Advances in understanding of the Greenland Ice Sheet's meltwater channels

Greenland_meltwater1LOS ALAMOS, N.M., Oct. 1, 2014—An international research team’s field work, drilling and measuring melt rates and ice sheet movement in Greenland is showing that things are, in fact, more complicated than we thought.

“Although the Greenland Ice Sheet initially speeds up each summer in its slow-motion race to the sea, the network of meltwater channels beneath the sheet is not necessarily forming the slushy racetrack that had been previously considered,” said Matthew Hoffman, a Los Alamos National Laboratory scientist on the project.

A high-profile paper appearing in Nature this week notes that observations of moulins (vertical conduits connecting water on top of the glacier down to the bed of the ice sheet) and boreholes in Greenland show that subglacial channels ameliorate the speedup caused by water delivery to the base of the ice sheet in the short term. By mid summer, however, the channels stabilize and are unable to grow any larger. In a previous paper appearing in Science, researchers had posited that the undersheet channels were not even a consideration in Greenland, but as happens in the science world, more data fills in the complex mosaic of facts and clarifies the evolution of the meltwater flow rates over the seasons.

In reality, these two papers are not inconsistent – they are studying different places at different times – and they both are consistent in that channelization is less important than previously assumed, said Hoffman.

The Greenland Ice Sheet’s movement speeds up each summer as melt from the surface penetrates kilometer-thick ice through moulins, lubricating the bed of the ice sheet. Greater melt is predicted for Greenland in the future, but its impact on ice sheet flux and associated sea level rise is uncertain: direct observations of the subglacial drainage system are lacking and its evolution over the melt season is poorly understood.

“Everyone wants to know what’s happening under Greenland as it experiences more and more melt,” said study coauthor Ginny Catania, a research scientist at the institute and an associate professor in the University of Texas at Austin’s Jackson School of Geosciences. “This subglacial plumbing may or may not be critical for sea level rise in the next 100 years, but we don’t really know until we fully understand it.”

To resolve these unknowns, the research team drilled and instrumented 13 boreholes through 700-meter thick ice in west Greenland. There they performed the first combined  analysis of Greenland ice velocity and water pressure in moulins and boreholes, and they determined that moulin water pressure does not lower over the latter half of the melt season, indicating a limited role of high-efficiency channels in subglacial drainage.

Instead they found that boreholes monitor a hydraulically isolated region of the bed, but decreasing water pressure seen in some boreholes can explain the decreasing ice velocity seen over the melt season.

“Like loosening the seal of a bathtub drain, the hydrologic changes that occur each summer may cause isolated pockets of pressurized water to slowly drain out from under the ice sheet, resulting in more friction,” said Hoffman.

Their observations identify a previously unrecognized role of changes in hydraulically isolated regions of the bed in controlling evolution of subglacial drainage over summer. Understanding this process will be crucial for predicting the effect of increasing melt on summer speedup and associated autumn slowdown of the ice sheet into the future.

The research letter is published in this week’s Nature magazine as “Direct observations of evolving subglacial drainage beneath the Greenland Ice Sheet.” The project was an international collaboration between the University of Texas at Austin, Los Alamos National Laboratory, NASA Goddard Space Flight Center, Michigan Technological University, University of Zurich, the Swiss Federal Institute of Technology and Dartmouth College.

This project was supported by United States National Science Foundation, the Swiss National Science Foundation and the National Geographic Society. The work at Los Alamos was supported by NASA Cryospheric Sciences, and through climate modeling programs within the US Department of Energy, Office of Science.

Also see “Geoscience: The plumbing of Greenland’s ice.”

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Latitude

Greater melt is predicted for Greenland in the future…..even though all of the temp stations in greenland have recorded temps falling since 1930’s

joelobryan

“This subglacial plumbing may or may not be critical for sea level rise in the next 100 years, but we don’t really know until we fully understand it.”
Send us money.

Steve Crook

Harsh. These days if do don’t do the climate change angle you’re probably gonna struggle for funding. In any event the way the ice behaves is interesting and it’d be cool (pun intended) to know more…

george e. smith

What happened to the ‘observe first, then try to explain’ method of scientific study.
Seems like these guys concocted a catastrophic scenario, and then went looking for support, and found none for a process that isn’t even happening.
Sort of a rerun of ‘black body’ radiation science. No such thing exists or has ever been observed, yet it is a cornerstone of modern physics. We keep hanging our hats on it (and it is a useful concept) even though we know for sure, that no such thing can exist, at least in our universe.
So Greenland ice is in fact not shussing uphill and over the mountains into the sea. Who could have guessed that ??

cynical scientst

The “observe first then try to explain” method is a myth. In reality scientists have always observed and conjectured at the same time. Scientific research is a two way feedback process whereby the conjectures help suggest what you should be observing, and the observations help constrain and improve the conjectures.
Where climate science is broken is in trying to lock in an explanation. “The science is finished!” I actually have trouble believing that this line got any traction whatsoever. The science is never finished.

george e. smith

Why the cynicism ? I asked plainly what happened to that methodology; observe, then try to explain those observations. Where did you read that I excluded the possibilities of other methodologies ?
So Einstein predicted from his “general relativity theory”, that light passing close to the sun would deflect. Whoopee ! How many such successes are there, for each case of an observer noticing a bright start, and wondering what it was.
As a WAG, I would venture that we seek to explain that which we observe, more often than we look for something we thought up.
But that’s a conjecture. The scientific method would challenge you to disprove my conjecture.

