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Ice Shelf Instability

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Guest contribution from Mauri S. Pelto

Ice shelves are floating platforms of ice fed by mountain glaciers and ice sheets flowing from the land onto the ocean. The ice flows from the grounding line where it becomes floating to the seaward front, where icebergs calve. For a typical glacier when the climate warms the glacier merely retreats, reducing its low elevation, high melting area by increasing its mean elevation. An ice shelf is nearly flat and cannot retreat in this fashion. Ice shelves cannot persist unless the entire ice shelf is an accumulation zone, where snowpack does not completely melt even in the summer.



Ice shelves have long been recognized as keys in buttressing Antarctic Ice Sheets. In turn ice shelves rely on pinning points for buttressing. The pinning point are where the floating ice shelf meets solid ground, either at lateral margins or a subglacial rise meets the bottom of the ice shelf causing an ice rise on the shelf surface.

The recent collapse of Wordie Ice Shelf, Mueller Ice Shelf, Jones Ice Shelf, Larsen-A and Larsen-B Ice Shelf on the Antarctic Peninsula has made us aware of how dynamic ice shelve systems are. After their loss the reduced buttressing of feeder glaciers has allowed the expected speed-up of inland ice masses after shelf ice break-up. (Rignot and others, 2004).

Several recent papers examine the causes of breakup of both Larsen B and Wilkins Ice Shelf, which prompts a closer look at the role of surface melting, structural weakness development and ice shelf thinning in this process.

In 1995 a substantial section of the northern Larsen Ice Shelf broke up in a few days. This was the first glimpse at a rapid ice shelf collapse. The breakup followed a period of warming and ice shelf front retreat, prompting (Rott and others, 1996) to observe that “after an ice shelf retreats beyond a critical limit, it may collapse rapidly as a result of perturbated mass balance”.

During the austral summer of 2001/02, melting at the surface of Larsen Ice Shelf in the Antarctic Peninsula was three times in excess of the mean. This exceptional melt event was followed by the collapse of Larsen B Ice Shelf, during which 3,200 km2 of ice shelf surface was lost. That meltwater was playing a key role in collapse was underscored by the unusual number of melt ponds that existed that summer and that the new ice front after collapse close to the limit of surface meltponds seen in images leading up to the March event (Scambos and others, 2003).

The ice shelves actually collapse via rapid calving, and the physics connecting meltwater to calving is its ability to enhance crevasse propagation. When filled 90% or more with meltwater a sufficiently deep crevasse can overcome the overburden pressure that tends to close the crevasse at depth (Scambos and others, 2000). Days before the final Larsen break-up, it is evident that the crevasses cut through the entire ice shelf. It also appeared that large meltponds contracted indicating that they were beginning to drain though the crevasses to the sea (Scambos and others, 2003).

As scientists it would have been easy to close the book on the issue after identifying the meltwater process. However, detailed examinations have continued identifying other key elements in the tale of collapse. The decade prior to collapse the Larsen-B Ice Shelf had thinned primarily by melting of the ice shelf bottom by 18 m (Shepard and others, 2003). This preconditions the ice shelf to failure by weakening its connection to pinning points as the shelf becomes more buoyant. This goes back to the critical limit mentioned by Rott (1996).

Glasser and Scambos (2008) reexamined the Larsen Ice Shelf breakup for structural weaknesses and observed the following. They noted that the rifting and crevasses parallel to the ice front crosscut the meltwater channels and ponds, hence, post dating them. The number and length of the rifts increased markedly in the year before collapse. Substantial rifts also existed between tributary glaciers feeding the ice shelf as far as 40 km behind the ice front. Enlargement of and development of new rifts in these regions occurred in the year prior to collapse. Downstream of the tributary glacier junction there are no evidence of relict rifts, illustrating that these rifts are a feature of the last 20 years. After ice shelf collapse the ice front receded to the pre-existing rifts, and the pre-existing rifts defined the area of collapse. In this case the structural weaknesses preconditioned the ice to rapid breakup. Rift formation occurred in areas of velocity differences and natural weaknesses Velocity differences are largest between tributaries and near the ice front.

