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Greenland Glaciers — not so fast!

Filed under: — eric @ 15 May 2012

There have been several recent papers on ice sheets and sea level that have gotten a bit of press of the journalistic whiplash variety (“The ice is melting faster than we thought!” “No, its not!”). As usual the papers themselves are much better than the press about them, and the results less confusing. They add rich detail to our understanding of the ice sheets; they do not change estimates of the magnitude of future sea level rise.

One of these recent papers, by Hellmer et al., discusses possible mechanisms by which loss of ice from the great ice sheets may occur in the future. Hellmer et al.’s results suggest that retreat of the Ronne-Filchner ice shelf in the Weddell Sea (Antarctica) — an area that until recently has not received all that much attention from glaciologists — might correspond to an additional rise in global sea level of about 40 cm. That’s a lot, and it’s in addition to, the “worst case scenarios” often referred to — notably, that of Pfeffer et al., (2008), who did not consider the Ronne-Filchner. However, it’s also entirely model based (as such projections must be) and doesn’t really provide any information on likelihood — just on mechanisms.

Among the most important recent papers, in our view, is the one by Moon et al. in Science earlier this May (2012). The paper, with co-authors Ian Joughin (who won the Agassiz Medal at EGU this year), Ben Smith, and Ian Howat, provides a wonderful new set of data on Greenland’s glaciers. This is the first paper to provide data on *all* the outlet glaciers that drain the Greenland ice sheet into the sea.

The bottom line is that Greenland’s glaciers are still speeding up. But the results put speculation of monotonic or exponential increases in Greenland’s ice discharge to rest, an idea that some had raised after a doubling over a few years was reported in 2004 for Jakobshavn Isbræ (Greenland’s largest outlet glacier). Let it not be said that journals such as Science and Nature are only willing to publish papers that find that thing are “worse than we thought”! But neither does this new work contradict any of the previous estimates of future sea level rise, such as that of Vermeer and Rahmstorf. The reality is that the record is very short (just 10 years) and shows a complex time-dependent glacier response, from which one cannot deduce how the ice sheet will react in the long run to a major climatic warming, say over the next 50 or 100 years.

These new data provide an important baseline and they will remain important for many years to come. We asked Moon and Joughin to write a summary of their paper for us, which is reproduced below.

Guest Post by By Twila Moon and Ian Joughin, University of Washington

The sheer scale of the Greenland and Antarctic ice sheets pose significant difficulties for collecting data on the ground. Fortunately, satellites have brought in a new era of ice sheet research, allowing us to begin answering basic questions – how fast does the ice move? how quickly is it changing? where and how much melting and thinning is occurring? – on a comprehensive spatial scale. Our recent paper, “21st-century evolution of Greenland outlet glacier velocities”, published May 4th in Science, presented observations of velocity on all Greenland outlet glaciers – more than 200 glaciers – wider than 1.5km [Moon et al., 2012]. There are two primary conclusions in our study:
1) Glaciers in the northwest and southeast regions of the Greenland ice sheet, where ~80% of discharge occurs, sped up by ~30% from 2000 to 2010 (34% for the southeast, 28% for the northwest).
2) On a local scale, there is notable variability in glacier speeds, with even neighboring glaciers exhibiting different annual velocity patterns.

There are a few points on our research that may be easy to misinterpret, so we’re taking this opportunity to provide some additional details and explanation.

Melt and Velocity

The Greenland ice sheet changes mass through two primary methods: 1) loss or gain of ice through melt or precipitation (surface mass balance) and 2) loss of ice through calving of icebergs (discharge) (Figure 1) [van den Broeke et al., 2009]. It is not uncommon for people to confuse discharge and melting. Our measurements from Greenland, which are often referred to in the context of “melt”, are actually observations of velocity, and thus relate to discharge, not in situ melting.


Figure 1. Components of surface mass balance and discharge. Most components can change in both negative (e.g., thinning) and positive directions (e.g., thickening).

When glaciologists refer to “increased melt” they are usually referring to melt that occurs on the ice sheet’s top surface (i.e., surface mass balance). Surface melt largely is confined to the lower-elevation edge of the ice sheet, where air temperature and solar radiation can melt up to several meters of ice each year during summer. Melt extent depends on air temperatures which tend to be greatest at more southerly latitudes. Meltwater pools in lakes and crevasses, often finding a path to drain through and under the ice sheet to the ocean. Glaciologists and oceanographers have found evidence for notable melt where the ice contacts ocean water [Straneo et al., 2010]. So, when you hear about ice sheet “melt”, think surface lakes and streams and melting at the ends of the glaciers where they meet the ocean.

So, why focus on velocity instead of melt? Velocity is more closely related to the discharge of ice to the ocean in the process of which icebergs break off, which float away to melt somewhere else potentially far removed from the ice sheet. You can picture outlet glaciers as large conveyor belts of ice, moving ice from the interior of the ice sheet out to the ocean. Our velocity measurements help indicate how quickly these conveyor belts are moving ice toward the ocean. Given climate change projections of continued warming for the Greenland ice sheet [IPCC, 2007], it’s important to understand at what speeds Greenland glaciers flow and how they change. On the whole, the measurements thus far indicate overall speedup. It turns out that on any individual glacier, however, the flow may undergo large changes on an annual basis, including both speeding up and slowing down. With these detailed measurements of glacier velocity, we can continue to work toward a better understanding of what primary factors control glacier velocity. Answers to this latter question will ultimately help us predict the ice sheet’s future behavior in a changing climate.

Sea Level Rise

Translating velocity change into changes in sea level rise is not a straightforward task. Sea level change reflects the total mass of ice lost (or gained) from the ice sheet. Determining this quantity requires measurements of velocity, thickness, width, advance/retreat (i.e., terminus position), and density – or, in some cases, an entirely different approach, such as measuring gravity changes.

