RealClimate logo

Ice hockey

Eric Steig

It is well known that ice shelves on the Antarctic Peninsula have collapsed on several occasions in the last couple of decades, that ice shelves in West Antarctica are thinning rapidly, and that the large outlet glaciers that drain the West Antarctic ice sheet (WAIS) are accelerating. The rapid drainage of the WAIS into the ocean is a major contributor to sea level rise (around 10% of the total, at the moment).

All of these observations match the response, predicted in the late 1970s by glaciologist John Mercer, of the Antarctic to anthropogenic global warming. As such, they are frequently taken as harbingers of greater future sea level rise to come. Are they?

Two papers published this week in Nature Geoscience provide new information that helps to address this question. One of the studies (led by me) says “probably”, while another (Abram et al.) gives a more definitive “yes”.

The somewhat different details of the two papers appear to have hopelessly confused many journalists (though the Christian Science Monitor has an excellent article, despite a somewhat misleading headline), but both are really just telling different aspects of the same story.

There is already strong evidence that anthropogenic forcing has played a significant role in the collapse of ice shelves on the Antarctic Peninsula, cause by significant melting at the surface during summer. The warm summer air temperatures have been related to an increase in the “Southern Annular Mode” (SAM), essentially the strength of the circumpolar westerlies. Increased CO2 is clearly part of the forcing of the observed positive trend in the SAM, though a larger player is likely to be ozone depletion in the stratosphere. Nevertheless, the short length of the observations – of both the ice sheet and climate – make it difficult to assess to what extent these changes are unusual. There is evidence for one ice shelf that a collapse like that observed in the 1990s has not occurred since at least the mid-Holocene, but comparable evidence is lacking elsewhere.

The connection between climate change and glacier response is more complex for the West Antarctic Ice Sheet than the Peninsula. As on the Peninsula, temperatures over the WAIS have risen significantly in the last few decades, but this is a symptom, rather than a cause. For WAIS, the culprit for the rapid thinning of ice shelves is increased delivery of warm ocean water to the base of the ice shelves. This isn’t due to a warming ocean (though the deep water off the Antarctic coast line is indeed warming), but to changes in the winds that have forced more circumpolar deep water onto the continental shelf. Circumpolar deep water, at about +2°C, is very hot compared with the in situ melting point of glacier ice. In a series of papers, we’ve shown that the warmer temperatures observed over the WAIS are the result of those same atmospheric circulation changes, which are not related to the SAM, but rather to the remote forcing from changes in the tropical Pacific: changes in the character of ENSO (Steig et al., 2012; Ding et al., 2011; 2012).

As on the Peninsula, there is evidence of anthropogenic forcing for the WAIS too: anomalous conditions since the 1980s in the tropical Pacific are characteristic of the expected fingerprint of global warming (e.g. Trenberth and Hoar, 1997; Collins et al., 2010). Still, as on the Peninsula, the short length of the instrumental observations make it difficult to say anything very definitive about long term trends.

Both our paper and that of Abram et al. add to our understanding of recent climate, glacier, and ice sheet changes in Antarctica by placing them into a longer-term context. Amidst the continuous chatter in the blogosphere about the strengths and limitations about “multiproxy” studies, these studies may be a refreshing return to simpler methods relying on just one type of “proxy”: data from ice cores. While ice core data aren’t perfect proxies of climate, they come pretty close, and aren’t subject to the same kinds of uncertainties that are unavoidable in biological proxies like tree rings.

Our study is the culmination of about a decade of ice core drilling and analysis in West Antarctica, through the ITASE program and the WAIS Divide ice core project. I’m the lead author on the paper but the author list is rightfully long; a lot of people have been involved in drilling and analyzing cores all across Antarctica.

The only “proxy” we use are oxygen isotope ratios. Oxygen isotope ratios (δ18O) in polar snow are well known to be correlated with temperature, and the underlying physics of the relationship is very well understood. In our study, we compile all the available δ18O data from high-resolution well-dated ice cores in West Antarctica and take a look at the average variability through the last 200 years. We also include data from the new WAIS Divide ice core that goes back 2000 years (actually, this core goes back to 68,000 years, and is annually resolved back to at least 30,000 years, but that’s a story for another time).