The latest example of a collapsing ice shelf is Wilkins Ice Shelf (WIS), which lost 425 km2 in late February and early March 2008. The dynamic nature of the WIS is examined by Braun, Humbert and Moll (2008), their findings are summarized below. WIS is buttressed by Alexander, Latady, Charcot and Rothschild Island and by numerous small ice rises, indicating subglacial contact. Recent history indicates that WIS experiences no continuous ordinary calving, but single break-up events of various magnitudes. They further show that drainage of melt ponds into crevasses were of no relevance for the break-up at WIS. On WIS the evolution of failure zones is associated with ice rises. Analysis of rifting indicated that in 1990 the central area of WIS did not have any substantial rifts. In 1993/94, rift formation started to expand at the northern ice front. Today, the central part of WIS is intersected by long rifts that formed in and around ice rises. The rifts can cover tens of kilometers. The evolution and coalescing of the rifts are followed by break-up events at the ice front. Hence, the connection of rift systems seems to be the trigger for collapse. The recent break-up has left a narrow 6 km wide; already fractured connection to Charcot Island in a sensitive area that is stabilizing the northern part of the ice shelf. A new rift connection formed between already existing fractures, crosses almost the entire northern shelf, which makes WIS even more fragile and vulnerable. This area of interconnected rifts is 2100 km2. An additional 3000 km2 of the 13 000km2 of WIS, is at risk if this connection to Charcot Island is lost as rifts around the Petrie Ice Rise indicate an area of weakness. The conclusion for WIS is pre-conditioning of the ice shelf by failure zones occurring at ice rises and triggered by break-up events are leading to a sequence cascade of failure.

Below you can see the evident rifts near Charcot Island in this March MODIS image and the narrow connection of the ice shelf to this pinning point. The lack of sea ice on the north facing ice front is also noteworthy.

It appears that ice shelf thinning is the key pre-conditioning factor for collapse. The mechanisms for ice shelf thinning include basal melting, meltwater production and rift development. These are interrelated mechanisms that pre-condition the ice shelves to collapse. This will be a key area of continued investigation to understand this critical process for the Antarctic Ice Sheet. At the moment it seems that the key process to rapid calving events is the rift development. Rift development is observed to begin at points of natural weakness. For both ice shelves prior to collapse an expansion of the area where rifts exists has been observed. In both cases this seems to result from pre-conditioning via thinning due to basal melt and surface melt. Rifts development is accentuated by water filling crevasses. A new study will be looking at the impact of reduced sea ice at the front as well (Scambos and Massom, 2008). It is obvious that the glaciologic community will be watching the Wilkins Ice Shelf next Austral summer.

References:

Rignot, E., Casassa, G., Gogineni, P., Krabill, W., Rivera, A., and Thomas, R. (2004). Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B Ice Shelf. Geophysical Research Letters 31: L18401, doi:10.1029/2004GL020697.
Scambos, T., Hulbe, C., Fahnestock, M. and Bohlander, J. (2000). The link between climate warming and break-up of ice shelves in the Antarctic Peninsula. Journal of Glaciology 46: 516–530.
Scambos, T., C. Hulbe, and M. Fahnestock (2003). Climate-induced ice shelf disintegration in the Antarctic Peninsula. In: Domack, E., Leventer, A. Burnett, A., R. Bindschadler, R., P.
Vaughan, D. G., Marshall, G. J., Connolley, W.M., Parkinson, C., Mulvaney, R., D., Hodgson, D.A., King, J.C., Pudsey, C.J. and Turner, J. (2003). Recent rapid regional climate warming on the Antarctic Peninsula. Climate Change 60: 243-274, 2003.

Reader Interactions

177 Responses to "Ice Shelf Instability"

  1. Re: #144

    Nick,

    You probably are already aware of this but, the remnants of the multi-year ice have already moved south towards Banks Island and have melted there (or the bit north of Greenland went out past Svalbard).

    This leaves just the saltier and weaker single-year ice, and it is going fast.

