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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. 101

    “..conceptual aliasing due to imaginary originality is fairly large..”

    Apt, given that aliasing often arises from undersampling.

  2. 102
    MS says:

    >Hank #98
    Looking for newer results I only found this
    Ancient Biomolecules from Deep Ice Cores Reveal a Forested Southern Greenland .
    NGRIP is not mentioned, but material from other ice cores is used.

  3. 103
    TimD says:

    Dbostrom, A fair snipe, I suppose. But if you read the article I referenced, you would find quotes of arctic scientists expressing shock at the ACTUAL PHENOMENON of rapidly increasing methane emissions in the shallow Arctic Ocean. I am not claiming authorship of the phenomenon, and it was that I was referring to, not a “new thought”. My apologies if it was new to you. I don’t think social calibration is required to make the true statement I made, except perhaps in your mind. And so nice to meet you.

  4. 104
    dbostrom says:

    Tim, for which phenomenon are you not claiming authorship? Clearly it was not that of the release of methane in Arctic seas either in observation or commission, as you duly note. Meanwhile I was not a party to your conversation with your friend wherein you proposed your hypothesis about bulk methane liberation and transport, so I must bow to the narrative of that exchange as you related it and your own conclusion about its significance.

    Anyway, it does seem you’ve a thick skin and need no help. :-)

  5. 105
    flxible says:

    But if you read the article I referenced, you would find quotes of arctic scientists expressing shock at the ACTUAL PHENOMENON of rapidly increasing methane emissions in the shallow Arctic Ocean.

    I believe most here read that article some months ago. If you read the realclimate post here and the following comments, you might see that “vaunted scientists” aren’t too agape while awaiting further actual data publications by those “shocked” scientists investigating the situation.

  6. 106
    Susan Anderson says:


    I was so happy to find real science being discussed, but I cannot in good conscience not say what I think about your “insular little group” comment – NOT! For a good perspective, you might check out this interview, and then read the comments, which are characteristic of the unending attacks on unwelcome but honest science prevalent in even the rarefied reaches of a New York Times blog:

    I enjoyed the presentation by Gavin of the genesis of this group there.

    My contributions there were treated far less politely and relevantly than yours have here, for example:
    I was also instructed to have my eyes checked, typical misdirection. I could go on, but best not.

    One of the problems with science is that it is a meritocracy. It is not easy to qualify for a good high-level education, and years must be spent by the best of the best to get to the level you find here. The pay is not comparable to that offered by the fossil fuel industry to the anyone willing to cater to the PR doubt and delay industry. This has been going on for decades: you could do good work, where have you been? Why are you joining the circular firing squad instead of working to get real knowledge publicly acknowledged outside these rarefied circles?

    On the other hand, I have not seen anyone censoring your comments. In fact, I read them with interest because you clearly know more about science that I do; once you went down the same path your hosts did you came to the same conclusion. But once you’ve seen the same tired old arguments repeated a few hundreds of times, it does get a bit old; why blame the victims? It is depressing that people facing this situation and trying to do something about it have to deal with political arguments that have nothing to do with the facts.

    This is the situation they are doing their best to ameliorate:

  7. 107
    David B. Benson says:

    wili @90 — Rather amazingly some methane comes out of bedrock almost everywhere. Where is is cold enough and with enough pressure it forms methane cathrates, methane ice. Once the pressure is lessened or the temperature rises the cathrates disassociate to express the methane. In my amatuer opinion there isn’t enough of it to be concerned about although some climatologists will certainly like to have good estimates of methane expression.

  8. 108
    TimD says:

    dbostrom, of course, not being a god I cannot claim any authorship of any natural phenomena. My little joke. Perhaps, being of Nordic descent, you missed it. But we are cool and I can understand your mis-perception of my comment. Honestly, I really would prefer to discuss the science, its implications to policy and to future research, and that is what I have been trying to discuss, but not so successfully given the many slings and arrows that seem to be flying around everywhere. But I guess that is the nature of public fora on climate change. For the record, I would favor more aggressive moderation to eliminate the bickering, but I also understand that it is difficult to walk that “free speech” tightrope in a hurricane of skeptic hot air.

    flxible, given that the author of the subject article here claims to have no time to read even these responses, I hope you will forgive me for not having digested the months of archived comments here. If many or most of the folks in this thread had read the article or a similar one, I hadn’t seen much evidence of that in the thread given the amount of time spent on discussing organic matter trapped in the ice sheet, which certainly will not become a significant source of methane. I thought it useful to try to re-direct to topic to a similar but much more pertinent topic to the issue of climate change.

