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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. 201
    Ray Ladbury says:

    Jim Larsen,
    There is nothing magic about the number “30” or the number “17”. They just happen to be the numbers of years required to have a given level of confidence that we can detect a rising signal in a noisy dataset. Signal. Noise. The particular number for another dataset would depend on the relative magnitudes of each.

  2. 202
    Hank Roberts says:

    Dan H.: what does “significant” mean to you, as you use the word?

    > Dan H. says:
    > 29 May 2012 at 9:04 AM
    > Many papers choose to describe the ice mass loss as linear,
    > not because it was the best fit to the data, but
    > as a general descriptor.
    > None of the data presented show a statistically significant trend.

    Above, you used the word this way, filling in Jim’s blank:
    >> 10 years of data are plenty to reasonably estimate the long-term trend
    >> … because ___
    > 31 May 2012 at 8:36 AM
    > by preconceived notions about what I feel should be happening.

    Is that what “significant” means to you? Personal opinion?

  3. 203
    dbostrom says:

    Dialing the base period to the absolute minimum, it’s nice and toasty in Greenland just now.

    More to the point of this thread, interesting stat for 2011 from the end of Masters piece: Total ice sheet mass loss in 2011 was 70% larger than the 2003 – 2009 average annual loss rate of -250 gigatons per year.

  4. 204
    Unsettled Scientist says:

    Hank Roberts said: “because it follows the long term trend in atmospheric CO2.

    Don’t just look at the shape of the lines that can be drawn through the various dots on various charts.
    Look at the physical change in the atmosphere causing the change in the ice.”

    Dan H. said: “because it follows by preconceived notions about what I feel should be happening.”

    That pretty much sums up the difference between how these two people analyze data and choose quotations. It doesn’t matter to Dan H. what Ian Howat actually said, it matters what he “could” have said. That’s precisely my point. He doesn’t care what the scientists are actually saying, he wants to use their credibility to bolster his preconceived notions about what he feels should be happening. So he cherry picks part of Ian Howat’s quote because the rest doesn’t fit with what he wanted. He is trying to steal his credibility to say something completely the opposite of what Howat actually said. To me that is dishonest, and I am not backing down from that. I thought I might overreacted until Dan H. literally put his own words into Howat’s mouth. Words that were 180 degrees from the truth, and he sees nothing wrong with that.

    This has been going on for months. Ian Howat was good enough to quote when Dan H. could support his idea, but once it is shown that Ian Howat was saying the opposite, Dan H. literally just changes the quote to suit his needs. The PDSI is used to support Dan H.’s idea, until we point out that it’s the exact opposite of what he thinks and then he calls its credibility into question.

  5. 205
    Dan H. says:

    You seem to have a difficult time deciphering the difference between scientific conclusions and personal opinion. I quoted Howat’s scientific conclusion, and you quoted conjecture. Which do you think carries more weight?

    Funny, how you think it is cherry picking, when I quote the conclusion, but leave out the conjecture. There is a difference, in case you did not notice. At no time did I incorrectly state what Howat said.

    Also, I see that sarcasism is completely lost on you.

  6. 206
    Charles says:

    I have been following this bantering, and cannot help but be reminded of the All in the Family episode where the trade apprentice (young black boy) is eating an apple with a small paring knife, and Archie claims that he was waving a machette in front of him, while Michael claims they was no knife at all. They are both embarassed when Edith pulls out the small knife.

  7. 207
    dbostrom says:

    Time series:

    Elastic mind attempts to slip one past, 30 May 2012 at 2:58 PM:
    Only the glaciers in the SE and NW averaged the 30% increase. The rest showed little movement. The curve seems to have gotten quite a bit smaller, according to Twila.

    Her co-author, Ian Howat is quoted as saying, “There’s the caveat that this 10-year time series is too short to really understand long-term behavior,”

    CS points out truncation of the above quotation, 30 May 2012 at 4:18 PM:
    Ian Howat’s entire quote was, “There’s the caveat that this 10-year time series is too short to really understand long-term behavior, so there still may be future events – tipping points – that could cause large increases in glacier speed to continue. Or perhaps some of the big glaciers in the north of Greenland that haven’t yet exhibited any changes may begin to speed up, which would greatly increase the rate of sea level rise.”

    Elastic mind disavows complete version of truncated quote, necessitating trashing of formerly convenient source, 30 May 2012 at 9:54 PM:
    “This is pure speculation on Howat’s part. There is no reason to expect that only the worst case events could occur.”

  8. 208

    Quite frankly, the continued tolerance of Dan H’s behavior by the proprietor’s of this blog now constitute tacit approval of his dissembling. As such, the veracity of this whole blog is now in question.

