2010-12-11 16:31:16Confusing paper on Greenland melt
Albatross
Julian Brimelow
stomatalaperture@gmail...
199.126.232.206

Robert way, mspelto, 

 

A penny for your thoughts and insight please? ;)

 

http://www.nature.com/nature/journal/v468/n7325/full/nature09618.html

http://www.publicaffairs.ubc.ca/2010/12/08/greenland-ice-sheet-flow-driven-by-short-term-weather-extremes-not-gradual-warming-ubc-research/ 

2010-12-13 22:05:42
Rob Painting
Rob
paintingskeri@vodafone.co...
118.93.251.166

Alby, not that I have access to the full paper, but I don't think there anything greatly controversial there.

Typically what happens with a steady deterioration of the ice sheet over time, is a widening of the main drainage channels. So these channels allow better drainage and their water flow increases, however they effectively drain all the smaller tributary channels leading to a drop in water pressure and lubrication under the ice. The end result with increased tunneling is a slow down in ice flow, due to this decrease in lubrication area (does that sound rude?)

See this full paper for details  - Magnusson 2010 - http://www.the-cryosphere.net/4/13/2010/tc-4-13-2010.pdf. 

When rapid inputs of rain occur the drainage system can't accommodate this extra water flow, the channels and their tributaries fill up, water pressure increases and away goes the ice again.  

So there should be seasonal/annual variation in the rate of ice flow, slower when there's no rain, and faster when there's rain. A slow gradual deterioration due to global warming, overlaid with a pattern of fits and starts due to weather (rain).

That's my take anyway. Oh, and I suspect the skeptics will eventually latch onto this one (it's not global warming it's the weather yada, yada), but given that more rain for Greenland is a near certainty, it doesn't sound like good news to me. 

     

 

 

 

2010-12-14 03:36:17
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
68.188.192.170
I sent an email to Mauri asking him about this
2010-12-14 04:17:02Greenland retreat
Albatross
Julian Brimelow
stomatalaperture@gmail...
199.126.232.206

Hi Rob,

 

Many thanks for that explanation.  Makes sense, and seems to suggests that the ice sheets must be deteriorating for this phenomenon to become widespread beneath the ice?Perhaps I should not be so lazy and read the paper that you linked-- they probably address it there.

"but given that more rain for Greenland is a near certainty, it doesn't sound like good news to me."

That also occurred to me....any data/information out there speaking to that?

 

Cheers,

Alby

2010-12-14 07:30:38Mauri's response
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
68.188.192.170

I have reproduced Mauri Pelto's responses to both the Nature paper and the Magnuson 2010 paper in full:

 

Mauri:

Over at Skeptical Science some of the author's are discussing the significance of a recent paper on Greenland melt:

http://www.nature.com/nature/journal/v468/n7325/full/nature09618.html

http://www.publicaffairs.ubc.ca/2010/12/08/greenland-ice-sheet-flow-driven-by-short-term-weather-extremes-not-gradual-warming-ubc-research/ 

One of the authors has likened it to the effects shown in Magnusson 2010:
http://www.the-cryosphere.net/4/13/2010/tc-4-13-2010.pdf



I don't know if John Cook has given you admin access, but if so, here's the relevant discussion thread:
http://www.skepticalscience.com/thread.php?t=494&r=1

If you have the time, could you share your insights  on it with us?

Thanks!

Daniel Bailey (the Yooper)

 

Response 1:

 

Dan I do not have access to the SS thread.  However below are my thoughts.   

First we know that the key cause of acceleration is the change in the dynamic force balance at the calving front.  That a reduction in back pressure due to thinning and then less frictional forces with the sidewalls and bottom.    We know this because the acceleration of the outlet glaciers is not seasonal, it is not short term.  The Zwally effect is season and is shorter term and has been quantified to be significant but minor compared to what has been observed.  The below mechanism is again true and applicable, but if the crucial ingredient would lead to considerable short term flow variations.  Is that what we observe for the big outlet glaciers?  No. For Jakobshavn there is plenty of water and plenty of basal water pressure, always.  High rates of flow can open the channels as noted, however, with rapid motion and thick ice the channels would not survive long at all, in slower moving areas, yes channel development is key.  So it comes back to where you are, not the same mechanism is  important in all locations of the GIS.  This mechanism cannot be key in areas of fast flow. 

