RealClimate logo

Ice Sheets and Sea Level Rise: Model Failure is the Key Issue

Filed under: — group @ 26 June 2006

Guest post by Michael Oppenheimer, Princeton University

A plethora of research articles has appeared over the past year reporting new observations of the Greenland and West Antarctic ice sheets along with associated modeling results. RealClimate has reviewed the issues raised by these articles and attempted to clarify the sometimes conflicting inferences about the current mass balance of the ice sheets, as well as their future contributions to global mean sea level rise (see here and here).

Nevertheless, the issue still seems to perplex many journalists and others because there are two entirely distinct aspects of the sea level rise problem that are emphasized, depending on which scientists are speaking. On the one hand, these ices sheets are large enough to ultimately raise sea level by 7m and about 5m, for Greenland and West Antarctica, respectively. On the other, the recent observations that caused such a stir report a current contribution to the rate of sea level rise not exceeding ~1mm/yr from both ice sheets taken together. If this rate were maintained, the ice sheets would make a measurable but minor contribution to the global sea level rise from other sources, which has been 1-2mm/yr averaged over the past century and 3mm/yr for 1993-2003, and is projected to average 1-9mm/yr for the coming century (see IPCC Third Assessment Report).

The key question is whether the ice sheet contribution could accelerate substantially (e.g., by an order of magnitude) either in this century or subsequently. Sea levels were indeed much higher in the distant, warmer past but the timing of earlier sea level rise is very uncertain. From the point of view of societal and ecosystem adaptation, the timescale over which ice sheets might disintegrate, which may be on the order of centuries or millennia according to the two extremes posited in the literature, is crucial.

The complexity of bridging the gap between past and future trends is familiar to the climate community, which has dealt with the same issues with regard to global mean temperature. Ice sheets aside, continuation of past warming trends based on the roughly 100-year temperature record (0.05-0.1ºC/decade) would pose a significant but manageable problem for most countries. Projected future warming (0.15-0.55ºC/decade) based on increasingly reliable general circulation models, poses much more serious, even unmanageable challenges. But the state of ice sheet modeling is far different from the state of atmosphere-ocean modeling, as underscored by the recent observations. At this juncture, numerical modeling simply does not provide a credible basis for quantitative projection of ice sheet behavior in a warmer world.

The limitations of ice sheet models were revealed starkly by the collapse of the northern sections of the Larson B ice shelf in 1998 and 2002. Glaciers bounded by the landward edge of the ice shelf accelerated toward the sea while glaciers bounded by the more southerly section of the ice shelf, which remained intact, didn’t. Apparently, backpressure on glaciers from the abutting ice shelf provides a significant portion of the restraining forces keeping land-based ice in place, at least in some instances. The recent behavior of glaciers farther south in West Antarctica, and in Greenland, points to a similar dynamical response to ice-shelf fragmentation.

Many glaciologists regarded these observations as a clear test of the ability of ice sheet models to forecast dynamical changes in a warming ice sheet, a test the models failed. The long-standing inability of ice sheet models to reproduce the ice streams of West Antarctica, unexplained dynamical contributions to the mass balance of the Greenland ice sheet during the late 1990s and its apparent basal response in one location to surface melt-water reinforced this skepticism. The problem is threefold: the physics in the models is incomplete, the numerical problems are very difficult particularly in the neighborhood of the grounding lines where the land-based ice begins to float (Vieli and Payne, 2005), and observations remain sparse. It may take more than a decade, perhaps much longer, to bridge the gap in the model world because human and financial resources dedicated to the ice sheet problem are woefully inadequate (see Kintisch, 2006: Science 312, 1296, for a discussion of problems with the planned National Polar-Orbiting Operational Environmental Satellite System (NPOESS), a proposed platform for crucial future ice-sheet observations).

We might also look to our experience with GCMs for some guidance as to how to evaluate the situation while we await an improved basis for numerical projection. GCMs are anchored in a broader range of observations than are ice sheet models at both the process and synoptic levels. But what would we do today if GCMs had failed several critical tests? With the climate already changing, we would likely not throw up our hands and say “let’s come back and reassess the situation once our numerical tools have improved”. More likely, we would increase our attention to paleoclimate analogs, a standard test bed for, and complement to, numerical modeling.

With the ice sheets already changing, the importance of analogs has come sharply into focus. Inferences from Eemian climate and sea level yield a wide range of estimates with regard to the climate changes that might result in widespread deglaciation of either or both ice sheets. Polar warming less than 5ºC (Overpeck et al, 2006) and global mean warming of 1ºC (Hansen 2004) or 2ºC (Oppenheimer and Alley, 2005) above recent temperature have been cited. Such global mean warming would almost certainly occur during this century, given forcing within the range of IPCC scenarios. In the southern hemisphere, the requisite circumpolar warming may occur more gradually, but a northern hemisphere trigger for WAIS deglaciation during the Eemian has also been proposed (Overpeck et al 2006). The information on potential rates of deglaciation is extremely sparse and its relevance to the future unclear. Potential rates of sea level rise equivalent to 1m/century (10mm/yr) have been suggested based on paleoclimate analogs (Overpeck et al, 2006) and by comparison to current ice discharge from West Antarctica (Oppenheimer 1998).

Filing the gap in knowledge between the risk (a significant probability of many meters of sea level rise) and the current reality (rapid local ice responses to local warming but small aggregate effect on sea level rise so far) will require a sharpened focus on all three fronts: observations, modeling, and paleoclimate assessment. Currently, the resources to do any one of these at the appropriate level are lacking. And because greenhouse gas concentrations and ice sheet loss are effectively irreversible, policy decisions need to be made based on the information in hand, which argues that deglaciation could be triggered by a modest warming.


E.Kintisch, Science 312, 1296-1297, 2006.
J.Hansen, Climatic Change, 68, 269-279, 2005.
M. Oppenheimer and R.B. Alley, Climatic Change, 68, 257-267, 2005.
M. Oppenheimer, Nature, 393, 325-332, 1998.
J.Overpeck et al, Science 311, 1747-1750, 2006.
A.Vieli and A.J.Payne, J. Geophys. Research, 110, F01003, doi:10.1029/2004JF000202, 2005.

