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wili

Great video. The full screen icon seems to be just off the screen. Any way to correct that?

wili

Also, Greenland doesn't really come up till the third video. Is there any way to rearrange them so the sequence starts with that one?

wili

Apologies for multiple posts. I think it may be the video at this link that you want at the top:

http://www.yaleclimatemediaforum.org/2013/07/this-is-not-cool-yale-forum-video-details-melting-of-greenland-ice-sheet/

wayne

The greatest impact of less or more sea ice is the temperature of the Atmosphere as a whole, which causes different pressure heights, with a lot of sea ice you have a colder atmosphere, the pressure height of a cold atmosphere compared to the much warmer (higher) one further South causes the very existence of the jet stream. Even with rapid AGW the tropics warm less than the Arctic, the jet streams become more jagged and less continuous (slower in some streams).

We witness the impacts of this right now, because of frequent persistent recent Polar cyclonic events, there is less ice in the North Atlantic Arctic ocean area. The cold temperature North Pole has shifted more frequent over Northern Ellesmere (normal area) as opposed with last summer when the CTNP hanged about North of Spitzbergen. The result is a different jet stream pattern affecting weather over a wide part of the Northern Hemisphere.

The Arctic gets overlooked by most scientists, Dr Trenberth focusses too much on atmospheric system heat injections, without looking at the pressure height impacts. But he is right with negative feedbacks driven by the latest adiabatic causations created by thinner ice.

Susan Anderson

Wonderful. I hope this collection gets pushed out as much as possible. Bigger picture is very useful. Rich! Thanks.

Neven
Also, Greenland doesn't really come up till the third video. Is there any way to rearrange them so the sequence starts with that one?

That's weird. Somehow the link I embedded kept getting converted to 'playlist', but I think I got it now. I also adjusted the width of the video.

Thanks for letting me know.

Larsboelen

Does anybody know if Newtons Laws of Motion also apply to Ice Sheets?
If the whole Ice Sheet gains momentum, top to bottom, wow, what is going to bring that to a halt?

Bob Bingham

The Greenland ice sheet is a big worry as it has a lower latitude than the Antarctic and is also prone to more pollution. If melt water turns the interior of the ice into a rotten core slush it could disintegrate quite quickly and put us al at risk.

Blaine

Lars: Of course Newton's laws apply, but the momentum term is negligibly small, simply because ice moves very slowly and the stress terms it's compared to contain terms with the weight of the ice sheet times years in them.

What they're talking about is a heat diffusion thing. It takes many thousands of years for heat to diffuse deep into or out of the ice sheet. If it's still cold, as most of it is today, that means there has been very little meltwater there for the entire Holocene. We now have moulins dropping meltwater into much of the area where it hasn't been before during the Holocene. This will quickly increase the temperature of the ice (up to freezing), resulting in a step function increase in deformation motion. Much of the base will also unfreeze, resulting in sliding motion as well, which friction will tend to keep going once it starts.

The "news" is this press release is detection of a small amount of increase in glacier motion which would have been shocking had it not been detected. The increase in the ice sheet area producing moulins last year and in other recent years was the truly shocking development.

Glacial flow will be very much faster, but we're still talking about >1000 year time scales, because the ice sheet is huge, and glacial movement is still, well, glacial.

Andrew Dodds

Larsboelen -

Newton's laws apply to everything.. but the ice sheet does not have much momentum. In that way it's a bit like continental drift - if you calculate the momentum you find that it's so small that, for instance, the human population of a continent could stop it by all pushing together. It's the continually-acting forces that give the *appearance* of a process with momentum.

(This can be counter-intuitive..)

Personally, I find the situation of the WAIS more worrying in terms of rapid sea level rise.. although I suspect that the southern GIS is already doomed as well. It really is a case of working out 'how quickly' more than 'if it melts'.

Ian Allen

25.9C recorded yesterday at Maniitsoq Mittarfia (Sukkertoppen Airport) SW Greenland. Looks like a national record to me, does anyone know for sure?

Doomcomessoon

According to DMI, the warmest temperature ever recorded on Greenland (records going back to 1958) is 25,5 degrees. So I think we may be looking at a new record.
http://www.dmi.dk/dmi/en/index/klima/klimaet_indtil_nu/temperaturen_i_groenland.htm

Kevin McKinney

"Glacial flow will be very much faster, but we're still talking about >1000 year time scales, because the ice sheet is huge, and glacial movement is still, well, glacial."

