Guest Commentary by Mauri Pelto
The net loss in volume and hence sea level contribution of the Greenland Ice Sheet (GIS) has doubled in recent years from 90 to 220 cubic kilometers/year has been noted recently (Rignot and Kanagaratnam, 2007). The main cause of this increase is the acceleration of several large outlet glaciers. There has also been an alarming increase in the number of photographs of meltwater draining into a moulin somewhere on the GIS, often near Swiss Camp (35 km inland from the calving front). The story goes—warmer temperatures, more surface melting, more meltwater draining through moulins to glacier base, lubricating glacier bed, reducing friction, increasing velocity, and finally raising sea level. Examining this issue two years RealClimate suggested this was likely the correct story. A number of recent results suggest that we need to take another look at this story.
Jakobshavn Glacier, West Greenland, retreated 30 km from 1850-1964, followed by a stationary front for 35 years. Jakobshavn has the highest mass flux of any glacier draining an estimated 6% of the GIS. The glacier terminus region also had a consistent velocity of 19 meters/day (maximum of 26 m in glacier center), from season to season and year to year, the glacier seemed to be in balance, as I noted in a 1989paper. This is the fastest glacier in the world, no steroids needed. After 1997 it began to accelerate and thin rapidly, reaching an average velocity of 34 m/day in the terminus region. The glacier thinned at a rate of up to 15 m/year and retreated 5 km in six years. Jakobshavn has since slowed to near its pre-1997 speed, the terminus retreat is still occurring, but likewise is.
Helheim Glacier, East Greenland had a stable terminus from the 1970’s-2000. In 2001-2005 the glacier retreated 7 km and accelerated from 20 m/day to 33 m/day, while thinning up to 130 meters in the terminus region. Kangerdlugssuaq Glacier, East Greenland had a stable terminus history from 1960-2002. The glacier velocity was 13 m/day in the 1990’s. In 2004-2005 it accelerated to 36 m/day and thinned by up to 100 m in the lower reach of the glacier. Helheim and Kangerdlugssuaq combined drain 8 % of GIS. Hence, they are more than canaries in the coal mine. In 2006, the velocity of Helheim and Kangerdlugssuaq decreased to near the 2000 level, the terminus of Helheim advanced a bit (Howat et al., 2007).
The first mechanism for explaining the change in velocity is the “Zwally effect”, which relies on meltwater reaching the glacier base and reducing the friction through a higher basal water pressure. A moulin is the conduit for the additional meltwater to reach the glacier base. This idea, proposed by Jay Zwally, was observed to be the cause of a brief seasonal acceleration of up to 20 % on the Jakobshavns Glacier in 1998 and 1999 at Swiss Camp (Zwally et al., 2002). The acceleration lasted two-three months and was less than 10% in 1996 and 1997 for example. They offered a conclusion that the “coupling between surface melting and ice-sheet flow provides a mechanism for rapid, large-scale, dynamic responses of ice sheets to climate warming”. The acceleration of the three glaciers had not occurred at the time of this study and they were not concluding or implying that the meltwater increase was the cause of the aforementioned acceleration. However, many others have made this assertion and are investigating (Stearns and Hamilton, 2007). Examination of recent rapid supra-glacial lake drainage documented short term velocity changes due to such events, but they had little significance to the annual flow of the large glaciers outlet glaciers (Das et.al, 2008).
The second mechanism is a “Jakobshavn effect”, coined by Terry Hughes, (1986), where a force small imbalance of forces caused by some perturbation can cause a substantial non-linear response. In this case an imbalance of forces at the calving front propagates up-glacier. Thinning causes the glacier to be more buoyant, even becoming afloat at the calving front, and is responsive to tidal changes. The reduced friction due to greater buoyancy allows for an increase in velocity. This is akin to letting off the emergency brake a bit. The reduced resistive force at the calving front is then propagated up glacier via longitudinal extension in what R. Thomas calls a backforce reduction (Thomas, 2003 and 2004). For ice streaming sections of large outlet glaciers (in Antarctica as well) there is always water at the base of the glacier that helps lubricate the flow. This water is, however, generally from basal processes, not surface melting.