The average of the records for the last 50 years looks very much like temperature records from the last 50 years, with scaling of about 0.5‰/°C, exactly as expected, providing yet another piece of evidence that recent warming in West Antarctica has been both rapid and widespread (see the figure below). A critical point, though, is that it isn’t necessary to use the δ18O data as a proxy for temperature. Because the physics controlling δ18O is well understood, and we are able to implement δ18O in climate models, we can actually just use δ18O as a proxy for, well, δ18O. This simplifies the problem from “how significant is the recent warming?” to “how significant is the recent rise in δ18O”? We’ve shown previously, and show again in this paper, that δ18O in West Antarctic precipitation reflects the relevant changes in atmospheric circulation just as well (if not better) than temperature or other conventional climate variables do. Putting δ18O into a GCM and using the same experiments that reproduce the observed warming over West Antarctica also produces the observed δ18O increase in the last 50 years.

Figure 1. (a) Comparison of averaged δ18O (blue) across West Antarctica with the recent temperature record of Bromwich et al. (2013) from central West Antarctica (yellow). The light blue background is the decadal smoothed values +/- 1 standard error assuming Gaussian statistics. (b) Number of records used, and probability that the decadal average is as elevated as the 1990s (green).

Data sources: Most of the data for this figure have been available at for some time. There’s a new location (which will link to the old one) where more recent data sets will be placed, but it’s not all up yet:

Our results show that the strong trend in δ18O in West Antarctica in the last 50 years is largely driven by anomalously high δ18O in the most recent two decades, particularly in the 1990s (less so the 2000s). This is evident in the temperature data as well (top panel of the figure). The 1990s were also very anomalous in the tropics — there were several large long-lived El Niño events with a strong central tropical Pacific expression, as well as only very weak La Niña events. As in the tropics, so in West Antarctica: the 1990s were likely the most anomalous decade of the last 200 years.

Our results thus show that, indeed, recent decades in West Antarctica, which have been characterized by very rapid warming, and very rapid loss of ice from the West Antarctic Ice Sheet, are highly unusual. Nevertheless, some caution is in order in interpreting this to mean that current rates of rapid ice loss from West Antarctica represent a long term trend. What we’ve observed is unusual, but it is also dominated by decadal climate variability, and can’t be considered “unprecendeted”. Furthermore, our statistical confidence that recent decades are truly exceptional is low. Our data suggest that there is about a 30% chance the 1940s were just as anomalous as the 1990s, and the 1830s have about a 10% chance of being like the 1990s. Based on the relatively small amount of available evidence from the tropics, both the 1940s and the 1830s were similarly characterized by long-lived El Niños. Looking at the very long term record from the WAIS Divide ice core, it appears that similar conditions could have occurred about once per century over the last 2000 years. Hence our answer to the question, “are the observations of the last few decades a harbinger of continued ice sheet collapse in West Antarctica?”, is tentative: “Probably”.

Anyone expecting a more dramatic result need only turn to the other new ice core paper in Nature Geoscience. Last year, Rob Mulvaney and others from the British Antarctic Survey (BAS), along with French, American, and German colleagues, reached a very similar conclusion to ours, from an ice core from James Ross Island, on the northern Antarctic Peninsula. We discussed that paper at Realclimate last year. With δ18O data alone, it was possible to demonstrate only that recent warming on James Ross Island was “unusual”. The new paper, led by Nerelie Abram, adds a record of melt layers in the ice core to the assessment. The findings: a veritable Antarctic ice hockey stick.

Figure 2. δ18O (scaled to temperature) and melt layer frequency from the James Ross Island ice core.

Abram et al.’s paper is elegant in its simplicity. The key thing that matters to the ice shelves on the Antarctic Peninsula is how much melting occurs in summer, and this is almost exactly what Abram et al. are looking at. I say “almost” because formation of melt layers requires both that melting occurs and that it gets preserved, which depends a bit on the snow structure, the previous winter temperature, etc. But the results are unequivocal: there’s about 5 times the fraction of melt layers in the core as there has been on average over previous decades, and at least twice the maximum of any time before about the 1950s. The amount of melting occurring now is greater than at any time in the past 1000 years. If there has ever been a question about whether the “hockey stick” shape of Northern Hemisphere temperatures extends to at least some areas of the Southern Hemisphere, this record provides a decisive and positive answer.