    Sad, the descent from valued resource to atavism. It didn’t have to be this way.

    [Response: Oh please. People asking that other commenters be banned is tiresome. No-one is obliged to respond to everything other people say (and not feeding the trolls being a very good maxim), and of the responses that have been made there are plenty that demonstrate clearly that Dan H. is a consistent mis-interpreter of the science – and people will draw their own conclusions about his credibility. We have a comment policy which we try to stick to. We moderate for impoliteness, excessive repetition, long-debunked talking points etc.- not whether someone annoys you, and not because they don’t agree with us. – gavin]

  9. 209
    Susan Anderson says:

    After TimD’s personal, fierce, but quite relevant takedown, I’m returning to spending more time trying to learn than distracting you all from the more interesting parts of what is said.

    While the controversy is continuously annoying, I think perhaps the best thing is for you all to go on discussing real science and ignore the distractions … please? Don’t like it? Don’t feed it.

    But I can’t help wondering why in discussions of time intervals such as 17 and 30, nobody ever mentions that they are in the context of a fairly complete record that goes back to about 1800, or at least 1850, and a fascinating and skilled attempt to reconstruct and quantify the record for a long time before that instrumental record. Nobody takes any particular 17 or 30 years in isolation. The denial/fake skeptic community seizes on these items and takes them out of context, and you all are so used to the context you don’t mention it.

    It is all too easy to forget that while individual events can be treated as noise – distractionalists discredit observable trends by saying, oh, that one was exceeded by a record in [name that year] – it’s foolish to ignore the overall level of excess bunched together in a shortish time period. The recent northeastern heat (shared by Greenland and Scotland) has passed, but that does not erase the excess.

  10. 210
    TimD says:

    dbostrom @203: Thanks for the Wunderground post. It’s a great summation of the current situation. I think these blogs tend to be addicting. We all think our individual ideas are oh, so important and a site like this is a big megaphone as we stand on our soapboxes. I am trying to give it up since my multiple suggestions to the principals here to correct the obvious errors and horrible tone of the root article has been totally ignored, but can’t help checking in on the back and forth. It is a most disheartening thing to see this site lob a slow ball right over the plate for the denialists at bat, but that is what has happened.

    The back and forth about exponential vs. linear, signal to noise, etc., are amusing, but now getting pretty tedious. As the link by dbostrom shows, the Greenland ice sheet mass loss has been accelerating over the entire record of the decade of GRACE measurements and the latest data shows it continues to accelerate. S/N? The overall signal we see in the GRACE data graph dropping like a big rock, is about 2200 gigatons. The system is clearly very easily resolving the seasonal melt/buildup signal which has an amplitude of less than 200 GT. This seasonal signal is pretty easily removed, since it is highly periodic with a period of precisely one year, but it makes it most obvious that the resolution of the system is well under 50 GT which indicates that the signal to noise ratio is at least 40 to one. There are other sources of noise in the climate system itself, but they are small. There are now about 120 high resolution measurements of a high signal to noise signal. How many points of data do you need to determine the trend of such a data set? A lot less than 120.

    Here’s a fun analogy: Imagine a cartoon of couple of WWII GIs slogging through the mud. All around are signs that say “MINEFIELD!!” or “MINEN”. The winding road ahead has been cleared of mines, but one of the dogfaces says, “Hey, we can save a lot of time by taking a shortcut!” As they slog through the minefield, there is a bright flash under the foot of one of the soldiers. Since this is a cartoon, time can be conveniently stopped and the GIs can take stock of their situation at the instant that the mine is detonated. One asks the other; “Do you think that white hot gas is expanding linearly or exponentially?” The question is kinda moot. They are in a minefield, and in a millisecond or so, the boot of the unlucky soldier will be sailing past his head, with his foot still inside and in a few seconds his body will land in the mud with blood gushing, if his heart is still beating.

    Now, to further clarify the analogy, the current earth’s climate system is like a minefield because, as the Hansen and Sato paper shows ( ), when large ice sheets are present at the poles, the system is dominated by positive feedbacks, and small energy forcings like the orbital variations are hugely amplified. Warming phases in these oscillations are particularly unstable and rapid compared to the cooling phase (it takes longer to plant a mine than it does for it to explode). Once the mine begins to explode, or the Greenland ice sheet begins to collapse, you have very little time to debate the details. It may be too late to avoid the tipping point of ice sheet collapse, but really, we are in ultra slow mo compared to the unlucky soldier and could still be able to turn around and get back to the safe road. That clear road (rapidly cutting GHG emissions) is longer and requires some added effort to walk, but we can’t rationally continue walking in the minefield of hitting an intrinsically unstable climate system with the largest and fastest climate forcing spike in millions of years.