    The only thing that is slowing it down, compared to last year, is the change in the PDO that has meant that the water flowing into the Arctic from the Pacific has not been as warm as it was last year.

  2. Nothing to worry about, just a natural fluctuation:

    No ice at the North Pole
    Polar scientists reveal dramatic new evidence of climate change
    By Steve Connor, Science Editor
    Friday, 27 June 2008

    It seems unthinkable, but for the first time in human history, ice is on course to disappear entirely from the North Pole this year.

    The disappearance of the Arctic sea ice, making it possible to reach the Pole sailing in a boat through open water, would be one of the most dramatic – and worrying – examples of the impact of global warming on the planet. Scientists say the ice at 90 degrees north may well have melted away by the summer.

    “From the viewpoint of science, the North Pole is just another point on the globe, but symbolically it is hugely important. There is supposed to be ice at the North Pole, not open water,” said Mark Serreze of the US National Snow and Ice Data Centre in Colorado.

    If it happens, it raises the prospect of the Arctic nations being able to exploit the valuable oil and mineral deposits below these a bed which have until now been impossible to extract because of the thick sea ice above.

    Seasoned polar scientists believe the chances of a totally ice-free North Pole this summer are greater than 50:50 because the normally thick ice formed over many years at the Pole has been blown away and replaced by huge swathes of thinner ice formed over a single year.

    This one-year ice is highly vulnerable to melting during the summer months and satellite data coming in over recent weeks shows that the rate of melting is faster than last year, when there was an all-time record loss of summer sea ice at the Arctic.

    “The issue is that, for the first time that I am aware of, the North Pole is covered with extensive first-year ice – ice that formed last autumn and winter. I’d say it’s even-odds whether the North Pole melts out,” said Dr Serreze.

    …

  3. Tenney Naumer @ 151: I saw the pack north of the archipelago was breaking up, in late May/early June (e.g. in fig 5 of the NSIDC update) – that was very disturbing, but I haven’t watched it since then. Can you point us to online images showing it heading west (to the open water by Banks Island) and melting? I’m surprised at your assertion that the ice from north of Ellesmere has moved that far – over a thousand kilometres – in only a month; I thought that drift speeds for the denser ice were generally no more than a few kilometres per day.

  4. Dear Nick,

    There are two different directions of circulation up there, separated by a deep sea ridge (don’t have time right now to look up all the correct names) — one sends the ice out to the Atlantic, and the other (the Beaufort Gyre) towards Banks Island (ice west of Ellesmere).

    I watched the ice begin to crack up in April — very interesting to see the water vapor rise from the fissures. Initially, it moved quite slowly, but gathered speed as the days passed.

    And, well, it is just not that thick, anymore.

    I saved a lot of the satellite images on my harddrive. Give me some time, and I can probably post some on my blog that would demonstrate what happened. I am surprised that no one seems to be talking about it, because it was very easy to observe.

    One of the things that shocked me the most while I was watching was that the point where the ice was thickest (right off the Canadian islands) is exactly where a very long fissure grew, meaning that it sure as hell was gonna go.

    Have a look here:

    http://climatechangepsychology.blogspot.com/2008/06/arctic-sea-ice-well-on-its-way-to.html

    But for a really good idea, have a look at the satellite image in this post:

    http://climatechangepsychology.blogspot.com/2008/05/three-in-five-chance-of-record-low.html

  6. Tenney Naumer (150) — Not in detail, but it is thought that during the Eemian interglacial period that WAIS contributed about half of the 4–6 meter sea highstand compared to now.

  9. Re: #147

    Dear Barton,

    I received a partial financial settlement last September which has allowed me to take a ‘leave of absence’ from my regular work. However, it has now been exhausted, and I will need to reduce my current activities. I hope lots of people will take up the slack. I certainly appreciate your comments. Thanks!

  10. Re: #149

    Dear Ron,

    Thank you so much for your comment. I feel that I am getting behind because so much new research has been coming out in the most recent months — in contrast to when I began back in the fall last year.

    It seems that a critical mass is close to be reached where the denialists will finally be swept away, but the issue should be kept in the public’s mind from now until the next president takes office so that less time will be lost on taking the necessary actions.