Our study does not include many of the measurements that are a part of determining total mass balance, and thus total sea level rise. In another paper that we highlight in our study, Pfeffer et al. [2008] used a specifically prescribed velocity scaling to examine potential worst-case values for sea level rise. The Pfeffer et al. paper did not produce “projections” of sea level rise so much as a look at the limits that ice sheet dynamics might place on sea level rise. It is reasonable to comment on how our observations compare to the prescribed velocity values that Pfeffer et al. used. They lay out two scenarios for Greenland dynamics. The first scenario was a thought experiment to consider sea level rise by 2100 if all glaciers double their speed between 2000 and 2010, which is plausible given the observed doubling of speed by some glacier. The second experiment laid out a worst-case scenario in which all glacier speeds increased by an order of magnitude from 2000 to 2010, based on the assumption that greater than tenfold increases were implausible. The first scenario results in 9.3 cm sea level rise from Greenland dynamics (this does not include surface mass balance) by 2100 and the second scenario produces 46.7 cm sea level rise by 2100. The observational data now in hand for 2000-2010 show speedup during this period was ~30% for fast-flowing glaciers. While velocities did not double during the decade, a continued speedup might push average velocities over the doubling mark well before 2100, suggesting that the lower number for sea level rise from Greenland dynamics is well within reason. Our results also show wide variability and individual glaciers with marked speedup and slowdown. In our survey of more than 200 glaciers, some glaciers do double in speed but they do not approach a tenfold increase. Considering these results, our data suggest that sea level rise by 2100 from Greenland dynamics is likely to remain below the worst-case laid out by Pfeffer et al.

By adding our observational data to the theoretical results laid out by Pfeffer et al., we are ignoring uncertainties of the other assumptions of their experiment, but refining their velocity estimates. The result is not a new estimate of sea level rise but, rather, an improved detail for increasing accuracy. Indeed, a primary value of our study is not in providing an estimate of sea level rise, but in offering the sort of spatial and temporal details that will be needed to improve others’ modeling and statistical extrapolation studies. With just ten years of observations, our work is the tip of the iceberg for developing an understanding of long-term ice sheet behavior. Fortunately, our study provides a wide range of spatial and temporal coverage that is important for continued efforts aimed at understanding the processes controlling fast glacier flow. The record is still relatively short, however, so continued observation to extend the record is of critical importance.

In the same Science issue as our study, two perspective pieces comment on the challenges of ice sheet modeling [Alley and Joughin, 2012] and improving predictions of regional sea level rise [Willis and Church, 2012]. Clearly, all three of the papers are connected, as much as in pointing out where we need to learn more as in indicating where we have already made important strides.

Alley, R. B., and I. Joughin (2012), Modeling Ice-Sheet Flow, Science, 336(6081), 551-552.
IPCC (2007), Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, S. Solomon et al., Eds., Cambridge University Press, ppp 996.
Moon, T., I. Joughin, B. Smith, and I. Howat (2012), 21st-Century Evolution of Greenland Outlet Glacier Velocities, Science, 336(6081), 576-578.
Pfeffer, W. T., J. T. Harper, and S. O’Neel (2008), Kinematic constraints on glacier contributions to 21st-century sea-level rise, Science, 321(000258914300046), 1340-1343.
Straneo, F., G. S. Hamilton, D. A. Sutherland, L. A. Stearns, F. Davidson, M. O. Hammill, G. B. Stenson, and A. Rosing-Asvid (2010), Rapid circulation of warm subtropical waters in a major glacial fjord in East Greenland, Nature Geoscience, 3(3), 1-5.
van den Broeke, M., J. Bamber, J. Ettema, E. Rignot, E. Schrama, W. Van De Berg, E. Van Meijgaard, I. Velicogna, and B. Wouters (2009), Partitioning Recent Greenland Mass Loss, Science, 326(5955), 984-986.
Willis, J. K., and J. A. Church (2012), Regional Sea-Level Projection, Science, 336(6081), 550-551.


269 Responses to “Greenland Glaciers — not so fast!”

  1. 1
    Phil Mattheis says:

    I’ll apologize for re-posting this link just sent to the unforced variations thread – I had linked directly to the jpeg, which does not include the accompanying narrative from NASA that explains the image:
    http://apod.nasa.gov/apod/astropix.html

    As I said, Greenland ice is not just a drop in the ocean, but water is just a drop on Earth…spread thin and vulnerable.

    [Response:That’s a very compelling image. I would love to see the same but for a) a water drop that represents the glaciers and b) a water drop representing all the unfrozen fresh water on earth. You’d have to zoom in a lot more! –eric]

  2. 2
    jaykimball says:

    Phil,

    I thought the same thing when I saw that image. I would love to see a water sphere for fresh water – surface and aquifer.

    Jay Kimball
    8020 Vision

  3. 3
    Rick Brown says:

    Eric – an image including a representation of your b) [all fresh water] is available at http://www.whoi.edu/page.do?pid=80696&i=7301. (From a link in the explanatory text for the image Phil provided.)

  4. 4
    BillS says:

    ” On a local scale, there is notable variability in glacier speeds, with even neighboring glaciers exhibiting different annual velocity patterns.”

    Here is a wonderful photo demonstrating entirely different behavior of three neighboring glaciers:

    http://www.slocanlake.net/congruent_glaciers.html

  5. 5
    Doug Proctor says:

    The 30% rise as measured for Greenland:
    A 2X rate increase by 2010, staying that way to 2100, results in 9.3 cm of ocean rise. Even if it rose to 2X by 2100, the average would be much less than 2X, and the resultant rise would be less than 9 cm.

    The concluding statements are nicely put. While saying 10X is implausible, and 2X might occur, the reader is still left with the idea that a crisis of ocean rise is possible or probable. The number these researchers are speaking of is considerably less than “crisis”, but by not giving a number, they cannot be criticized – by either side.