If the Zwally effect is the key than since meltwater is a seasonal input, velocity would have a seasonal signal. If the Jakobshavn effect is the key the velocity will propagate up-glacier, the terminus velocity will be impacted by tides, and there will be no seasonal cycle.
On Jakobshavn the acceleration began at the calving front and spread up-glacier 20 km in 1997 and up to 55 km inland by 2003 (Joughin et al., 2004). On Helheim the thinning and velocity propagated up-glacier from the calving front. Each of the glaciers fronts did respond to tidal variations indicating they had become afloat, detached from their bed (Hamilton et al, 2006). This had been the case at Jakobshavn for the last 50 years, but not for Helheim or Kangerdlussuaq. In each case the major outlet glaciers accelerated by at least 50%, much larger than the impact noted due to summer meltwater increase. On Jakobshavn the acceleration was not restricted to the summer, persisting through the winter when surface meltwater is absent.
As a result of the above Luckman et al. ( 2006) concluded:
“The most plausible sequence of events is that the thinning eventually reached a threshold, ungrounded the glacier tongues and subsequently allowed acceleration, retreat and further thinning. It is reasonable to believe that the 1998 Jakobshavn speed-up, also following a long period of stability, was triggered by the same processes of thinning but occurred earlier and after a shorter period of thinning because the tongue was already afloat.”
Examination of the acceleration of other glaciers such as the Petermann Glacier indicate a much smaller acceleration than that observed on three glaciers we have focused, and indeed it is in the summer and of a magnitude that the Zwally effect could explain (Rignot, 2005). Other large outlet glaciers such as the Rinks and Daugaard-Jensen have been stable since 1960 (Stearns et al, 2005). Many other lesser outlet glaciers have accelerated substantially.
That each of the three glaciers has a reduced velocity in 2006 and 2007 despite some exceptional melt conditions in 2007 further suggests that meltwater is not the dominant driver of the acceleration of the main outlet glaciers. Temporarily, there appears to be a force imbalance at the glacier fronts. This will reduce the annual contribution to rising sea level from glacier dynamic changes. The bad news is that the degree of acceleration that can occur via the Jakobshavn effect is greater in these cases than that from the Zwally effect. The Zwally effect is nonetheless real and also implies a direct sea level impact of greater melt.
The Jakobshavn is of particular importance as it has a bed below sea level for at least 80 km inland from the terminus. In this reach there are no significant pinning points, or abrupt changes in slope or width (Clarke and Echelmeyer, 1996) that would help stabilize the glacier during retreat. It is the only outlet glacier of GIS to lack these, and can then (via backforce reductions) tap into the heart of GIS. We know that surface melting is a slow process for raising sea level. but as Greenland’s major outlet glaciers have recently shown, rapid acceleration can quickly deliver large volume of ice to the ocean. The pace of change is not glacial.
Clarke, T.S. & Echelmeyer, K. 1996: Seismic-reflection evidence for a deep subglacial trough beneath Jakobshavns Isbræ, West Greenland. Journal of Glaciology 42(141), 219–232.
Hughes, T. (1986), The Jakobshavn effect. Geophysical Research Letters, 13, 46-48.
Pelto, M.S., Hughes, T.J. & Brecher, H.H. 1989: Equilibrium state of
Jakobshavns Isbræ, West Greenland. Annals of Glaciology 12, 127–131.,
Thomas, R. H. Abdalati W, Frederick E, Krabill WB, Manizade S, Steffen K, (2003) Investigation of surface melting and dynamic thinning on Jakobshavn Isbrae, Greenland. Journal of Glaciology 49, 231-239.