    Susan Anderson, I really do appreciate your comments and I will take them as a belated welcome to the group. I have no problem with you taking exception to my little barb about this being an “insular group”, but I hope you can see from the history of this discussion that I was really trying to discuss science and then was instantly caught in a barrage of floggings of a commenter. That was my perspective, and I acknowledge that it came from a small sample of experience here. I appreciate your advice about this being a possible “circular firing squad” and you should know, especially given that I am new here, that I really do work on projects that put my money where my mouth is. I am presently working to set up a solar thermal/algae biofuel hybrid system and just today was talking to a Chinese scientist about using super-critical CO2 from power plant exhaust as a fracking and hydrocarbon solvent for tertiary recovery and carbon sequestration. So don’t worry about me getting lost in conversation here. I have a day job. But these are things that I don’t discuss much in public at this point, so don’t bother trying to find those projects on the web, at least at this point.

    And if this website has been discussed in detail before, please forgive, but these folks seem to be trying to do something about the arctic methane fast feedback issue. I will readily admit that this link is the only substantive part of this post.

    Peace, y’all!

  9. 109
    tamino says:

    Re: #108 (TimD)

    Methane release in the Arctic was indeed a topic of some discussion here not too long ago. I’m certainly no expert on the subject, but the general consensus seemed to be that although the recent observations are troubling, the observed increase in methane concentration — both worldwide and in the Arctic — doesn’t yet justify declaring an emergency. I posted about the issue recently, here:

  10. 110
    TimD says:

    Tamino, I understand your perspective. It would be very costly to actually do something about methane release, and rapid increases in release seems to be a very recent phenomenon, given that we recently went through a period where atmospheric methane stabalized. But, again, when you are at the beginning of what looks like exponential change, you had better figure out the mechanism before very many doublings so you can make a better decision as to how big the emergency is, if at all. It would appear very reasonable that the rapid increase in methane release that the Guardian article discussed is driven by SST increases, which would, in turn, suggest that it is a big deal in the Joe Biden sense. So I would strongly argue that arctic shelfal methane release from clathrates is really an emergency, at least in the sense that we desperately need to throw some real, hard science at it, and fast, to better understand the mechanism so that prediction becomes something more than curve fitting.

  11. 111
    flxible says:

    TimD@108 – No, I won’t forgive you for interpreting the responses of 2 of the principals here as being they “have no time to read even these responses”, when you in turn state the same as they did, which was actually that they have day jobs too, and feel that what they decide to do with their time is more relevant than what you think they should be doing. Not to mention that they in fact as moderators here likely read every comment.
    The link I provided you is not “months of archived comments”, but a previous article on the subject you allowed the comments of this article to sidetrack you into, methane emissions in the Arctic. If you’re going to jump into a comment thread with opinions about things that the original blog post didn’t concern, I’d think you would attend to previous posts that did deal with that off-topic subject. To lift your phrases, I “thought it useful to try to re-direct [you] to a similar but much more pertinent topic on [your] issue“, about Arctic methane

  12. 112
    dbostrom says:

    Tim: Honestly, I really would prefer to discuss the science…

    Good on you, and as you say the air is rather heated regarding this topic, radiating brightly at various wavelengths illuminating and otherwise. My fault for swerving the conversation.

  13. 113
    TimD says:

    flxible, your forgiveness, in truth, means very little to me, but the truth means a lot. When Eric said in response to #42 ” I’m not bothering to read all the comments, but I will just remind people there is a difference between watts and milliwatts” I think that it is clear that my interpretation of that was wholly correct, and I could add that his remark was dismissive and snooty, especially since the origin of the topic mentioned was only a few comments above. Eric may be a god in these parts, but in my opinion he could well learn some manners so that he doesn’t contribute to the unnecessarily belligerent tone of many of the posts. And your first reference in your #105 comment was to an RC archive from January, so it could only have been found by me by searching “months of archived comments”. You should try to tell the truth, especially about things that are so easily refuted. Now anybody want to talk about the science of climatology? I sure do!