Increased ice velocities in Greenland1 are contributing significantly to eustatic sea level rise. Faster ice flow has been associated with ice–ocean interactions in water-terminating outlet glaciers2 and with increased surface meltwater supply to the ice-sheet bed inland. Observed correlations between surface melt and ice acceleration2, 3, 4, 5, 6 have raised the possibility of a positive feedback in which surface melting and accelerated dynamic thinning reinforce one another7, suggesting that overall warming could lead to accelerated mass loss. Here I show that it is not simply mean surface melt4but an increase in water input variability8 that drives faster ice flow. Glacier sliding responds to melt indirectly through changes in basal water pressure9,10, 11, with observations showing that water under glaciers drains through channels at low pressure or through interconnected cavities at high pressure12, 13, 14, 15. Using a model that captures the dynamic switching12between channel and cavity drainage modes, I show that channelization and glacier deceleration rather than acceleration occur above a critical rate of water flow. Higher rates of steady water supply can therefore suppress rather than enhance dynamic thinning16, indicating that the melt/dynamic thinning feedback is not universally operational. Short-term increases in water input are, however, accommodated by the drainage system through temporary spikes in water pressure. It is these spikes that lead to ice acceleration, which is therefore driven by strong diurnal melt cycles4, 14 and an increase in rain and surface lake drainage events8, 17, 18 rather than an increase in mean melt supply

 

Response 2:

 

I reviewed the Magnusson paper.  Remember this is a lot of water, but also look at the velocity “At that time the fastest part of the cross-section flowed at 0.25md−1

compared to a typical winter velocity of 0.4md−1

in 1995–1996,”  That is not rapid flow.

 

 

 

2010-12-14 09:09:17Mauri's response
John Cook

john@skepticalscience...
121.222.210.74
That's a terrific response from Mauri. Daniel, if you want to adapt his response into a blog post, I'll also add this as a skeptic argument and add your post as the rebuttal. Mauri's response is also a little dense for public consumption, would need to be a little broader for a general audience.
2010-12-14 11:28:05As long as he's OK with it
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
68.188.192.170
I'd be happy too, but I want to run it past him first.
2010-12-14 19:41:35
Rob Painting
Rob
paintingskeri@vodafone.co...
118.93.202.99
Cheers for that Yooper. 
2010-12-15 00:24:11
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
68.188.192.170

While I'm waiting for Mauri to get back to me, can anyone with access to the Nature paper email me a copy?

yooper49855@hotmail.com

or

daniel.bailey@abbott.com

 

Thanks!

2010-12-15 00:45:57Provisional thumbs up
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
68.188.192.170

Mauri said he's going to tweak his response a bit and then get it to me later today or so.  Gave OK to then use that for a post.

 

BTW, snowblowing sucks (8 times in the last 4 days; only had to do that 8 times all last winter), as do 3-day blizzards.

 

 

2010-12-15 06:15:30Nature paper
Albatross
Julian Brimelow
stomatalaperture@gmail...
199.126.232.206

Daniel,

 

Do you have the Nature paper yet?  I can send that along if you need it.  Just let me know.

 

Sorry to hear about the snow/blizzard.  Looks to get wintry here too.  Only about a week to solstice then we are almost over the hump...

2010-12-15 07:58:50Please send the Nature paper; don't have it yet!
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
68.188.192.170

If you could be so kind as to send a copy of the Nature paper, it'd be greatly appreciated!

yooper49855@hotmail.com

or

daniel.bailey@abbott.com

 

Abbott, while an understanding employer that gives me access to many pub's, doesn't give me access to Nature or GRL (for some reason they let me order stuff from the Royal Society - go figure).

 

Every day without snow is a day closer to spring!!!

 

Down to just normal lake effect today.  Warm (20degrees F) wind out of the north, off of Superior.  No freeze-up this winter unless we get prolonged Arctic flows out of Alberta for much of the winter (unlikely with the northward migration of the polar jet[even in winter it's been hanging out to the north side of Superior] and the expansion of the Hadley cells).

 

Fun stuff, this warming...

 

Thanks!

2010-12-15 09:10:27Nature paper
Albatross
Julian Brimelow
stomatalaperture@gmail...
199.126.232.206

Hi Dan,

 

Check your email :)

 

Cheers,

Albatross

2010-12-15 16:41:33Rough draft updated
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
68.188.192.170

http://www.skepticalscience.com/Schoof_et_al_rebuttal_2010.html

------------------------------------------------------------------------------------------


Wednesday, 15 December, 2010

Greenland Ice Sheet Melt: Whether Warming Withers or Wetter Weather Wastes?