Why the difference between the Peninsula and the WAIS? After all, both locations are warming at about the same rate. We could speculate that if there were melt layers in the WAIS cores, they would also show a significant increase like the James Ross Island core does. (It’s too cold at all the WAIS sites to have summer melting at all, so such information isn’t available.) I don’t think that is likely though. More important is the specific location of James Ross Island, on the eastern side of the Antarctic Peninsula. On the western Antarctic Peninsula, temperature trends are greatest in winter and spring, just as they are over the WAIS, and we’ve argued elsewhere that the causes are similar: changes in regional circulation forced by anomalous conditions in the tropics (Ding and Steig, in press). But it is on the eastern Peninsula that the most rapid summer warming has occurred, and where the surface-melting has caused ice shelf collapse (indeed, James Ross Island wasn’t really an island until 1995, when the Prince Gustav ice shelf collapsed). Both statistical assessments and modeling results show that the trend in the SAM accounts for this warming trend. As I noted in the introduction to this post, the SAM trend is partly explained by ozone depletion in the stratosphere, and the most clearly anomalous melt in the James Ross Island core occurs after the late 1970s, about the time the ozone hole appeared. But the melt data also show that melting has increased nearly monotonically since the 1930s, well before the advent of the ozone hole. As in West Antarctic δ18O, there was a bit of an increase in melt in the 1830s and the 1940s at James Ross Island, perhaps also ENSO-related, but these little bumps pale in comparison with the amount of melting occurring since the 1950s.

So what does all this mean for the fate of Antarctic Peninsula glaciers and the West Antarctic ice sheet? Both our paper and the Abram et al. paper add substantial new evidence that something rather unusual is occurring in Antarctica. It is not just happenstance that rapid ice sheet, glacier, and ice shelf changes are occurring now, when we have finally begun to observe them closely. Rather, these changes are occurring along with what is happening to the rest of the planet. That said, it appears that we not have yet driven West Antarctic climate (nor West Antarctic glaciers) definitively beyond what might be expected from natural variability alone. In particular, I won’t be surprised if continued decade-to-decade variability in atmospheric circulation results in more, and less, intrusion of circumpolar deep water onto the continental shelf, and to more, and less, rapid thinning of ice shelves in West Antarctica*. On the Peninsula, though, it seems very clear that we have already pushed the system well beyond “normal”, and into conditions reminiscent of the mid-Holocene. I don’t think we’re going to see a return to “normal” conditions any time soon. It’s worth noting that most model projections suggest that the SAM trend may level off for a while as the ozone hole gradually declines, but those same model projections suggest the SAM trend will recover as CO2 continues to rise. See. e.g. Thompson et al. (2011).

The real take home message here is that the ice loss from the WAIS and from the Antarctic Peninsula that have been observed in the last few decades are indeed likely to be harbingers of things to come. The very rapid rate of change in West Antarctica that we’ve seen over the last few decades is clearly overprinted by substantial decadal variability, so caution is in order in projecting that rate forward in time. The magnitude of the century scale trend will depend quite a bit, in my view, on what happens in the tropics over the next century. The sign of the trend, however, is clear. On the Peninsula, it’s crystal clear.

Note: An excellent summary of these two papers by Tas van Ommen will appear in Nature Geoscience in the May issue.

*I’ll have much more to say about this in a future post, but this is work in preparation at the moment.

49 Responses to “Ice hockey”

  1. 1
    Hardy Cross says:

    Would the westerly winds also affect the ice core data giving a false reading of temperature? Windward cores may look different than leeward cores even though they share the same climate. The windward core tops would be older than the leeward core tops due to transport of snow and ice. How is that effect controlled?

    [Response: Westerly doesn’t equal windward in these locations. But more to the point, wind redistribution in the area of our cores isn’t significant relative to the annual snow accumulation. Also, read the papers! The result of neither depends at all on an estimate of temperature.–eric]

  2. 2
    sidd says:

    Thanks for the review and the mention of Mercer. He was one of the luminaries at the Byrd Polar research center, formerly the Institute of Polar studies, which I see, supported Mercer with the magnificent sum of 16K odd US$ as early as 1963 for the study of S. American glaciers. Prof. Jason Box and Prof. Lonnie Thompson lurk around there too and they have a nice web site.

    I have a soft spot for the Byrd center, having both stolen and lent equipment in the misty past …

    Let me ask you for a personal opinion: Do you believe that we can avert Eemian scale collapse of WAIS and GRIS ?


