Mechanical erosion of ice sheets

[Gray-Wolf]

Do Ice sheets respond in the same way Ice dams? To my way of thinking the two things must overlap with an ice sheet being a larger version of an ice dam.

Ice dams can catastrophically fail due to the erosional force of supercooled water mining at their bases, is this also true of ice sheets?

I ask the question as, for me, alarm bells ring every time time I'm told that the Greenland ice sheet will take many decades to ablate completely. In a period when we are starting to uncover alarmingly fast periods of climate change (both positive temps and negative temps) we need to fully understand what drives these changes.

From my limited understanding of ice sheets I was already aware that melting (due to friction and pressure) at the base allowed for basal erosion but can this fataly affect the ice sheet itself?.

With ever larger meltwater lakes draining into the ice over the summer melt the potenial for deep 'supercooled lakes' to form within/at the base of the ice must exist (the pressure/friction at lower levels effectively 'warm' the ice at lower levels in the way rock 'heats up' with depth/pressure) and they in turn exploit any fractures that ice sheet movement creates at depth within the sheet.

Recent recordings of mag. 5 tremors within the greenland ice sheet make me think that large failures (due to faster ice flow at the glacier fronts 'draining' the ice sheet) are accelerating in formation within the ice sheet giving greater areas for the supercooled water to perculate into.

At present, I imagine, these areas of honeycomb ice filled with supercooled water at pressure are not easily noticed as they will show a similar plot on radar scans (being of the same substance/density) to the surrounding ice. It would not be hard to see some mis-interpretation of plots occuring when the folk taking measurements are expecting to see partial melt/plastic deformation at the base of the structure.

My concerns are that when a suitable conuit to the outside world is found, either by the retreat of glacier snouts or failures within the ice sheet itself, then the supercooled water (at pressure) will explode from the base and drain the honeycombe structures at the ice sheet base(an event likely to cause massive localised erosion of the ice). This would lead to a catastophic collapse of the ice sheet above exposing larger surface areas of ice to the atmosphere and thus accellerating the ablation of the ice sheet, topside.

This event (if comparable to ice dam failure and glacial lake drainage) could take as little as 6 months to complete, a long way short of the slow (many decade) ablation we are promised at present.

Do I join the lunatic fringe now or could their be cause for concern?

[Dawlish]

HI GW,

No, definitely not. You are far removed from the lunatic fringe!

The processes you describe so well are, not surprisingly as they happen at great depth, not well understood. I see nothing in your summary to warrant white coats at all, it is just whether those processes are occurring on the scale that you theorise they may be. I tend towards a catastrophic view of ice processes, at the end of a long period of ablation/erosion, rather than the ablation/erosion being the only possibility for their demise. The evidence is there in sediments and features in the NE of the USA. It has happened before in the North Atlantic, several times, in the last 14,000 years, with the breaking of ice dams, the reslease of Lake Iroqois's waters about 13,400 years ago is one example and there are other dams which broke as the ice sheets retreated. This will probably have disrupted the THC of the North Atlantic, with the release of a massive amouth of fresh water and climates will probably have been affected.

The obvious difficulties, as I'm sure you have figured yourself, for Greenland today, are: to what extent is the ice sheet being depleted, or undermined at depth; when could any catatrophic failure occur; and how big would the failure be?

6 months, should everything be in place, could be accurate. It may not be, of course, but would that 6 month catastropic period be more likely to occur in our lifetimes, or in 2540? It's just impossible to predict. The bulk of scientific opinion would go for "not in our lifetimes", but who is really to say? Even that is a calculated guess. When will the Eastern flank of Cumbre Vieja, in the Canaries, slide into the sea and swamp us in the tsumnami that will follow? When will the Caldera in Yellowstone collapse and spell the end for the USA's global dominance (and, possibly, the end for most of the human race!). The answer is that they will happen, but the probability is that they will not affect anyone alive for many, many, generations, maybe tens/hundreds of thousands of years. Perhaps millions! This example is not, however, in that long a timescale.