    [Response: Which is precisely right of course, because the reality is that this new paper provides no real guide here. It simply says “here are the data” and if we MUST say something about sea level, here’s what we can say. It may be boring, but it’s right. By the way, a crisis of ocean rise is actually CERTAIN already if you live in particular places. The question is how many places that will wind up being, and over what timescale.–eric]

  6. 6
    MMM says:

    Doug Proctor: The question is whether a 30% increase over a decade is a 30% increase to a maximum speed, or whether it heralds continuing 30% increases. If the former, then we’re looking at 1.3 over 2 time 9.3 or 6 cm of sea level rise by 2100 from Greenland. On the other hand, 30% increase every decade means about a 5x average speed over the century, or about 25 cm of sea level rise from Greenland alone. Not as bad as the 10x future, but solidly in the middle of the original range.

    (remember also that Greenland is not the only source of sea level rise: thermal expansion and land-glacial melt are fairly certain to contribute tens of centimeters, and Antarctica is a dark horse)

    [Response: Yes, all correct –eric]

  7. 7
    John Mashey says:

    Thanks for this useful summary …
    and for the single most appropriate use I’ve seen of a common phrase:

    “…our work is the tip of the iceberg…”

  8. 8
    TimD says:

    The ice mass loss in the Greenland region as measured by the GRACE satellite system clearly shows an accelerating rate of loss with a doubling time of less that ten years. If the arbitrary and unsupported “assumption” that a 10X speedup in mass loss turns out to be wrong, it seems we will know it pretty soon. The discussion in the article does not adequately cover the nonlinear behavior of ice and partially melted ice as it rapidly warms to near the melting point. As the ice slides away on the edges of the sheet, the slope increases, flow rates increase, viscous heating increases with flow rate, viscosity decreases, flow rates increase, partial melting occurs in the high flow rate areas, viscosity drops precipitously… RAPID COLLAPSE.

  9. 9

    The question I’ve had for the last few years is:

    What proportion of the mass balance loss is due to glacial outflow and what proportion to melt runoff and how are these proportions changing over time?

  10. 10
    jaykimball says:

    For those that are wondering what size sphere would be required to hold the fresh water accessible to humans, I updated the USGA image and posted it here: http://8020vision.com/2012/05/15/how-much-water-is-on-earth/

    Rick, the image you link to doesn’t look right. The sphere is too small. Perhaps they are not including ground water. They link to their definition of freshwater, but it is a dead link. So hard to know what their assumptions are.

    The USGS says that about .75% of the earths water is freshwater. That yields a sphere with about 20% of the diameter of an “All Water” sphere.

    Jay Kimball
    8020 Vision

  11. 11
    Martin Vermeer says:

    Great story, thanks Eric.

    Nit:

    s/Jakoshaven Isbrae/Jakobshavn Isbræ/

    (If that’s too difficult to pronounce, try ‘Sermeq Kujalleq’ :-)

    [Response:Will correct! –eric]

  12. 12

    The link in #1 has now ‘moved on’ a day, and so no longer reflects the topic. The correct image can be found in the archives, here:

    http://apod.nasa.gov/apod/ap120515.html

    It is indeed striking.

  13. 13
    Dan H. says:

    TimD,
    I am not sure where you are getting your GRACE data, but a recent study does not comfirm your statements.

    http://hal.ird.fr/docs/00/66/40/41/PDF/Bergmann_et_al_GPC_2012_corr.pdf

  14. 14
    Susan Anderson says:

    Thanks to Tenney and particularly TimD @~8:

    The ice mass loss in the Greenland region as measured by the GRACE satellite system clearly shows an accelerating rate of loss with a doubling time of less that ten years.


    Typical: 3 guesses and the first 2 don’t count: Hal(.ird.fr) is an outfit that either allows self-publishing or uses an internal biased so-called peer review in favor of an agenda. Distracting from what should be a real conversation again, and I’m one of the first sinners there …

  15. 15
    Hank Roberts says:

    Susan, look at the linked page, that’s a copy at a repository for open access docs. The source is identified at the top of the linked PDF:
    Bergmann et al.: Global and Planetary Change 82-83 (2012) 1-11
    DOI : 10.1016/j.gloplacha.2011.11.005

    It’s on the list of GRACE-related publications here:
    http://op.gfz-potsdam.de/grace/references/sort_date.html

    Full cite from that list:
    Bergmann, I., Ramillien, G., and Frappart, F., 2012. Climate-driven interannual ice mass evolution in Greenland. Global and Planetary Change, 82:1–11, doi:10.1016/j.gloplacha.2011.11.005.

    As GavinEric wrote earlier:

    “… the reality is that this new paper provides no real guide here …”

    That would apply for “TimD” who cited nothing and Dan H. who cited an irrelevant paper.

    If there are papers out on ice sheet collapse, and our hosts want to go into it, it’d be interesting — the ANDRILL work might give some paleo data as it’s beginning to be published, for the Antarctic. But that’s not the topic.

  16. 16
    BillS says:

    Tenney at #9

    It may be the case that more mass is lost via basal melting than through calving or surface runoff for marine terminating glaciers north of about 74-75 deg. North latitude on both the east and west coasts. In Fig. 1 above that melting would occur in the area between where the base of the glacier rests on the bed (the grounding line) and the front of the glacier.

    A search at scholar.google.com on “runoff, greenland” will lead you to any number of accessible papers. I think you will find that the proportions vary with regions.

  17. 17

    #14, 15 and previous–

    I scanned the Bergmann et al paper and it seemed reasonable to these amateur eyes. It found a slowing of mass loss rates in 2009-10, which seems entirely reasonable in conjunction with the low 2009 SSTs which we already knew about.

    That slowing is the basis of Dan H’s comment that acceleration was not supported by GRACE data in recent years. True enough, I suppose, but then with study pretty much limited to 2002-2010, it’s pretty early innings for *any* firm conclusion about the evolution of loss rates.

    The *fact* of ice mass loss, though, seems a pretty darn firm conclusion; the present study has the lowest rate found so far (which would account for why Dan likes the study), but that’s still 32 GT/year, over the full span from 2002 to 2010. (Their highest number was 92 GT/year.)