    [Response: I agree with Chris’s response below. I do think we are likely to push both WAIS and Greenland to contribute several meters of sea level ice over the next 1000-2000 years. Whether that is a catastrophe depends a bit one whether one takes a long view of things or not. I’m confident we’ll continue to sea ice sheet contributing significantly to sea level, but over the next 100 years, small glaciers are going to be just as big a contributor. I’m not a believer in 2 m sea level rise by 2100, but nor do I think it’ll be much less than 1 m.–eric]

  3. 3
    Tom Scharf says:

    Shouldn’t an acceleration in ice melt show up in the sea level altimeter data? Where is all the melt going?

    [Response: As indeed it has. –eric]

  4. 4
    Paul says:

    Dr. Steig,

    1. Would it be correct to say your δ18O data indicates a decline over the past 2,000 years, thus the WAIS has cooled over the past 2,000 years?

    2. The abstract says, “However, δ18O anomalies comparable to those of recent decades occur about 1% of the time over the past 2,000 years.” This appears to be only in relation to the declining δ18O trendline. Would it be correct to say that if compared to the mean δ18O of the past 2,000 years, the anomalies of recent decades would actually still be negative in comparison to the mean?

    [Response: Yes and yes. It is a bit tiresome answering questions whose answers are given unambiguously in the paper. Read it, please! The mean cooling is consistent with Milankovitch forcing, and is not particularly relevant to the question of atmospheric circulation and glacier anomalies.–eric]

  5. 5
    David B. Benson says:

    sidd @2 — In my amateur opinion, no. Indeed, probably more SLR than during the Eemian…

  6. 6
    Chris Colose says:

    sidd (#2)

    My own amateur opinion has to disagree with David. Sea level rise during the Eemian was something on the order of 6-9 meters higher than present (see e.g., Kopp et al., 2009). No one credibly has sea level exceeding 1-2 m around 2100, and even on longer timescales it may be hard to get to that level, but there’s a lot of uncertainty. Paleoclimate inferences probably don’t probe the full span of uncertainy associated with forcing and rate-dependent mechanisms of melt, and the hysteresis associated with ice sheets. The Eemian forcing is ~40 W/m2 in summer high-latitudes, order of magnitude more than that of doubled CO2 globally. The long-term sea level rise will depend critically on the cumulative carbon emission pathway humans follow, which determines the sustained global warming that can be maintained for centuries to millennia. I cannot believe we have committed ourselves to an Eemian degree of sea level rise already, so since you asked whether we could avoid it, I have to say yes.

    In any event, there is unequivocal geologic evidence for parts of the GIS still in tact during the last interglacial, and Northern Hemisphere ice core records (see NEEM) now go back that far, which rules out ice-free conditions at the time in the NH. This also means West Antarctica contributed a substantial amount. The GIS differs from the WAIS, however, because the former is now largely land-based and the latter marine-based, which potentially makes the WAIS more vulnerable to an abrupt and much less predictable onset of retreat. So Greenland is probably a bit easier in that it melts when it gets warmer, but not enough melt in the near future to give you Eemian-like conditions. But I think 6+ meters is pushing it anytime soon.

  7. 7
    Fredt34 says:

    Can you please tell us a word about the loss of Larsen A and B ice shelves? How do they relate with these studies?

    [Response: Those ice shelves are just south of James Ross Island, and collapsed due to summer warming. Good discussions on this abound — e.g.; or this paper: and references therein–eric]

  8. 8
    Lennart van der Linde says:

    Chris #6,
    According to Jim Hansen the Eemian forcing was smaller than now, or this century, averaged over the entire globe. And during the Holsteinian with insolation more similar to today sea level also rose up to maybe 10 meters above the present level. So meters of SLR per century seem quite possible to Hansen, who I think we would ignore at our peril.

  9. 9
    Lennart van der Linde says:

    Eric #2 (inline response),
    About one meter by 2100 could mean about 3m by 2200 and 5m by 2300, right? At least this seems to be the case according to the risk estimate of the Dutch Delta Committee, for example. Jim Hansen seems to think it could be about 10m by 2300.

    How hard or easy do you think it would be for cities/countries/islands to adapt to 5 or 10 meters of SLR by 2100? You say it depends on how long term your view is, but if this would come to pass, how damaging do you think this would be to the people involved, locally and globally?