Blaine, there is a lot of sense in this comment--the ice sheet IS huge, and movement is still slow--but can we really assume that "we're still talking about >1000 year time scales?" These dynamic effects are not well understood, AFAIK--this field work is presumably going into the mixer to develop ways to model them usefully--so how can we quantify the evolution of the ice sheet?

Our ASIB friend, Aaron Lewis, would no doubt point out in this connection that this process of deglaciation could be *highly* non-linear. Non-linear enough to see significant effects by, say, 2100? I don't know. But it's intriguing to note the movement of the station documented via GPS. 2% per year? I'd like to understand better how he meant that.

Jai Mitchell

if it is 2.3% per year acceleration than 10 year value of speed is 26% higher than the start.

Of course there is no absolute value given so it doesn't really say what he means. Just a ratio.

Now if he can equate this to increased melt in cubic meters per year, well that would say something!!!

Hans Gunnstaddar

One of the most interesting parts of that series of videos for me was the fact that Greenland is an oval shaped series of islands in which ice formed in the middle, rather than an ice covered land mass. This would seem to reduce the concern for slippage, i.e. a sudden cataclysmic giant ice chunk sliding off is less likely.

But slippage aside, going from .5mm ocean rise contribution to 1mm in just 10 years is a doubling in a geological blink if the eye. Also, ocean level rise of 1 foot since the dawn of the industrial revolution actually seems like a lot since the greatest melt and warm up has occurred since 2000.

Greenland's melt this year is not near as much as 2012, and one person said we'll have to wait to see if that was a one time event. Ah, excuse me, but if we've learned anything from 2012's arctic melt passing 2007's, is this situation is just getting worse not better. The Greenland melt is bound to increase over time (with increasing GHG emissions) albeit in an uneven process.

Hans Gunnstaddar

Edit: .5mm 'per year' rise...

Allen McDonnell

In the millennial length future we need to member isostatic rebound, a lot of the center of Greenland that is currently below sea level is depressed by the weight of the ice. Take away the ice and the whole center will spring up just like Canada around Hudson's bay did 12000 years ago and is still doing at a slow rate. It is debatable if our distant descendants will find Greenland to be an atoll surrounding a lake or if it will be a flat plain ringed by high mountains.

Yes I believe it will all melt.

Martin Gisser

1) I'm still looking for a good map of the bedrock. How far and how much sea water can intrude? With all the moulins, what about bottom melt lakes destabilizing the ice? Perhaps GIS disintegration will turn from glacial flow into breakup and crumbling and then the chunks being flushed out.

2) What are the latest decadal estimates of ice loss? Some years back I extrapolated assuming exponential decay and got 1m-4m SLR by 2100 (huge error bars due to nonlinearity). Gavin Schmidt declared that impossible (arguing like Morgenstern's Palmström that this can't be because it would be catastrophic). Meanwhile it seems a mainstream estimate.

wili

Hans, it's true that the ring of relatively high elevation bedrock around the GIS does mean that the whole thing won't easily just all slip right into the ocean. But do note that there are places where there are breaks in the circling ridge, some of them below see level.

So there are passages for flow in and out. And, as one of the commenters said iirc, the consistency of the GIS is approaching that of warm butter--Flow is likely to keep accelerating at exponential rates till it's all gone.

Allen, good point about isostatic rebound. Surely that will start before all the ice is gone. It might be quite a minor and slow effect, but I imagine that as the bedrock rises, it will add to the acceleration of the overlying glacier toward the sea--even slightly tipping a table that has marbles on it will set those marbles in motion.

jonthed

Everybody must surely agree, experts included, that the dynamics of an ice sheet as thick as the GIS when subjected to a change in weather as severe and abrupt as what it is experiencing, are highly complex and unknown.

But so many people's approach seems to be fairly simplistic and dismissive, 'glacial pace' crops up all the time.

But with more and more warm melt water working its way over, through, and under the ice, surely the effect is not as simple as surface melt and glacial calving.