90 Responses to “Ice Sheets and Sea Level Rise: Model Failure is the Key Issue”

  1. 1
    S Molnar says:

    Hansen’s article in the current New York Review of Books gives a worst case historical rise of one meter per 20 years. How does this relate to your one meter per century number? Are they different events or different interpretations of the same event(s)?

  2. 2
    Eli Rabett says:

    Thanks for a timely answer. I think you could better state the problem with uncertainty, the way Hansen does: (hope you don’t mind a long quote, but I think Hansen states the problem as well as possible)

    ” Time constants: the slippery slope.

    Three time constants play critical roles in creating a slippery slope for human society: T1, the time required for climate, specifically ocean surface temperature, to respond to a forced change of planetary energy balance; T2, the time it would take human society to change its energy systems enough to reverse the growth of greenhouse gases; T3, the time required for ice sheets to respond substantially to a large relentless positive planetary energy imbalance.
    T3, the ice sheet response time, is the time constant of issue. I argue that T3 is of the order of centuries, not millennia, as commonly assumed. Growth of ice sheets requires millennia, as growth is a dry process limited by the snowfall rate. Ice sheet disintegration, on the other hand, is a wet process that can proceed more rapidly, as evidenced by the saw-toothed shape of glacial-interglacial temperature and sea level records.
    It seems inescapable to me that the time constant T3 is measured in centuries, not millennia. I would be surprised if T3 exceeded 1-3 centuries. Ice sheet models will not be capable of providing a good assessment of T3 until they are driven by all anthropogenic forcings, incorporate realistically all significant processes and feedbacks, including those discussed above, and demonstrate the ability to simulate realistically rapid nonlinear ice sheet disintegration as occurred during meltwater pulse 1A.

    The likelihood that T3 is comparable to T1 + T2 has a staggering practical implication. T3 >> T1 + T2 would permit a relatively complacent “wait and see” attitude toward ice sheet health. If, in the happy situation T3 >> T1 + T2, we should confirm that human forcings were large enough to eventually alter the ice sheets, we would have plenty of time to reverse human forcings before the ice sheets responded.

    Unfortunately, T3 ~ T1 + T2 implies that once ice sheet changes pass a critical point, it will be impossible to avoid substantial ice sheet disintegration. The reason for this is evident in the definition of the time constants. The comparability of these time constants, together with the planetary energy imbalance, make the ice sheets a ticking time bomb.

    If, as I have argued, T3 indeed is not very much larger than T1 + T2, it becomes of high priority to detect as early as possible beginnings of ice sheet disintegration. High precision measurements of ice motion and sea level change are needed for early detection of any acceleration in the global rates of ice movement and sea level rise.”

  3. 3
    Ike Solem says:

    I was happily surprised to come across this very interesting article in the press that also covers this issue, and gives a nice flavor of what earth science field research is like (100 proof peppermint schnapps is also good for a quick warmup): LA Times report on ice sheet dynamics research.

    The fact that models missed the dynamics issue, instead focussing on the latent heat of melting, should be a cautionary note for policymakers, as well as for students of the earth sciences. There may be other trigger-like booby traps in the climate system; the carbon trapped in methane ice clathrates and in the northern permafrost zones is also a cause for concern. I’ve also heard reports of anomalously high CO2 emissions over the past couple years; “sink exhaustion” could also be a concern – i.e., the possibility that the mechanisms that naturally remove CO2 from the atmosphere could become “saturated” or could themselves be negatively affected by climate change or human activities: see for example Letter on tropical deforestation to Science

    Along the lines of models and data, this Brevia in Science is interesting. The authors consider the effects of multidecade trends in warming on the mid-troposphere by examing the ‘following northern summer’ effects of El Nino. Maybe this is more relevant to the previous article on the Walker circulation, though a warming mid-troposphere would also have effects on the ice sheets:
    Enhanced Mid-Latitude Tropospheric Warming in Satellite Measurements
    The poleward shift of the jet streams would lead to less shear in the atmosphere; shear impedes hurricane formation. The widening of the tropical Hadley circulation due to warming – there seems to be an issue in whether or not this is captured in climate models. Increased hurricanes and altered rainfall patterns could have societal impacts, sure.

    The Anasazi culture is widely appreciated for their beautiful cliff dwellings in the American Southwest – but those cliff dwellings were the result of brutal wars of extermination brought on by climate change and the resulting limitations of essential resources. The single ladder leading up was easily defended from raiders. Wouldn’t it be nice to avoid such scenarios?

    This site really seems to be having a positive informational effect on media coverage of climate science.

  4. 4
    pat neuman says:

    I think ice sheet failure is the key issue in sea level rise, along with a less rapid rise due to thermal expansion of ocean waters. I also think that the significance of latent heat for snowmelt, which was described by Dunne and Leopold (1978):

    “If water from moist air condenses on a snowpack, 590 calories of heat are released by each gram of condensate. This is enough energy to melt approximately 7.5 gm of ice, which when added to the condensate yields a total of 8.5 gm of potential runoff”,

    is important. I brought the Dunne and Leopold (1978) reference into my article titled: Earlier in the Year Snowmelt Runoff and Increasing Dewpoints for Rivers in Minnesota, Wisconsin and North Dakota
    September 11, 2003, at:

    I would also like to insert a thank you note here to John McCormick for his compliment in 43 to my post in 42 at:

    Thank you John.

  5. 5
    cwmagee says:

    Is the Eemian a sub-division of the Pleistocene? When was it?