    [Response: I think the chances of 5 or 10 m of SLR by 2100 is zero. The rates of rapid rise Jim Hansen talks about occurred when large ice sheets covered Canada and the Antarctic ice sheet extended to the edge of the continental shelf. Obviously, adapting to such a rate of sea level rise would be very difficult, but it ain’t gonna happen. Let’s stick to science rather than speculation. For more on this, read Tad Pfeffer’s view from an earlier RC post.–eric]

  10. 10
    Lennart van der Linde says:

    Eric #9 (inline),

    Oops, typo, I meant 5 or 10 m by 2300, not 2100, which is what I said in the paragrapfh before. It may not be likely, but it does seem to be a risk that part of the scientific community considers real. Does this clarification change your reply?

  11. 11
    Pete Dunkelberg says:

    As mentioned by Chris Colose in #6, Earth’s orbital tilt in the Eemian brought a lot of sunlight to the Arctic in the summer even our planet’s overall solar input was not changed. Our current climate forcing is global, and well directed to warm the global ocean. At the same time, much of the ice reduction so far is due to melting from below. The topographies of Antarctica and Greenland give warming seas plenty of opportunity to get at the ice. How much ocean heat can the great ice masses stand up to?

  12. 12
    sidd says:

    Thank you for the response Prof. Steig.

    Mr. Dunkelberg writes on the 18th of April, 2013 at 1:18 PM:

    “How much ocean heat can the great ice masses stand up to?”

    1mm SLR is approx 1.2e20J into ice melt
    1 m SLR 1.2e23J
    Balmaseda et al. (doi:10.1002/grl.50382) show OHC above 2000m rising at approx 1e22 J/yr

    How much heat gets into the ice ? St. Laurent et al, in the last WAIS workshop estimate 1Gw/Km today into the Antarctic coast.

    Antarctic coast is about 2e4Km, 6e20J/yr. This seems too large (5mm/yr SLR)

    Less in Greenland, i imagine looking at topo.


  13. 13
    prokaryotes says:

    Great read! Are there any studies done on past ice sheet collapse and resulting tsunami impacts? Are there any studies done to model a worst case ice sheet collapse, such as WAIS collapse causing a mega tsunami?

  14. 14
  15. 15
    Isotopious says:

    Figure 2, Is the temperature uptick at JRI during the 20th century, as shown by the think averaged green line, influenced in any way by the fact that it is at the end of the series. For example, if the series ended at 1600, would the thick average change character at all.

    It’s an obvious question since the dramatic” uptick is at the end of the series! It probably doesn’t change….

    [Response: Just look at the raw data, also shown.–eric]

  16. 16
    Hank Roberts says:

    The ice sheets are floating, remember — no big wave when one breaks off.

    [Response: The ice shelves are floating. The ice sheets (by definition) are not. But in any case they don’t “fall into the sea” as that commenter I guess imagines. Locally big waves when a calving event occurs! Watch Chasing Ice film!–eric]

  17. 17
  18. 18
    prokaryotes says:

    Hank Roberts, i especially refer to IS which are grounded and up to 2 km high, as outlined by Richard Alley in this video

  19. 19
    Thomas says:

    sidd @12.
    I imagine that any loss of ice shelf ice results in a positive feedback. The grounding line moves inland. The possibility of breakup increases as the ice shelf thins. I believe these make the ice more susceptible to breaking off adn floating towards warmer waters. So there is probably an amplification factor greater than 1.

  20. 20
    Pete Dunkelberg says:

    sidd @ 12, Thanks for that analysis. Unfortunately it does not lead to a reasonable upper bound on sea level.

  21. 21
    Isotopious says:

    What about sampling error. What would happen if you removed random data points to the 20th century uptick. I’m sure you could randomly chip away at the magnitude /significance of the uptick. Surely the raw data has higher resolution at the most recent part of the core. That could mean there is “missing data” the further you go back in time. If the resolution does change (assumption), will adding random data to older parts of the series, to simulate missing data, create any significant upticks. The difference between a significant trend or not could simply be missing data, or poor resolution. What random tests were conducted with regards to this?