The whole ice sheet could (will?) become riddled with cracks, holes, gullys, caverns, etc. not to mention the base lubrication, all of which massively increase the amount of surface area exposed to warmer water, and the melt and erosion that that will bring. And this is deep into the core of the ice sheet. It will be eaten away in 3 dimensional space, not just the surface. Like a bone with osteoporosis. Its integrity will weaken and parts will start crumbling.

Whilst calving might speed up with the added lubrication and melt water action, I see the largest threat to the ice sheet as the melt water itself. Eating away and destabilising the ice sheet form the inside. Particularly if we are to see larger melt areas more frequently as the weather patterns change.

I'll be very interested to see how measurements of melt water runoff and ice sheet mass loss develop in the next few decades.

Sure it might all be a bit unknown, and hard to model or predict. But we can't just assume a simple model and be satisfied with that.

wili

Good points, jonthed. Simple, and even sophisticated, models have proved fantastically inaccurate in predicting sea ice melt. I expect that we can expect the same types of surprises here. IIRC, raised levels of CO2 itself has a slightly corrosive effect on ice structure (sorry, no time to find the link now--feel free to post it if anyone has it on hand.)

Warmer ice also seems to change albedo since it becomes more hospitable to biological activity.

There are lots of major and minor feedbacks going on here, and they all make the whole system devilishly difficult to model accurately, I imagine.

Kevin McKinney

"One of the most interesting parts of that series of videos for me was the fact that Greenland is an oval shaped series of islands in which ice formed in the middle, rather than an ice covered land mass. This would seem to reduce the concern for slippage, i.e. a sudden cataclysmic giant ice chunk sliding off is less likely."

Mmm, I don't think it's quite that simple. As already mentioned, there are channels to the sea. Moreover, glacial movement isn't controlled primarily by the subsurface; the ice thickness also has a lot to do with it. That's illustrated, for example, by the observation that the terminal regions under a number of glaciers (such as Antarctica's PIG) have a negative slope for some distance inland.

That means that ice, to a point, can and does flow uphill. You just need ice piled sufficiently high on the downhill side to force the flow up and over.

And there's quite a lot of gravitational potential for such a thing, considering that:

"The mean altitude of the ice is 2,135 metres (7,005 ft). The thickness is generally more than 2 km (1.24 mi) and over 3 km (1.86 mi) at its thickest point."

http://en.wikipedia.org/wiki/Greenland_ice_sheet

Compare that to this elevation map of the bedrock:

(Hope that image embed works!)

More technical, but less directly relevant:

http://www.cpom.org/research/jlb-jgr.pdf

Kevin McKinney

Argh.

Or, alternately:

Kevin McKinney

I also wonder about what happens to the outlets, if slippage increases? Do they get cut deeper, as would seem reasonable, and if so, how fast?

I'd tend to think "not very," on 'human timescales'--but still, probably quite a bit faster than isostatic rebound.

Hans Gunnstaddar

Kevin quote from my previous post: "This would seem to reduce the concern for slippage, i.e. a sudden cataclysmic giant ice chunk sliding off is less likely."

Mmm, I don't think it's quite that simple.

Actually it is, because what I thought before was Greenland was a big land mass, and as such steep grades could potentially result in huge chunks of ice sliding, crashing into the ocean causing tsunami's. In this case ice may move quickly through gaps in the ring of islands but the scenario I thought was possible isn't.

But thanks all for the added information.


Lennartvdl

Kevin,
Bamber et al 2001 has been recently updated by Bamber et al 2013:
http://www.the-cryosphere.net/7/499/2013/tc-7-499-2013.pdf

It's not clear to me if they think the Jakobshavn glacier extends all the way inland below sea level or not, but it seems it might.

Jai Mitchell

It seems to me that the movement he is talking about is due to plastic deformation, not "slippage" in the classic sense.

The observed flowrates from 2012 are here:


The critical juncture happens when the strain of rapid plastic deformation away from the center causes the equivalent of a brittle fracture all along the rim of the Greenland land mass.

Not sure if this is even a likely situation, but it seems that if the center ice does warm considerably with melt moving down though the glacier then this plastic deformation would increase exponentially.

Jai Mitchell

bah! what is the code for height and width at 360?

img src="http://web.ornl.gov/info/ornlreview/v45_3_12/images/a08_p20_lrg.jpg" height="360" width="360"

Paul Pentony

Apart from isostatic rebound, the loss of Greenland's ice sheet and its gravitational pull would result in a lowering of the sea level around Greenland even as average global sea levels rise. This would be countered to some extent by loss of ice in Antarctica.