    Velicogna (2009) had the highest number so far–230 GT, for the span 2002-(Feb)2009.

  18. 18
    TimD says:

    DanH,Kevin M, The most recent Grace data I had seen (http://www.skepticalscience.com/latest-grace-data-record-ice-loss-in-2010.html), which doesn’t discuss methodology for data reduction, but which is consistent with the earlier results doesn’t show a slowdown as the Bergmann paper does. Honestly, I will need a bit more time to digest that paper, but it clearly is a review of the data using different reduction methods and seems to be an outlier. But even if it isn’t, it’s results are clearly highly smoothed and show a slowdown over an insignificant amount of time. The result I showed clearly illustrates seasonal variation, so it is resolving the mass changes well, and this is not decelerating. It would be interesting to look at the GRACE data in conjunction with ENSO and the North Atlantic cycles. If the slowdown suggested in the Bergmann paper are supported by further work, it would be coincident with the La Nina phase we have experienced for the last couple years and may not be significant in the long term. The point of my previous post was that there are plenty of nonlinearities in ice physics near the melting point that are generally poorly represented in ice sheet models, explaining the current and, no doubt, future surprising behavior of ice sheets.

  19. 19
    Aaron Lewis says:

    At the heart of of this model is a glacier sliding down hill from a colder environment into a warmer environment (see Fig, 1 above). In this model, the rate of downhill movement is faster than the conduction of heat into the core of the glacier and the core of the glacier remains cold enough to provide some structural strength to the ice. Where the ice warms to the core, a “calving face” forms.

    However, much of the Greenland ice is not sliding down hill, rather the environment around it is warming, and heat is being advected down into the ice. Thus, it is not just the calving face that is warming and losing structural strength, it is the the entire volume of ice affected by moulins carrying melt water from the surface.

    This is not something we have seen (on this scale) before. Icesheet breakup is a non-linear process. Extrapolation of recent trends tell us nothing. GRACE data gives the mass of the structure, not structural strength of the ice. We have to calculate the mechanical and structural stresses, sources of heat, the Gibb’s energy, the structural strength of ice, and trust the result. It is not pretty.

    The result is that the massive core of Greenland’s ice is supported by gently sloping ice bulwark or buttress. When the top surface of this is “bulwark” ice is above the melting point, pools of melt water and condensate form on the surface, then form moulins, and the melt water drains through the moulins. Large volumes of ice are warmed and weakened simultaneously. (When one gram of water condenses, it releases enough heat to melt 7.5 grams of ice resulting in 8.5 grams of “runoff”. Thus, water vapor from an ice free Arctic sea, will dramatically affect Greenland.) Then, the cumulative weight of the ice structure blows the weakened bulwarks outward, followed by the progressive collapse of the ice core (as it is no longer supported by the bulwarks).

    With the loss of Arctic sea ice, more latent heat is available to melt the surface, cold dry air is less common around the shores of Greenland, and warm moist air is more common around Greenland. Loss of Arctic sea ice will dramatically accelerate the decay of the GIS.

    After a lengthy period of warming and weakening, the progressive collapse happens rather fast. This does not often happen in glaciers moving down slope because the uphill ice is colder and structurally stronger, and there is less potential energy involved. The glacier model does not work for ice sheets.

    Published? Naw, this is just basic undergrad physics. As such it is very crude. However, it has other implications. For example, the wave benches on Lake Missoula were the result of super-glacial ponds, and Missoula Floods were the result of the progressive structural collapse of an ice structure. Likewise, some of the Lake Agassiz floods were the result of the rapid progressive collapse of ice structures rather than the failure of ice dams holding lakes of liquid water. The truth is that no ice dam can support a head of more than ~6 meters of liquid fresh water. Ice in contact with sea water can be colder and withstand higher pressures.

  20. 20
    Hank Roberts says:

    Ice sheet system model links:
    http://issm.jpl.nasa.gov/publications/

    and ‘Google Scholar’ comes through with
    Sell Me Your Beach House, Please!
    PJ Michaels – The Tea, 2012

  21. 21
    TimD says:

    Aaron L, I like your analysis. In the near term,I think the big question is; “Is this recent acceleration in ice mass loss in Greenland a short term thing due to sub-sea melting affecting the grounding of ice streams, or is this likely to continue over several doublings because the acceleration is reflecting deeper non-linear fluid effects?” Things can change rapidly in this environment because loss of sea ice is rapidly warming surface waters, which, being saline, can slip below fresh meltwaters and very quickly transfer heat by mass tranfer of heat deep beneath the ice sheet. And, of course surface melt waters, which have increased quickly during the summers, have reduced the albedo of the melt water lakes and when they frack their way through the sheet, the heat from that water is transferred quickly through the sheet, reducing its strength. If a number of melt water lakes were to hook up, a very large calving surface could form, and lead to a propagating collapse front. But I can pretty well guarantee that these sorts of effects are not included in the ice sheet dynamic models. In paleo data we lots of indications of rapid sea level rise due to ice sheet collapse, but the time resolution of that data isn’t sufficient to tell if the changes occurred in months or centuries, which is a pertinent uncertainty in the context of the current situation. A good review of pleoclimatology related to current global warming is Hansen, et. al.: http://www.columbia.edu/~jeh1/mailings/2011/20110118_MilankovicPaper.pdf Hansen has done some modeling that suggests that ice mass loss could continue to double until so much ice is unleashed to the north Atlantic that there would be an end to sea ice melt in the summers and the climate could jerk back into cooling, but throwing the climate into a chaotic mess.

  22. 22
    Tom Scharf says:

    It should probably be mentioned that instead of all these complex studies and inaccurate methods to attempt to determine the exact melting rate of a particular glacier, it is seemingly simpler and more accurate to measure sea level via satellite measurements.

    Although total SLR has other factors than just this, my understanding is these ice sheet melts are seen as the primary driver for any future high level SLR.