    [Response: The resolution is very very high in this core all the way through the last 1000 years. Look at the supplemental data. I will re-read this myself, as you do raise a legitimate question about sampling (thinking entirely abstractly), but I am pretty sure this is a non issue for this core.–eric]

    [Response: Update. Actually, it’s totally obvious from looking at the graph that the sampling is quite uniform. –eric]

  22. 22
    sidd says:

    A useful concept here is Volume Above Flotation, VAF. Consider a block of ice 1Km^2, 2Km top to bottom, sitting in 1Km water. It will not float until the height above sea level (freeboard) is 0.1 Km. Initial VAF is 0.9Km^3. Final VAF at flotation point is zero. No surface melt in Antarctica to speak of. So the 0.9 Km^3 VAF must mass waste from below, from circumpolar deep water (CDW) Only after it floats and comes unstuck can it wander out to the warm embrace of the ocean.

    Complications are the Weertman instability from retrograde submarine bed, and gravitational driving stress increasing as slope of surface increases as ice melts at edges, subglacial hydro and tidal pumping CDW in and out, ice shelf buttressing failure, sea level rise, and a buncha other stuff. Most of these increase mass waste. The wais workshop pages i linked to earlier are quite good.


  23. 23
    sidd says:

    P.S. i should mention that VAF is the only bit that contributes to SLR. After the thing is floating, no more contribution to SLR.


  24. 24
    Phil Scadden says:

    Isotopious – it seems to me that you are confusing the issue in Marcott et al (proxy drop out) with the data and methodology here. Easily dismissed from looking at the data.

  25. 25
    AntonyIndia says:

    The James Ross Island is one of several islands around the peninsula known as Graham Land, which is closer to South America than any other part of that continent. The form James Ross Island is used to avoid confusion with the more widely known Ross Island in McMurdo Sound
    This island @ 63.50S / 58.15W not a very good location for monitoring Antarctic continental ice sheets.

    [Response: Agreed. But it’s a very good location for monitoring the glaciers and ice shelves of the Antarctic Peninsula. –eric]

  26. 26
    toby says:


    Have you read this paper by Frezzotti et al, getting some attention. It reviews surface mass balance changes over the last 800 years or so.

    Does htis complement your own work?

    [Response: They find no signif. trend in surface mass balance (net snowfall) for most of Antarctica, but significant trend in coastal regions. That’s consistent with our findings yes. It’s been argued a significant trend is expected under a warming climate, but the signal-to-noise ratio is still too low in most places in Antarctica, even where the warming trend (e.g. WAIS) is quite large. An exception again the Peninsula where, at least at the Gomez ice core site at the base of the Peninsula — the increase in snowfall has been very large — see Thomas et al., 2009. It isn’t yet clear (at least not to me) why. –eric]

  27. 27
    Isotopious says:

    Now having looked at the data of abrams, I now realize the thick green line in figure 2 is a filter rather than an a more simpler average. This filter highlights the the fact that the series (which is an 11 year running average) has a significant recent warming uptick, and that During the same time this uptick occurred, variability was significantly less than normal when compare with the rest of the series. So the magnitude of warming is not what is significant (disregarding the melting threshold), rather it’s that this warming has occurred with far less noise then usual.

    It is very plausible that the coupling of a steep warming trend with less than normal variability is simply due to chance (especially since not only does the uptick start at a historically low point, the magnitude itself is not that great. It would be unusual for these “usual events” not to occur in a 1000 year series).

    [Response: I’d like to see that math that gives this result, and then the “chance” that it happens at just the right time! –eric]

  28. 28
    Chris Dudley says:

    “Let’s stick to science rather than speculation.”

    I notice that there is tendency here at realclimate to say insulting things about James Hansen without actually reading his work. Hansen, addressing the question “How good are assumptions about the slow response of ice sheets to warming?” looked at rates of sea level rise in the paleo-record and found that a rate of 5 m per century has occurred in the past. That rate is not consistent with a top down melting model and implies dynamic response of ice sheets to warming. To me, that is science, not speculation. The observational record contradicts the simplifying assumptions used in current models of ice sheet response.

    It could be that ice sheets, through dynamical behavior, are not on a 3000 year clock that straight melting implies but rather respond with much less delay to warming. If so, then it would matter if a 5 C warming occurred in 100 years or 3,000 years. A smaller ice sheet extent might still respond with the observed high rate of sea level rise (5 m per century) if the warming is much more rapid than when ice sheets were more extensive.

