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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. 251
    TimD says:

    Hank @246, I guess I am saying that seeing signs of exponential growth is important in this context, basically because it is saying that something very interesting is happening to the system experiencing that type of growth. The growth of the Chinese GDP, which has a growth rate similar to GRACE data of Greenland’s ice loss, of about 10% per year that has been maintained for some 30+ years, is not a function of any intrinsically exponential activity. Chinese economic growth is, in fact, due to trillions of individual economic transactions every year, that individually, don’t have any real relationship to “exponentiality”.'s_Economic_Growth_and_the_Environment_Fa_08
    But it is very clear that the overall behavior of that system is very interesting and something we need to understand, since they are totally kicking our ass in the economic sphere. That is, in the same way, why I have said several times, that the appearance of exponential growth is an alarm bell that says something interesting is happening to the growing mass losses in the Greenland ice sheet system that needs to be studied with urgency and understood. Hansen fit an exponential curve to the data when it was only available through 2008. That curve still fits through 2011. When someone in a prominent public position claims that the exponential growth is not exponential and that the issue of rapid exponential growth “has been put to rest” that person is acting irresponsibly by arguing against the urgency to rapidly come to understand the system. I’m talking to you Eric.

  2. 252
    Hank Roberts says:

    Tim, if you’re talking about that Hansen paper — note the discussion there around Fig. 7.

    “Alley (2010) reviewed projections of sea level rise by 2100, showing several clustered around 1 m and one outlier at 5 m, all of these approximated as linear in his graph. The 5 m estimate is what Hansen (2007) suggested was possible under IPCC’s BAU climate forcing. Such a graph is comforting – not only does the 5-meter sea level rise disagree with all other projections, but its half-meter sea level rise this decade is clearly preposterous.
    However, the fundamental issue is linearity versus non-linearity….”

    Point of Fig. 7 is that the “Exponential (10-Year Doubling)” is much _slower_ to rise — sea level barely changes up through 2050 or so — then takes off.

    And Fig. 8.: Greenland (a) and Antarctic (b) mass change deduced from gravitational field measurements by Velicogna (2009) and best-fits with 5-year and 10-year mass loss doubling times.

    These are hypotheticals — spooky for sure. He’s saying this could happen, we can’t yet tell if it’s happening, and pouring CO2 into the atmosphere as we are means there’s no reason it wouldn’t be happening.

    If we come across some idiot throwing fuel onto a fire, we don’t argue about the rate of change and whether the fuel’s going to make the problem significantly worse.

  3. 253
    David B. Benson says:

    dbostrom @248 — For micro-organism growth, fit an S-shaped curve such as the logistic function. That appears exponential, approximately, up to the inflection point. I opine that the same is, approximately, the case for Greenland ice.

  4. 254
    dbostrom says:

    Thank you, David. S-shaped is indeed what I was picturing.

    Swerving into repeating myself, I sure hope we’ve slotted in a replacement for the GRACE twins when the one of the pair inevitably shuffles off this mortal coil. They’ve proven wildly successfully for all sorts of applications, far beyond expectations. What a bummer if the austerity fad means we blind ourselves.

  5. 255
    Ray Ladbury says:

    Tim D., I never fit a single curve to any dataset that I am serious about analyzing. Multiple fits give you a lot more information–even if none of them is the “correct” fit.

    Given the current data, it is irresponsible to say that the increase IS exponential for the same reason that it is proper to be concerned that it COULD BE exponential–namely because of the implications of such growth. The fact remains though, that I know of no period in paleoclimate where we saw such catastrophic collapse. I’m a wee bit more concerned about the possibility of it here than I am about, say, the simultaneous release of all the Arctic’s methane as mooted by our last catastrophist. The reason is that the rate of warming could play a much bigger role for the ice sheets. Still, I think sustained catastrophic collapse of the Greenland ice sheet by the end of this century is unlikely.