    The pundits in the denial-sphere have a point when they speculate on the possibility of confirmation bias when some studies (not these specifically) conclude the melting rate of an ice sheet / glacier is “worse than we thought”, accelerating, doubling, etc.

    The obvious question is where is all this melt going, because it doesn’t appear to be going into the sea according to satellite measurements.

    I don’t think this conflict with observations is a minor point, but is a bit too easily dismissed as somehow unimportant. I think many would trust the reliability of satellite measurements over these type of models or estimates, and the alarm-o-sphere inevitably picks up on any whiff of speculation that things are “worse than we thought”, and pretending that universities don’t engage in throwing out some red meat for the MSM with their press releases is a fantasy in my view.

    For the record, I thought this piece was balanced overall.

  23. 23
    TimD says:

    Tom S – Just for the record, I wouldn’t worry about your post being balanced as much as well supported scientifically. We don’t try to balance discussions of geology with flat earthers, we shouldn’t do the same with politically motivated denialists either. But to your points, I believe that total ice mass due to losses from glaciers world-wide is about .5 mm/yr SL equivalent, while annual variability due to precipitation is on the order of 20 mm/yr. So the problem is seeing the signal through a lot of noise. This ref has recent data and a good discussion of SL change factors. Over the last few decades it seems that higher global precipitation due to warmer sea surface temperatures and increased water sequestration in large dams has pretty well balanced the small ice melt contributions. The long term trend is pretty linear: http://sealevel.colorado.edu/

  24. 24
    wili says:

    Tom, why do you think a measure of sea level would autmatically tell us more about Greenland melt than direct measures of that melt itself? You do know that sea level has its own dynamics, it can temporarily be (and has recently been) affected by increased evaporation from increased global temperatures, don’t you?

    Back to the issue at hand–How much of a factor will the rebounding land beneath the ice (as the mass above it decreases) have on the rate of glacier movement over the next hundred years or so?

    It seems to me that even a slight change in slope as the interior bedrock rebounds could cause dramatic changes in rate of movement. After all, if you tip a table only slightly, a marble on it will start to move even though it had been stationary before. I have not read all of the recent studies. Has anyone seen this calculation included? Is it too minor to mention? Or too uncertain to predict?

    (reCaptcha: John, bettcom)

  25. 25
    Hank Roberts says:

    Just as a reminder how little we knew two years ago, which may suggest how little we still knew: do you know how fast drumlins form? Used to be thought they were a slow process, til about 2010:
    http://www.terradaily.com/reports/Drumlin_Field_Provides_Answers_About_Glaciation_And_Climate_999.html

    —-excerpt follows—-

    ICE WORLD
    Drumlin Field Provides Answers About Glaciation And Climate

    This is a drumlin by the Mulajokull glacier on Iceland. The ravines cutting into the drumlin have given researchers an opportunity to study its structure. Credit: Mark Johnson
    by Staff Writers
    Gothenburg, Sweden (SPX) Nov 18, 2010
    The landform known as a drumlin, created when the ice advanced during the Ice Age, can also be produced by today’s glaciers. This discovery, made by researchers from the University of Gothenburg, Sweden, has just been published in the scientific journal Geology.

    Drumlins generally consist of an accumulation of glacial debris – till – and are found in areas that were covered by ice sheet. As the ice advanced, it moved rocks, gravel and sand and created tear-shaped raised ridges running parallel with the movement of the ice.

    “Until now, scientists have been divided on how drumlins were created,” says Mark Johnson from the Department of Earth Sciences at the University of Gothenburg.

    “Because they are formed under the ice, it’s not an observable process. Drumlins are common almost everywhere the Ice Age ice sheets existed, but they’re almost unknown with modern-day glaciers. Now, though, we’ve found a new drumlin field by the Mulajokull glacier on Iceland. It’s quite unique.”

    The melting of glaciers reveals drumlins
    The melting of glaciers as a result of climate change has helped the researchers to study this geological phenomenon. The drumlin discovery on Iceland has presented unique opportunities to study their structure.

    “One of the drumlins we found was sliced through by erosion. This gave us an opportunity to study it layer by layer, and it was clear that it had been built up only recently. In other words, the glacier has not just retreated to reveal old drumlins, but is continuing to create new ones.”

    There are currently multiple theories about the origins of drumlins. The Gothenburg researchers’ discovery shows that they can form within two kilometres of the edge of the ice.

    “A surging glacier can move 100 metres a day, as opposed to the more normal 100 metres a year. If we can link drumlins to fast-moving glaciers, this would mean that the ice sheet advanced much more quickly than scientists currently believe.”

    Can effect climate research
    The link between drumlins and rapid ice movements is important for climate research. When modelling climate change, we need to know how high and how cold a glacier was in order to understand the last Ice Age. A glacier that moves quickly will not be as thick. This discovery could therefore affect how scientists approach climate modelling.

    Solving the riddle of the drumlin is a longstanding dream for Mark Johnson:

    “We discovered the drumlin field while flying in towards the edge of the glacier to do a completely different study. It was the most exciting thing I’ve been involved in during my research.

    “All geologists know about drumlins, and when I began to study geology in Wisconsin in the 1980s, many people would come there to study the drumlins in the area. Coming up with a theory for how they formed was a big question even then.”

  26. 26
    Edward Greisch says:

    20 Hank Roberts: Thanks.

  27. 27
    Dan H. says:

    Tim,

    Here is a paper showing a longer term correlation between Greenland (albeit, only one glacier) and North Atlantic SST. The most recent results correlate reasonable well with the GRACE measurements. The study found that short term measurements correlate with NAO.
    http://www.nature.com/ngeo/journal/v5/n1/full/ngeo1349.html

    Other studies found similar comparison:
    http://www.whoi.edu/cms/files/straneo_usclivar_63163.pdf
    http://www.arctic.noaa.gov/reportcard/greenland_ice_sheet.html

    Also compare the much larger ice mass loss in 2010 with the much lower in 2011. Many of the analyses showing accelerated ice mass loss were performed before the inclusion of the more recent data. All in all, this shows the difficulty in projecting long term trends based on short term data.