John

Yes, this is good science. Don’t mean it is necessarily right, rather that they are measuring things they previously made assumptions about.
Previous assumptions appear to be that more meltwater in summer caused glaciers to flow more quickly to the sea, via lubrication.
If that is true, this this study appears to contradict the previous assumption, as this is what they found:
“…they determined that moulin water pressure does not lower over the latter half of the melt season, indicating a limited role of high-efficiency channels in subglacial drainage.
Instead they found that boreholes monitor a hydraulically isolated region of the bed, but decreasing water pressure seen in some boreholes can explain the decreasing ice velocity seen over the melt season.
“Like loosening the seal of a bathtub drain, the hydrologic changes that occur each summer may cause isolated pockets of pressurized water to slowly drain out from under the ice sheet, resulting in more friction,” said Hoffman.”
Decreasing ice velicity du to more friction….the opposite of previous suppositions. That is why we do science.

Joel O'Bryan

From their paper:
“At our primary field site in the ablation zone of Sermeq Avannarleq in western Greenland (Fig. 1; 69-27′ N, 49-53′ W; Extended Data Table 1), we drilled seven boreholes to the bed using a hot water drill and instru- mented three of these with pressure transducers in 2011 (ref. 7) (Methods). The ice thickness in our instrumented boreholes was between 614 m and 624 m, and each borehole either drained slowly or did not drain before closure. ”
Read about this interesting glacier here:
http://whc.unesco.org/en/list/1149
This was a location for one of the Norse colonies during the MWP, then abandoned as the LIA set in. Likely a more hospitable back then to Viking cultures and their livestock.

Leon Brozyna

So, in short, the movement of glaciers is more complicated than at first assumed.
Next thing you know, climate scientists will confess to not having a clue as to what the climate’s doing.
Before you know it they’ll be ditching the models and will start to *gasp* collect data!

Mark Bofill

but we don’t really know until we fully understand it

Why is it I don’t hear this more often from climate scientsts? Listen to that, doesn’t it sound, refreshing?

sleepingbear dunes

Agreed. Totally refreshing. Had I seen more of this humility, I might have had more respect for the science.

Bill Illis

The temperature of the ice in Greenland’s glaciers is about -31.0C down to 2000 metres after which is starts to warm-up as it goes deeper as a result of bedrock heating. Has anyone ever actually proven that these moulins make it all the way to the bottom of the ice sheet through -31.0C ice and all that additional pressure. It seems extremely unlikely to me.

tty

No it is a lot more complicated. The Greenland ice is polythermal, i e there are both areas where the ice is frozen to bedrock and areas where the bottom temperature is above the pressure melting point (though not necessarily above zero degrees centigrade). There is a good description of glacial temperatures here:
http://people.ee.ethz.ch/~luethim/pdf/script/pdg/chapter6.pdf
By the way, here is a quote from another chapter in the same book “Physics of Glaciers”:
“Basal motion of Alpine glaciers is highest in spring and early summer (May-June) when meltwater production is increasing. Despite higher meltwater input, sliding speeds are generally lower in summer (July-August). Röthlisberger and Aellen (1967) explained this observation with the principle that not the amount of meltwater, but the water pressure at the glacier base is important. The water pressure
is highest in spring when the subglacial drainage system is not yet well developed (Röthlisberger, 1972).”
Sounds rather familiar eh? There is a lot of “me too”-research in ´climate science´.

Billy Liar

Looks to me from Figure 6.6 in the book extract that the Jakobshavn Isbre was for a very long time creating a narrow fjord that ran from a point at least 40 nautical miles to the east of the current ice front and that the ice cap remained 1,400 meters lower than its current height whilst that fjord was being created. When was that?

tty

Billyliar says:
“When was that?”
Could have been a very long time ago. Fiords often follow river valleys that existed before the glaciation. I haven’t seen any dating evidence for the fiords in Western Greenland, but in Scoresby Sound on the east coast there are Pliocene marine deposits right inside the fiord which shows that it already existed in essentially its modern form by then.
While the main icecap on Greenland is probably only about 2 million years old, montane glaciation is very much older. IRD (Ice-rafted Debris) in offshore deposits show that there were tidewater glaciers in Greenland by the Oligocene c. 35 million years ago. Montane glaciers that didn’t reach the coast must have existed even earlier. It is even conceivable that Greenland has never been entirely unglaciated since it separated from Europe and North America. The climate was subtropical during the Eocene Climate Optimum, but the mountains (which reach 11,000 feet now) were probably higher then, so ther may have been tropical glaciers even at that time.

Billy Liar

tty,
Many thanks for your reply. I always enjoy your posts.

Sequestration of ocean surface water by the Gulf Stream
Anthony, not entirely unrelated to your post!