 

 

 

Greenland Ice Sheet outlet glaciers ice loss: an overview

 

The mass loss from Greenland's ice sheet has been well documented over the years.  In recent years, the rate of loss has accelerated.

 

Figure 1: Greenland ice mass anomaly (black). Orange line is quadratic fit (John Wahr).

To understand the causes of the acceleration we must examine how they vary in time and from glacier to glacier.  It also must be recognized that the same processes will not have the same level of impact on each glacier.  The two key mechanisms are the Zwally Effect and the Jakobshavn effect.  Let's take a closer look at those.

The Zwally Effect

This mechanism relies on meltwater reaching the glacier base via moulins and reducing the friction at the base of the glacier. This mechanism has been examined in detail and has yielded short term accelerations in the 10-20 % range (Zwally et al, 2002) (Das et al, 2008), but is of little significance to the annual flow of the large glaciers outlet glaciers.  A recent paper by Schoof et al (2010) further examines this issue.  They conclude that rapid changes in the basal water pressure that is key:  long periods of sustained melt may lead to reductions in basal water pressure as the channels that drain the meltwater at the glacier base mature. 

A mature channel system would successfully remove the meltwater, instead of having the meltwater fill the channels and spread out as a lubricant over more of the glacier bed.  This is not a new concept, having been observed on Ryder Glacier during a surge after a lake outburst in 1995.  This mechanism has been observed in Iceland and Bering Glacier in Alaska as well. 

The mechanism does have limitations, however.  First of all it would be short lived, as it is only at times of rapid change in the amount of available meltwater that acceleration would occur.  The examinations of increased speeds from glacial lakes in Greenland to the base fit the pattern noted.  It is not a continuous summer long acceleration necessarily; it is often a short term rapid flow increase that is also localized.  Thus, this mechanism falls within the domain of the observed meltwater driven accelerations.  For this to be the star player, we need a glacier area where flow is slow enough for channels to develop and where basal water pressure is often limited. 

This is not the case on the rapid flowing marine terminating outlet glaciers.  The mechanism of a meltwater impulse driving short term acceleration would then be most important in regions where flow is slower and basal meltwater production not persistent.  Joughin and others, 2008 observed that seasonal drainage of meltwater to the glacier bed induces a uniform acceleration of 50–150 meters/year over a ~300 km long section of the West Greenland margin that is not drained by outlet glaciers, causing a large fractional acceleration of the interior ice sheet but a small fractional change in the speed of fast-moving outlet glaciers.  This suggests that over the process of glacier acceleration due to changes in meltwater flux tend to not lead to localized accelerations that generate a different overall velocity.

The Jakobshavn Effect

We are still left with the main cause of glacier acceleration in Greenland resulting from  dynamic thinning of the terminus zone of the marine terminating outlet glacier reducing the effective bed pressure, allowing acceleration – the Jakobshavn effect. The reduced resistive force at the calving front due to the thinner ice, now experiencing greater flotation, is then propagated “upglacier” (Hughes, 1986; Thomas, 2003 and 2004). This type of acceleration has a limited seasonal signal, and propagates upglacier from the terminus. 

Howat and others (2010) examined changes in terminus position, surface elevation and flow on 32 glaciers along the southeast coast of Greenland from 2000-2006. They affirmed that speedup results from loss of resistive stress at the front during retreat. Many retreats began with an increase in thinning rates near the front in the summer of 2003, a year of record high coastal-air and sea-surface temperatures.

This indicates again the importance of preconditioned thinning via melting.  The mass balance at the calving front is the sum of the ice flux from upglacier, the rate of melting above and below the waterline and the iceberg-calving rate. Mass balance transfer to the calving front is a slow process with a large lag time (centuries) and is not capable of playing a meaningful role in the recent relatively large and sudden glacier accelerations (Pfeffer, 2007).

Surface ablation and basal ice ablation are determined by the climatic and oceanographic conditions at near the glacier front. Increased ablation even in a single summer will cause thinning near the ice front.  This will reduce the effective pressure at the glacier bed, reducing friction and encouraging acceleration.  Acceleration at the calving front will then effectively pull on the ice upstream, stretching it causing further thinning and acceleration.  This is how the marine terminating outlet glaciers can respond rapidly to climate conditions.  Howat and others (2008) in observing the seasonal flow rates of 32 outlet glaciers concluded that  the presence of a seasonal oscillation in speed in was ambiguous. On average, the glaciers show a difference in summer (faster) and winter (slower) speeds on the order of 10%.
 

How is does this play out on various glaciers?