    [Response: I can assure you that no one thinks of me as a god, nor would I want them too. And I apologize for the appearance of snootiness. Problem is a) too busy and b) the "it's not CO2, it's volcanoes" comes up so often that I thought that that's what it was. My apologies. --eric]

  14. 114
    Susan Anderson says:


    Without entering into much discussion about your recent kerfuffle, you should know that you and I are exactly equal in our participation here – that is, we are posters on somebody else’s website. My humility about the subject matter comes from not being a scientist. My irritation at people’s assumptions about their “rights” to say anything and everything about and to their hosts – especially given the subject matter and the consequences of promoting conflict in this particular arena – is not humble. Once again I have to wonder where you’ve been if you don’t know things are already so rife with conflict that a little irritability is hardly out of place. You might enjoy this as a sterling example of unrestrained language and his characterization of many scientists is spot on:

    You can find plenty of courtesy at some denier sites (though they are rude to those promoting reality) but not much good information. I prefer the truth.

    As for methane, I’d suggest you all move that to the open thread linked below; in January there were a couple of posts about that as well which I found through a simple search on realclimate methane. There was a lively to and fro there about AMEG and a read through might give you some background. Now the Brits have postponed even the geoengineering experiment due to politics, it looks like its going nowhere fast.

    [Please do NOT reply here about this as the topic is Greenland glaciers; I'll take a look there for any replies since I brought it up.]

    I had to hunt around to find the aforementioned comments, which was here, not in the January methane articles:

  15. 115
    Hank Roberts says:

    > it could only have been found by me by
    > searching “months of archived comments”

    That’s no burden.

    Did you try?

    Put the words:

    methane emergency
    in the search box.
    Upper right corner of the page.

    You find too much?
    That’s because it’s
    much discussed for
    many years here.

    Too much to read?
    Use more search terms.

    Narrow it down.

    ameg release depressurize

    The “emergency” is to
    capture and burn the methane
    as fuel for sale

    Before people realize
    that either it’s
    a) escaped,
    b) not likely to escape

    Odds favor the latter in
    the short term during which
    most of us will be alive.

    It’s not the most urgent

    What is?
    Stopping burning carbon.

  16. 116
    Susan Anderson says:

    Hank Roberts, right you are. In my case I was advantaged because I knew to add John Nissen to the search so it showed up immediately. But the RC methane articles in January were very easy to find. I still think this discussion should be moved to Unforced Variations, as the discussion about Greenland’s ice had gotten more like what I enjoy on RealClimate, expertise from people who know what they’re talking about, not conflict promotion ad nauseam.

    Also, this is an educational site for those like me who are interested but lack knowledge and skills. Being too lazy to read through means one should not comment.

    Here, that is:

  17. 117
    Susan Anderson says:

    My emphasis was wrong. I was talking about its usefulness to people like me, not the purpose of RC which is none of my business.

  18. 118
    TimD says:

    Eric, I do so much appreciate your apology and it wasn’t that big a deal. I am both happy and sorry that you are so busy. It is a bit hard to imagine putting the considerable work you did into the root article and not being very curious about what people are saying, even if you have to read the posts as you fall asleep at night. You have my apology for being a bit demanding and rude to get you to respond to my points. Let us all strive for professional civility and focus on the pertinent scientific questions.

    Along those lines, I am curious as to how you reconcile the ice stream info, which doesn’t look particularly exponential, with the Grace data which REALLY does: (fig HTC19). The mass loss has been consistently accelerating in a weirdly smooth fashion for ten years now and the data is exquisitely accurate and well sampled. What is your favorite mechanism? To me it has to be basal melting driven by SST increases and when you are talking about basal melting, the slickest analogy is cold butter on a hot flat griddle. It doesn’t require very high degrees of melting to make the slab of butter slide off the edge of the griddle.

    I thank you in advance for your response and for your efforts to educate the public about this, perhaps the greatest threat to humanity since Toba.

  19. 119
    TimD says:

    Susan and Hank, please note that Eric has graciously apologized and I did so in return, then I returned to the subject at hand. Also note that your effort toward my education in your posts were utterly, and rather rudely, off topic. Hank, I know how to search the internet, but you should ask yourself, what was my incentive to do so? Why should I care what self-absorbed blatherers were saying five months ago? I wanted to discuss the Greenland ice sheet with real scientists here, and instead I am treated rudely and dragged into old battles that I have already said I have no interest in. Susan, you claim to dislike “conflict promotion ad nauseam” but on this thread, you were the one who instigated the flogging of the roundly hated commenter that devolved the discussion into a communal beat down. Note also, that I did not bring up the methane issue and before I joined the extensive discussion on that topic, I noted it was off topic, but that, being new, I did not know the customs in that regard on this site. Again, I would, if I intended to continue posting on this site, strongly argue for better moderation of the egotistical nonsense that seems to make up the bulk of the comments. You folks have most effectively convinced me that I have better things to do. I would hope that the moderator would allow this admittedly off-topic post for 24 hours or so then remove it and the posts it was responding to and similar off-topic nonsense. Hasta luego… P.S. reCapcha totally sucks.