Dan Ellis-Jones

Hans - I can see what you're getting at, but I'm not as sure as you are (but of course respect your opinion!). If the ice sheet is 2km-3km thick, and the ground underneath is only, say 500m high, then there is a huge amount of ice sitting on top, which COULD go crashing into the sea, causing a gigantic wave to funnel through the CAA, or across the NA and funnelling up the Scandinavian fjords – now that would be interesting!

That's assuming that I think that a massive chunk of ice would break and fall into the sea. I'm not so sure, again. It would take very quick and very extreme warming to provide such instability. I have no scientific back-up on that, just using my logic!

I completely agree with jonthed. If we've learnt nothing else from the scientific modelling and predictions over the last few decades, is that natural systems are fiendishly complex and will do what they will - more often than not quicker than we expected. Mere super-computer models can go jump. This would be far more comforting if there was a higher percentage of the models OVER-estimating the impacts – but I feel they mostly under-estimate them.

I think the future of the GIS is best found in the Paleo-glaciology books - What happened to glaciers under similar conditions in the distant past? This gives a higher chance of telling us the outcomes of the multitude of positive and negative feedbacks that are in play, instead of just guessing (informed or otherwise) about their outcomes now.

The fatal flaw in that is the likelihood that we are really in unprecedented times, so we have no paleo records that align with anything like todays conditions (with such a massive and rapid rise in GHGs – I think the rate of change is not discussed enough). But I also warn that we (as a species) tend to over-emphasise the current time, and think 'today' is unique, for instance believing we are the cleverest incarnation of humans ever - which I very much doubt!

Espen Olsen

New temperature record (25,9 C)for Greenland.

From DMI but in Danish:

http://www.dmi.dk/nyheder/arkiv/nyheder-2013/groenland-saetter-temperaturrekord/

Fairfax Climate Watch

hydrofracking, popular with natural gas extraction companies and melting glaciers, but not so popular with computer models of glaciers

Lennartvdl

From Bamber et al 2013 (linked above), pp.506-507:
“In the Jakobshavn catchment, there is a dendritic channel system extending for about 325 km from the current grounding line into the interior almost as far as the ice divide. It seems likely that this is a palaeo-fluvial feature that predates ice cover in Greenland and may be important for subglacial water routing… The width of the trough 3–4 km and the region of fastest flow coincides fairly well with the location of the deepest ice. The trough is 1366m below sea level at its deepest point compared to a maximum depth over the entire region of 556m below sea level in the older dataset. The main trough of Jakobshavn Isbrae is not continuous in the new dataset, disappearing around 100 km and reappearing at about 140 km. This does not imply that the trough is discontinuous, but only that there are insufficient data to confirm the trough’s presence or otherwise in this region.”

So maybe warming ocean water could in time eats it way into the interior bottom of the ice sheet?

Kate

Grief, the clouds around GL right now are looking awesome!

The cyclonic pattern is so clear, and strong. It's really pretty. And I dare say carrying warm humid air from the US.

I would love to see a huge chunk of ice carve itself from GL. The latest sat pics of the east coast show huge long trailing melt ponds that must be affecting glacial flow rate.

The effect of all this will hit us in the near future. If the rate of warming is what we think it is, we will see physical changes, that would normally be seen over centuries, in the space of years.

Kate

And there is a new circulation in the CAB :/

Kevin McKinney

"bah! what is the code for height and width at 360?"

I think your code is fine, but is overridden by the Wordpress settings. You have to resize the image separately before posting, I think.

There are quite few free sites letting you do that online, like this one:

http://www.picresize.com

(NB-Haven't tried this one, just the first search result.)

Mdoliner43

Would it be possible for warm water to slip into the inland lake to hollow out a huge ice dome from which waterfalls from the melt ponds above would pour forth. Cool!

Jai Mitchell

Thanks Kevin!

Pete Williamson

I don't get what the guy in the hat in the video is trying to say here, there seems to be some wonky logic.

"Starting to slip" suggests it wasn't slipping before, backed up by "did nothing before" and "frozen at the base". Then he's talking about "acceleration of 2-3%". Can you measure an acceleration from 0 to something in terms of %? I actually thought that pretty much the whole of the ice sheet is on the move.