Petermann Glacier is a much different glacier than the large fast flowing marine terminating glaciers above. Its extensive ice tongue, the largest in the Northern Hemisphere, makes it particularly susceptible to basal melt processes, due to the area and duration of exposure of the glacier base.  Its velocity of 2-3 m/day is much lower than 10-30 m/day observed on the other marine terminating outlet glaciers.

Petermann is located on the northwest corner of Greenland and certainly experiences less melting and less snowfall. The lower 80 km (in length) and 1300 km2 (in area) of the glacier is afloat. This makes it (by area) the largest floating glacier in the Northern Hemisphere. The ice front is not impressive, unlike the faster outlet glaciers. The calving front protrudes a mere 5-10 m above sea level, reflecting the fact that the ice at the front is only 60-70 m thick.

Further upglacier, the ice at the grounding line is 600-700 m thick. The combination of velocity and thickness yield the volume of material calved each year. Petermann Glacier calves 0.6 km3 (Higgins, 1990), whereas Jakobshavn yields close to 40 km3. The thinning between the grounding line and the calving front is mainly via melting as the snowline is at 900 m. The low slope leads to very low velocities, giving the low-lying floating section plenty of time to melt, and surface melt ponds are common.  The glacier flow in the long terminus section is not susceptible to basal water pressure changes.

Left panel denotes ice velocities; the right panel shows changes in velocities
 
Figure 2. The Panel on the left shows ice velocity; the right, changes in velocity.  Areas in black are sea ice and open water.
 

Humboldt Glacier is much different as the lack of confining topography prevents the development of the strong ice stream flow we see on Jakobshavn Glacier or the weaker ice stream flow of Petermann Glacier and its subsequent long floating tongue.  This glacier could then be more susceptible to changes in meltwater flux.

 

Figure 3. Humboldt Glacier profile
 

Ryder Glacier is much different: Howat and others (2010) note that Ryder Glacier, North Greenland, accelerated by 300% over a 7 week period following drainage of a supraglacial lake in 1995.  This indicates the ability of an unusually large sudden discharge of water can increase basal water pressure dramatically and enhance basal sliding.  This glacier is in the north of Greenland and has an order of magnitude less melt than Jakobshavn and would be more susceptible to such a sudden meltwater pulses.


Figure 4. Horizontal velocity field of the Ryder Glacier. Contour interval is 20 m/yr (cyan) for velocity less than 200 m/yr and is 100 m/yr (blue) for values greater than 200 m/yr. Red arrows indicate flow direction and have length proportional to speed.
 

With the warming of the globe (land + ocean) continuing apace, the mass loss of Greenland’s outlet glaciers is not only expected to continue but to increase their acceleration as well.  Current events surrounding increased oceanic heat around ice sheet margins in Antarctic are expected to play a dynamical role in marine terminating glacial ice loss acceleration there as well.  Stay tuned...

 

The vast majority of this blog post was contributed by glaciologist Mauri Pelto.  Examples of his work can be found at From A Glaciers Perspective and at RealClimate.

2011-01-16 05:01:42New information?
John Hartz
John Hartz
john.hartz@hotmail...
98.122.68.19

There a re a couple of papers cited in the article, "Last chance to hold Greenland back from tipping point" published in the Jan 5 edition of New Scientists magazine.   

2011-01-16 16:07:51No access
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
68.188.192.170

Free access expired yesterday; go figure.

 

What were the papers & pubs?

 

Thanks!

2011-01-16 21:25:10
Riccardo

riccardoreitano@tiscali...
93.147.82.121

First paragraph of the Zwally effect: "large glaciers outlet glaciers"

In the aknowledgment of Mauri contributions i think it would be fair to add that any error is yours ;)

2011-01-17 01:07:45New Scientist Article
John Hartz
John Hartz
john.hartz@hotmail...
98.122.68.19

 

Last chance to hold Greenland back from tipping point

New data and models show that Greenland's ice cap, the world's second largest, is on track to hit a point of no return in 2040

ON 4 AUGUST 2010, the Petermann glacier in Greenland sounded a warning. A gigantic slab of ice broke off and the glacier retreated 15 kilometres, leaving it further inland than it has been since observations began a century ago.

That warning went unheeded at the UN climate talks in Cancún, Mexico, last month. Delegates left without agreeing drastic cuts in greenhouse gas emissions, leaving the planet on course for 3.2 °C of global warming, and Greenland - the world's second largest ice cap - heading for a point of no return. The suggestion is that Greenland will reach a tipping point in the early 2040s. After that no amount of action on our part can save the ice sheet. Unless governments dramatically up their game, the only thing that will change that date is natural variations in the climate, which might either hasten or delay the tipping point.