  20. 120
    wili says:

    And to add to TimD’s analogy, even a slight tilt of the griddle will greatly accelerate how quickly the melting butter slides off the edge. Hence my interest in isostatic rebound.

    Thanks to all for checking on references to organic matter beneath the icesheet. David at 107, while your “amatuer opinion” may lead you to the conclusion that there is not enough methane to worry about, the scientists in the article came up with the figure 1,200 Pg, nothing to sneeze at, given that all atmospheric methane comes to 5Pg. But they didn’t break out what part of that was specifically under the Greenland ice sheet, so that figure could be smaller. Hanks article about forests that once covered southern Greenland is suggestive.

  21. 121
    Susan Anderson says:

    TimD, I’d best apologize and retire, though you do, now, after thanking me for the “welcome” earlier (@108 re 106), put the blame on me, and seem overready to find fault. The order of comments supports your takedown so I’ll try to take it to heart and try to do better in future. I was interested in the work you do* and wished to point you at discussions that were relevant to that, but phrased it poorly. I have always felt I am only here on sufferance as I cannot contribute to the scientific discussions, only observe and learn. I meant to point out that I do not in any way speak for “the management” nor do many others in comment sections anywhere.

    The discussion about AMEG went on for days and was at the link I provided, and the January methane articles are easy to find.

    * I really do work on projects that put my money where my mouth is. I am presently working to set up a solar thermal/algae biofuel hybrid system and just today was talking to a Chinese scientist about using super-critical CO2 from power plant exhaust as a fracking and hydrocarbon solvent for tertiary recovery and carbon sequestration. So don’t worry about me getting lost in conversation here. I have a day job. But these are things that I don’t discuss much in public at this point, so don’t bother trying to find those projects on the web, at least at this point.

    And if this website has been discussed in detail before, please forgive, but these folks seem to be trying to do something about the arctic methane fast feedback issue. I will readily admit that this link is the only substantive part of this post.

  22. 122
    Dan H. says:


    I think you argument for exponentially increased Greenland ice mass loss is not well supported? Box, et. al. writing in State of the Climate in 2010, noted that “Glacier ice area elsewhere in 2010 (i.e. excluding Petermann Glacier) remained near the average loss rate of 121 km2/yr observed since 2002.”

    While the GRACE measurements support an increase in annual ice mass loss in its first half of its operation, no such increase can be detected since (which includes the Petermann calving). Again, this is still very short-term results from GRACE, but compared to other long-term measurements, there does not appear to be an exponential increase.

    [Response: You can't have it both ways. Either the record is too short to tell, or it isn't too short to tell and it either does or does not show an exponential increase. Logic dictates that you need to decide on one, not two, of these options. Of course, rhetoric dictates that one mangle these things up to ensure the minimum of informational transfer, but I'm hope that's not the goal. - gavin]

  23. 123
    Dan H. says:

    I have no idea what you are talking about. My contention is that the GRACE data does not support claims of exponential increase in Greenland ice mass loss.

    [Response: piece of data X may not provide support for proposition A, because X => not(A), or X implies neither A nor not(A). But it can't do both at once. Having said that, GRACE estimates of Greenland mass loss do show an acceleration. - gavin]

  24. 124
    Dan H. says:


    Looking at the actual numbers from GRACE (and I am fully aware that there are several different estimates), one cannot conclude that mass loss is accelerating. From the following, the ice mass loss increased from 84 GT in 2003 to 201 GT in 2005, but there is no significant trend since: 2006 – 177 GT, 2007 – 244 GT, 2008 – 287 GT, 2009 – 190 GT, and 2010 – 266 GT. A better analysis would be that ice mass loss has held constant at ~230GT / yr as Rene Forsberg concluded. There is simply too much scatter in the data to conclude that ice mass loss is accelerating.