The calculation below the fold suggests that the Gulf Stream sequesters the equivalent of the surface waters (333m layer) of the whole Atlantic Ocean once every decade. 380 years for The Gulf Stream to cycle a volume of water equivalent to all the world’s oceans.

iSchadow

Envisioning the kayak ride of a lifetime, albeit a short lifetime.

Gamecock

“Understanding this process will be crucial”
No, not really.

Gary Pearse

Geothermal flux through the earth’s crust warms up the bottom of glaciers in both Greenland and Antarctica. They recently drilled an unfrozen lake in East Antarctica and found life forms in it!
http://www.smithsonianmag.com/science-nature/thousands-microbe-species-live-buried-antarctic-lake-180952415/?no-ist
Having not mentioned geothermal heat, are these people saying that surface water was thought to be a big player in the movement of the ice sheet and now they have disproved it?
Let me fix this for them: “Retreat in understanding of the Greenland Ice Sheet’s meltwater channels”

http://www.vukcevic.talktalk.net/G-S-ridge.jpg
The above is image of the Greenland Scotland ridge. Iceland is a relatively young island, it appeared some 20 Ma ago (?), it sits on the mid-Atlantic ridge. It is is built by the magma eruptions and it is one of the most active volcanic regions on Earth. Iceland has only 2% of the world’s active volcanoes, but it is estimated that since 1600 they produced 30% of the total tephra.
Image would suggest that Greenland may be a much larger and much older ‘version’ of Iceland, built on the same principle, but once the crust achieved a certain thickness and weight, the mid Atlantic ridge eruptions moved further east. If so then remnants of the ‘plume’ may be still present providing sizeable degree of bedrock heating.

tty

“Image would suggest that Greenland may be a much larger and much older ‘version’ of Iceland, built on the same principle”
No it isn´t. Greenland is a continental block consisting mostly of Precambrian igneous rocks, like Scandinavia and Canada with which is was once contiguous. There was some volcanic activity when Greenland split from North America and Europe. There is an area of Paleogene volcanic rocks in the Disko area on the west coast and a larger area on the eastern side on the Blosseville coast which was originally situated on top of the Iceland hotspot (the other half of this igneous province is found in Northern Ireland and Western Scotland).
Recent research also suggests that there may be some subglacial intra-plate volcanic activiy in northwestern Greenland.

Thanks, the length of Reykjanes ridge with Iceland appears to be equal to that of Greenland, the ‘stone age’ spear shape, sharp end to the south west, rounded to the north east, with the almost parallel axial orientation, it appears convincing to a non-professional, but I am happy to accept your view.

Resourceguy

Or maybe the bathtub shape of the basement rock under the ice sheet has some impact on the partial findings to date and complexity issue.

tty

The “bathtub shape” is originally due to the presence of rift valleys on both sides of Greenland as it split away from both Europe and North America. The southern part of Africa is rather similar for the same reason, with a raised rim and an escarpment all along the coast. In Greenland’s case this concave shape has been accentuated by glacial erosion and the weight of the icecap, and the “bathtub shape” in turn almost certainly contributes to the exceptional stability of the Greenland icecap which has not melted completely for almost 2 million years, as shown by the Kap Köbenhavn formation of Northern Greenland.

mpainter

The abstract gives the author’s conclusion: the study was inconclusive.
Regarding the press release: I hope that the authors did not have to pay for it. If they did, they wuz robbed.

Anything is possible

“things are, in fact, more complicated than we thought.”
======================================
If you’re a climate scientist, remember this well. It is your “get-out” clause.

Billy Liar

Does that animation look unphysical to you?

tty

It is probably roughly correct. Pine Island and Thwaites glaciers are after all the only ones in Antarctica with any appreciable mass loss. The to-and-froing in the animation is probably just measurement error, but the concentration of the mass loss to the center of the ice streams in the 2009-12 average seems reasonable. However the mass loss from the Getz ice-shelf seems odd. It is hard to find a mechanism that will melt the inner part of the shelf but not the outer (unless geothermal energy is involved, this part of West Antarctica is a major volcanic province).

u.k.(us)

They say during the recent ice age the glacier over Chicago was about 1 mile thick.
The evidence shows it scoured the landscape.
So how high was the origin that caused the flow, it must have been much higher ?

tty

The Laurentide Ice Sheet is usually modelled as being 10-13,000 feet thick at the center, but even for the relatively simple case of modelling an an ice-sheet there is a lot of uncertinty involved. Note that the ice in the Midwest terminated on land and melted in place rather than calving into the sea. Such ice.sheets have much steeper profiles than maritime ones, For example the Greenland Ice-sheet that was considerably smaller during the previous interglacial actually seems to have been slightly thicker at the center than today due to this effect.

Jeff Alberts

I hope those folks in the pic realize it’s not a urinal.

RoHa

“its slow-motion race to the sea”
Who or what is it racing against? The answer seems to be “nothing and no-one”. If so, it isn’t a literal race, so “race” must be a metaphor for “rapid movement”. But this is slow-motion rapid movement.