  6. 256
    Ray Ladbury says:

    Indeed to amplify on what David said above, I can’t think of a single instance of sustained exponential growth in nature–be it if bacteria, cancer cells, nuclear fission…

    Exponential growth always leads to catastrophe–in the sense that the physics of the system changes dramatically. One interesting system was the inadvertent creation of a pulsed neutron generator by Japanese nuclear workers.

    Running behind schedule mixing solutions of enriched uranium, they doubled the amount in the mixer, causing the system to go critical (exponentially at first). The heat of the fissions heated the solution, causing it to expand, decreasing the rate of fission, cooling it… This bit of ingenious stupidity caused the untimely death of the two workers, but I don’t think it ever got its well deserved Darwin award.

  7. 257
    flxible says:

    When someone in a prominent public position claims that the exponential growth is not exponential and that the issue of rapid exponential growth “has been put to rest” that person is …
    …. Talking about something other than what TimD has hijacked this thread over. I would expect that I’m not the only one here to take exception to TimD’s repeated “calling out” of our host, using a misinterpretation of the introduction to the post [by invited guests], which concerns Greenland’s ice discharge, NOT the mass (loss). In addition, the entire paragraph from that introduction makes it entirely clear what is under discussin in the post, regardless of TimD’s pet fixation, particularly the entire phrase, which includes “speculation of monotonic or exponential increases …” and “… one cannot deduce how the ice sheet will react in the long run to a major climatic warming”, although TimD appears to think he can make that deduction. Maybe TimD could submit his analysis to Science as a response to the Moon et al paper so we can put the exponential digression here to rest.

  8. 258
    Hank Roberts says:

    “has been put to rest” is a quote from where?

    Look at Hansen’s Fig. 7 – “exponential” growth there is pictured — it’s _slower_ than linear growth up through around 2050, and until 2100 not as much melted as with linear growth.

    It sounds like you think “exponential” has to mean “rapid” and that’s not necessarily so.

  9. 259
    TimD says:

    Hank, I don’t see much that we disagree about. I like your parable and would add that if we saw a man beginning to throw fuel on a small fire consuming a wooden house, we would think an onlooker completely crazy or an accomplice to the arson if he were to say “Oh, the fire is not so fast.”

  10. 260
    flxible says:

    Hank @258 – See comment by TimD @22 May 2012 @ 12:51 AM with Erics response, and his further misinterpretation @ #78. Maybe TimD should investigate Erics “day job”.

  11. 261
    David B. Benson says:

    Ray Ladbury @255 — 8.2 kya event. From memory only, the largest of the various Laurentide proglacial lakes suddenly drained through the Lauerntide ice sheet and out to sea via Hudson’s Bay. It just occured to me it would be of some paleoclimatological interest to have an estimate of how much SLR was due to that single event (as it was over in a matter of at most a few weeks). Unfortunately I haven’t the time to attempt to find an estimate these days, being intensely busy with something else.

  12. 262
    TimD says:

    flxible@257: The exact quote is “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”. My paraphrase is reasonable although I will admit I should have included a “…”. Given that both Eric and the guest authors discuss melt and the relevance of Greenland ice loss in general on sea level rise, it is important to reconcile the very significant discrepancy between their study, which shows a 30% increase in ice stream velocity/discharge while the GRACE data, which is intrinsically more relevant, complete and accurate, shows about a 120% increase in the rate over the same period. By not discussing that discrepancy Eric has no business at all in saying that anything regarding ice mass loss, including discharge, has been “put… to rest”. That sort of language is a sweet slow ball for the denialists, and that is my primary beef with Eric. I asked him nicely to discuss that discrepancy and he has consistently refused to get involved. As far as I am concerned, if he is not prepared to take the time to discuss and clarify his positions relevant to inflammatory statements in his work product on this site, he should delegate that responsibility. This is a prominent public forum in the global warming debate and it should serve to inform the public at large about important aspects of climate studies. His article grievously misses the most important aspect of Greenland ice loss and his sloppy language damages the cause that this forum claims to serve. And you are flat out lying when you say that I claim or “appear” to claim to know how the mass loss will proceed into the future. I have, in fact, consistently stated that it is, instead, a call to redouble our efforts to understand the underlying mechanisms of that observed, very disturbing loss record. Tell the truth!

  13. 263
    Hank Roberts says:

    > the most important aspect of Greenland ice loss

    Measures vary, and we don’t know how it’s going to proceed. Glaciers vary and their responses to the varying weather over the past few decades is much more sensitive than had been thought. Nobody’s missed this.

    Read Joe Romm on how climate discussions get taken over by the alarmists and denialists, who between them claim the entire conversation and stifle scientists who won’t take either side and shout down those trying to learn rather than proclaim they know the truth.

    > pulsed neutron generator

    The Japanese event wasn’t the first — a similar criticality happened June 16, 1958, at the Oak Ridge Y-12 Plant.

  14. 264
    Brian Dodge says:

    “…it would be of some paleoclimatological interest to have an estimate of how much SLR was due to that single event…”

    “We present a high-resolution early Holocene sea-level record from the Mississippi Delta that documents a distinct sea-level jump, marked by a characteristic stratigraphic succession that is corroborated by paleoenvironmental reconstruction. The 0.20–0.56 m local sea-level jump occurred within the 8.18 to 8.31 ka (2σ) time window and is attributed to the final drainage of proglacial Lake Agassiz– Ojibway (LAO). Since the timing of the sea-level jump is indistinguishable from the onset of the 8.2 ka climate event, this study provides compelling evidence for a nearly immediate ocean–atmosphere response to the freshwater perturbation. In addition, the total inferred eustatic sea-level rise at 8.2 ka (after correction for gravitational effects) amounts to 0.8 to 2.2 m, considerably higher than previous estimates for the final stage of LAO drainage.” Synchronizing a sea-level jump, final Lake Agassiz drainage, and abrupt cooling 8200 years ago, Yong-Xiang Li a, Torbjörn E. Törnqvist, Johanna M. Nevitt, Barry Kohl

  15. 265
    David B. Benson says:

    Brian Dodge @264 — Thank you vary much. This may well resolve an issue regarding the almost complete disappearance of near-shore (tidewater) ocean organisms on along the east coast of South America at some (rather uncertain) time betwen LGM and the Holocene. As I understand it, this coast was only repopulated around or after HCO.

    The other possiblity is Meltwater Pulse 1A. However, despite the rather large SLR, this event is spread over a millenium or so and therefore one supposes the tidewater organisms could keep up with the change. It hadn’t occured to me until yesterday to consider the 8,2 kya event. So thank you again for your research.

  16. 266
    Dan H. says:

    Reference to this discovery with the article abstract was posted on another thread. It seems more appropriate to the discussion here, especially concerning the effects of increased SST on glacial loss.

  17. 267
    Hank Roberts says:

    > 266
    More info and links on the paper Dan mentions — it’s the one on the historical archive of glacier photographs — posted above, earlier in the thread at 30 May 2012 @ 10:43 AM

  18. 268
    Unsettled Scientist says:


    Some points I found interesting from the article linked in 266:

    “From 1943-1972, southeast Greenland cooled – probably due to sulfur pollution, which reflects sunlight away from the earth.

    “The important point is not that deadly pollution caused the climate to cool, but rather that the brief cooling allowed researchers to see how Greenland ice responded to the changing climate.”

    And a quote from Box in the article:

    “From these images, we see that the mid-century cooling stabilized the glaciers,” Box said. “That suggests that if we want to stabilize today’s accelerating ice loss, we need to see a little cooling of our own.”

  19. 269
    Dan H. says:

    Also noteworthy, is that the higher land temperatures of the 1930s resulted in greater melting of the land-terminated glaciers, while higher Atlantic ocean temperatures recently has resulted in greater melting of the marine-terminating glaciers.

    [Response: It’s worth pointing out that the glaciers are substantially more ‘melted’ now than they were in the 30s in both domains. – gavin]