    And now, we should let Dr. Pelto have his post back.

  11. Yes, I saw the coastal polynya grow along the archipelago and thought “well, at least my money with Stoat is safe”. And the weather reports from everywhere around the Arctic Ocean continue to be disturbing (checking just now I can’t find a single place below freezing point; Resolute is at 11C).
    Has anyone heard from Wayne Davison lately?

  12. By the way, the two gyres are the Beaufort gyre and the Barents gyre, and the central motion is the Transpolar Drift.

  13. Re #140

    Dear Tenney,

    I am certainly for all efforts that help get relevant understanding out to the public. As you can see, a lot of people here support your efforts. I don’t agree that we should just ask people to ‘believe’ as that merely gives fuel for the opposing arguments. The better we illustrate the science in context, the faster people will get it right.

    I just thought it was strange that you came in to RC apparently trying to make scientists feel guilty if they did not come over and blog on Dot Earth and basically, albeit lightly, accusing them of not doing enough. If it weren’t for them we would not know what we know now, so I think it is odd to say they are not doing enough.

    Of course you also have pointed out multiple times that you are doing all this for free. Believe me, you are not the only one.

    I’m glad you dedicated the last 8 months to this issue. I’m glad many others have been dedicated to the issue longer than that. I just disagree with the tactics of trying to guilt people into helping you on the Dot Earth blog because you work for free, and coming into RC and saying we are out of time. Everyone knows we are essentially out of time, especially the scientists doing the research on a daily basis. This is probably the last place on earth you need to tell anyone about that.

    DOT EARTH:

    I posted several times and the more important posts I made that pointed to the evidence links refuting what some of the posters were saying… well those posts did not show up.

    Andy Revkin wrote me and said that it was a problem in the posting system. But the reality is that those posts still did not get up where they need to be to help people understand.

    The signal to noise problem is real on that blog for multiple reasons. Some are technical.

    Until they fix the problem with posting links to the science it will be very difficult to get people to understand the truth of it all. If you only post a few links you can get a post in. but lists of links that cover a subject appropriately or thoroughly don’t get through. So, mainly it is used for people to spout opinion, which of course can go in circles forever if the facts are not well understood.

    I hope they fix the problem with the links. It is probably a spam bot engine. But for my money, this is the best place to send people still for reference in the debate. At the same time I do my best to interpret what is said and get it out there in understandable chunks.

    Again, I approve of all that are trying to help but I’m saying that better informed opinions will win over lesser informed opinions if presented reasonably, even though it takes time.

    Continued best wishes in your efforts, we really are all in this together,
    John

    PS If you wish to discuss issues and tactics, you can find me on skype just look up my name. I’m always open for communication. Let’s help each other learn about science. You can also reach me through the http://www.uscentrist.org site.

  14. Dear John,

    The effort needed to save our species will require much more of us than we imagine.

    RealClimate is a wonderful means for scientists to teach and learn from each other.

    It also represents a certain “comfort zone,” and I was seeking to “prod” knowledgeable and respected voices to move beyond it.

    My words, here, cannot make anyone feel guilty unless they already have something to feel guilty about.

    If you wish to post comments with many links at Dot Earth, then you can e-mail your comment to Andy, and he will certainly post it.

    Using tiny urls also works.

    Let me just add that as the non-denialists have improved on Dot Earth, so too have the industry-paid denialists. In the past few days, there have arrived many new and very subtle posters, much more insidious than in the beginning.

    Andy says that there have been more than 200,000 hits per month on that blog; therefore, it is very important to continue efforts to beat down the noise.

    No, I do not receive any monetary recompense for this particular work I am doing, nor would I accept any — I absolutely do not want my motives questioned. But I will tell you anyway what my motive is — I have a daughter, and every instinct in me tells me to do what I am doing in order to do everything in my small sphere of power to secure her future.

    Once again, my apologies to Dr. Pelto for this off-topic comment.

    John, please contact me via my blog if you would like to continue this discussion.

    Thank you.