The answer to your excellent questions lie in probabilities. There is a high probability that the processes are happening, a lower probability that they are happening on a widespread honycombe scale, a lower probability still that they will cause catastrophic collape and an even lower probability that this collapse will affect the THC, but what are the ratios of those probabilities and what is the resultant probability of the original processes causing the last event?

I wish I could be certain, but I have to shake my head, along with everyone else and say "I dunno for sure"! I'd lay odds on it not happening in our lifetime, but odds have that little "1" at the end of them and I wouldn't like the "1" to begin to show itself tomorrow!

Regards, Paul

Edited November 5, 2006 by Dawlish

[parmenides3]

G-W, if you click on the 'Arctic and Antarctic' link on RealClimate.org, you'll find a good article by Rasmus Benestad on Greenland's ice sheet.

I can't answer your question about ice dams, but I can say that the rate of loss in Greenland is too slow for the sort of massive ablation you suggest, and that its' contribution to sea levels and freshwater levels is relatively small.

:)P

[Gray-Wolf]

G-W, if you click on the 'Arctic and Antarctic' link on RealClimate.org, you'll find a good article by Rasmus Benestad on Greenland's ice sheet.

P

Thanks for the pointer P3 but sadly (on your advice) I've been ploughing through lots within RealClimate and had already read and tried to digest the article you have guided us too.

Many of the papers released on both Antarctica and Greenland seem to take pains to point out that their modeling is based on what little we know about the dynamics of ice ablation and its geophysical mechanisms and that there is much we do not yet understand of the processes/timescales.

Over the past 10 to 15yrs their have been many revelations around the current rates of ablation at both top and bottom of the planet that have then been retrospectively analysed/explained. As such we seem to be at a place where thinking around the issue can still bring forward explanations/mechanisms to deal with what is being better observed of our ice bound regions.

Remote sensing techniques are now being applied to ice sheets but both areas of plastic deformation (where only part of a solid is 'melted' at any one time but allows 'fluid movement' within the solid) and 'veins' of supercooled, high pressure water are not easily differentiated from the surrounding ice as they are ,effectively, all the same substance with similar density and pressure responses.

I would dearly like to believe both Paul and P3 regarding the likelihood of us having a very rude awakening one spring time soon but having a subject painted as 'understood' when you know full well that it is not doesn't tend to promote reassurance in this or that 'finding'.

If catastrophic failure of ice sheets were purely an object of amusement/debate then things wouldn't matter so much but if sudden catastrophic failure occured, at either end of the planet , then we very quickly be dealing with an event that could doom all attempts to mitigate climate change.

[parmenides3]

As you rightly say, this is a very complex subject made more difficult by the relative lack of data. However, on the issue of ice sheet collapse, I can suggest a couple of pointers.

First off, the Greenland Ice sheet isn't vulnerable to a mass 'collapse' as a whole, because the main part of it, in the centre, is at a lower altitude than the edges; it sits in a large hollow, effectively. The edges of the ice, however, are much more vulnerable, as they lose mass through glacier outflow and, to some extent, evaporation, if the conditions are right. This is why, even should mean temperatures rise by four degrees, which no longer seems unlikely, it would still take 2-3000 years for all of the ice to be lost. The concern that some climatologists have is that the process will be irreversible, so effectively the world would be changed for thousands of years.

In the Antarctic, the concern is over the stability of the West Antarctic Ice Sheet [WAIS]. It has been suggested that, through the processes you describe above, this huge mass of ice could run into the Southern Ocean over a few hundred years. But for this to happen, the Ross Ice Shelf has to first collapse. Is this possible or likely? The research I've read says yes, if there is no effort made to curb warming, but we are still talking 1-200 years. But the uncertainty over this is much greater than the situation in the Arctic. I suppose it is not impossible that the Ross Shelf could give way this century. This alone would raise global sea level by some metres. Following that, the calving from the WAIS glaciers would continue to add to sea level rise for centuries to follow. I can look for the paper on this if you are interested.

So, in response to your suggestion of a 'rude awakening'; I suppose, as time goes on, it becomes more likely. It all depends on what you mean by 'soon'.