Starting to slip???? Surely it's (almost) always slipping????

Allen W. McDonnell

@ Pete,

I think what he is trying to say is the ice where they put the GPS tracking tower was frozen all the way to the base and not moving when they first set it up. In the last five years melt water has penetrated all the way to the base in that area and it has started moving, gaining a couple percentage points of speed each melt season since it became unstuck. I have seen similar remarks from other researches on the GIS, not sure if they are anywhere near this location though.

TenneyNaumer

Rather than worry about glacier outflow and collapsing this that and the other, wouldn't it be better to focus on the portion of the ice sheet in southern Greenland, which is going to melt like crazy and run off in great streams? How much sea level rise would that cause?

Fairfax Climate Watch

Here's something to think about: the meltwater is likely entering the groundwater underneath the ice sheet in those places where there is highly permeable bedrock. But eventually those systems of groundwater pores will fill to equilibrium pressure with the ice overhead and stop accepting water. Then if the pressure drops (e.g. from thinning ice overhead), that pressurized groundwater which will also now be geothermally heated would start to actually come back out.

Craig Merry

So could meltwater actually work more towards the center of Greenland and not out to the ocean? I mean- I usually think of GIS as one thing sitting in a bowl without much meltwater underneath or should I say- beyond where melt ponds form.

I'm wondering if its possible that say the southern tip starts melting so much faster than can escape that it causes adverse affects to the GIS.

I really don't know that much, but a simple "no- because" is awesome - thanks!

Lewis Cleverdon

Does anyone have a topographic map showing access channels from the sea to the interior of the GIS ? If so, please could they get it posted ?

The one above kindly posted by Kevin clearly shows 3 low points in the mountain ring, but no means of sea access. Large surface melt pools within the mountains' internal watershed (which increase in number annually as they gain altitude each year) have been observed to drain very fast once their moulin gets 'unplugged', indicating an extant arterial system of a large capacity, which must logically form a riverene pattern flowing to the lowest point in the bowl.

From this it follows that melt water from 100s of kms south must feed through arteries to the lowest point, before meeting arteries from just inside the watershed at Jacobshaven, which are now acting as outflow channels.

This would imply that each melt season a watertable rises above the level of the 3 outlets, lubricating their glaciers but also softening the interior ice by raising its temperature.

Can anyone clarify whether or not interior collapses are to be expected ? Their blocking of the arterial system would greatly raise the watertable in summer, degrading ice higher in the sheet. On being flushed out, increased internal voids holding additional meltwater would be formed, raising the annual heat transfer.

With the GIS being over 100 to 1 in width to height, and with surface melt pools occurring further inland each year, an inland collapse of the sheet into an open annually expanding lake on a latitude with Jacobshaven would seem the logical outcome of that hypothetical internal collapse process.

A critical set of data that would at least clarify the rate of increase of meltwater input is the area within the ring watershed where melt-pools are forming. If anyone can post a link, I'd be most grateful.

Regards,

Lewis

Fairfax Climate Watch

...As the ice sheet surface warms, meltwater hydraulically fractures into the sheet. That same process plus bedrock porosity means the water also travels into and out of the bedrock through both hydrofracking and pressurized pore flow.

Martin Gisser

CC, those bedrock maps aren't detailed enough. Are there channels from the sea to the interior? E.g. below Peterman glacier: The wikipedia map suggests there might be something (interpolate possible errors). But I've also seen something (forgot link) that suggests a barrier not far behind the terminus. Similarly for Jakobshavn.

Meanwhile I strongly suspect these questions are still open.

Lennartvdl

Look at figures 3, 4 and 5 in Bamber et al 2013, as linked above earlier:
http://www.the-cryosphere.net/7/499/2013/tc-7-499-2013.pdf

On pp.506-507 they say, as also cited above:
“In the Jakobshavn catchment, there is a dendritic channel system extending for about 325 km from the current grounding line into the interior almost as far as the ice divide. It seems likely that this is a palaeo-fluvial feature that predates ice cover in Greenland and may be important for subglacial water routing… The width of the trough is 3–4 km and the region of fastest flow coincides fairly well with the location of the deepest ice. The trough is 1366m below sea level at its deepest point compared to a maximum depth over the entire region of 556m below sea level in the older dataset. The main trough of Jakobshavn Isbrae is not continuous in the new dataset, disappearing around 100 km and reappearing at about 140 km. This does not imply that the trough is discontinuous, but only that there are insufficient data to confirm the trough’s presence or otherwise in this region.”