    Does it really matter if the century that has a sea level rise of 5 m starts in 2030 or 2090 or 2150? The point is that it has happened in the past and the models are not yet sophisticated enough to rule out it happening again. The current rate (over the last 2 years) is about 1 m per century and we still have a lot more warming to cause in a BAU scenario.

    [Response: No one at RC has every said anything insulting about Jim Hansen, and yes, we have read his work. The dynamics of large of sheets is of course a possible unknown, but no one has yet be able to come up with a *mechanism* for 5 m of sea level rise in 100 years given current ice sheet configurations. It is on that basis that I find Hansen’s suggestion of this possibility to be implausible.–eric]

  29. 29
    Lennart van der Linde says:

    Perwis #17,

    Yes, thanks, I read those papers a few years ago. But I’m wondering what Eric estimates as the (local and global) risks and adaptation chances of a scenario with 5-10m of SLR by 2300.

    I know Pfeffer’s position and what Hansen thinks of that position. For now I consider 2m by 2100 as a worst-case scenario, although the risk of 5-6m between 2100 and 2150 is maybe not zero. Or at least it’s maybe too early to know for certain, as Richard Ally seems to argue, not to speak of Hansen.

    So a more thorough follow-up to the Atlantis-project and the Delta Committee would seem advisable to me.

  30. 30
    Hank Roberts says:

    My mistake using the wrong word above — but as Eric says the ice sliding off the continent makes splashes locally (I have seen _Chasing_Ice_ and recommend it highly).

    It would be interesting to compare the numbers, if any, for volume and time span of an ice sheet failure (are there any for how fast it could happen)? to the numbers published in the estimates for volcano collapse, e.g. Cumbre Vieja

  31. 31
    Hank Roberts says:

    I try to stay away from Huffpo — too much wackywoo — but that “ice sheet collapse … megaflood” story does give some numbers: “sea level rose about 45 feet (14 meters) in less than 350 years.”

    A rate you can walk away from, so not a tsunami. Yes, it’s a huge big deal for humanity. But it’s slow small steps for the individuals living at the seacoast.

    [Response: That’s Meltwater Pulse 1a (see e.g. paper by Weaver et al., 2007) that you’re referring to. This is the same event that gives Hansen his example of super-fast sea level rise. It occurred when Canada and most of Europe were covered by ice sheets, and when the Antarctic ice sheet extended to the edge of the shelf. If Antarctica tried to dump that much ice out now, all the icebergs would ground on the continental shelf. It, like Greenland, is in a far more stable configuration now than it was then. The conditions that produced Meltwater Pulse 1a ares simply not a good analogy for the present.–eric]

  32. 32
  33. 33
    prokaryotes says:

    MacAyeal, D R., D S Abbot, and Olga V Sergienko, August 2011: Iceberg-capsize tsunamigenesis. Annals of Glaciology, 52(58), 51-56.
    [ Abstract ]

    Calving from the floating termini of outlet glaciers and ice shelves is just the beginning of an interesting chain of events that can subsequently have important impacts on human life and property. Immediately after calving, many icebergs capsize (roll over by 90◦) due to the instability of their initial geometry. As icebergs melt and respond to the cumulative effects of ocean swell, they can also reorient their mass distribution by further capsize and fragmentation. These processes release gravitational potential energy and can produce impulsive large-amplitude surface-gravity waves known as tsunamis (a term derived from the Japanese language). Iceberg-capsize tsunamis in Greenland fjords can be of sufficient amplitude to threaten human life and cause destruction of property in settlements. Iceberg-capsize tsunamis may also have a role in determining why some ice shelves along the Antarctic Peninsula disintegrate ‘explosively’ in response to general environmental warming. To quantify iceberg tsunami hazards we investigate iceberg-capsize energetics, and develop a rule relating tsunami height to iceberg thickness. This rule suggests that the open-water tsunami height (located far from the iceberg and from shorelines where the height can be amplified) has an upper limit of 0.01H where H is the initial vertical dimension of the iceberg.

    Because ice sheet disintegration is non-linear, the potential SLR adjusts accordingly.

  34. 34
    Lennart van der Linde says:

    There is less ice now than during the last deglaciation, and the ice may be more stable now, but the forcing is/will be much stronger, right? So couldn’t that compensate each other and still produce (very) high rates of SLR, if not in the coming century then in the following centuries?