  11. 211
    TimD says:

    Dear Susan @209: I didn’t mean to be fierce. Stern? Sorry, and didn’t mean to hurt your feelings. But we are, in reality, on the same page and both wishing to discuss the science, so we should be friends. Peace.

  12. 212
    Susan Anderson says:

    TimD, you were right, that was the important part, and it’s likely others were having the same problem but were too polite to say so. The truth can be painful but it is also liberating. I have an odd relationship to this site – some know about my Dad (PW; please leave it alone, thanks) and I taught drawing at MIT to a lot of scientists – aging faculty brat? I dropped out of MIT absorbing differential equations which leaves me hanging, but my tire-kicking instincts are pretty good and weather is always fascinating:
    (front page particularly vivid today)

    Once I got out of the way, IMHO it turned out you are a valuable addition to the conversation.

    Masters at Wunderground is terrific. So is Neven and a good few others who hang around here.

  13. 213
    Susan Anderson says:

    oh sigh, *without* absorbing diffyQ’s

  14. 214
    tamino says:

    Re: that guy.

    Gavin, it’s your blog and you get to decide. There is undeniable merit in not moderating just because people are annoying, or because they disagree with you.

    But I urge you to consider the purpose of this blog. It’s to spread information and combat misinformation. Dan H. has been nothing but a source of misinformation. He hasn’t just disagreed — he has actively distorted the truth. He claims Forsberg says one thing when the opposite was stated, he pronounces as fact the lack of significant trend when it’s easily shown to be present, he quotes selectively even though context contradicts him, then when the full context is posted he disparages the very source he misused. These aren’t mistakes — they are a deliberate pattern.

    You should also consider that, to some small degree, this is our blog too. We work hard — yes, we put real time and effort into it — to further the purpose. We spread information, we combat misinformation. It’s a sacrifice of our time and energy, but we believe in the cause and its importance so we’re happy to help. God knows there’s enough misinformation to combat. Adding to the load is a step backward. It’s a bitter pill to think that his efforts are actually working — both by wasting our time and by sewing doubt in the minds of those (especially the new) visitors who don’t ingest all the details or see all the refutations.

    The stated purpose of the Bore Hole is for comments “that would otherwise disrupt sensible conversations.” So I ask you in all honesty: if Dan H.’s comments don’t fit that description, what does?

    Whatever you decide, we’ll remain loyal to this, the best climate science blog on the internet. And we’ll continue to do our best to spread information and combat misinformation. The continued presence of Dan H. makes it harder.

    [Response: With all due respect I think you are over-reacting here. The rebuttals to Dan H.’s comments have been valuable – people learned what is actually happening in Greenland, what the sign of PDSI represents, the difference between TCR and ECS etc. All issues that have come up elsewhere and where they haven’t necessarily been competently rebutted. Too many of the comments in response to Dan H. have been related to his motives, character and suitability as a commenter – and frankly that is far more disruptive of conversation than either simply ignoring him, or quoting his purported sources to show that they actually contradict the position he was putting forward. Some of his stuff has been bore-holed, some was moderated for cause – as have intemperate or ad hom responses to his posts. But continuous discussions of blog comment policy are tedious, and I’m loathe to continue this one. This stuff is requires quick judgement calls, and I haven’t got the time or energy to justify every single one that is made on a daily basis. Thus there are basically three options – either we drop comments altogether, let them all though, or we continue to moderate in the mostly the same fashion we have been doing – but for that to be sustainable, we can’t spend too much time on it or discussing it. Note that this is a function of traffic as well – if there was less, it might be easier, and if there was much more than it would be more difficult – but these are judgements that we need to make. Please just let this drop and let us get on with it as best we can. There are far more interesting conversations to be had. – gavin]

  15. 215
    dbostrom says:

    Somebody (Tim?) mentioned albedo here. Just ran across another of mindblowing statistic of the size only available via planetary-scale tampering:

    Darkening of the [Greenland] ice sheet in the 12 summers between 2000 and 2011 permitted the ice sheet to absorb an extra 172 quintillion joules of energy, nearly 2 times the annual energy consumption of the United States (about 94 quintillion joules in 2009).

    Via a post at Meltfactor where there’s more fascinating stuff on albedo of Greenland.