  28. 28
    Susan Anderson says:

    Hank Roberts, thanks for all, and my apologies to anyone paying attention. Given my lack of expertise, I should never butt in here unless I have checked things, particularly when exhausted. I promise to read carefully through now.

    That said, I am heartsick at the way Dan H. is now using these comment sections to feed his fans and getting better all the time, thanks to your tolerance, at appearing plausibly scientific. His arrogant tone and the occasional conversation across comments with those who approve his work are IMO a reason to stop giving him a platform. I do not have standing to say this but cannot in good conscience not do so.

  29. 29
    Ray Ladbury says:

    Susan Anderson,
    I would note that while Dan H. continues to improve his “scientist” imitation, we also learn how to deal with the likes of Dan H. It is rather like the first battle Grant fought against Robert E. Lee. Upon overhearing some of his staff speculating as to what Lee had in store for them, Grant said sternly, “Don’t tell me what he’s going to do to you. Tell me what you are going to do to him.”

    Dan H. is not the only anti-science type who has started trying to imitate science. You see is not just in climate science, but evolution, the anti-vaxxers and so on. The imitation is flattering, but we have to find a way of distinguishing science from anti-science in the mind of the public.

  30. 30
    SecularAnimist says:

    Ray Ladbury wrote: “… we also learn how to deal with the likes of Dan H. …”

    Hmmm, I don’t really see that “learning” going on.

    On the contrary, I see people continuing to engage with Dan H. as though he were a legitimate commenter, posting in good faith — rather than treating him as the deliberately, repetitively and blatantly dishonest time-wasting troll that he is.

  31. 31
    TimD says:

    Dan H. – Thanks for the refs. Will check them out. Sounds like you have made some enemies here. I am a newbie, so I am unaware of the issues, but as long as you support your arguments with good refs, I’m good.

    Back to the issues, if ice loss responds strongly to SST warming, then that is useful in that it indicates that heat transfer by sea water flow under the ice streams is important as a mechanism, and I think that is where things are going. Given SST warming is a strong feedback effect of global warming associated with sea ice loss, it is still fundamentally an anthropogenic effect. The Hansen paper I cited shows clearly how polar ice strongly amplifies small forcings and that human contributions to GHG growth in the atmosphere is orders of magnitude larger than natural sources over the long term.

    I sure don’t see any sign of a slowdown in ice mass loss from the Grace data from John Wahr at Boulder – http://www.skepticalscience.com/greenland-cooling-gaining-ice-intermediate.htm is the latest I have found and it clearly shows continuing loss acceleration through ’11. I tend to be suspicious of amazing new methods of data reduction as is the case of the Bergmann paper. His results looks more like a filtering edge effect to me. I have recently had an email conversation with Dr. Wahr and I find his data reduction methods to be widely accepted and robust. Since Grace can see the entire change in mass over the region with good spatial and temporal resolution, it is a far superior method than those dependent on observing individual ice streams.

    Wili, isostatic rebound after glacial unloading is an important effect that is taken into account in some detail in the analysis of the Grace data set. While that effect is significant in that regard, since it is due to flow in the upper mantle and the viscosity of the upper mantle is pretty high, it is a slow process in human terms. The Hudson Bay, which lost its thick ice sheet suddenly around 10,000 years ago, is still rebounding strongly. Some have suggested that the mantle flow associated with rebound could go strongly non-linear and produce volcanic activity, but I don’t see any volcanoes around the Hudson Bay.

  32. 32
    TimD says:

    Dan H. – Maybe I am seeing what those folks are talking about. A quick review of one of your refs (http://www.arctic.noaa.gov/reportcard/greenland_ice_sheet.html) clearly states that ’11 was a high melt/mass loss year for Greenland, opposite of your statement “compare the much larger ice mass loss in 2010 with the much lower in 2011.” Note the “Highlights” of that article:

    “A persistent and strong negative North Atlantic Oscillation (NAO) index was responsible for southerly air flow along the west of Greenland, which caused anomalously warm weather in winter 2010-11 and summer 2011.

    The area and duration of melting at the surface of the ice sheet in summer 2011 were the third highest since 1979.
    The lowest surface albedo observed in 12 years of satellite observations (2000-2011) was a consequence of enhanced surface melting and below normal summer snowfall.
    The area of marine-terminating glaciers continued to decrease, though at less than half the rate of the previous 10 years.

    In situ measurements revealed near record-setting mass losses concentrated at higher elevations on the western slope of the ice sheet, and at an isolated glacier in southeastern Greenland.

    Total ice sheet mass loss in 2011 was 70% larger than the 2003-09 average annual loss rate of -250 Gt y-1. According to satellite gravity data obtained since 2002, ice sheet mass loss is accelerating.”

    You need to read your own refs…

  33. 33
    simon abingdon says:

    #28 Susan Anderson “His arrogant tone and the occasional conversation across comments with those who approve his work are IMO a reason to stop giving him a platform”. Quite so Susan, can’t abide skeptics meself.

  34. 34
    Susan Anderson says:

    Thanks Ray.

    However, my point is that he is talking past you to his fans at this point and likely planning to reference his acceptance here elsewhere, hence the condescending tones. I am well aware of the sciencey imitations which have been around for a while; the cosmetics get better all the time. I’m glad you guys are addressing the actual issues, but the way this particular conversation is morphing is something different. I got my training in anti-science elsewhere, and when the blog starts to be used by its non-audience to talk past the blog owner, it is no longer serving the function you are talking about.

  35. 35
    Hank Roberts says:

    I do wonder what the four or five real scientists here think, as Dan. H. takes over another topic, pretends to answer questions, his new friends gathering ’round, and he takes on the host role.

    I imagine the scientists think, oh, well, it’s a chatroom, why bother.