Thomas RH (2004), Force-perturbation analysis of recent thinning and acceleration of Jakobshavn Isbrae, Greenland, Journal of Glaciology 50 (168): 57-66.
221 Responses to "Moulins, Calving Fronts and Greenland Outlet Glacier Acceleration"
Hank Roberts says
> a reality TV show about glaciers?
That’s a scary thought. I’m old enough to remember when “watching glaciers melt” was a simile for something very slow and very boring, like “watching the grass grow.” Now, you’re right, people would tune in to watch. Good grief.
Here’s one for the paleo-Arctic:
Megatides in the Arctic Ocean under glacial conditions
Several named effects have been discussed in this thread, although there is a further named effect concerning the ice sheets which may be worthy of note. The following is an extract from an as yet unwritten draft of the sea level rise section of the IPCC’s 10th report on climate change, expected to be published some time around 2060. There was a bit of a delay because the authors couldn’t agree on the wording. An extract from the technical section on sea level rise reads:
“Each of the ice caps sat on a pole
All the world’s governments and all the world’s industrialists couldn’t put the icecaps together again.”
After a lot of deliberation they did agree on a name for the effect, but for the moment it is being kept secret, although one suspects it might get leaked.
As the icesheets slip off their perch and crash into lots of little pieces (just like an egg that falls – or is pushed? – onto the ground), it is about time some the world’s top industrialists had the courage to convene through a globally broadcast internet conference and tell the world’s public what they are going to do specifically about the icesheets.
#194 “My guesstimate is that the perennial sea ice has dropped by ~half this winter.”
Josefino C. Comiso
Cryospheric Sciences Branch, NASA Goddard Space Flight Center
Journal of Geophysical Research
22 February, 2008.
“Arguably, the most remarkable manifestation of change in the polar regions is the rapid decline in the Arctic perennial ice cover. Changes in the global sea ice cover, however, have been more modest, being only slightly negative in the Northern Hemisphere and even slightly positive in the Southern Hemisphere”
This paper confirms that there is at least some thickening ice in the Antarctic :
H. Jay Zwally
Cryospheric Sciences Branch, Goddard Space Flight Center
Journal of Geophysical Research
19 February, 2008.
Ron Taylor says
If you want to so something “chilling,” watch this Japanese video of polar satellite images, showing the disintegration of multi-year ice during last winter.
From the video:
The image is a low-resolution reproduction of a sequence of satellite images of Arctic ice this past fall and winter. The sequence runs in a continuous loop from October 01, 2007, to March 15, 2008. A link to the high-resolution video file is also provided.
Note the stream of multi-year ice flowing out of the Arctic basin down the east coast of Greenland at one o’clock in the image. As of the middle of March, most of the basin, including the pole itself, appears to be covered only by seasonal ice.
Hank Roberts says
> This paper confirms that there is … some thickening … Zwally
“May–June (fall) and October–November (late winter) of 2004 and 2005” and “0.28 m in 2004 and 0.29 m in 2005” — but are you claiming a trend based on these numbers? The paper’s not about that.
Phil. Felton says
Nice video similar to the Quikscat one I showed in #194 (interesting that the Japanese look at it upsidedown ;) ) The shots of the ice breaking up and squeezing through the Nares strait are really good.
The paper referred to sea level contribution, and post #194 speculated on the loss of sea ice. I posted a link to a paper that indicated that net loss of global sea ice was slight. That paper made the, perhaps surprising to some, that Antarctic sea ice had increased, so I looked for and found a confirming paper.
No I’m not claiming a trend.
It is interesting, though, that the Arctic and Antarctic often seem to go in different directions.
links for sea surface temperature anomaly plots to go with the satellite images linked in post 204 can be found here
Tenney Naumer says
Keep looking into things and you will rapidly find information that will tell you that it is not at all surprising that the Arctic and the Antarctic would tend to go in different directions with regard to sea ice extent (at the present time, but perhaps not so much in the future when the ozone hole over the Antarctic is repaired).
some interesting data on bursts of seismic waves in an ice stream (60 miles wide and one-half mile thick)
” … combined seismological and global positioning system (GPS) analyses to reveal two bursts of seismic waves from an ice stream in Antarctica every day, each one equivalent to a magnitude seven earthquake”
An interesting title for the article “Something’s shaking in Antarctica”
I recall reading somewhere that ice quakes don’t occur in Antarctica. Well that’s another tick in the nonlinearity box.
