Marchant from Boston University believes this may have happened before…..
    —–end excerpt—-

    While there’s a link, it’s only to a teaser online:
    http://www.newscientist.com/channel/earth/mg19225801.800-hidden-antarctica-what-lies-beneath.html

    (PS, I pointed back to this thread and your summary, in a recent addition to that Stoat thread)

    —-So, again, if this is off topic, file for later?

    I’m still hunting for those ice/radar pictures I recall seeing that showed the under-ice topography of Antarctica and the surrounding ocean floor.

  38. 188
    Hank Roberts says:

    Ok, this looks like part of the image I remember
    http://www.jsg.utexas.edu/images/feats/crystal_ball/WAIS_bed595.jpg

    — you can see the fairly sharp topography inside the line that marks the edge of the ice cap, and the very soft contours outside that. I imagine the accumulating ice pressing down on the middle of the continent and slowly extruding all the sediment out the underlying drainage channels to the surrounding seafloor.

    Articles with this image:
    http://geology.com/research/west-antarctic-ice-sheet.shtml
    http://www.jsg.utexas.edu/news/feats/2007/crystal_ball.html
    http://news.bbc.co.uk/2/hi/science/nature/4505330.stm
    larger:
    http://news.bbc.co.uk/nol/shared/spl/hi/pop_ups/05/sci_nat_enl_1133909571/img/1.jpg

    And a bit more:
    http://www.ig.utexas.edu/research/projects/agasea/

  39. 189
    Nick Barnes says:

    Aaron Lewis @179:
    You entirely miss my point @175. The potential energy of surface water, turning into heat as it falls into the ice, will always be negligible compared to the latent heat of that same water. There’s a factor of at least 10. The latent heat is the biggie, and can warm a lot of the interior and base of the ice sheet as the surface water re-freezes.
    It might not be able to melt much of the ice (because of course to melt a kg of ice requires the latent heat of a kg of water), but it can raise its temperature significantly. I guess the limiting case here is when the ice sheet is all at melting point.
    Of course, if the water leaves the ice without re-freezing (i.e. if it drains into the sea or into lakes under the ice) then it retains its latent heat.

  40. 190
    Nick Barnes says:

    Mauri@181: Advection of what? Of heat, by the draining of surface water into the ice sheet? It might not have been demonstrated, but the physics makes it obvious that this happens, and lets us put limits on the amount of heat which can be transported in this way. I’m entirely ready to believe that it hasn’t been a dominant process in the past, and even that it is still not so today, despite the rather alarming pictures of huge moulins and talk of arctic Niagaras. However, now that the bedrock topography has been clarified for me, some sort of advection of heat to the interior and the basement is the only mechanism I can see for catastrophic collapse of the GIS (about which I am getting more sceptical by the day, as can be seen in comments). Conduction surely isn’t going to do it, as the colossal latent heat of the ice sheet will keep surface temperatures at or below melting point until there’s no ice sheet left.
    There are various ways in which this heat advection through the ice sheet can become much larger. One is a great deal of rain. Another is a great increase in surface melting.

  41. 191
    Nick Barnes says:

    Phil Felton@184: That’s a great pair of images; the Beaufort Gyre as plain as day. But the main contrast with a year ago is surely the area around the pole, and in the eastern Arctic basin. I recommending loading this pair of images into adjacent browser tabs and flicking back and forth:

    http://manati.orbit.nesdis.noaa.gov/ice_image21/2007/D07119.NHEAVEH.GIF
    http://manati.orbit.nesdis.noaa.gov/ice_image21/D08119.NHEAVEH.GIF

  42. 192
    Ike Solem says:

    On the Larsen B collapse:
    http://adsabs.harvard.edu/abs/2005AGUFM.C21B1102H
    Oceanographic Conditions at the Larsen B Ice Shelf Front Before and After the 2002 Breakout,
    Huber, B.; Domack, E.; Padman, L.

    The Larsen B ice shelf in the Northwest Weddell Sea experienced a catastrophic breakout in early 2002, resulting in an open-water embayment in the region formerly covered by shelf ice, with remnant ice fronts and exposed tidewater glacier regimes. Oceanographic station data occupied at the Larsen B ice front in December 2001 and at the remnant fronts in March 2005 are examined for changes in water mass properties which might be related to the ice shelf breakout of 2002.

    The region is characterized in both years by a dominance of near-freezing point remnant Winter Water capped by a warmer seasonal mixed layer. At several locations along the ice fronts there is evidence of sub-freezing Ice Shelf Water (ISW) emanating from beneath the ice shelf. In the 2005 observations near the remnant tidewater glacial fronts, some plumes of ISW are marked by significant reductions in optical transmittance, implying that the plumes may be carrying glacial debris.

    Modified Warm Deep Water (MWDW) is present in only small quantities at some stations, consistent with published observations of conditions at the Larsen C ice shelf front to the south.

    See also:
    http://iri.columbia.edu/~goddard/EESC_W4400/CC/rignot_etal_2004a.pdf

    “Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B ice shelf, E. Rignot, G. Casassa, P. Gogineni, W. Krabill, A. Rivera, and R. Thomas GRL 2004

    Warmer air temperatures increase surface melt water production, which may reach the bed and increase basal lubrication near grounding lines [e.g., Zwally et al., 2002]. Similarly, ice shelf thinning and accelerated flow and calving reduce ice shelf buttressing, which allows faster flow. It is not clear a priori which process exerts the greatest control on ice flow.

    We note however that most glacier equilibrium lines in the eastern Peninsula are close to grounding lines [Morris and Vaughan, 2003], so surface melt should not be a major factor in the glacier acceleration because it is mostly confined to the ice shelves.

    Furthermore, there has been no marked change in air temperature in the last two years compared to the prior decade that could explain a sudden and widespread glacier acceleration from enhanced surface melting. Indeed, Flask/Leppard glaciers have remained more or less steady although they experience the same climate conditions as Crane, Evans, Green and Hektoria. In contrast, the widespread glacier acceleration coincides with the ice shelf removal, and glaciers still buttressed by an ice shelf did not accelerate.

    These results are in broad agreement with the theory of ice shelf buttressing. The most likely explanation for the glacier acceleration is a major reduction in ice-shelf buttressing. . .

    Further south, glaciers drain larger reservoirs of ice, and the thinning of Larsen C [Shepherd et al., 2003] may trigger an even larger contribution to sea level. These observations are also particularly relevant to the evolution of ice streams and glaciers draining West Antarctica. Although the climate conditions of the Antarctic continent are colder and drier than in the Peninsula, ice shelf thinning could be caused by a warmer ocean instead of warmer air temperatures.

    A news release on more recent work is here:
    http://www.sciencedaily.com/releases/2008/02/080210100441.htm

    The theme seems to be that rising ocean temperatures are playing at least as large a role as air temperature increases in the breakup of Antarctic ice sheets, but that the subsequent acceleration of glacial outflow is not a temperature-driven phenomenon.

  43. 193
    Hank Roberts says:

    > ome plumes of ISW are marked by significant reductions
    > in optical transmittance, implying that the plumes may
    > be carrying glacial debris.

    Intriguing. One of the articles I noticed recently suggested that in past cycles glaciers were grinding away a layer of soft sedimentary rock, and eventually that gets removed and a later glaciation will be working on a harder, older, deeper layer of igneous or metamorphic rock on which its behavior will change.

  44. 194
    Phil. Felton says:

    Re #191

    Nick, agreed the loss from North of Greenland via the Fram Strait this winter has been dramatic, it’s hard to think that the sea ice can survive another year or two of this. My guesstimate is that the perennial sea ice has dropped by ~half this winter. This movie is pretty impressive and it still falls short of the present situation!

    http://arctic.atmos.uiuc.edu/cryosphere/multiyear.ice.quikscat.mov

  45. 195
    Aaron Lewis says:

    In 171 ice flows to fill in moulins and in 181,it is noted “that even warm ice is not weak.” If this seems a bit confusing, try http://www.tms.org/pubs/journals/JOM/9902/Schulson-9902.html
    Ice is the most interesting stuff in the world.

  46. 196
    Aaron Lewis says:

    re 189
    Given a moulin, if the water in it freezes, it delivers 10X heat, but it plugs the moulin. If the water does not freeze, then maybe 11X units of water can flow, and deliver their units of heat in the course of a melt season. Thus, ultimately more heat is advected if the water does not freeze. Since I tend to take worse case, I assumed that a lot of water flowed through without freezing. It helps that I first thought about this back when I worked just downstream of the Priest Rapids hydroelectric dam

    We need a field trip with lots and lots of expensive toys. I think this would be very tough thing to measure in the near future.

  47. 197
    Nick Barnes says:

    Mauri@186 and elsewhere, you say that the base of the ice is at melting point (unlike the middle depths of the ice, which are much colder). Why is this? Is there a source of heat (geothermal?) which keeps the ice so warm?

  48. 198
    Nigel Williams says:

    Now all that’s quite interesting, because from
    http://en.wikipedia.org/wiki/Vostok,_Antarctica

    It looks like ice temperature near the surface of the Antarctic ice sheet around Vostok Station is typically around -55 C. However Vostok Station altitude about 3490 m. The surface of Lake Vostok (or the ceiling of the ice above or confining the lake) is at about 3750 below Vostok Station and so 260 m below sea level. The usable core only extends down to 3310 m, which is to a position 180 m ABOVE sea level. Below that the cored ice consisted of refrozen waters from Lake Vostok. Lake Vostock ranges in depth between 400 m to 800 m with an average depth of about 340 m.

    Presumably the water in Lake Vostok is around 0 C, and the ice above it (at least for the first 440 m) is ranging from ice at 0 C above the lake surface towards the -55 C found near the top of the ice sheet. Since the lake is probably connected via other water ways to a discharge/recharge channel to the ocean, then pore pressure will be forcing water up through the ceiling towards sea level, and beyond by capillary action.

    So here at the Pole of Cold we find that the ice sheet is at around 0 C at its base, that the bottom 10% of the ice sheet is comprised of refrozen lake water – and the temperature gradient runs from 0 C at the base to -55 C at the surface. This refrozen water tops at 180 m above sea level, so the ice sheet is poised above sea level on a 400 m thick wad of refrozen lake ice which in turn is sitting above a vast lake of water at 0 C. The proximity of the interface between old ice and refrozen ice to existing sea level is also disconcerting since Vostok is 1400 km from the coast. That’s hardly the sort of structural base I would like to think is supporting 50 metres worth of sea level rise.

  49. 199
    Andy Siebert says:

    I’m an environmental studies major at UMKC so I have a bit of general background; but upon reading others’ comments to this blog, I’m sensing that many people reading this know a great deal more about glaciers than I do, as I had to consult Dictionary.com for “moulins” and went to Wikipedia for a better understanding of “calving fronts”. I found this very informative, especially the information about the Zwally and Jakobshavn effects. This is a bit specialized for my level of expertise however.

    I wonder if there’s a single repository of stockpiled glacier data: a “Glacier Watch” forum where data is compiled and contrasted, including analysis of historical patterns and also future projections. I’ve found many individual glacier resources, but I didn’t find one website to link them all… I think that would be great if it doesn’t already exist.

    I’d like to re-ask Mauri’s question posed in comment #15: I don’t understand the sentence “Jakobshavn has since slowed to near its pre-1997 speed, the terminus retreat is still occurring, but likewise is.” I thought maybe the sentence would be continued somewhere.

    I also was hoping to find more elaboration on Alestair’s comment #12:
    “There are two positive feedbacks which have not been mentioned. First, as glaciers melt sea level rises and grounding lines retreat speeding the remaining glaciers’ advance into the sea. Second, as the glaciers melt their height reduces bringing a greater proportion of the glacier below the snow line. Sea level rise has a similar but lesser effect. Both of these cause more melting. Both could end with a runaway melt.” Sounds like this chain reaction could accelerate or at least complicate calculations.

    I’ve read a lot of these discussions including calculations and attempts at drawing conclusions. I believe a consensus exists that regardless of the specific anticipated rate of glacial melting, the consequences aren’t good. What about some solutions? I think this is a fascinating and ingenious approach to the problem:
    http://videos.howstuffworks.com/reuters/2105-saving-a-glacier-with-a-huge-blanket-video.htm
    While it may be slapping a bandage on a compound fracture, at least I can derive a sense of hope from this.

    I’m just gonna put this crazy idea out there: what if there was a reality TV show about glaciers? Public interest in nature is growing, as evidenced by the box office success of March of the Penguins and Disney’s recent addition of a nature documentary department. What better way to relate to the “common folk” than with a popular “reality” format? Or: imagine a mock commercial with a “receding glacier” as the symptom and some a pr3scr!p+!0n for greenhouse gas reduction techniques?

    My simple theory is this: if we want to “save nature” we have to really relate to “human nature” as it has become. At least in my experience, many Americans are generally reactionary and don’t go out of their way to acknowledge problems of this magnitude, especially if there’s any room to hide in ambiguity or debate. But if we pump the entertainment industry full of images they can relate to, say: instead of keeping track of c3lebr!+!3s’ life… we might just find more voices, more signatures on petitions and subsequent laws or policies, more trend altering behaviors…

    [some words include symbols in place of letters due to over-active spam filter]

  50. 200
    Mauri Pelto says:

    The base of the ice sheet is at the pressure melting point because of geothermal heat. The heat travels up the thermal gradient to the base of ice sheet. Sorry, about the discontinued sentence, I think I just failed to remove the “but likewise the”. From “12 it is true that sea level rise shifts the grounding line, but this is insignificant over the time span of a decade or two for the Jakobshavns. More important for ice shelves that rely on pinning points, and lack good lateral margin pinning points. It is true that surface lowering at the equilibrium line would cause it to expand inland. The thinning has not been large enough to cause any significant shift to date. But during many rapid calving retreats this does occur.


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