  6. 6
    Blair Dowden says:

    Re #1: Hansen’s New York Review of Books article (supposedly a book review although he barely mentions the books in question) is referring to an event at the end of the last ice age known as Meltwater Pulse 1a (see this summary or the Science paper). This occured when the Earth’s ice caps were much larger and extended much further south, and with a 5 degree (C) rise in global average temperature. It is unlikely that melt rate could occur with today’s smaller ice caps and two or three degress of warming. But clearly, the IPCC 2001 projections for sea level rise in the 21st century were too low. I suspect the 2007 report will be different in this area.

    Re #5: The Eemian is the previous interglacial period, about 120,000 years ago, similar to the one we are currently experiencing. Global average temperature was about 1 degree C higher than today, and sea levels were 4 to 6 meters higher.

  7. 7
    llewelly says:

    Wikipedia has a nice entry on the Eemian
    The Eemian is the second-to-latest interglacial period. It occurred during the Pleistocene.

  8. 8
    cwmagee says:

    I thought that was called stage 5.

  9. 9
    Bananus says:

    An, in my opinion important paper, you’ve missed in discussing the Greenland ice sheet is

    Fettweis, X., Gallée, H., Lefebre, L., van Ypersele, J.-P. (2005) The 1988-2003 Greenland ice sheet melt extent by passive microwave satellite data and a regional climate model. Climate Dynamics DOI: 10.1007/s00382-006-0150-8

  10. 10
    llewelly says:

    I thought that was called stage 5.

    See Timeline of glaciation . The Eemian is equivalent to MIS 5e .

  11. 11
    john mann says:

    re 8: Are you thinking of Marine Isotope stages which are marked in the north atlantic by D-O (I have to abbreviate, I can’t spell the names!) events? Eemian refers to a glaciation period in Europe (ISTR it’s been called the Ipswichian in lil’ol Britain). I think there is some difficulty in precisely relation the marine isotope stages with events in europe as the former were driven by glacier advance/melt in the Laurentide ice sheet in North America.

  12. 12

    An interesting article but will it make any difference?

    It concludes:

    “And because greenhouse gas concentrations and ice sheet loss are effectively irreversible, policy decisions may need to be made based on the information in hand, which argues that deglaciation could be triggered by a modest warming.”

    This equivocation may be suitable when addressing fellow scientists on a new hypothesis, but is it likely to rouse a sleeping public, or alter the policies of an intransigent administration? The ending ought to have read:

    “Greenhouse gas concentrations and ice sheet loss are irreversible. Therefore policy decisions must be made based on the information in hand, which argues that deglaciation is being triggered by a modest warming.”

    But it is not just journalistic skills that scientists are lacking. They also seem to be incapable of doing the engineering calculations which would show the desperate plight we are in. The speed at which the glaciers move into the sea depends on the resolution of only two forces: gavitation and friction. Friction depends on the coefficient of friction and the weight. With water perculating to the base of the Greenland ice sheet and reducing the coeffient of friction, the speed of the glaciers towards the sea will accelerate. But as sea levels rise and the grounding line of the Ross ice shelf retreats, the weight of the WAIS is being reduced as more of it floats. This will also reduce the friction and cause the WAIS to accelerate towards the sea. In other words, if one ice sheet surges, then it can cause all the others to surge too, just as is seen in paleo record.

    Steven Schneider famously said “And like most people we’d like to see the world a better place, which in this context translates into our working to reduce the risk of potentially disastrous climatic change. To do that we need to get some broadbased support, to capture the public’s imagination. That, of course, entails getting loads of media coverage. So we have to offer up scary scenarios, make simplified, dramatic statements, and make little mention of any doubts we might have.” These doubts are now receeding, but under the leadership of Sir John Houghton the media and the public have been bypassed and the scientists have directed their message to the “opinion formers”, who are though to be civil servants and politicians. Rupert Murdoch, Bob Geldorf, and Bill Gates are ignored. It is time that the scientists tell the journalists the truth, scary or not. Better late than never.

    [Response: Actually, the ‘decisions may need to be made’ was my editing oversight. Michael’s final draft was written to say ‘decisions need to be made’ but I messed up between versions. I have corrected it above. -gavin]

  13. 13
    Pekka Kostamo says:

    Trying to fathom the glasier issue as a layman, I came to the view that there may be three rather distinct processes (or stages) at the various altitudes, latitudes and weather circumstances.

    The dry process occurs in the interior areas, where temperatures remain solidly below zero at all times. Clean and dry snow ensures that radiation absorption is minimal, heat transport is by conduction only, and cracking of the ice is due to increasing weight and the ground slopes below. Overall, the glasier is growing.

    The surface melt stage introduces many changes. Energy absorption increases at the surface. Energy transport to the interior is also provided by water running into the crevasses; re-freezing in the still cold interior releases the water latent heat. Re-freezing also provides mechanical strains due to expansion at water phase change. Uneven small area heating contributes more strain due to thermal expansion of ice. Ice is broken into smaller blocks, sharp corners are rounded. Re-freezing also welds blocks together in some cases. Speed of flow increases. Some water run-off occurs on the surface, also changing the weight balance locally.

    At the wet stage the glasier internal temperature is close to zero. Liquid water penetrates through the cracks to the base and provides lubrication between ice and the bedrock. Friction is minimized throughout. Collisions between blocks cause further breakage and rounding of the blocks. Surface energy absorption remains high, heat transport to the interior is less efficient through water specific heat only (no latent heat release). Water run-off occurs both on the surface and through under-surface channels out to the sea, but local internal pools may also create hydraulic pressures. A rapid collapse is possible.

    Wet stage occurring in areas where the base is below sea level brings some buoyancy forces into the play as well.

    Some process to model, there being very little quantative observation data available.

  14. 14
    Philip says:

    I think the problem of sea level rising that we are facing is due to the ice sheet failure! If this continue persistently I am sure that our earth will submerged in water. We need to do something in advance so that we can save our earth!