So it seems there may well be a connection below sea level into the interior bottom of the ice sheet. Looking at their figure 3a this may also be the case from Petermann Glacier into the interior.

Espen Olsen

Speaking of slipping ice sheets, the next Petermann calving may be earlier than expected:

https://forum.arctic-sea-ice.net/index.php/topic,53.msg11365.html#msg11365

Espen Olsen

Talking about slipping, Petermann may calve earlier than expected, have a look for yourself over at the forum!

Susan Anderson

Martin Gisser:

You asked for a better topographic map of Greenland bedrock, and though this is not the one I remember, it's better than the standard one.

http://nsidc.org/cgi-bin/atlas_north?zoomdir=0&zoomsize=2&zoom_to=&glossary_term=&imgxy=250.0+250.0&imgext=-2429169.261523+-2544017.238477+-169958.261523+-284806.238477&layer=land&layer=coastlines&layer=copyright&layer=&layer=greenland_bedrock_elevation&layer=&layer=&layer=&layer=

I've left the long URL as I made some choices to get this map - you might like to fossick around. There's a legend on the left.

Susan Anderson

MG, if you haven't gone over to the forum (needs signin to look at images), please do. More topography, in detail.

Doomcomessoon

It seems to me that many people are not so worried about GIS because it is surrounded by mountains and therefore can't just slip into the ocean and disappear like the WAIS. However, I still think Greenland can melt really fast, first of all because GIS is not situated on top of a mountain range, like parts of East Antarctica, just surrounded by them. This means that the icecap can flow virtually freely towards any place along the western edge, as the ice melts there, because it don't have to flow through mountains, which again means that the rate of which GIS is going to melt is almost entirely determined by the melt on the edges and not the melt (or lack of melt) on summit camp and the central inland. In other words, the cap doesn't have to melt in place just because it can't all end up in the ocean, it will instead flow towards the warmest area inside the Greenland "mountain ring", and such hot spots are allready popping up on the western edge (Just look at Jai Mitchells map and see how the ice is allready flowing towards the south western edge even though there are no outlets into the ocean there).

What I think will eventually happen when the edges have torched for long enough, and when the flow away from the inland has been accelerating for a while, is that the central parts of GIS will start loosing the high altitudes that have kept it cool, because parts of its volume has slipped away (Mind you that only 100 meter loss of altitude leads to almost 1 C of warming). Combine that with some decades more of global warming and the melt will really take of.

Craig Merry

Truly fascinating. Horrifying also.

If there's a need for research support on Greenland- I'd do it.

Kevin McKinney

"...the icecap can flow virtually freely towards any place along the western edge, as the ice melts there, because it don't have to flow through mountains, which again means that the rate of which GIS is going to melt is almost entirely determined by the melt on the edges..."

_doomcomessoon

FWIW, I think that's a great observation. Greenlandic microclimate effects are huge, so if these dynamic flow rates keep accelerating melt could increase a whole bunch. And shouldn't the high melt rates we're seeing keep increasing said flow rates?

I think that this issue is just crying out for proper quantitative study. As I said above, I haven't a concrete idea about just what the flow rate are, and what their spatial structure might be. If someone asked me, I'd guess that all this is likely to be effective on decadal timescales, but 'guess' is the operative term.

Kevin McKinney

Also, FWIW, 'microclimate' wasn't really the right term. I'm really talking about the climatic variation between, say, the ice divide and the coast--a distance of hundreds of kilometers.