  15. The exchanges between Tenney and John, touching on the question of what we can expect from J Q Public, provoke me to remind that there are nearly 7 billion of us. What moves large numbers tends to be mysterious, and there’s room for many approaches, which have many targets, each different, each internally complex, each changing, and each feeding back changes to the others. Seeing how hard it is to know just what to do and say, I tend to be grateful for anyone pushing in what I judge to be the right direction, using whatever time, information, and passion that individual has.

  16. Dear Nick,

    I think we should move over to the North Pole post — saw Wayne Davidson there already.

    Thanks for the info — I have a couple of different maps of the currents that flow in and out of the Arctic Sea, but I do not consider any of them to be very good. I wish I had a more 3D type map to look at.

  18. Here’s an animation of sea ice in the Beaufort Sea from 2008-05-01 to yesterday:

    http://i27.tinypic.com/2qc3r79.gif

    Created from the asi-n6250-*-v5_nic.png images at

    http://iup.physik.uni-bremen.de:8084/amsredata/asi_daygrid_swath/l1a/n6250/2008/

    with these ImageMagick (and bash) commands:

    for f in asi-*20080[56]*; do d="${f:10:4}-${f:14:2}-${f:16:2}"; convert -extract 384x256+448+1024 -repage 384x256+0+0 -font Verdana -pointsize 22 -fill white -draw "text 250,20 '$d'" -type Palette -colors 256 -quality 105 "$f" png8:"$d.png"; done; convert -set delay 50 -delay 200 2008*.png +clone +swap +delete anim.gif

  20. #72 Ken Feldman:

    This is a bit stale by now, but I’ve been wading through the past few weeks’ threads here and the sea level rise discussion caught my eye.

    Chao and Wu recently (2007, AGU) presented data suggesting that for most of the 20th century construction of impoundments considerably delayed/slowed sea level rise. Scrutiny of this work reveals interesting periods during WWII and in the latter years of the 20th century when reservoir construction was either slowed by warfare or, later, by substantial saturation of available reservoir sites. Chao and Wu’s identification of the slackening pace of dam construction in the later part of the last century nicely dovetails w/the observed quickened pace of sea level increase.

    Chao & Wu:
    http://adsabs.harvard.edu/abs/2007AGUFM.H11C0670C

  21. Antarctic ice shelf ‘hanging by thread’: European scientists

    PARIS (AFP) – New evidence has emerged that a large plate of floating ice shelf attached to Antarctica is breaking up, in a troubling sign of global warming, the European Space Agency (ESA) said on Thursday.

    Images taken by its Envisat remote-sensing satellite show that Wilkins Ice Shelf is “hanging by its last thread” to Charcot Island, one of the plate’s key anchors to the Antarctic peninsula, ESA said in a press release.

    “Since the connection to the island… helps stabilise the ice shelf, it is likely the breakup of the bridge will put the remainder of the ice shelf at risk,” it said.

    Wilkins Ice Shelf had been stable for most of the last century, covering around 16,000 square kilometres (6,000 square miles), or about the size of Northern Ireland, before it began to retreat in the 1990s.

    Since then several large areas have broken away, and two big breakoffs this year left only a narrow ice bridge about 2.7 kilometres (1.7 miles) wide to connect the shelf to Charcot and nearby Latady Island.

    The latest images, taken by Envisat’s radar, say fractures have now opened up in this bridge and adjacent areas of the plate are disintegrating, creating large icebergs.

    …

  24. With regard to Alastair McDonald’s Post #110:

    “The question is what will happen when the ice reaches the tipping point at the end of June and the retreat steepens?”

    We are almost at the end of July now. How much worse is 2008 than 2007 in areal ice coverage?

    And on what date this year do you project the North Pole be ice free?

    Thanks!

    jas3

  27. My appologies if this is a bit off topic. My daughter and her husband are aboard a 47′ sailing vessel in route from the Falkland Islands to South Georgia. We are trying to locate real time reporting of ice bergs, flow ice, or any significant hazards that might pose significant threats to navigation. Weather data is available via satalite but does not seem to cover ice conditions. Any help would be appreciated.