  15. 15
    S Molnar says:

    Re #6: Thanks for the elucidation. To my layman’s eyes, it certainly makes sense that the rate of melting is, to a first order approximation, proportional to the surface area of the ice and to the temperature above zero (with both perhaps raised to a power not quite equal to one). I wonder if Hansen is engaging in a touch of hyperbole or if he just feels it’s prudent to assume the worst given that “model failure is the key issue”. As for his book review not being a book review, I can assure you as a long-time reader of the New York Review of Books that he has captured the style exactly – it grows on you after a while.

  16. 16
    Jud says:


    Does the breakup of an ice sheet such as the Larsen B contribute to increased sea level (other than what I presume is more rapid melting of the ice once it’s broken up and more of it is in contact with seawater)?

  17. 17
    Coby says:

    Re meltwater pulse 1a in #6:
    This occured when the Earth’s ice caps were much larger and extended much further south, and with a 5 degree (C) rise in global average temperature

    It is true there was alot more ice available for melting at that time, but it is not true that this happened in conjunction with 5oC rise, at least not from what I thought I knew or what I got from your own links. The rapid melting happened over only 500 years and temperatures were not rising 1o/century.

    So, more ice then but much faster temperature rise now makes it very plausible that we will experience similar SLR.

  18. 18
    pat neuman says:

    re 16.

    Jud, the breakup of an ice sheet such as the Larsen B does not contribute to increased sea level by itself, but may open up places near the land/sea boundary to allow meltwater runoff from land which may have been plugged to flow into the sea.

    re 6.


    … referring to an event at the end of the last ice age known as Meltwater Pulse 1a (see this summary or the Science paper). This occurred when the Earth’s ice caps were much larger and extended much further south, …

    I understand that glaciers extended much further south during the last ice age than now, but I don’t know what you mean by Earth’s ice caps were much larger then. Interior areas of Antarctic and Greenland have likely been accumulating ice since the Eocene.

  19. 19
    Steve Sadlov says:

    RE: #14 – The sky is falling, the sky is falling! The whole earth will be submerged!

    Seriously, this sort of thing does not help make the case for RC.

    [Response: Publication of a comment does not imply endorsement. After all, yours mostly get through… – gavin]

  20. 20
    Dragons flight says:

    Since you didn’t link to the data directly, here are a couple plots that may be useful.

    First, the IPCC sea level projections through 2100. These are of course model dependent results and as well discussed above should be regarded with healthy levels of caution.

    Secondly, the deglaciation of the last ice age. By eye, I make the sustained rate from 11 to 8 ka to be about 1.75 m/century. The average is more like 1 m/century over the entire deglaction, and could be considerably more during isolated events, such as the apparent Meltwater pulse 1A. Is there a reason that people prefer to talk about the 1m/century average rather than some of the demonstrated fluctuations? Admittedly we have less ice now, so maybe a slower rise is to be expected.

    I’ve seen claims that the Eemian deglaciation, which occured under considerably greater Milankovitch forcing than today, allowed sea level to peak ~4-6 m above present and occured significantly faster on average than the recent deglacition. Could someone comment on what is known about the rate of sea level change during the Eemian deglaciation?

    Also, many of the projected temperature scenarios seem to be suggesting that Arctic climate could end up in a regime by 2100 that has not occured for millions of years. While one should definitely be sceptical of the ice sheet models, one also has to worry about whether we have appropriate paleoclimate analogs for the changes that could be coming.

  21. 21

    Temperature of most important ice sheets on their surface and within, is not to my knowledge being measured on a grand scale, it would be difficult to model this important variable without some extensive observations.

  22. 22
    Ken Rushton says:

    May I point out an interesting display? This displays the current global ocean temperature anomalies. It is in good agreement with the others like it out there, but shows the data the best.
    The most intriguing sustained paradox has been the area around Greenland – it has been anomalously cold (the blue area), year-around, for as long as I’ve been looking at this chart, – over a decade, I recall. The unusually cool area, which used to be just along the south-east coast, is now a wide half-ring around the bottom of the island. This is in spite of the whole North Atlantic being anomalously warm.
    A dynamically stable long-term anomaly like this must have a power source. I suspect the increased flow of glaciers â?? but it would have to be a sustantial flow increase. HAve we seen this? Or is there another cause?

  23. 23
    Eli Rabett says:

    Just like to note an implication out that S. Molnar missed. Hansen points out (I am sure that it not original to him), both in his NYRB review and in the Climatic Change essay, (God, Alastair is right in #12, two clauses, two partenthetical remarks before I even get near the point) that increased meltwater at the top of the ice can percolate to the bottom and lubricate the flow of the glacier. What SM misses, is that this means that more ice will flow into the sea AS ICE. Of course, absent an intervening Titanic it then melts, but even if it does not, having flowed into the oceans, it will raise their level by displacement.

    Thus, the rate of melting beyond that which provides enough lubrication, is irrelevant to the amount of sea level rise.

  24. 24
    sidd says:

    Re: comment # 20

    the Overbeck reference in Science, 2006 discusses the last interglacial.


  25. 25
    Mark Shapiro says:

    Re 22: Ken Rushton’s question –

    “The most intriguing sustained paradox has been the area around Greenland – it has been anomalously cold (the blue area), year-around, for as long as I’ve been looking at this chart, – over a decade, I recall.”

    One hypothesis would be that the increased flow of cold meltwater from Greenland’s glaciers is displacing warmer water from the Gulf Stream that had been warming Greenland’s southern tip. It’s one mechanism by which global warming would lead to regional, temporary cooling.

    In any event, the question of one cooler spot on the globe pales next to the question of how fast, and how far, sea level is going to rise.

  26. 26
    S Molnar says:

    Re #23: I didn’t so much miss the implication as assume (perhaps improperly) that since the perimeter of a glacier is roughly proportional to the square root of the surface area and the thickness of the sheet tends to increase with more area, the effect scales up (roughly speaking) with more ice. Of course, that’s only true in cases like Greenland and West Antarctica where the edge is an ocean – an ancient glacier covering the northern half of North America would not disappear the same way. Actually, I’m still quite concerned about those magnitude 5 icequakes I asked about back on the “How much future sea level rise?” thread, but people tend to treat you like a nutcase if you ask whether a sudden ice discharge from Greenland can create a tsunami that will inundate New York.