    [Response: Are you even looking at the same graph? page 9 on that presentation - it is clear that a linear trend through the first half is shallower than a linear trend through the second half, and although I don't have the exact numbers, I would guess that the trends are significantly different. You could also fit a quadratic and show that the quadratic term is significantly non-zero. Comparing one year to another is not a statistically robust procedure.- gavin]

  25. 125
    tamino says:

    Re: #124

    In which Dan H. says

    A better analysis would be that ice mass loss has held constant at ~230GT / yr as Rene Forsberg concluded.

    and links to a presentation by Rene Forsberg. In that presentation, Forsberg himself states on the final (“conclusions”) slide:

    Current changes of Greenland ~ 230 GT/yr … accelerating, but slowly

    Dan, how did you get from Forsberg saying “accelerating” to your claim “held constant … as Rene Forsberg concluded.”

    Do tell.

  26. 126
    David B. Benson says:

    wili @120 — This is the wrong thread to go further. Use the Unforced Variations thread after reading the thread devoted to Arctic methane here on RealClimate and my comment over at Tamino’s. The links to both have been given previously on this thread.

    Vaguely related to the topic here are the buried forests in southern Greenland. Those grew during the Eemian (interglacial 2) and 100,000 years hasn’t been long enough to scrape all of that off.

  27. 127
    Hank Roberts says:

    > “held constant”

    Are those words a quote?

    Sounds like a modeling study, describing
    something held constant for a run.

  28. 128
    Unsettled Scientist says:

    Hank, “held constant” was not at all in the link provided by Dan H. He grabs the 230 number out of a link he provides, and then claims it shows something it doesn’t injecting his own “held constant” idea (or maybe he was mislead by a denialist blog and the idea wasn’t even his). My guess is that Dan H. didn’t read the whole thing (and it’s all pictures) and grabbed the 230 number off page 3 instead of page 15 where Forsberg put the conclusions including the point that the rate is accelerating as tamino pointed out. It could be a copy pasta mistake, or it could be him injecting his bias. Either way it’s just another example of a misrepresentation by this commenter.

  29. 129
    John E Pearson says:

    127 Hank asked was “held constant” a quote. No. Tamino quoted in 125:

    “Current changes of Greenland ~ 230 GT/yr … accelerating, but slowly”

    In other words the kindest way to interpret “held constant” is that it was due to a careless reading of Forsberg on the part of the poster. Given the poster’s past behavior I personally find it easier to believe that it was deliberate misinformation .

  30. 130
    MARodger says:

    Re Ice burden on Greenland.

    Using the word “acceleration” to describe variations in changing levels of something is fraught with ambiguity unless a pause is taken to define the term. Best to use a less ambiguous term.
    The word orignially bandied about here was “exponential increase” (and previously there has been talk of “doublings” of Greenland’s ice loss) These terms obviously refer to an increase (or in this case ‘decrease’) in level that is faster than linear.
    I have in the past found arguments that GRACE shows the loss of Greenland’s ice mass as increasing exponentially, that the rate is doubling per decade or quicker – such argument I have found entirely unconvincing in the past and have been happy saying so, even when I then am refuting statements from the likes of Jim Hansen.

    I would argue that the GRACE measurements presently suggest no more than that Greenland’s net ice mass loss is increasing linearly, at a rate of 50-60 Gt pa.

    Regarding the past few years annual GRACE data that shows a reduction in this rate of increase of net ice mass loss (eg – 3 year rolling average trends dropping from 60 Gt pa to 6 Gt pa), such assertions requires more supporting analysis & evidence rather than blindly serving up the numbers as some sort of difinitive proof. Indeed, the last couple of melt seasons had record areas of melt & other such extremes (For instance, 2010 see ), yet GRACE shows no spectacular net mass loss (although the quantity ‘max annual ice mass – min annual ice mass’ is starting to look interesting).

  31. 131
    Hank Roberts says:

    You know that slippery film that forms on, well, anything and everything — rocks in streams for example, or bathtubs and showers, or in distilled water lines in medical and science labs, or inside crude oil pipes, or lungs? It’s generically called a ‘biofilm’ — often a multilayered structure in which a variety of microbes participate, that excels in picking small amounts of nutrients out of flowing water.

    We used to have them so extensively in watersheds that the water in many streams and rivers was cleaned up enough to drink, tho’ we’ve rather overwhelmed that process in recent decades; biofilm ‘reactors’ are being built to replicate the way they clean dirty water: .
    Turns out biofilms grow underneath glaciers.