Aaron Lewis

1) A gram of water vapor condensing on the surface of the GIS melts 7.5 g of ice resulting in 8.5 g of melt water on top of the GIS.
2) Water falling down a moulin converts its potential energy to kinetic energy, and then to heat. The water is at 0C because it has been transferring its energy to the ice around it.
3) Large ice sheets are complex structures subject to huge gravitational forces. Suddenly form a cylinder of ice a kilometer tall, and the ice at the base would explode. This does not happen in ice sheets because the ice at the base is under compression supplied by an infinite series of buttresses formed by ice at lower elevations. As moulins perforate the buttresses this support is lost.
4) Ice sheets are not designed by engineers with some margin of safety. They are at structural equilibrium. Suddenly formed holes (moulins) in the foundation disrupt that equilibrium.
5)Once the potential energy of a column of ice exceeds the compressive strength of the ice structure at the base of the column, the ice structure at the base of the column fails rapidly. This adds to the stress on the ice structure at the base of adjacent columns of ice; they fail, and so forth.
6) The difference between a structurally competent column of ice and one that fails is likely a fraction of a degree in temperature. Once a progressive structural failure begins, it can propagate into colder, stronger ice. Rate of propagation is a function of the total energy of the ice column. The higher the total energy, the faster the propagation.
7)Water/ice slurry from ice sheet collapse cuts its own channels. Current fjord system under the GIS was likely formed this way. Such deep fjords dug by solid ice would be wider. No, those channels were dug by cavitation in water as a walls of ice collapsed into them.

I do not much worry about the kind of slippage we see in mountain glaciers. Nor, do I worry much about plastic flow as the ice warms and softens.

Ned Ward

Kevin McKinney writes: shouldn't the high melt rates we're seeing keep increasing said flow rates?

Not necessarily. The picture is a lot more complicated.

It's true that the delivery of meltwater from the surface to the bed through moulins etc. can lubricate the base of the ice and cause acceleration. This has been observed.

But the subglacial hydrologic network can evolve fairly rapidly to accommodate increased meltwater, becoming more efficient at routing water out from under the ice. That can then lead to a period where there's less rather than more movement of the ice, because instead of lubricating the base, meltwater is just rapidly being carried away through enlarged and better connected tunnels. This has also been observed.

Robert S

Seems like a lot of the discussion here is assuming that the ice cracking and shattering processes which occur at surface pressures would be replicated at the base of the ice sheet. However, at the enormous pressures at the bottom of 2 - 3 km of ice, aren't most of the processes much more likely to be plastic deformations? Any build up of water at the base of the sheet would have to be under enormous pressure - if it started "leaking" out at the sides it would come out with enormous force, and the icesheet would then deform downwards as the water drained.

It would seem likely that moulins in the main ice sheet don't necessarily drain right down to the base, but to some depth in the ice determined by a complex set of factors including flow rates and temperature, deformation rates, etc.

For the outlet glaciers, which are much less deep, probably the moulins could drain down to base rock - seems pretty clear that they do.

Ned Ward

Water does in fact make it to the base of the ice sheet, even in Greenland, especially at low to mid elevations on the ice.

Hans Gunnstaddar

All these great questions posters have about what may or may not happen as melt increases in Greenland, suggests it would be a great project for a University to set up a model of Greenland and then test different melt rates.

I know an artist in Idaho that can take topographical maps and have a computerized controlled router remove dense foam until it matches the terrain. The foam gets covered in a thin layer of clay, then molds are made and cast in bronze or whatever material seems appropriate. This could be done for the islands and their relative spacing.

Anyway, make a scale model, put it in a walk-in freezer add ice, put it in a tank of water (scaled to the depth of the surrounding ocean using a coil to keep the ocean at a certain temperature), then cycle through the seasons at a faster than usual pace to see how the ice sheet behaves.

If someone wants to run with the ball and needs my contact in Idaho, let me know and I'll get you an email address. Hans

Allen W. McDonnell

@ Aaron Lewis

Ice sheets are under massive compression and the basal layer is very plastic, it deforms to fill any gaps in a short period of time. This has even been demonstrated experimentally by drilling ice caves deep into the ice with steam lines and then observing the way the ice deforms to fill the air space back in within a couple of days.

@ Robert S.

The Moulins form only in the melt zones and the highest part of the sheet usually doesn't have any of them in evidence. If you dump melt water on the GIS in 1975 it would refreeze very quickly. Even in those cases where the water did find a crevasse and drop down into the ice sheet the surrounding ice was so cold the melt water would refreeze long before it made it to the base of the sheet. However as this process continued summer after summer the melt water transmitted an enormous quantity of thermal energy to the deeper ice, bringing its internal temperature much closer to zero C in the process. A decade or so ago it got close enough that melt water entering the glacier was no longer refrozen before it reaches bedrock. Now it flows along the bottom lubricating the area between its entry point and where it exits closer to the sea. By the time it travels that distance it has passed under thinner and thinner ice, by the time it gets to the edge the thin ice there doesn't need much pressure for the water to escape and it acts like any other river or stream.