  27. 27
    Hank Roberts says:

    A friend of mine recently pointed out that the removal of ice by melting through to the base rock is very similar to “tunnel gully erosion” — which I’ve noticed happening after forest fires on a restoration site in N. Ca. but is only described in the literature from New Zealand as far as I know.

    Take a layer of loosely consolidated or fragile material (loess, pumice, or I suspect glacial ice) on top of a sloping harder material (rock).

    Water cuts notches down through the overlying material then hollows out channels at the base that enlarge over time.

    Slumping of the upper material into the channels removes more and more until penetrations through to the surface occur.

    Collapse back to the ‘angle of repose’ occurs over and over as the material is eroded away at the base, so long as water continues to run.

    Aside: one Mars Rover photograph looks to me like a tunnel gully — a linear depression (crack dubbed “Anatolia”) punctuated by what may be sinkholes:

    Here’s a general search for “tunnel gully” +erosion, many of the articles are in PDF form.

  28. 28
    Ken Rushton says:

    Re #22, 25: This isn’t just “one cooler spot” on the globe – it’s the critical interface point between the Gulf Stream, the start of downwelling for the THC, and Greenland itself. What happens here can influence climate far afield.
    For one thing, enough freshwater (if that’s the cause of the temperature anomaly) will definately affect the THC.
    More importantly, from the scientific point of view, this should be a really good way to cross-check the recent satellite observations of the Greeenland icecap; “simply” (I know this is a loaded phrase) measure the temperature, salinity, and isotope changes in the water, and with a little bit of math we can say, “yes, here it is: prediction confirmed; call the press conference” or, “it’s a no show; better fix that &!@#@$ model”

  29. 29
    Eli Rabett says:

    My point was simply that there could be significant sea level rise by ice moving from the Greenland ice cap without actually melting the ice cap. We appear to agree on that. Yes, there are currently only two such ice caps, but one of them appears to be a LOT more subject to the process. If this is indeed the case, everything else is second order.

    If the rate discharge into the sea increases because of bottom lubrication you actually need little direct melting.

  30. 30
    Steve Bloom says:

    Re #27: Ice does not behave like that, at least on a small scale, until it’s warm enough to turn to slush, tending to move in discrete chunks instead. I don’t think either of these analogies would necessarily be very helpful in considering the movement of Greenland’s glaciers due to the way in which the ice there is squeezed into outlet glaciers as it moves toward the coast.

  31. 31
    Blair Dowden says:

    Re #17: Coby – Meltwater Pulse 1A occured after about six thousand years of sustained warming. It is not accurate to only look at the 500 years during which it took place. But it was also not accurate to count the entire 5 degress of warming that preceeded the interglacial, as I did. The best information I can find (this Science paper) suggests 2 or 3 degrees of warming before it took place.

    So there was a lot more ice, it extended much further south, and there was a similar degree of warming, though not nearly as rapid warming as is happening now. I do not think either Greenland or Antartica could produce a rate of sea level rise equivalent to mwp-1A, but there could well be very significant melting events in the next few centuries.

  32. 32
    Gareth says:

    Developing Eli’s point: is there any evidence of sea level rise during DO events, when there were lots of icebergs in the North Atlantic?

  33. 33
    Hank Roberts says:,0,7670457,full.story
    NOTE, comment software chokes on commas and fails to select rest of string; copy and paste full URL to see source.

    …. In an influential paper published in Science, Zwally surmised that the ice sheets had accelerated in response to warmer temperatures, as summer meltwater lubricated the base of the ice sheet and allowed it to slide faster toward the sea.

    In a way no one had detected, the warm water made its way through thousands of feet of ice to the bedrock � in weeks, not decades or centuries.

    So much water streamed beneath the ice that in high summer the entire ice sheet near Swiss Camp briefly bulged 2 feet higher, like the crest of a subterranean wave.

    “This meltwater acceleration is new,” Zwally said. “The significance of this is that it is a mechanism for climate change to get into the ice.”

    ….University of Texas physicist Ginny Catania pulled an ice-penetrating radar in a search pattern around the camp, seeking evidence of any melt holes or drainage crevices that could so quickly channel the hot water of global warming deep into the ice.

    To her surprise, she detected a maze of tunnels, natural pipes and cracks beneath the unblemished surface.

    “I have never seen anything like it, except in an area where people have been drilling bore holes,” Catania said.

    No one knows how much of the ice sheet is affected.
    —— end quote

  34. 34
    mauri pelto says:

    Bi-Polar Outlet Glacier Acceleration and the Jakobshavns Effect

    It is correct that glaciologic models do not work well enough to model the system correctly. Having used these models for 20 years I realize the uncertainties. A key problem is knowing the specific boundary conditions at the base of the ice sheet, and the temperature gradient in the lower section of the ice. These two items control much of the sliding and internal deformation. On the other hand the basic physics are understood. Thus, a rapid breakup of a marine based section of an ice sheet and its ice shelves have long been recognized as a scenario that has and can happen. The rapid breakup is borne out in the glacial geologic record for the rapid deglaciation via Hudson Strait of a Marine based ice sheet. Meanwhile land based ice sheets even with fjord connections such as the Cordilleran Ice Sheet along the BC and SE Alaskan coasts did not disappear very quickly. Nor of course did the main portion of the Laurentide Ice Sheet ending in the United States and retreating slowly north. This understanding does include the ability of a thin ice shelf such as the larsen Ice Shelf to rapidly breakup, though certainly not quite as fast as it did.

    Let us be clear the Greenland Ice Sheet is not marine based, and the speedup of a few outlet glaciers albeit draining a noticeable portion of the ice sheet, does not physically lend itself to a rapid collapse of the Ice Sheet as some have suggested. This Ice sheet more resembles that of the Cordilleran Ice Sheet and the Scandanvian Ice Sheet neither of which disappeared all that quickly. It is not like the West Antarctic Ice Sheet which is more like the Innutian Ice Sheet of the previous ice age over the Canadian Arctic, which did disappear quickly.