    One paper here on one glacier finds the stuff is nitrogen-limited.

    What difference will feeding that stuff make, if any? Nitrogen compounds are a significant pollutant falling out in rain — and running off in meltwater that would feed whatever’s living under the ice.

    Pure speculation from here on — is a change in the microbial population underneath glacial ice going to affect movement of the ice?

  32. 132
    wili says:

    Hank, that is really interesting. Not something I ever would have thought of.

    This is the kind of info and insight that keeps me coming back here, in spite of the DH’s and his feeders.

    David–Thanks. Will do.

  33. 133
    TimD says:

    MARodger; While I have pretty much sworn off this site, it is the weekend and I have some time to waste. But your statement that GRACE data doesn’t support the argument that Greenland ice mass loss is exponential is COMPLETELY OUTRAGEOUS NONSENSE! (I hope that is sufficient emphasis) How can you look at the figure in the latest available data ( fig. HTC19) and not see a continuously accelerating function? [edit - polite please]
    And please, good Tamino, fit a curve to that data and then tell us what the doubling time is of the best exponential fit. I know, Tamino, that you have the talent to do that fit before or after removing the seasonal (annual) component of the function. [edit]

  34. 134
    TimD says:

    P.S. This is what that curve fit will look like, but the article unfortunately does not give the parameters of that fitting equation. But, marodger, is that a continuously accelerating function or not?!!!! Jesu Christo.

  35. 135
    Unsettled Scientist says:

    Is anyone who is competent and honest looking at the GRACE data or Forsberg and concluding that it shows ice mass loss has held constant? It appears not.

    “These data records [more than just Forsberg] suggest that the rate of mass loss is increasing, indeed nearly doubling over the period of record, but the record is too short to provide a meaningful evaluation of a doubling time. Also there is substantial variation among alternative analyses of the gravity field data (Sorensen and Forsberg, 2010), although all analyses have the rate of mass loss increasing over the period of record.

    “Together the two satellite systems show a consistent picture of ice mass loss in marginal zones of the ice shees, and increasing trends in some central regions. …in Greenland chares are largest in SE Greenland, with mass loss trends currently increasing along the NW margin, consistent with the observed increase in flow speeds of major outlet glaciers.”

  36. 136
    Dan H. says:

    I have to agree with MA on this. The slight increase in the annual ice mass loss cannot be seperated from random noise. The first few years of GRACE data did show a greater annual increase, but the more recent years, have not. This does not preclude that ice mass loss could be increasing exponentially, just that the GRACE data we have to date, is too sparce to show such a trend.

    We seem to have a spilt here between those who would argue for an exponential acceleration of the annual rate of ice mass loss, and those arguing that ice mass loss is increasing only slightly or not at all.

    [Response: #logicalfail. You cannot argue both that the data are too uncertain AND that there is no increase in the loss. Only if the data were clear and unambiguously showing no acceleration or negative acceleration (which they do not), could you make your second point. Please at least try to remain consistent in a single comment. - gavin]

  37. 137
    MARodger says:

    TimD @133 & 134
    I do admit to some sloppy (potentially ambiguous) writing @130, a problem I was myself warning of as I am not alone in doing this.
    So here let the ‘Quantity In Question’ (QIQ) be defined. Greenland has ice cover. The mass of that ice cover is reducing annually. Let the rate of that reduction be defined for discussion here as QIQ.

    All the evidence (bar Forsberg’s numbers discussed below) shows that QIQ is increasing.

    The GRACE data graphed in Hansen & Sato 2010 ( This version of the paper presents graphs of our QIQ in fig 8c.) extends to 2009 & which is used to support the proposal that QIQ is increasing exponentially.
    I find their argument less than convincing & TimD @133 adds nothing extra except his link to Box et al 2011 provides a graph of data up to 2011. I contend that Hansen & Sato 2010 Fig 8c could as easily be fitted showing a linear trend in QIQ.

    The data presented on Page 9 of Forsberg (linked to by Dan H @124) does present numbers for annual ice loss based on GRACE data to end 2010. These numbers point to a reduction in QIQ over the last 2 years.
    The data presented by Forsberg is however of unknown calculation. Scaling measurements from Box et al fig HCT19, I see no sign of such a reduction in QIQ for the last 2 years from either changes in annual minimums or in changes in the annual max-minus-min values.

    While linearity in QIQ may prove simplistic, given the data available, I remain unconvinced of anything as dramatic as proposed by Hansen & Sato 2010.

  38. 138
    tamino says:

    Re: #136

    In which Dan H. says

    We seem to have a spilt here between those who would argue for an exponential acceleration of the annual rate of ice mass loss, and those arguing that ice mass loss is increasing only slightly or not at all.

    No, Dan. We have a split between you, who said “ice mass loss has held constant at ~230GT / yr as Rene Forsberg concluded,” and Rene Forsberg, who said “accelerating.”

    YOU are the one who brought this up. Are you now hoping it will go away? Either own up to your distortion of Forsberg’s conclusion, or you have no place in the discussion.

  39. 139
    Gene Goldring says:

    In debating for AGW an interesting topic came up. Groundwater pumping creating 0.05mmper year sea level rise. Yoshihide Wada, with Utrecht University in the Netherlands has commented on the study at Live Science’s article, “Groundwater Pumping Is Causing Seas to Rise”

    He also claims this has not been addressed in the IPCC reports.

    Any comment?

    [Response: Well, the issue of human water management - both from reservoir construction and groundwater depletion - affecting sea level is indeed discussed in IPCC (section and was taken into account in many recent estimates of future rise (see Rahmstorf and Vermeer for instance). These new estimates are higher than previous ones and so that is interesting to look into. I don't have any particular insight into the methodology though - perhaps some others might? - gavin]

  40. 140
    Unsettled Scientist says:

    Dan H.

    We’re not interested in your analysis anymore. You have too many times clearly stated the opposite conclusions of the references you provide. Now you are saying there is a split of people between those who think the ice mass loss rate is increasing and those how think it is not. That may be true among laymen, but we are concerned with the scientific understanding, not the laymen understanding.

    The link you provided shows accelertation of ice mass loss. I provided a link that shows that “all analyses have the rate of mass loss increasing over the period of record.” We don’t care what your opinion is, and nobody should care what my opinion is. This a blog about science. Please provide scientific references for your points because we know how bad you are at analysis after this, the PDSI, and beyond.

  41. 141
    Gene Goldring says:

    Thanks Gavin for the quick response. I’m a lay person that reads the articles here at RC frequently trying to understand climate science. RC has been a big help in this regard over the years.

    Thank you for all the hard work you folks put in explaining this rather complicated topic.


  42. 142
    Paul S says:

    Gene Goldring – Looking at the link, it specifically refers to the IPCC not including groundwater depletion in its sea level rise projections, which I think is correct. As Gavin says, AR4 did briefly cover it in terms of past changes but with huge uncertainties. Given that the IPCC reports are about climate change and its effects, it’s not immediately obvious that groundwater depletion should be included in forecasts.

    Wada et al. published a previous paper on this topic in 2010. Konikow 2011 found that there were significant problems with their approach. Presumably those criticisms have been addressed in this latest paper, but I don’t have access to the full text. It looks like they’ve compared their initial results against GRACE gravity data at least.

  43. 143
    Paul S says:

    Hmm.. looking closer at the numbers in the article it forecasts contributions of 0.8mm/yr by 2050, whereas the 2010 Wada et al. paper suggested a ~0.8mm/yr current contribution, so clearly the methods have changed somewhat.

  44. 144
    TimD says:

    MARodger, The quantity in question (QIQ, how clever) is not the rate of ice loss, since that is a single value defining a quantity of change withing a specific time period. These discussions have been about the rate of change of ice loss, which describes how the loss rate changes over time and in this case, is a simple mathematical function that best fits the observations. Your statement that the data in Sato and Hansen, and the more recent data I provided, “could as easily be fitted showing a linear trend” is absolutely factually untrue, if the quality of the fit is defined as the RMS error between the data and the fitted curve. Your opinion is of no concern or import, and you might as well keep it to yourself. The value of such curve fitting exercises is predictive, and curve fits found in the paper by Sato and Hansen predicted the continuing accelerating trend seen in the later data. But the important result of understanding the form of the mass loss function is that it has ramifications as to how to understand the underlying mechanisms of that loss. The root article pointed out that outlet glacier velocity may only account for some 30% of the mass loss increase over the decade of GRACE measurements, which shows an increase on the order of 100%. Increases in surface ice melt is significant, but not high enough to account for the losses. The only reasonable mechanism remaining to explain the accelerating ice mass loss is basal melt driven by increasing penetration of the ice that is exposed below sea level to the rapidly increasing sea temperatures, driven in turn by large and fast albedo reductions due to very rapid sea ice loss. Gravity measurements see this phenomenon very well, which is not nearly so apparent using surface measurements of velocity and melt. I strongly criticize the misleading title of the root article, which suggests that ice loss in Greenland is not accelerating when it clearly is, and especially the statement in the article that the results of a cited paper “put speculation of monotonic or exponential increases in Greenland’s ice discharge to rest”, when that is factually in error. I strongly believe that the author should correct that misstatement and the misleading title, which is already being utilized by denialists in their big energy business propaganda campaigns. The article by he guest researchers is also misleading in that they state flatly “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.” I would maintain that that statement is seriously in question based on the GRACE data and several other studies that show rapid increases in basal melting due to sea water infiltration. These statements contained in the root article are very irresponsible and, potentially, a huge disservice to the task of educating the public about the risks of rapid climate change. They should be changed in a prominent fashion as clear errata and by changing the title of the article.