Hans Gunnstaddar

One added note: The ice on the Greenland model could be added in the freezer a layer at a time using different colored dyes, so when an upper layer melts it is easily visible where that water is going.

Robert S

@Allen W. McDonnell

That sounds like an interesting physics model where either the hydraulic resistance of the flow layer +/- = the pressure under the ice, or (more or less equivalently), the flow layer undergoes classic complementary pressure/flow/volume changes as it moves toward the edge of the ice. Either way the physics could be an interesting clue to the impact of the melt water on the rate of energy transfer to the bottom ice.

Crozet Dutchie

This a recent upload on the Greenland Ice Sheet (Dark Snow Project): www.youtube.com/watch?v=qkpFNteryX8

Crozet Dutchie

Ha, silly me, please ignore, it is the same one as posted at the top of the blog... There is no feature to delete the comment myself, so Neven, please go ahead and remove.... I don't mind!

Gerald Hawkins

A wild card that has been mentioned only rarely is that as the ice sheet mass is reduced the potential for earthquake activity is increased. Even modest tremors associated with glacial (isostatic) rebound have the potential to accelerate the movement of the remainder of the ice mass.

Nigel Williams

The way Jakobshavn glacier is retreating is noteworthy, as it appears already to have moved from its narrow coastal channel to be calving on a widening line behind the island ring.

Now its calving face is propagating sideways fast, with the result that the length of the wall of ice at the calving face is offering opportunities for ice far left and right of the glacier fall line to simply fall off into the sea.

This sort of effect will likely occur all around Greenland, resulting in the present phase of the melt-out we have now (mostly through 'coastal' gaps in the island ring) being followed quite soon by the phase where there is open water inside the island ring lapping the unsupported face of the ice cap.

This will mean that the melt due to direct ocean/ice effects and the effect of unsupported ice faces with wet feet calving will change from occurring over the few hundred kilometres (at the most) width of glacier outlets there today, to over 4000 kilometres of calving face in the not too distant future.

looks to me like the potential for a non-linear event is pretty high...

Gerhard Trausner

..
One of the reasons why some glaciers faster, and other glaciers flow slowly, could also be that the lithosphere is thin beneath the ice sheet.
Characterized it has different temperatures
at the base of the ice sheet. Some areas have only 2 ° C at the base.
http://www.scinexx.de/wissen-aktuell-16521-2013-08-12.html

This fact should be taken into account in models.

Fairfax Climate Watch

this just out in Science: "Water formed by surface melting of the Greenland Ice Sheet is transferred rapidly to the underlying bedrock, but how the water is then dispersed is less clear. This question is important because how the ice-rock interface is lubricated affects how fast the ice sheet moves. Existing conceptual models are based on observations of mountain glaciers, but Meierbachtol et al. (p. 777; see the Perspective by Lüthi) now show that those ideas may not be applicable to the Greenland Ice Sheet. Measuring water pressures in a transect of 23 boreholes revealed that drainage structures differ between the edge, where large melt channels form, and further inland, where more distributed pathways are found." http://www.sciencemag.org/content/341/6147/777.abstract?sid=ea7a2ce2-47f3-4d8b-a78d-54ff121bb070

conclusion: water does not drain efficiently from the center of the ice sheet because subglacial cavities are not well-connected, or connected at all, except for at the very edges of the ice sheet.

Fairfax Climate Watch

Also, a new paper out on geothermal heat flux at the base of ice sheets http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1898.html

"Here we use a coupled ice–lithosphere model driven by climate and show that the oldest and thickest part of the Greenland Ice Sheet is strongly influenced by heat flow from the deep Earth. We find that the geothermal heat flux in central Greenland increases from west to east due to thinning of the lithosphere, which is only about 25–66% as thick as is typical for terrains of early Proterozoic age5. Complex interactions between geothermal heat flow and glaciation-induced thermal perturbations in the upper crust over glacial cycles lead to strong regional variations in basal ice conditions, with areas of rapid basal melting adjoining areas of extremely cold basal ice."

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