:)P

[Gray-Wolf]

In the Antarctic, the concern is over the stability of the West Antarctic Ice Sheet [WAIS]. It has been suggested that, through the processes you describe above, this huge mass of ice could run into the Southern Ocean over a few hundred years. But for this to happen, the Ross Ice Shelf has to first collapse. Is this possible or likely? The research I've read says yes, if there is no effort made to curb warming, but we are still talking 1-200 years. But the uncertainty over this is much greater than the situation in the Arctic. I suppose it is not impossible that the Ross Shelf could give way this century. This alone would raise global sea level by some metres. Following that, the calving from the WAIS glaciers would continue to add to sea level rise for centuries to follow. I can look for the paper on this if you are interested.

P

And then there is the 800m deep,50km long 'ruck' in the ice that BAS discovered in 2004. They seem to think that this shows very rapid ice movement but reassure us that responses of the sheet take many centuries to manifest and so any climatic influence happening today would take several centuries to 'work through' the system.

Well what if the 'ruck' (deep folding within the ice sheet) were a response to rapid 'blocking' of the ice sheets movement by the build up of ice along the coast forming the ice shelves? What happens when the shelves calve away into the Southern oceans and take off the handbrake? Are we looking at something similar to the release of tied up motion along conservative fault margins? Without the dating evidence of the ice folding we can only surmise the age via the depth but when this is distorted how can you be sure without ice core dating evidence?. I wish I could rest as easy on either the 'odds' of change or the current scientific opinions but I feel I have had my fingers burned quite badly in the past by my own poor researching and the overconfident (believable?)presentation of data to support certain personal 'pet' theories.

Many times the 'first look' at something leaves you with a 'gut feeling'. This is not a poor scientific response on our part but an acceptance that we are a fabulous piece of kit that can process so much more information than the consciousness can grasp at any one moment but unconsciously the thing that has caught our eye/mind and brought about our disquiet is slowly working to find a way out (the 'Eureka' moment).

This is how I 'feel' about catastrophic collapse of the ice sheets (Southern Greenland and the WAIS) I hope it is just the onset of paranoia

[parmenides3]

I don't know enough about the mechanics of the WAIS to say whether or not this is a likely scenario. I don't even think that it is necessary for there to be an ice dam effect for us to be severely affected by a colllapse of the Ross Ice Sheet and an opening up of the WAIS glaciers. I suspect that we'd be so much in schtuck if that happened, we might not even be in a position to notice if the whole lot came down.

My gut feeling? The Ross ice shelf will be at risk from about 2030 onward. Beyond that, I just don't know.

:)P

[Gray-Wolf]

I don't know enough about the mechanics of the WAIS to say whether or not this is a likely scenario. I don't even think that it is necessary for there to be an ice dam effect for us to be severely affected by a collapse of the Ross Ice Sheet and an opening up of the WAIS glaciers. I suspect that we'd be so much in schtuck if that happened, we might not even be in a position to notice if the whole lot came down.

My gut feeling? The Ross ice shelf will be at risk from about 2030 onward. Beyond that, I just don't know.

P

If we go with what your Adrenals for tell then we will have amassed enough data to be fully prepared for the eventuality. If we were to be faced with it over the next 15yrs then I seriously doubt our capacity to respond.

[parmenides3]

If we go with what your Adrenals for tell then we will have amassed enough data to be fully prepared for the eventuality. If we were to be faced with it over the next 15yrs then I seriously doubt our capacity to respond.

Fat chance! The level of funding for Antarctic research is pathetic, and the work done astonishing in spite of this. There is some ongoing work at NOAA/NSIDC with satellite measurement, and the Australians have a half-decent programme, but there is no way we'd be prepared for a Ross Shelf collapse at any time in the next 50 years; just think of all of the housing developments, power stations and cities being developed along the low-lying coastal areas of the world.

The kind of event you are contemplating appears to be the 'catastrophe'-type. I'm not accusing you of scaremongering, by the way, it's a proper term. The trouble with the mathematics of catastrophe is that the event itself is by definition unpredictable; we can see that its probability is moving along a curve towards an event horizon, but we cannot be certain when the timeline will cross it and become reality. This is one of the reasons why climate scientists like to use probability analysis when looking at future possible events; it is just about the only way they can relate the hypothetical event to the real world. The trouble is, when they give a number, people tend to focus on this alone, instead of understanding the nature of the event, and complain that the science 'got it wrong'. Then they get complacent and get caught out when it does happen.