Greenland's ice sheet holds enough ice to raise global sea levels by 7 metres. Ice melting at the surface and breaking off at the margins of the ice sheet is already adding up to about 300 gigatonnes each year. That accounts for about 25 per cent of the annual, global rise in sea levels.

Greenland's ice sheet holds enough ice to raise global sea levels by 7 metres

Last month's meeting of the American Geophysical Union in San Francisco highlighted the situation. Jason Box of Ohio State University in Columbus and colleagues listed Greenland's "biggest losers": the five glaciers and ice streams that lost the greatest area of ice in the past decade. The Petermann glacier topped the chart, with 500 square kilometres (see map).

But not all ice is created equal. Glaciers in the north like Petermann and Humboldt lost a lot of thin, floating ice that does not impede the outward flow of ice behind. That means the glaciers did not immediately surge seaward. But thicker ice was exposed to the ocean. Thicker ice acts like a cork in a bottle: take it away and the glaciers accelerate. "If we continue to lose ice, we'll start losing important ice," says team member Ian Howat, also at Ohio State University. "If these glaciers were to accelerate and mobilise the large amount of ice up in northern Greenland, it has the potential for a huge change."

It is the kind of change that has already been seen in Greenland, south of 70 degrees latitude. For instance, the speed at which the Jakobshavn glacier flows has more than doubled over the past 10 years. In July, its margin withdrew by about 1.5 kilometres, bringing its grounding line - where the glacier lifts off the bedrock and begins floating - to a knife edge, where bits can break off to form icebergs.

Beneath the ice, Greenland is built like a soup dish: the bedrock slopes down towards the interior and in the case of Jakobshavn bottoms out some 1600 metres below sea level. Jakobshavn's margin is now perched on the edge of that dish. If it breaks up further, it would end up on a downward slope, with nothing to stop it slipping 80 kilometres inland.

"It would cause a huge embayment into the ice sheet, something that we have never seen before," says Howat. Jakobshavn is one of many glaciers perched on similar topography. "Once a glacier hits this point, the dynamics of the ice take over. No matter what climate does, whether it gets warmer or colder, that glacier is going to keep [retreating]," says Howat.

One-way ticket

Other factors could also put glaciers on a one-way ticket to extinction. Kristin Poinar of the University of Washington, Seattle, and colleagues have been studying the bottom of Jakobshavn. The ice there is slushy due to the enormous friction and pressure at those depths: friction within the glacier and against the bedrock generates enormous amounts of heat. Studies show this "temperate ice" layer is about 270 metres thick and acts like a conveyor belt, helping the ice slip faster into the sea.

Not only that, it could give glaciers some form of "memory" of past warm events, says Poinar.In the 1990s, warm ocean waters caused Jakobshavn to speed up dramatically, creating more temperate ice, which could stick around for decades. That means the consequences of sudden changes like Jakobshavn's increase in speed in the 1990s could be felt for tens or hundreds of years, says Poinar.

Thousands of smaller glaciers are also showing dramatic declines. Sebastian Mernild of the Los Alamos National Laboratory in New Mexico and colleagues have been studying one in south-east Greenland, close to the Sermilik fjord. From photographs going back to 1931, the team calculated that the glacier has retreated by 17 metres per year on average, but in 2010 it lost ground by 35 metres. "The same trend is happening to all the glaciers in east Greenland," says Mernild.

Last year was also a bad one for the ice sheet as a whole. By combining observations and modelling, Mernild's team calculated that 52 per cent of the ice sheet experienced surface melting. Natural annual variability can't be ruled out, says Mernild, "but if you check the trends, surface melt has been increasing since 1972, all the way up to 2010, and 2010 was a record year". South-west Greenland saw a dramatic increase in the number of melting days, about 50 days more than the average for the past 50 years. And three decades of measurements from the Watson river drainage basin in west Greenland show that surface runoff in 2010 was 30 to 40 per cent higher than average (Cryosphere, DOI: 10.5194/tc-4-231-2010).

More melting is in store, warns Mernild. His team's models show that Greenland's glaciers haven't fully responded to the temperature rises. In other parts of the world, including Antarctica and the Himalayas, glaciers are about 25 per cent out of equilibrium, meaning that even if warming were to stop today, the glaciers would continue to melt further before stopping. But temperatures in and around Greenland have been increasing faster than elsewhere, and the glaciers there are 70 per cent out of equilibrium, says Mernild.