    So does the bi-polar acceleration of key ice sheet outlet glaciers that has been observed in the last decade pose similar possibilities. Pine Island and Thwaites Glacier West Antarctic Ice Sheet (WAIS) and Helheim, Kangerdlugssuaq and Jakobshavns Glacier in Greenland. Twenty years ago Terry Hughes proposed the Jakobshavns Effect (JE). The JE as explained by Hughes (1986) results from an imbalance of horizontal hydrostatic forces at the grounding line. With positive feedback mechanisms that sustain rapid ice discharge: ubiquitous surface crevassing, high summer rates of surface melting, extending creep flow, progressive basal uncoupling, lateral uncoupling, and rapid iceberg calving.

    Are the recent outlet glacier accelerations indicative of the Jakobshavns Effect at work is the reduction in back stress allowing the ice to be pulled out of the ice sheets, or is reduced basal coupling solely enhancing basal sliding? The following summarizes some key recent findings.

    As noted by Sterns and Hamilton (2005) Kangerdlugssuaq Glacier and Helheim Glacier, two fast-flowing tidewater glaciers in South-East Greenland, accelerated between 2001 and 2005 and retreated 3-5 km since July 2003. Together, the catchment basins of these two glaciers encompass ~10% of the area of the Greenland ice sheet. Helheim Glacier was flowing at ~8 km/yr in 1995 and 2001. In 2005, flow speeds were ~11.7 km/yr, a ~40% increase. The acceleration of Kangerdlugssuaq Glacier was more substantial. Portions of the main trunk that were flowing at ~5 km/yr in 1988, 1996 and 2001 were flowing at ~14 km/yr in summer 2005. The acceleration of these glaciers was synchronous with a rapid retreat of calving fronts, which had been stable, and a lowering of the ice surface by about 100 m. The rapid thinning, acceleration and retreat of these two relatively nearby glaciers suggests a common triggering mechanism

    de Lange, de Lange, Murray, Luckman, Hanna (2005) investigated the reasons behind the speed-up of Helheim Glacier. The results showed a dramatic increase in velocity during 2002/2003 but not during the 1990’s, although the glacier thinning rate observed by NASA of around 1.5 ma-1. They conclude that the recent speed-up does not coincide with high runoff, suggesting no direct link between surface runoff and velocity during the 1990’s or the recent speed-up event.

    The Jakobshavn Isbrae has long been viewed as the fastest sustained tidewater glaciers in the world. After nearly 50 years of stability (Pelto, Hughes and Brecher, 1990) a remarkable retreat of the ice front and an increasing flow velocity has been noted (Dietrich, Maas, Baessler, Ruelke, Schwalbe, 2005). In August 2004 they determined the flow velocity of the ice front and last 5 km of the glacier. They obtained velocities range up to 40 m/day, a dramatic acceleration from the 20-23 m/day that had been observed over a sustained period. (Mayer and Herzfeld, 2005) observed that in 2002, this ice stream with a 12 km long floating tongue, suddenly entered a phase of rapid retreat. The ice front started to break up, the floating tongue disintegrated, the production of icebergs increased. Thomas (2004) argues for acceleration via back force reduction. The thinning and acceleration of the glacier immediately following calving of about 4 km of its 15 km floating ice tongue, suggest that acceleration may have been initiated by the calving. He assumes that the force perturbation associated with such weakening is swiftly transmitted up-glacier, to a maximum point 10 km upglacier. The conclusion is that the initial observed changes in flow are consistent with the comparatively small perturbation associated with the calving. And would afterwards be sustained by thinning of the remaining ice tongue.

    Rignot (2002) has observed recent large changes on Thwaites and Pine Island Glaciers draining the WAIS into the Amundsen Sea. These glaciers because of their size and lack of large buttressing ice shelves have been referred to as the weak underbelly of the WAIS (Hughes, 1981). Rignot (2002) documents an 18% acceleration of Pine Island Glacier during the 1992-2000 period over a 150 km reach of the glacier. The glacier has thinned at 1.6 m/a and the grounding line retreated 5 km between 1992 and 1996. For Thwaites Glacier Rignot (2001) observes a 1.4 km retreat of the grounding line from 1992 and 1996 indicating a 1.4 m/a thinning of the lower glacier. There is insignificant surface melting on these glaciers, thus basal sliding cannot be enhanced via this mechanism. An examination of the forces acting on these glaciers by Schmeltz, Rignot,DuPont and MacAyeal (2002) led them to conclude that a reduction in buttressing due to a decrease in ice shelf area could cause a 70% acceleration, while basal shear stress would only increase it 13%. Shephard and others (2001) observed an inland thinning and acceleration of Pine Island Glacier that led them to conclude this glacier was responding to enhanced glacier bed lubrication. On Thwaites Glacier there is no reported acceleration above the grounding line.

    We return to the key question is the force driving these accelerations simply due to basal shear changes or to the more comprehensive change in dynamic forces of the Jakobshavns Effect? But even if we do the Greenland Ice Sheet based on glaciologic principles just is not capable of rapid collapse.

  35. 35
    Hank Roberts says:

    >30, 27
    Steve, “ice doesn’t behave like that” unless covered with surface water and slush apparently — see field reports via the first link in resp. 3 above; I tried a longer excerpt post but it isn’t showing up; so this short one:
    “…University of Texas physicist Ginny Catania pulled an ice-penetrating radar in a search pattern around the camp, seeking evidence of any melt holes or drainage crevices that could so quickly channel the hot water of global warming deep into the ice.

    To her surprise, she detected a maze of tunnels, natural pipes and cracks beneath the unblemished surface….”