    Boring.

    For myself, I’d sure rather be hearing what the scientists who came here to write about their work have to say — without the clutter.

  36. 36
    Hank Roberts says:

    Anyone invited the biologists here recently?

    http://epic.awi.de/30
    Nöthig, E. M. , Bauerfeind, E. , Haas, A. , Boetius, A. , Bracher, A. , Cherkasheva, A. , Kraft, A. , Metfies, K. , Peeken, I. and Schwarz, J. N. (2012): Variability of chlorophyll a distribution in the Fram Strait, Greenland Sea and Central Arctic Ocean , IPY 2012 Conference: From Knowledge to Action , Montreal, Canada, 23 April 2012 – 27 April 2012 .
    Cite this page as:
    hdl:10013/epic.39192288/

    “The Arctic Ocean is changing severely since one decade. Extremely low ice coverage in late summer has been recorded since 2007 with a new minimum in 2011. The zone of the receding ice edge is known for its high primary productivity in polar waters, but depending on light and nutrient availability productivity in the ice can also be high.

    We don’t know yet what the influence of less ice will be for the future Arctic Ocean ecosystem. Several scenarios are under discussion.

    Chlorophyll a is a measure of biomass standing stock of phytoplankton and can give information on surface as well as depth distribution of autotrophic biomass in the ocean. Chlorophyll a measurements also serve as ground truth data used to validate productivity estimates by remote sensing from space.

    Here we present a data set obtained since 1991 during several cruises carried out on RVs Polarstern, Lance & Maria S Merian to the Fram Strait, Greenland Sea and to the central Arctic Ocean including Laptev and Kara Seas. Almost every year samples have been taken from at least six different depth horizons …”

    [paragraph breaks added for online readability –hr]

  37. 37
    Hank Roberts says:

    > primary productivity in polar waters

    Just to make that clear; “primary productivity” is what we eat and breathe.

    What we breathe, and what we eat, starts with photosynthesis — chlorophyll — and mostly comes from polar waters around the edge of the ice.

    The ice that forms in winter, melting from the springtime on, supplies all the fallout from the winter months (good stuff, bad stuff, whatever is carried in and dropped) — in a short interval as the ice melts, mostly. Lots of stuff around the edge of the ice blooms and reproduces.

    Increasing sunlight, fresh water, minerals –> primary production
    The source where life is being made in excess — food and oxygen.

    It’d be a shame if anybody were to damage that.

    But on the other hand, of course right, now it’s not worth anything, economically.

    In fact, as there’s an excess of it in places getting in the way of a lot of petroleum and stuff on the seabed — it’s kind of a nuisance, isn’t it?

  38. 38
    Thomas says:

    Aaron. I thought I’d estimate a few numbers to put some context on your musings. Surface area of the GIS is roughly 1.7e6 km**2. Net mass loss of circa 200km**3, means an average thinning rate of 12cm/year. I would think changes in surface mass balance of 12cm/year is not so great. The average thickness is 2KM. Stress heating ultimately comes from gravity, so 2KM height loss translates to roughly 4.7 degrees C of heating. I think the surface temp in the interior is on the order of -20C, and this cold ice is advected downwards, and outwards (upwards in abalation areas), so the bulk of the ice should be at least -10C or colder, so without concentration of heating, flow shouldn’t cause melting of the bulk of the ice.
    How those surface ponds/lakes that drain affect ice temps. Once they get started, I think it is assumed they drill through to the bottom pretty quickly. The difference in hydrostatic pressure of a column of water versus ice is great enough that once the depth of the water column gets beyond a critical size, it can easily drill on down. If you assume these drainage pipes only have significant influence on ice temp within say ten meters of the conduit, conduits should be widely spaced, say kilometers apart, so the volume of ice heated by this mechanism should be very small compared to the mass of the ice. I don’t think the meltwater can heat the bulk of the deep ice by anything significant.

    I am far more concerned about how the ice sheet albedo will evolve. It can go from dry-snow, to wet snow, to wet-dirty snow, to wet clean ice, to wet dirty ice. How much dirt can accumulate year to year on the ablation surface? Do we have any way to estimate this effect?

  39. 39
    Kevin Hood says:

    I have two questions:
    1) I live in Juneau, Alaska near some of the southernmost tidewater glaciers in North America. The longterm rise in temperature here has been accompanied by an increase in precipitation. Is there any significance to the difference between glaciers melting more solely due to higher temps and those melting more due to higher temps combined with exposure to more rain for longer periods? For example, is there a negative feedback that has to be accounted for when accumulating snow converts to wasting rain? Or is all melt the same?

    2) I have been wondering if the cold temperature of the meltwater coming off of the ice sheets and glaciers could be adding to the oceans’ mass but not the volume – at least, until the melt water has mixed in with the sea water and heated up a bit. If this is true, wouldn’t this mean that there is some lag time between melting ice and observing the consequent contribution to sea level rise? I ask this mindful of how our bays/fjords with glaciers or glacial rivers are notably colder than bays/fjords without glacial run-off.

    Thanks for any enlightenment!

  40. 40
    TimD says:

    Thomas, the problem with your little diatribe is that in non-linear thixotrophic fluids (fluids with viscosities inversely related to strain rate) strain heating is concentrated, and strongly along slip surfaces that can and do develop in large ice bodies. In high strain rate zones, meltwater fracking through from the surface will tend to penetrate into the structure of the ice along sliding crystal interfaces and so better transmit heat from the surface to large volumes of ice with great efficiency, thereby reducing its viscosity and focusing strain heating. These effects are very difficult to model, so they typically are ignored even in some of the latest models, although things are improving all the time. Your back-of-the-envelope calculations show some talent, but don’t really cut it when considering the complexities of large, non-linear, mixed phase fluid bodies. You really need a super-computer to make any reasonable predictions.