  16. 216
    MARodger says:

    dbostrom @215
    The quote you present is fascinating stuff but also ambiguity in carnate.
    This extra energy absorbed equals the latent heat required to melt 500 Gt of ice and it’s safe to assume this is an annual figure but is it the average summer darkening or the increase in summer darkening? The paper doesn’t make this clear. I’m inclined towards the former as 500 Gt of extra annual melt seems rather too high, assuming all (or most of) the heat goes that way. On the other hand 500 Gt of annual summer melt is approaching the level of Greenland’s annual rain/snowfall which the laws of physiscs insist must be melted every year for the sole purpose of annoying Dan H.

  17. 217
    Lawrence Coleman says:

    What I gleaned from all your comments is that it is virtually impossible to predict future glacial retreat based upon past data. It is a very dynamic system and tipping points are/will play a crucial part in future predictions. Decrease of speed of the North atlantic overtuning circulation due to increased fresh water release and accelerating rates of methane release from the many Giga tonnes of thawing methane hydrides in the arctic sea and tundra regions and of course our old friend ice albedo and black soot particulates building up on the once pristine arctic to name the most obvious known to date make it impossible to accurately quantify any change in glacial melt and thus sea level rise. However put it this way the number positive feedback mechanisms currently at work are quite probably unprecedented in paleoclimatology.

  18. 218
    Lawrence Coleman says:

    Tamino: You have all heard the saying that your most valuable teacher can be your worst adversary. Boy! Politics wouldn’t get far if both sides agreed on all the issues the other was putting forward. Treat Dan H as your teacher and wield your keyboard with all the skill of a master sword fighter as you skilfully cut through his ignorance and misinformation with the ultimate weapon of scientific and irrefutable logic.

  19. 219
    Dan H. says:


    I endorse your comment #209. I know we have not seen eye-to-eye on many aspects, but I agree with you here. The 17-yr period was particular annoying, as people took Santer’s analysis of the satellite data, and applied it to just about everything else. This was not limited to your denial/fake skeptic community (whatever that is), but was used to show that the 10/12 year temperature hiatus was insignificant.
    In those areas, where we have much more data, it should be used – as you put forth. Where we only have a decades worth of data (i.e. GRACE), that is all we can work with. We can compare it to other methods, such as was the original intent of this thread, and we have proxy measurements dated hundreds and thousands of years prior. Short-term trends are useful, in light of a certifiable forcing. However, we must be careful in extrapolating the short term into the long term, as errors can propagate.

  20. 220
    Dan H. says:

    I remember that episode, and it is a good analogy. Taking a step back, I recognize my part in the whole fiasco, and hope others do also. That said, the Meathead would like to offer his apology to the Archie Bunkers here.

  21. 221
    Susan Anderson says:

    re @209, Dan H. misleads again. The 17 and 30 year periods have more meaning in the context of the record; the way he puts it unconstructs my intention. Ben Santer, a top best scientist, has vast experience with phony skepticism and its evil effects on our future if we choose inaction. I believe he does not dismiss the effort to extend the record by whatever means our real world makes available, sediments, ice cores, tree rings, the lot, with more than adequate clarification of the uncertainties involved. There are a whole lot more lawyers than scientists in government, and they skilled at flooding people with real work to do with phony well-sounding demands. Exploiting the meaning of uncertainty and the complexities of scientific efforts to reveal the past in our physical world beyond what is available using instrumental records is a favorite trick to obfuscate. Saying we can’t expand knowledge is not helpful. In addition, the instrumental record would never have been created with this know-nothing argument, which is political rather than scientific.

    When somebody consistently returns to misstate the facts, they are not doing anybody any favors. Visitors inclined to believe these distortions will cite RealClimate as the source of the disinformation. Not all the sources he misconstrues are here to undo the damage, which appears to be far from innocent.

    I get this crowdsourcing stuff from Andy Revkin, and it does not take into account the amount of work it takes to track down the sources and correct the misinformation, as well as the persistent trickle of fuel provided to those looking to discredit scientific findings rather than tease out the truth. In addition, it lays the person doing the work open to attack.

    And of course this has once again detracked the discussion of Greenland glaciers.

  22. 222
    Unsettled Scientist says:

    > Where we only have a decades worth of data (i.e. GRACE), that is all we can work with.

    “We can’t look at one glacier for 100 years, but we can look at 200 glaciers for 10 years and get some idea of what they’re doing.” – Ian Joughin

    > We can compare it to other methods

    …such as other satellite data such as MODIS or QuickSCAT. Luckily GRACE isn’t our only bird in the sky.