  36. 36
    George A. Gonzalez says:

    In the _New York Times_ today there was a very interesting article on geoengineering to counter the effects of climate change emissions. It is entitled “How to Cool a Planet.” In this article it was explained that there is a great of opposition/skepticism in the scientific community to studying geoengineering approaches to global warming. I suggested in a thread on RealClimate that geoengineering approaches to climate change should be explored. My suggestion was rejected out of hand. In light of the planetary emergency we are currently facing, is such opposition/skepticism not dangerous?

    As pointed out in the piece that initiated this current thread, “ice sheet loss[es] are effectively irreversible.” We know that global warming is occurring and accelerating. The solution currently being embraced/championed by most scientists and the environmental lobbying community is not working (i.e., the reduction of greenhouse gas emissions). Looking at the U.S., China, and India, we know that global climate change emissions are set to increase. It should also be stressed that it is very possible that humanity has already put enough greenhouse gasses into the atmosphere to trigger catastrophic warming, and it is only a matter of time before this full warming occurs.

    We have modeled business-as-usual enough! It is time to thoroughly model and explore geoengineering responses to our planetary crisis. The failure to do so will likely lead to our collective demise.

  37. 37
    Leonard Evens says:


    I read the same article. It seems to me that it is difficult enough to make projections of future climate without adding other large scale perturbations. Of the methods proposed, the only one that seemed even faintly plausible to me was the injection of aerosols in the stratosphere. This at least has some observational support from what we have seen happen from very large volcanic eruptions. But allowing greenhouse gas emissions to rise at an accelerated rate and at the same time trying to counteract their warming effect with such a mechanism seems like a hire wire act with the state of the world in balance. So, geoengineering on a large scale without first trying to control emissions seems like a bad idea. Also, if you think it is difficult to get international agreement to limit emissions, imagine how much more difficult it would be to agree on such projects. Perhaps modest measures along these lines could be tried in the future as an adjunct to emission controls, but not as a substitute for them.

  38. 38
    Hank Roberts says:

    Geoengineering to block sunlight won’t slow the acidification of the ocean as CO2 increases. Collapse of the ocean food chain by about 2100 is the near term problem, little discussed because it’s unthinkable.

    I’d like to see Paul Creutzen invited here to discuss his suggestions.

  39. 39
    Nigel Williams says:

    OK. So rate of ice movement and loss is the â??criticalâ?? issue of the moment.

    With present poor coverage of glacier movement and ice loss by conventional systems, is it not time for IPCC to seek a UN mandate to deploy a vast array of sensors so that we â?? the people of the earth â?? can get a clear idea of what is happening to our planet.

    The design of a simple probe to deploy would be trivial.

    Probe say 1.5m long in a bit of aluminium or HD PVC pipe. Three 1-wire temperature chips – bottom (for â??deepâ?? temperature) 1m up (for â??surface ice temp) and top (for ambient temperature). Add a GPS encoder for location data, a timer, a unique ID chip, a battery, and a satellite phone dialer. All these components come off the shelf. The probe would be configured to float top-up, so when the ice melts we then get sea temperatures and ocean current information until the battery dies.

    Get the navies of the world to deploy these using helicopters flown of boats, or whatever. With a simple electric drill a guy stands on the skid of a helicopter, drills a 1m deep x 25mm diameter hole into the ice, and drops in the probe, and flies to the next site. A helicopter could deploy 100 probes in a day.

    Within a few months we could have 10,000 to 100,000 probes monitoring ice conditions world wide.

    Within a year we would have good data and a very strong story to tell.

    Or we can simply keep on guessing. Whatever.

  40. 40
    sidd says:

    Re: comment #34: Thank you, Mr. Pelto, for a learned review. Would you care to comment on the observations of accelerations in ice streams following the Larsen shelf collapse ? I believe these results were from the British Antarctic Survey.


  41. 41
    sidd says:

    Re: comment # 34: Pine Island and Thwaites

    Is it possible, given the topography of the bedrock below the ice, that sea water intrusion plays a role in the acceleration of these glaciers ?


  42. 42
    Hank Roberts says:

    The LA Times article (link in #3) mentions sea level in relation to the ice cap twice; apparently Greenland like Antarctica is actually several islands:

    “Mile upon mile of the steep fjord was choked with icy rubble from the glacier’s disintegrated leading edge. More than six miles of the Jakobshavn had simply crumbled into open water.”

    “… The ice is so massive that its weight presses the bedrock of Greenland below sea level, so all-concealing that not until recently did scientists discover that Greenland actually might be three islands.”

  43. 43
    Ben Coombes says:

    Thanks for the informative post RealClimate. I’ve read the Hansen paper and he certainly is a very charismatic writer. However, as he points out, he is no glaciologist and so I wondered if any of arguments he puts forward in the piece are patently wrong?
    If not it seems to me that he makes a very strong case for framing the argument in terms of uncertainty rather than minimal 21st Century sea level rise as suggested by the IPCC.

    Thanks, Ben

  44. 44
    John L. McCormick says:

    Geoengineering of the type and scale discussed briefly in #36 was somewhat dismissed in #37 for reasons that are readily accepted when one considers trying to counterbalance increasing energy consumption and its emissions with injecting aerosols into the stratosphere. Sounds like a tail-chasing venture and one that the international community will debate forever.

    However, Mr. Gonzalez has a valid, larger point, if not the details, to challenge RC contributors.

    Assume base-case projections will be scrapped when positive feedback is finally detected and accepted. What then? Does the developed world pull up the ladder and focus on survival? That is a likelihoood where borders are essentially closed by wide seas. But the Ganges Delta, for example, has no borders and provides topsoil and agriculture for about 400 million people and two of its neighbor nations have nuclear arsonals.

    My view of geo- and bioengineering has everything to do with rapid deployment of highly drought, pest and heat resistent crop varieties and and augmented water supplies, (please, no disucssion on nuclear powered desalination plants) new and effective vaccines stockpiled in strategic sites around the world, a new look at forestry management in a warmer, drying world and an international effort to find a truly rational substitute for gasoline and diesel powered vehicles that does not require turning our food producing topsoil into fuel refineries.