Glaciers haven't fully responded to temperature rises. Even if warming stops, melting will go on

This worries Mernild. His team modelled the fate of Greenland, using a scenario for future human development outlined by the Intergovernmental Panel on Climate Change. The scenario assumes rapid economic growth, a global population that peaks in 2050, and rapid adoption of new, efficient technologies for energy use and generation. Given the outcome in Cancún last month, it is a likely scenario for the future.

Mernild's models show that if it does play out, Greenland will reach a tipping point in about 30 years. After that nothing will prevent the ice cap from eventually vanishing entirely (Journal of Hydrometeorology, DOI: 10.1175/2009JHM1140.1).

"We can see which way the trend is going," says Mernild. "It doesn't look nice".

2011-01-17 17:12:42
citizenschallenge
Peter Miesler
citizenschallenge7@gmail...
166.164.173.93

Daniel Bailey's rough draft was quite readable, very nice.

Regarding the "tipping point" in Greenland, what specifically is meant by that?

Is it that once coastal 'buttresses' collapse there will be an unstoppable draining of the inland bowl through the handful of drainage valleys?

2011-01-18 11:42:59
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
66.96.96.66

From a geomorphological (structural) perspective, Greenland is a large "bowl": a rim of mountains inside of which is a depression (with its bottom well-below sea level), piled high with ice.

In order for glaciers to grow (or at least hold their own), the depositions in the accumulation zone (above the melt point) must exceed losses in the ablation zone.  One way to interpret Mernild's comment about Greenlands' equilibrium is that (due to the temperature increases) the sheet  is losing about 70% more mass from its edges than it is gaining in its interior.

The tipping point reference is about physics:  given enough melt, the marine-terminating glaciers like Jakobshavn that penetrate the coastal range represent an Achilles heel for the ice sheet as a whole.  Once those glaciers retreat off their terminal ground moraines/grounding points, they recede into the significantly deeper channels and then back into the Greenland bowl proper.  This will allow the much warmer waters of the Atlantic to penetrate into the soft underbelly of Greenland, unchecked.  Once the central mass of the ice sheet begins to sag into those oceanic waters, physics takes over and the tipping point is passed: no amount of temperature rise cessation (or even dropping) will stop the loss of the sheet at that point.

Game over.  The only question left unanswered at that point will be how long it will take.  We may be surprised by the answer to that.

2011-01-18 20:50:07
Riccardo

riccardoreitano@tiscali...
192.84.150.209
Be carefull when you read about any kind of tipping point. The common sense associates it with an abrupt and fast event, like an object in unstable equilibrium that rapidly falls.
Often, though, it is used to describe the point at which "physics takes over", in Daniel's words. In other words I'd say that it is when the system proceeds to a new equilibrium (if any) even if the initial forcing stops. If I hammer a nail in my water tank, the tipping point is when the last diaphragm breaks; from then on water will flow out even if a stop hammering. Maybe slowly, but I'll loose my water.
How fast the process will proceed depends on the physical characteristics of the system. And on if I stop hammering more nails ...
2011-01-19 08:56:05
citizenschallenge
Peter Miesler
citizenschallenge7@gmail...
166.164.130.228

DB: "given enough melt, the marine-terminating glaciers like Jakobshavn that penetrate the coastal range represent an Achilles heel for the ice sheet as a whole.  Once those glaciers retreat off their terminal ground moraines/grounding points, they recede into the significantly deeper channels and then back into the Greenland bowl proper. This will allow the much warmer waters of the Atlantic to penetrate into the soft underbelly of Greenland, unchecked."

Oh wow, I didn't realize the topography of some of those channels/valleys... The sceptics never mentioned that when taking about the stability and growth of the ice in the central bowl of Greenland.

A new ice thickness and bedrock dataset for the Greenland ice sheet: part I.
J.L. Bamber, R. L. Layberry, Bristol Glaciology Centre, School of Geographical Sciences,  http://www.google.com/search?q=bedrock+elevation+of+greenlands+interior&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

 

I was trying to find some topo map of Greenland, under the ice, but didn't have any luck. Do you know of one that's available?

2011-01-19 11:42:36This one's pretty cool
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
68.188.192.170

 

http://membrane.com/sidd/greenrockturn.gif

2011-01-19 11:44:59This one's from Icesat
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
68.188.192.170

 

http://earthobservatory.nasa.gov/images/imagerecords/5000/5118/greenland_ice.gif