    It is important to look at the totality of evidence, and not just one source of data. We shouldn’t place too much emphasis on any newly published paper. It is the coherence between many avenues of investigation that make AGW clear. Similarly with Greenland’s accelerating ice mass loss, we look at the totality of evidence. Sadly, the rate of ice mass loss is still increasing and even GRACE doesn’t have a single negative year (i.e. one in which rate of ice mass loss didn’t speed up).

    These new papers “add rich detail to our understanding of the ice sheets; they do not change estimates of the magnitude of future sea level rise. The bottom line is that Greenland’s glaciers are still speeding up.” -OP

  23. 223
    Dan H. says:

    You may need to check the reports a little closer on the GRACE data. The Forsberg data showed a slowdown from 2005 to 2006, and from 2008 to 2009. The Joodaki paper showed the two previous slowdowns, with an additional decrease from 2003 to 2004. Combining these results with the Bergman paper and the Moon and Joughlin paper, yields ample evidence that the rate of increase is decreasing.

    [Response: Science is not a salad bar where you can pick a years worth of data from here or there, spice it up with a creamy dressing and present it as if it was something substantial. It has been demonstrated above that you are simply wishing the acceleration away. Enough with the made up stuff please. – gavin]

  24. 224
    dbostrom says:

    MARodger says: 1 Jun 2012 at 3:53 AM

    Read the post I linked; obviously you did not, or you’d know that the additional energy absorption happened over the span of ten years. Indeed, it does not seem you even read my comment, where that is fairly clear.

    The distribution of albedo loss is explained in detail and you can learn about it if you suppress your “no” reflex for just a few short minutes.

  25. 225
    dbostrom says:

    Susan Anderson Jun 2012 at 10:44 AM

    A concise, accurate synopsis. I’d add that the the misinformation is artfully crafted; what appears to be quotation is rearrangement, findings are modified and even reversed while brazenly accompanied by citations and there is no accompanying invective or sprinkling of insulting adjectives to trigger BS detectors.

    Quality stuff, in its own way. As to stopping it Gavin’s explained why that can’t work, so treat the problem as a phenomenon for study and this venue as a lab for developing clinical solutions, as if the problem were malaria or some other subtle parasitic infection.

  26. 226
    Dan H. says:

    Your response appears to be precisely what I have trying to tell other people here. On several occasions, someone has simply pointed to start and end points, and claimed an x rate of acceleration. Moon and Joughlin have pointed it out to start this thread (a point you have constantly refuted). Others have tried to find significance, where none exists. In most of the references cited here (Hansen excepted), acceleration of ice mass loss was greatest in the earlier years, diminshing or disappearing in the following years. How you can continue to content that the ice mass loss is accelerating exponentionally in the light of all the evidence presented here is beyond me? The data which refutes your position, is not simply “made up.” It exists in the scientific literature referenced here (and elsewhere).

    Enjoy your salad.

    [Response: You are a funny guy. But a clearer case of projection is hard to find. – gavin]

  27. 227
    sidd says:

    I think Dr. Bindschadler pointed out a while ago that the marine terminating glaciers of Greenland might retreat to escape warming ocean, but that there was no such escape for W. Antarctica. (I note that some large sections of E. Antarctica are also bedded submarine, and most of these Antarctic areas are exhibiting increased ice velocities and mass loss.)

    Now the rate of albedo reduction on Greenland is another positive ablation factor that might make Dr. Bindschadler’s point irrelevant. Greenland can melt in place and drain away, and has as complex hydrology in and below the icesheet as is found in Antarctica. Ice discharge is not the only road to high water.

    And I repeat my plaintive request for a breakup of precip over Greenland into snow and rain.


  28. 228
    dbostrom says:

    How you can continue to content that the ice mass loss is accelerating exponentionally in the light of all the evidence presented here is beyond me?

    Yeah, don’t believe your lying eyes…

  29. 229
    TimD says:

    dbostrom@228; yeah, a picture is worth… a lot. Nice use of html. Slightly more recent data shows that the acceleration continues throughout ’11. I do think we need to be clear as to why the GRACE data set is so special. The thing that we are interested in in this discussion is the overall change in the mass of the Greenland ice sheet. Since gravity is directly related to mass and distance from that mass, if you can measure gravity really, really accurately, you can see pretty small changes in mass. The GRACE mission is very important because it represents a quantum leap in accuracy and resolution, since it makes very, very precise measurements very quickly, all over the world. When I was an undergraduate and first used the then state-of-the-art Worden gravity meter, I found that I could accurately measure the difference in the earth’s gravity from the floor to a table top, due to the change in distance from the center of the earth. The GRACE system is hundreds of times more accurate than that, and it can essentially measure the change in gravity continuously as it orbits the earth in a way that guarantees complete coverage of the earth every few days. This new data has revolutionized geodynamics because of the truly exquisite precision and resolution of the information. Check out the two gravity maps of the earth in this file:
    There is a large negative gravity anomaly just south of India that was thought to be due to the earth’s upper mantle flowing north past the thick Indian continent. With GRACE data you can easily see the flow lines of the mantle moving around India like the bow of a ship. The force of this mantle flow is what is continuing to push India into China, constantly renewing the Himalayan Range.

    Measuring changes in the motion of individual ice streams is hard and only measures surface features. The efforts of glaciologists as in the root article are clearly important, but when it comes to the question at hand (Greenland ice mass loss) the GRACE data is much, much better. Given that the graph in dbostrom’s post averages all of the contributions of moving and calving ice over the whole of the region, we are seeing the integrated effect of both warming water and air from top to bottom of the entire sheet. It is a very scary picture, but thank NASA, we have it. It shows conclusively that the statement made in the root article here that “the results put speculation of monotonic or exponential increases in Greenland’s ice discharge to rest” is absolutely false and a disservice to climate science and should be prominently disavowed by the author. I am talking to you, Eric.

  30. 230
    Susan Anderson says:

    Thank NASA, full of GRACE. Would we were not losing our satellite edge:

    Heidi Cullen seems to have missed your point about GRACE and Greenland?

    Understanding maths is the most important higher power needed for our civilization’s future.

  31. 231
    Susan Anderson says:

    aargh, *an* important, not *the most* important …

  32. 232
    Ray Ladbury says:

    The problem with designations like “exponential increase” is exactly the same as that of periodicity–it is rare that you have sufficient data to really definitively call the increase exponential. If you don’t have several doubling periods, all you can really say with confidence is that the increase is “nonlinear”–unless you have physics that supports an exponential increase. Dbostrom’s graph certainly shows nonlinearity. Do we have physics favoring exponentiality.

  33. 233
    MARodger says:

    dbostrom @224
    I would not do you the discourtesy of commenting in such a manner without having first read the linked papers. You see no ambiguity there. Fine. But I see ambiguity aplenty & have sent an enquiry to the author asking for clarification.

    Re comment @ 228, 229 & 232.
    This is well trampled ground being re-visited. Rather than look for a monotonic, exponential or whatever form of increase within the curvature of a graphed line, a plot of the rate of change may be more easily handled. Hansen & Sato 2010 have one figure 8c in this version of the paper. & I have grafted on the Box et al data bringing it up to 2011 See here (usually requiring a second click to ‘download your attachment.’.

  34. 234
    dbostrom says:

    MARodger says: 2 Jun 2012 at 10:13 AM
    I see ambiguity

    You do?

    From the post:

    “Satellite observations from the NASA Moderate-Resolution Imaging Spectroradiometer (MODIS) indicate a significant Greenland ice sheet albedo decline (-5.6±0.7%) in the June-August period over the 12 melt seasons spanning 2000-2011. According to linear regression, the ablation area albedo declined from 71.5% in 2000 to 63.2% in 2011 (time correlation = -0.805, 1-p=0.999). The change (-8.3%) is more than two times the absolute albedo RMS error (3.1%). Over the accumulation area, the highly linear (time correlation = -0.927, 1-p>0.999) decline from 81.7% to 76.6% over the same period also exceeds the absolute albedo RMS error.

    Because of extreme 2010 melt and little snow accumulation during the melt season (Tedesco at al., 2011) and afterward, the ice sheet albedo remained more than two standard deviations below the 2000-2011 average in October. Like year 2010, 2011 albedos are more than 1 standard deviation below the 2000-2011 average.

    Darkening of the ice sheet in the 12 summers between 2000 and 2011 permitted the ice sheet to absorb an extra 172 quintillion joules of energy, nearly 2 times the annual energy consumption of the United States (about 94 quintillion joules in 2009).

    This decline is not only over the lowest elevations, but occurs high on the ice sheet where melting is limited.

    A significant albedo decline of 4.6±0.6% in the 2000-2011 period from a year 2000 value of 83.0% is observed for the accumulation area, where warming surface temperatures are enhancing snow grain metamorphosis.”

    Cut and dried. Personally I wouldn’t waste the author’s time just for the purpose of dramaturgy here but you’ll notice at the meltfactor site you can simply ask the author your questions in the comments section of the post.

    As to the graph at 228, sure, it’s an ephemeral curve (really must be because there’s not an infinite supply of ice) but as that curve is what’s actually visible in data, wishing the curve away without fresh data to support curve-free dreams requires speculation more fanciful than the facts we have.

  35. 235
    dbostrom says:

    Susan Anderson says: 2 Jun 2012 at 8:07 AM
    Thank NASA, full of GRACE.

    Nice turn of phrase!

    Sadly GRACE is an experimental mission, is not classed as an Earth observation satellite. As is so often the case the mission is still functioning at over twice its nominal lifespan. As GRACE is an experiment it does not seem as though replacement will be automatic.

    This is akin to considering the oil pressure warning lamp on one’s dashboard to be “an experiment” wherein if the lamp or circuit fails it is not replaced because “the experiment” is over.

    I don’t understand how it is that missions such as OCO and GRACE are not considered vital instrumentation as opposed to “experiments.” OCO spacecraft were twice attached to salvaged ICBM boosters and subsequently lost even though it’s arguable that these spacecraft are equally as life-safety critical as a “human rated” booster intended for transport of human cargo.

    If I were a researcher working with GRACE data I’d consider no investigation project complete until I’d peppered legislators with copies of resulting publications. Similarly the rest of us armchair researchers should make a point of pestering our lawmakers about this. Seeing this data truncated when some part of one of the GRACE twins breaks will be a bitter pill to swallow.

  36. 236
    Hank Roberts says:

    “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.”

    — from the original post

  37. 237
    TimD says:

    Ray Ladbury@231; “Do we have physics favoring exponentiality?” Yeah, but what we don’t have is good specificity of the mix of positive feedbacks that would allow better predictive models. It is the clear presence in the system of so many potential positive feedbacks in a melting ice system that ensures that the ice loss is accelerating. That the best curve fit equation that effectively de-trends the data has an exponent is the test that the system has an exponential, or accelerating, component. That is clearly the case with the GRACE Greenland data. The obvious positive feedbacks in this system are:
    1) The loss of sea ice, where we observe a large decrease in albedo that leads to increased sea surface temperature that increases melting.
    2) Melt water lakes at low elevations that reduces albedo, again accelerating melting and lubricating ice streams at their base when the water breaks through to the base of the sheet.
    3) Thawing of tundra releasing GHGs that increase temperatures that increases thawing
    4) Reduced ice albedo due to soot that increases melting period that in turn reduces the albedo by eliminating high albedo snow cover for longer periods
    5) Increased basal melting due to infiltration by warmer sea water that increases the capacity of basal ice channels, that increases infiltration rates
    A less obvious, but potentially very important feedback, is non-linear behavior of ice as a fluid. As the body of ice warms due to infiltration of melt and warm sea water, the viscosity of the ice drops, flow rates increase leading to increases of viscous heating, potentially leading to a thermal runaway. There are others. The empirical evidence that there are positive feedbacks in ice sheet systems is the paleoclimate data that shows how small orbital variations in forcing are greatly amplified when ice sheets form in the Antarctic, then the Arctic in the late Tertiary. You can see this in the Sato, Hansen paper previously cited. You can also note that warming phases occur faster than cooling.

    It is the interaction of these positive feedbacks that will allow a reasonable model that could predict how long the accelerating trend will continue. It is obvious that the observed acceleration cannot continue through the end of the century, because we would run out of ice before then.

  38. 238
    Ray Ladbury says:

    Tim D.,
    Yes, those are positive feedbacks. No, they do not necessarily indicate exponential decay. Specifically, is there some reason to think dx~axdt? That is what is required for a truly exponential system. Nonlinearity does not equal exponentiality.

  39. 239
    Hank Roberts says:

    > the end of the century, … run out of ice

    That would be rather fast.

    It would be interesting to see a chart of the rates published over time and how those have changed. Kind of a meta-study.

  40. 240
    TimD says:

    Ray, I think you are putting too fine a point on it. If there are positive feedbacks dominating in a dynamic system, that system will tend to accelerate and its average behavior will take the form of an accelerating function that can be fit with an exponential curve, which is clearly the case with the GRACE Greenland data. The important point is that when we see this sort of behavior in a system dominated by positive feedbacks, we need to quickly determine what positive feedbacks are most important and create models that accurately duplicate past behavior based on that understanding and then may allow us to compute the range of possible future states. At present, we don’t really know whether the current behavior of Greenland’s ice sheet is a minor decadal bump or a major tipping point that will lead to rapid coastal flooding and constitute the greatest disaster in human history. Inquiring minds want to know!

  41. 241
    TimD says:

    Hank @239: “That would be rather fast.”