    When the engine room damage report comes up to the bridge, we expect the captain to have a plan in mind to save the crew. That is not the time to experiment with a plan to flip the ship over so the water does not come in through the torpedo hole.

    Adaptation is not at the cost of mitigation efforts. It is a part of getting the next generation prepared for the destruction we have brought to their lives.

  45. 45
    George A. Gonzalez says:


    Geoengineering and increasing climate change gasses are both dangerous. It needs to be stressed, however, that political and economic elites have refused to abate climate change emissions. (For a discussion of why see Gonzalez, George A. 2005. “Urban Sprawl, Global Warming, and the Limits of Ecological Modernization.” _Environmental Politics_ 14, no. 3: 344-62.) In light of this, it appears that a rational course of action for the scientific community is to explore/analyze the plausibility, effectiveness, and risks of geoengineering responses. In the current context this may be scientists only course of action.

  46. 46
    Eli Rabett says:

    I second Sidd’s post thanking Mauri Pelto.

  47. 47
    Craig Duncan says:

    I’m just curious what reception James Hansen’s recent New York Review of Books article has had (as well as earlier articles of his that argue similarly). It seems his main purpose is to get us to take seriously, by way of historical analogy, the possibility of a rapid sea level rise, such as 1m every 20 years. I have little sense of whether Hansen’s argument is representative of increasing alarm among the majority of climate researchers, or is still at the moment judged to be an outlier position among climate researchers. Any comment?

    Also, can anyone recommend a book on glaciology for newcomers–e.g. an Intro to Ice 101? Ideally such a book would explain the difference between ice sheets, ice shelfs, glaciers, icebergs, ice streams, ice caps, ice domes, sea ice, ice floes, etc.–to the extent that these are different (perhaps some of these phrases are synonyms?) E.g. I’m dimly aware that it is significant that the WAIS is anchored below water, as this would make it easier for it to come unanchored and slip away (right?) But what portion of the WAIS rests on land below seal level? Just the outer edge of it? And at what depth at the deepest? To what extent does this matter (viz. portion and depth)? Are we talking about just the portion that is marine anchored breaking off and slipping away, or is there a chance it would take the rest of the WAIS with it? I find that hard to imagine. But as I said, it would be nice if there were an accessible book to introduce the WAIS, GIS, etc. to newcomers (Or even a textbook for science undergrads, which I could probably understand.)

  48. 48
    Mauri Pelto says:

    Elaborating on #40 Ice shelves buttress the glaciers that feed them slowing them down considerably. This has long been understood. However, our first chance to observed this directly is with Larsen B. Satellite images from before, during and after the break-up of the Larsen B Ice Shelf in March 2002 illustrate the acceleration. To feeder glaciers Crane Glacier and the Hektoria-Green-Evans Glacier sped up. Crane Glacier increased from 1.7 meters/day to 3.1 meters/day in April through December of 2002, and then to 4.1 meters/day between December 2002 and February of 2003.

    With respect to #41, takes me back to my first job as a scientist, mapping the ocean floor in front of Thwaites and Pine Island Glacier back in the early 1980â??s. These glaciers have relatively limited floating tongues, that are really just extensions of the land based glaciers. They have generated fairly substantial deep troughs immediately beyond the glaciers. This suggests to me a more limited circulation of water beneath these ice shelves than others. However, we have no real measurements. The sliding is being enhanced on Pine Island Glacier and there is no surface melting, but nor is there a connection to the ocean on the land based section. Under the ice shelf, warmer water would enhance melting, which would thin the tongue, which would reduce the buttressing and allow acceleration. But the bottom of the glaciers are truly a land of unknowns.

  49. 49
    pat neuman says:

    re: 47.

    When might the collapse of these sheets will begin?

    Excerpt: … The business-as-usual scenario, with five degrees Fahrenheit global warming and ten degrees Fahrenheit at the ice sheets, certainly would cause the disintegration of ice sheets. The only question is when the collapse of these sheets would begin. The business-as-usual scenario, which could lead to an eventual sea level rise of eighty feet, with twenty feet or more per century, …

  50. 50

    There is geoengineering to remove CO2, which is a dubious proposition energetically but which would be absolutely fantastic news if someone could somehow get it to work.

    And there is geoengineering to deflect solar radiation. My impression is that it’s almost certainly a bad idea. If someone could get that to work, it could prevent the mean surface temperature of the earth from rising, but it could not balance out the local forcings (nor the chemical disequilibrium in the carbon cycle, as Hank Roberts has already pointed out).

    Anthropogenic global change is the problem, and global warming is only a symptom.

    The idea of crudely cancelling out a growing perturbation with another one seems to miss the point, as well as being fraught with peril, like treating alcoholism with aspirin.

    As symptomatic relief, the cost of colossal panteary beach umbrellas of various sorts should not be compared with the cost of reducing emissions or the cost of increasing sequestration. If a case can be made that such an action would protect the ice caps preferentially, for instance, its cost might be compared against the cost of sea level rise. But stabilizing greenhouse gas concentrations stabilizes the system. Adding a contrary perturbation does not.

    We’d still be kicking the climate system with increasing perturbations, and we’d be moving farther away from any paleoclimate analogs, so we’d be increasingly less certain of what we were doing. And if people took this as a license to keep emitting greenhouse gases, the solar perturbation would have to keep increasing apace, continually increasing the risk.

    Global mean temperature is a symptom, albeit an important one. Nobody lives in a global mean climate, so stabilizing global mean temperature isn’t in itself a useful goal; it in no way guarantees an end to anthropogenic climate change.

    Perhaps this is another side effect of the use of the inappropriate name “global warming” to describe our problem. It would certainly be not just a tragedy an absurd tragedy if global warming in a literal sense, i.e., mean surface temperature increase, were eliminated at great expense while anthropogenic perturbation of the earth system would continue to accelerate, simply because we decided to name our problem “global warming”, confusing the symptom for the disorder.