Sorry, I'm waffling.

:)P

[Gray-Wolf]

After looking at the Meto's CET archives we have ,historically, hit the 10*c yearly CET on regular occasions. If we have this 7 to 14 year 'warm spot' then this could provide a mechanism for short term upward lurches in temperature every so often leading to very exceptional years which would then trigger proposed 'tipping points' in the Antarctic nudging us towards catastrophic failures.

[parmenides3]

After looking at the Meto's CET archives we have ,historically, hit the 10*c yearly CET on regular occasions. If we have this 7 to 14 year 'warm spot' then this could provide a mechanism for short term upward lurches in temperature every so often leading to very exceptional years which would then trigger proposed 'tipping points' in the Antarctic nudging us towards catastrophic failures.

With the uncanny internet synchronicity factor going into overdrive, I find this evening three pieces on this subject. There is no question, G-W, that you should follow these ones up. The first reference was linked to from a post by Andrew Dessler about 'tipping points', which can be found on 'Gristmill.com'. The other two are the latest offerings from RealClimate.org, who are on good form this week: So; 'Runaway tipping points of no return'; How much CO2 is too much?' and the title of the third one eludes me for the moment. For the first: http://www.realclimate.org/index.php/archi...s-of-no-return/ , for the other two, the first two posts on; http://www.realclimate.org/ . Make sure you click the 'more' link to read the full story.

Included in the third is a discussion of rapid climate change recorded in ice-cores, Dansgaard-Oeshger events (rapid warmings), and the possible causes of abrupt climate shifts. Enjoy.

Edit; That's it; 'Revealed; secrets of abrupt climate change'. Could we possibly ask for better?

:)P

Edited November 8, 2006 by parmenides3

[Gray-Wolf]

Though I've read (and digested?) the papers you highlighted none really soothes my concerns as I believe we've (the planet) never been where we are today. If we are to accept that we (humans) are influencing the workings of our climate then it doesn't take a great 'leap of faith' to see we are entering unchartered territories as far as climatic responses are concerned.

If I were to make reassuring noises about what the picture in a jigsaw showed with only 3 pieces of a 1,000 piece jigsaw in you'd think me mad. At times, as 'new evidence' supersedes old, I get the impression that this is how climate change is being understood. I know we have to give it our best guess on the evidence we do have but to phrase it in a way that paints it as the 'full picture' not only is misleading but also diminishes the impact of any further 'discoveries'. Like food/health scares in the media people start to think/say 'but they said...........last time' and the new information is discarded.

If the 'natural cycle' brigade are as right as the AGW brigade then maybe we have 2 problems that are effectively augmenting the effects of one another leading to a much more rapid development of the effects of GW. If we take this as a possibility then not only do we need to understand how our planet used to work but also what our influence can do to these 'natural cycles'

The fact that past warming takes a while to circulate through the THC before presenting in the Southern oceans could mean a very unfortunate coincidence of natural warming surging our AGW to a point where 'tipping points' are breached. Once certain events have occurred then even if temps reduce the damage will have been done and the changes will be unstoppable.

[Guest Viking141]

With the uncanny internet synchronicity factor going into overdrive, I find this evening three pieces on this subject. There is no question, G-W, that you should follow these ones up. The first reference was linked to from a post by Andrew Dessler about 'tipping points', which can be found on 'Gristmill.com'. The other two are the latest offerings from RealClimate.org, who are on good form this week: So; 'Runaway tipping points of no return'; How much CO2 is too much?' and the title of the third one eludes me for the moment. For the first: http://www.realclimate.org/index.php/archi...s-of-no-return/ , for the other two, the first two posts on; http://www.realclimate.org/ . Make sure you click the 'more' link to read the full story.

Included in the third is a discussion of rapid climate change recorded in ice-cores, Dansgaard-Oeshger events (rapid warmings), and the possible causes of abrupt climate shifts. Enjoy.

Edit; That's it; 'Revealed; secrets of abrupt climate change'. Could we possibly ask for better?