Arch Stanton says
>”…Well that’s another tick in the nonlinearity box.”
Doubtful, the article implies that the quakes were previously undetected, not that they are new.
I propose another mechanism which needs to be considered regarding ice flows. Glacier ice is under extreme pressure, and it is also just slightly below freezing. It should not be considered solid like a diamond, but rather solid like a candle. Waxy. When an individual molecule of water happens to have enough energy to jump to a neighboring location it may do so. Remember, the ice is not a rigid crystaline structure, but an amorphous solid. It is possible for a molecule to ‘wiggle around’ in this solid. When the headend pressure is relieved, the pressure differential (head to tail) is increased, this will accelerate the flow rate and increase the thinning. One must consider the pressure, temperature, viscosity, and perhaps chemical composition of the particular ice to properly model this.
Ray Ladbury says
Steve, the physical properties of ice are known over a broad range of properties that encompasses all conditions occurring terrestrially. Were the ice homogeneous (and yes, amorphous solids can be homogeneous) and the terrain it advances over even moderately smooth, modeling would be a tractable problem. As it is, you have a variety of processes going on, and which are most important is still moot.
Hank Roberts says
Excerpt follows, click link for full article from New Scientist.
Cite to Science magazine is at end of excerpt.
Van de Waal believes that the channels that carry the meltwater out to sea freeze up during the winter months. In summer, pulses of water rushing down the moulins to the bedrock overwhelm the narrowed channels, and the increased pressure lifts the ice sheet off the rock, enabling it to move faster.
However, after a few days the channels are forced open by the water, and it drains away from the glacier. As a result, the ice grinds back down against the bedrock and the lubricant effect is lost.
Van De Waal says this indicates that, overall, meltwater has a negligible effect on the rate at which the ice sheet moves.
Not all scientists agree. Jay Zwally of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, US, says that averaging data over the last 17 years does not make sense because the most rapid melting at the edges of the ice sheet did not start until recently.
“It’s only in the last five years or so that the warming signal has really been visible,” he says.
Zwally told New Scientist that unpublished data from the eastern edge of the ice sheet suggests between 3% and 5% more ice is being lost because of lubrication than would otherwise happen. That is less than the 25% that was previously calculated, but still significant, he says.
Journal reference: Science, vol 321 p 111
[Response: Mauri Pelto discussed this exact issue here in one of his guest posts a couple of months back. – gavin]
Tenney Naumer says
Does anyone know what is going on at Jakobshavn? Between the 28th and 29th of July, the satellite images appear to show that something massive occurred (large white mass representing ice) in the waters at the outlet point.
Sometimes, you have to start at this link to get to the composite satellite images:
On the 28th the waters were clear, on the 29th the little bay is full. Anyone know anything about this? Thanks
Click on the farthest left image, which will take you to the pages of the latest satellite photos.
Tenney Naumer says
Oh, and the images from the 28th and the 29th are no longer available, but if anyone wants them, I saved them and can e-mail them. In any case, today’s image shows that the bay is full.
Tenney Naumer says
Well, obviously my knowledge of the geography of Greenland is pitiful. In fact, the area I am talking about is to the east of Qeqetarsuag. Sorry.
Tenney Naumer says
OK, I finally figured it out — it is the outlet for the Ilulissat glacier. Anybody know what is going on there?
Tenney Naumer says
OK, I know I can be a complete dummy — in any case, I cropped the images from the 28th and the 30th and posted them on my blog so anybody can see what I am going on about: