Greenland Glaciers — not so fast!

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.

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269 comments on this post.
  1. TimD:

    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. Hank Roberts:

    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. David B. Benson:

    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. dbostrom:

    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. Ray Ladbury:

    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. Ray Ladbury:

    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. flxible:

    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. Hank Roberts:

    “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. TimD:

    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. flxible:

    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. David B. Benson:

    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. TimD:

    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. Hank Roberts:

    > 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. Brian Dodge:

    “…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. David B. Benson:

    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. Dan H.:

    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. Hank Roberts:

    > 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. Unsettled Scientist:


    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. Dan H.:

    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]