:)P

Hi P3

Interesting you bring up Dansgaard-Oeschger events, something which, Im sure you'll recall, I discussed with you several months back. The speed of abrupt climate change is stunning, it can happen in a matter of a decade or so. Also interesting to note however, is that the follow-on from a Dansgaard-Oeschger event is a sustained and often severe period of cooling, including glaciation, and not a run-away greenhouse effect.

Edited November 22, 2006 by Viking141

[Gray-Wolf]

Hi P3

Interesting you bring up Dansgaard-Oeschger events, something which, I'm sure you'll recall, I discussed with you several months back. The speed of abrupt climate change is stunning, it can happen in a matter of a decade or so. Also interesting to note however, is that the follow-on from a Dansgaard-Oeschger event is a sustained and often severe period of cooling, including glaciation, and not a run-away greenhouse effect.

The DO events, being so sudden, upset the Eco-system by there impacts on the climate. Unfortunately in todays world any cooling would lead to further increases in greenhouse gas production as people turn up the central heating (especially all the newly fledged 'moderns' in China/India) so I don't think we can draw parallels from past events.

The other thing of course is the fact that in a warming world the signals that promote DO events seem to be swamped and so have no climatic effect (and have not had any effect, that can be gauged from paleo-climatic data, for many 'cycles').

However........in a system that is now being 'forced' who knows what 'straw' will impact this 'climate camel' to the point of major climatic upsets? As posted above I feel that what would once have been a minor fluctuation could, in todays warming world, have a major impact on some of the more 'finely balanced' climatic systems leading to DO scale events.

[Stratos Ferric]

It's an interesting thread, and as others suggest, probably hampered above all else by the lack of data.

I know that in Iceland (with Europe's largest permanent ice sheets) there are sub sheet volcanoes, one of which went off at the time I was there a couple of years back. As an analogue for what you're talking about this is a crude model, but better than nothing. The eruptions do not seem to destabilise the ice sheet, which speaks volumes for the insulating properties of ice, but they do cause huge basal melt, leading to enormous floods out of the mouths of the tributary glaciers; the last huge eruption was around 20 years or so ago as I recall.

The ice sheets are so deep that the base ice will be under huge pressure, and must be super cooled to not be melting simply under pressure alone, other than at the very surface. This would, to my mind, generally legislate against honeycombing, but surface meltwater could easily find ways to exploit flaws, just as rainwater does in rock (which is much denser and less soluble), so some kind of flawed superstructure becomes possible.

The theory about a tipping point is very easy to accept. Anyone who has watched a snow covered mountainside clear of snow on a sunny day can appreciate this, how once a small part of the surface is exposed the albedo drops and rapid warming occurs, so speeding up the melting, and giving a +ve feedback loop.

[MAF]

Hello G-W, thought this may add to the debate

http://news.bbc.co.uk/1/hi/sci/tech/6171053.stm

[Gray-Wolf]

Thanks for that! It may be a product of my early conditioning but I do get the feeling that we do not have all the 'pieces' of the climate change jigsaw and some of the missing parts are crucial to our understanding of how our planet reacts to 'forced warming'.

The Polar ice sheet is just another factor in the equation yet it seems to keep surprising us with its behaviours (some natural/cyclic , some not).

So far as it's mechanical erosion is concerned I can see that not only warm water currents and raised ambient temps are at work here. This Autumn, with it's Atlantic activity, illustrates well how initial ice gains can be whittled away by both the warm air atlantic storms push northwards and also the warm waters that get driven northwards by the winds field (measured increased speeds in the northern most section of the N.A.D. this autumn). You must also consider the swells that lift, strain and break the seaward edges of the ice pack. Whilst watching the storms roll through we've all witness 'Magic Seaweeds' swell predictions so we know how big the seas have been around our coasts so what of further north? (esp. when you've a 932mb storm off Greenland). The second polynya that opened up this season did so in November showing that warm water upwelling happens far under the ice pack even well into the 'cold season' if the right synoptics prevail so how far does this 'basal erosion' project under the pack weakening it's integrity as a solid sheet?

When an ice sheet is broken into sections it presents a larger surface area to the ablating forces and so the 'melt' is accellerated. How long befor the sections of Polar ice sheet facing open ocean are ablated completely leaving only a yearly build up to melt the following spring? Eventually you can envisage a time when a reduced ice pack is able to be driven by the wind to force the 'landward edge' into shallower, warmer waters each year eating further into the ancient ice pack and further reducing it's stability.

[Dazza]

Not wanting to hijack your thread G-W but I was reading this article this morning and thought it was relevant to the topic.

[Gray-Wolf]

Not wanting to hijack your thread G-W but I was reading this article this morning and thought it was relevant to the topic.

Now I have a strange sense of Deja-Vu

[Dazza]

Now I have a strange sense of Deja-Vu

Oops, apologies G-W, didn't relise Mick had got in there first

[Gray-Wolf]

Does anyone have any info on the current SST anomalies around NZ? I can only assume that this is connected with the Antarctic summer and the amount of cold water flowing into the southern oceans. Am I wrong in my thinking?

Edited January 6, 2007 by Gray-Wolf

[Mondy]

Really don't know, Gw. Trying to help you so dunno if you've clicked on the NZ MetService yet for assistance?

Edit: Played about with that site, not much help with Southern Ocean SST's..

Edited January 6, 2007 by Mondy

[Gray-Wolf]

Cheers Mondy! I'd already rumaged around down there but all I can 'discover' is a load of scientists/meteo types 'pouring cold water' on global warming (their words). Made me smile with the current SST's and their coldest christmas on record down there........maybe it wasn't the right time for them to release their paper?

NZ is to the NW of the Ross ice shelf and the expanse of sea ice that surrounds it . The sea ice seems to be in the last stages of melt but a few weeks back the melt started from the inside (a polynya formed) which made me think that the melt and outflow from the Ross area was actively eroding the sea ice from below. I guess the next two months will show how grounded both my fears and the Ross ice shelf is!

Current Sat photo of ice melt in front of the Ross Ice sheet and my trace (in yellow) of meltwater outflow/cold water anom.

Ross

My outline of what may be an outflow of cold meltwaters from beneath the Ross Ice shelf/interior.

This is what I would have expected to see if and 'ice dam' below the Antartic Ice cap had given way releasing 'backed up', subterranean waters (the reason for the post origionally) into the southern oceans. For the past 2 months the sat images seem to portray just such a huge flow of cold waters into the summer southern oceans (I mean, just look at the scale of this thing!!!)

Awaiting reassurrance that I am mistaken/mad.

Edited January 7, 2007 by Gray-Wolf

[parmenides3]

I think you are misunderstanding the dynamics of the Antarctic melt process slightly, G-W: it is normal for the Antarctic to melt both from the edges and the coasts at the same time. There is a discussion of this on the MMAB sea-ice pages, with some nice maps, and similar on the NSIDC pages.

My guess would be that the anomalies, as opposed to the temperatures, are as much a function of timing as anything; if the melt season is a couple of weeks 'off-schedule' it will come up as a strong short-term anomaly. I have also read in a recent paper about Southern Ocean cold water upwelling (vertical flux), so this may also be playing a part in the current anomalies.

For SST discussions, your best bet might be NOAA, in the first instance.

:)P

[Gray-Wolf]

For SST discussions, your best bet might be NOAA, in the first instance.

:)P

Cheers P3!,

I shall have a gander. The NZ islands are part of a ridgeline that breaks surface there so any basal flow of 'cold' water would be forced higher in these areas.

As you point out my understanding of the mechanisms there is in it's infancy but I would imagine that this is the kind of thing we would see if a large area of water (subterranian lakes beneath this ice sheet) broke the final obsticles and flowed out into the oceans there. If you watch the past 30 day simulation of ice melt from Cryosphere today you can see the ice fronting the Ross ice sheet ablates in the general direction of NZ (the NW portion of sea ice melting in advance of the rest). We should also be open to the partial break up of the front of the ross ice shelf this year.

Last years anoms 3/1/06

This years anom 5/1/07

A very coincidental 'upwelling'?

Edited January 7, 2007 by Gray-Wolf