  45. 145
    David B. Benson says:

    What Gene Goldring said.

  46. 146
    MARodger says:

    Further to my whitterings @137 about GRACE data & ice loss from Greenland:-

    Curious about what looked like discrepancies, I plotted Forsberg’s data of increasing Greenland mass loss along side Hansen & Sayo 2010′s data I had hiding on a spreadsheet. Then I scaled the recent Box et al data & grafted it on top (which is not the best of fits but for plotting the trend is likely adequate).

    The resulting graph pretty much reconciles Forsberg’s numbers with the rest. Link to graph here (usually requiring two clicks to get “your attachment”.)

    Concerning comment @136. It is strange that the comment begins “I have to agree with MA on this.” but then goes on to present statements I almost wholly disagree with.

  47. 147
    TimD says:

    MARodger, a nice graph, but your linear fit is mis-labeled. The linear fit line should be labeled “33 GT pa^2, since the quantity plotted is the annual rate of change in the mass anomaly, and the curve fit to that data is the rate of change of that quantity, otherwise known as acceleration. So your linear fit is consistent with a constant acceleration of the ice mass anomaly, which in turn is an exponential increase of the mass loss. Are we clear now?

    Please also note that the rate of loss has doubled from 150 to 300 gt/a between 2003 to 2010, which gives us a doubling time in annual mass loss rate of some 7 years. As I previously stated, the really pertinent question here is for how long will that acceleration remain high and nearly constant, and therefore, how many doublings of the loss rate will we see before negative feedbacks kick in? To begin to answer that question, we need to understand the mechanism of the increase. If the acceleration stays constant only until mid century, the loss rate will be some 60 times greater than today. That is something we need to understand, with great urgency. The root article severely downplays that urgency and is highly irresponsible as it stands.

  48. 148
    MARodger says:

    TimD @147

    It is nice that you like the “nice graph”. The trend line indeed should be in Gt pa^2. The trend line is more illustrative than predictive and, yes, presents roughly a 7 year doubling (170 Gt to 350Gt in 7.25 years). As you note, it is achieved with a constant ‘acceleration’. In my speak, as defined @137, dQIQ/dt = constant.

    Given this situation dQIQ/dt = constant, in the next 7.25 years, the annual ice loss will increase by another 180 Gt pa, yielding QIQ = 530 Gt pa. This is not, as you suggested, a further doubling of QIQ over even the 7.25 year period and without such a periodicity for doublings in QIQ, I fail to see how this represents an “exponential” increase.
    Hansen & Sato 2010 do propose an “exponential” increase which is why their figure 8c that graphs my QIQ (in this version of the paper) has a cirving trend.

    You do in your comment @147 address the major problem I have with Hansen & Sato 2010 as you see the process as limited by feedbacks.. In my view, any ‘doubling’ mechanism will quickly begin to impact on the climate system inducing the strongly negative feedbacks that you mention..

  49. 149
    john byatt says:


    Church et al 2011 is a relevant paper.

    Revisiting the earth’s sea-level and energy budget from 1961 – 2008


  50. 150
    Hank Roberts says:

    > the comment begins “I have to agree with MA on this.”
    > but then goes on to present statements I almost wholly disagree with

    Debate coaches have a name for that, it’s a tactic.

    > 33 GT per annum

    Out of what total?
    What would that be as a percentage per annum?
    Got a snowfall accumulation in the total?

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