    Right. That is the nature of exponential growth. It quickly gets ridiculous. If the cumulative loss continues to double every 7 years or so, there are about 12 doubling periods to the end of the century. Starting with 1400 GT lost in the first 7 year period of Grace data, you would be, at that rate of acceleration, get rid of some 6.16 million km^3 by the end of the century, more than the total volume, estimated at 2.85 million km^3. That’s why I said that we can’t go through too many 7 year doubling periods before negative feedbacks inevitably kick in to change the trend. But that also means that the earth’s climate will be very much different than the average Holocene conditions and we will all, effectively, be living on a different planet.

  42. 242
    David B. Benson says:

    TimD @240 — Ray Ladbury has the right opf it. A so-cfalled positive feedback does not necessarily imply exponential growth. Read any text on linear system analysis.

  43. 243
    Ray Ladbury says:

    Tim D., The defining character of an exponential is that the change in the quantity is proportional to the quantity at a given time.

    Exponential=/=rapidly increasing.

  44. 244
    TimD says:

    Ray, Exponential growth doesn’t have to be fast, and can be negative. Here is a good exponential curve fit for the GRACE data from dbostrom’s graph @228.
    M(T)= M(T-1) – (135 * (1.105 ^(T – T(0)))) Where M(2002)=800 and T(0)=2002

    It has a doubling time of about 7 years and fits the data through 2011. Plug it in to a spreadsheet and see how long the ice loss stays exponential. The value exceeds the total Greenland ice mass before 2080.

  45. 245
    Ray Ladbury says:

    Tim D., I have a passing familiarity with the exponential function. I also know that for short time series (less than 2-3 doubling times) it is very hard to distinguish exponential growth from, say, a power law–or, if the time series is short enough a linear trend + a sinusoidal variation.

    Curve-fitting is fun. By itself, it isn’t science.

  46. 246
    Hank Roberts says:

    Tim, nobody’s saying the ice just decided it’s going to change on its own.
    It’s changing as the result of forcings; the question is how sensitive is the ice, how quickly does it show the effects and what changes happen. Look at the recent work with old photographs — the ice — in that area, at this time — may be a more sensitive indicator rather than the last thing to start changing when the climate warms. It’s a local effect of multiple causes.

    Nobody’s claiming it’s a specific mathematical curve.

  47. 247
    TimD says:

    Ray, How very heavy of you. Please feel free to fit any curve you wish. There is a reason to fit an exponential curve to the GRACE data that seemed compelling to perhaps the most accomplished climate scientist in the world, one J. Hansen. That is because systems dominated by positive feedbacks, when strongly forced (which is the current situation in the arctic) frequently experience periods of exponential growth, typically when that system is beginning to approach a tipping point and basically blow itself apart. Please feel free to ponder the science behind a sinusoidal curve fit, but I don’t think I need your help.

  48. 248
    dbostrom says:

    Dumb question from a relative ignoramus: Does exponential growth axiomatically imply growth without limit? Is it considered cricket that growth may be considered exponential for some limited period of time?

    Looking (advisedly) at Wikipedia, I see these examples:

    “The number of microorganisms in a culture both will increase exponentially until an essential nutrient is exhausted. “


    “Heat transfer experiments yield results whose best fit line are exponential decay curves.”

    The curve in the graph above is bent, not linear, so surely describing the curve must involve an exponent, implying that for the period of time observed loss has grown exponentially?

    My naive interpretation is that “exponential” need not lead to no ice; right now loss of ice is swiftest at the margins of Greenland’s ice sheet where due to geometry the area encompassing loss is greatest. It’s not surprising to see a curve in the affair.

    Looking at the physics of situation (albedo loss at margins, attack by warmer water also at margins) linear loss early in the game seems the least likely scenario. Presumably we’ll eventually see the curve in the graph above reverse itself because the conditions of loss right now are limited in their scope.

    Again, opinions of a naif. Perhaps the discussion here of “exponential growth” is on a much more formal level.

  49. 249
    Hank Roberts says:

    Would you cite exactly what Hansen says and where?
    See also: (page down a few times to get to the content)

  50. 250
    Hank Roberts says:

    PS, Tim, are you referring to this?
    Paleoclimate Implications for Human-Made Climate Change
    James E. Hansen, Makiko Sato
    (full text pdf link at the web page)

    “… These data records 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.
    We conclude that available data for the ice sheet mass change are consistent with our expectation of a non-linear response, but the data record is too short and uncertain to allow quantitative assessment. A 10-year doubling time, or even shorter, is consistent with the gravity field data, but because of the brevity of the record even a linear mass loss cannot be ruled out. Assessments will rapidly become more meaningful in the future, if high-precision gravity measurements are continued.”