  41. 41
    Dan H. says:

    Tim,
    Read the reference more carefully. The “area and duration” were the third highest. These were lower than 2007 and 2010, but higher than 2008 and 2009; neither accelerating, nor decelerating. Part of the reason that the ice mass loss was 70% higher than the 7-year average (2003-2009), is that Greenland experienced some very low ice mass losses during several of those years, and the average does not include the high ice loss year of 2010. Compared to the first four years, the ice mass is accelerating, compared to the last four, it is not. These are the trials of a short-term data base. These were based on the GRACE calculations.

    Additionally, visible imagery (MODIS) showed a decrease in area loss of marine-terminating glaciers. This was largely due to increases in some larger glaciers (Granted the gain in the Petermann glacier was expected after the rather large calving in 2010).

    Much of this is perspective. Since 2002, the ice mass loss has accelerated, but much of that is based on the large jump that occurred between 2006 and 2007. A more appropriate statement would be that ice mass loss accelerated (past tense) from 2006 to 2007. This coincides with the large decrease in Arctic sea ice measured at the same time, which reinforces the theory that warm North Atlantic waters have enhanced melt.

    http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/seaice.anomaly.arctic.png

  42. 42
    Dan H. says:

    Tim,

    This is the only paper that I know that shows Arctic volcanoes.

    http://www.nature.com/nature/journal/v453/n7199/abs/nature07075.html

    I have been around long enough to know that we scientists tend to overstate the importance of our own theories and findings, and minimizes others that contradict us. Consequently, tempers flare – we are human, afterall. Science thrives when people push the envelope into new horizons. Scientific research will then determine the validity of these new theories. Oftentimes, these new theories are quickly disproven, but other times, we get scientific breakthroughs. Neils Bohr and Albert Einstein argued fervently for years over quantum mechanics (not that any of use can be compared to either great physicist).

    [Response: How on earth did we get onto the subject of volcanoes? I very much hope no one is suggesting changes in Greenland are driven by volcanism?! I’m not bothering to read all the comments, but I will just remind people there is a difference between watts and milliwatts. -eric]

  43. 43
    wili says:

    eric, by not consigning Dan H and his ilk to the bore hole, you are colluding in destroying this site. When every thread becomes a troll-feeding session, this very promising site can quickly become worse than useless. Everyone but the die-hards will just stop coming here. What exactly is the bore hole for, anyway?

  44. 44
    Dan H. says:

    Kevin,
    Your first question would need to be answer on a case-by-case basis. Oftentimes, higher precipitation will result in an increase in snowfall in the higher elevations, compensating for the losses in the lower elevation due to higher temperatures/rainfall. Mt Shasta in California is an extreme example, where all 7 glaciers have been growing due to increased snowfall overcoming increasing temperatures. I do not know of a study comparing the glacial loss in the ablation zone due to solar insolation compared to rainfall.

    Your second question is basically no. The volume increase would be slightly less due to the lower density of fresh water compared to seawater. The equivalent volume of fresh will decrease by 2.8% once salinity increases to the levels found in the oceans (nutrient-rich runoff would experience less of a volumetric change). Consequently, a liter of pure fresh water from glacial runoff would result in an oceanic volumetric increase of 973 ml. This change in density is at least an order of magnitude higher than changes due to temperature.

  45. 45
    Dan H. says:

    Eric,

    See response #31. I do not think anyone is suggestion that volcanoes are influencing the Greenland glaciers.

  46. 46
    Ray Ladbury says:

    Dan H. “I have been around long enough to know that we scientists tend to overstate the importance of our own theories and findings, and minimizes others that contradict us.”

    Bullshit. Pure, irremediable, unadulterated bullshit. Remember those Bohr-Einstein debated, do you?

    Dan, I’d be willing to bet you couldn’t even competently sketch out the positions taken by Bohr and Einstein and why they took those positions. You’ve been around enough to be a wannabe…a pudknocker. That’s all.

  47. 47
    Hank Roberts says:

    > I have been around long enough to know that we scientists ….

    “we scientists” is it now?

    Do the RealClimate hosts know who Dan H. is, and have some reason to respect him despite the errors he posts?

    If y’all vouch for the guy being real, and the posts coming from a real person, fine — you’ve got your reasons, you needn’t tell us who a pseudonymous person is.

    Dan H. makes claims often implied, not stated explicitly.
    And often enough wrong when checked.

    Is he a scientist? Can anyone we have reason to trust confirm his “we scientists” claim?

    Or … http://newsgroups.derkeiler.com/Archive/Alt/alt.usage.english/2007-11/msg05570.html

  48. 48
    SecularAnimist says:

    Dan H wrote: “we scientists”

    Has Dan H ever, even once, provided the slightest evidence to support his prima facie laughable claim to be a “scientist”?

    Because each time he says that, I get the feeling that it’s his little way of sneering and thumbing his nose at the real scientists who run this site.

    I agree with wili. The RealClimate comment pages have basically become “The Dan H Show”.

    I appreciate that the moderators likely delete and/or send to the Bore Hole a whole host of garbage that gets thrown at this site, with the result that there is a “better class of trolls” here — e.g. those who are more or less polite and able to write more or less “sciencey-sounding” stuff.

    But they are still trolls. And they are still out to dominate the discussion with propaganda, and they are all too often succeeding.

  49. 49
    tamino says:

    Dan H. is a persistent annoyance to the regulars here, and decreases the value of the site. But far more important than annoying us, is his turning every discussion thread into what wili (#43) aptly describes as “a troll-feeding session.” If the moderators allow him to continue, then the regulars here won’t like it — but we can take it.

    But I urge the mods to consider, very seriously, the non-regulars, especially first-time visitors and uncertain fence-sitters. Seriously guys, what influence do you think the trolls have on them? Aren’t they more in need of your protection from misinformation than those of us who actively seek to refute it for ourselves?

  50. 50

    From the peanut gallery…The RC team’s posts are must-reads, but threads quickly devolve once the current tribe of trolls start posting. Dan H.’s garbage in particular makes me yearn for the Rod B. era.


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