    Hansen seems to argue for a maximum rate of SLR, under BAU forcing, of at least 4-5 meters per century, somewhere in the coming centuries (including a negative feedback he calls the ‘ice berg cooling effect’). What in your opinion is the maximum rate of SLR humanity will have to deal with over the coming centuries under BAU?

    Are you saying 4-5 m/century is not physically possible/plausible? If so, then what rate do you think is possible/plausible? And what does that imply for flooding risks and adaptation options for coming generations?

  35. 35
    Chris Colose says:

    Lennart- Globally the forcing will be much bigger this time around than the Eemian, but ice melting is largely a local threshold process, so comparison with the high-latitude summertime insolation during the Eemian is more appropriate in this context.

    It is not implausible that on the multi-century timescale we can melt enough of Greenland and parts of Antarctica to give us over 6 m of sea level. But the confidence in these long-term projections of sea level is low.

    Regarding all these hypotheticals of Earth-ssytem timescale feedbacks, etc- before results are brought forward with high confidence and reach a level of minimal academic disagreement, they should be understood physically, be exhibited in a range of models from simple to complex, begin to emerge in observations against natural variability, are shown to be robust to methodological choices and interpretation, and are borne out paleoclimatically. If these pedestals are all present, and have been studied by multiple groups and published in several sources, in addition to having undergone assessment by the IPCC, National Academies, etc, and have high predictive and explanatory power, then you can have substantial confidence in the conclusions. This is the case with e.g., the existence of a water vapor feedback, that Arctic sea ice melts with warming, that Milankovitch forcing is important over the Quaternary, etc. But I think you need to assign substantially less confidence to conclusions which are reliant only on paleoclimate, particularly times with much different boundary conditions and hysteresis relevant for the ice sheets. So while times like the Eemian or Pliocene or Miocene may be interpreted as a warning call in some respects, it is not really good to determine threshold policy targets, or give substantial weight to answering big questions like future sea level change, based solely on these analogs.

    In any event, I feel I’ve distracted from the main purpose of Eric’s post, which is really about putting contemporary Antarctic changes in a millennial context, as well as highlighting interesting tropical-high latitude connections in weather and climate. In particular, it is not self-evident that future Antarctic climate change may be partially slaved to the spatial structure of tropical SST warming.

  36. 36
    Tom Dayton says:

    Lawrence Sullivan has a new [insert adjective here] on the Financial Post site, about these two Nature Geosciences article. I’ll not link to it here, but you can search for “History Trumps Climate Scientists.”

  37. 37
    Lennart van der Linde says:

    I agree with you that much more scientific work is needed to determine if we can have confidence is Hansen’s suspicions on the risks of SLR.

    However, from a societal risk perspective I think it would also be wise to take scientific suspicions such as Hansen’s more seriously than is currently being done, since we cannot confidently conclude that those suspicions are wrong either. Or can we?

    Compared to the Eemian the local insolation forcing may be much smaller, but the global forcing impacting the Arctic and Antarctic through various processes, thru the ocean e.g., may still add up to have comparable or stronger effects as during the Eemian. At least that seems to be Hansen’s argument, which doesn’t look too unreasonable to me.

    Can we afford to underestimate the risk of high and fast SLR, since it will be impossible to prevent once we can have enough confidence in its eventual occurrence. We shouldn’t err on the side of least drama in this case, is my impression.

  38. 38
    Pete Dunkelberg says:

    As human beings we wish for an assessment of risk. That is evidently not forthcoming here. As adults we do not risk extreme harm just for sport. We are mindful that there is no Planet B. But what is the risk?

    Hansen mentioned faster sea level rise as a possibility based on the idea that as the temperature rises, the rate of heat transfer to ice rises. What is implicit, I think, in all this “wait for the models” talk is that there is no model for the ocean heat to contact so much ice in just eighty years. The topography of Antarctica which I linked above is a model of a sort, but it is not a GCM. Since we have no data from the future, most climate scientists will speak only of what the GCMs say, yet as humans we have learned from the Arctic that melting does not wait on models.

    CO2 is evidently going well over 450. After the Revolt Against Coal we will lose our sulfate aerosol shield. We will be also be well into Significant contribution to climate warming from the permafrost carbon feedback, nicely covered by Kate.

    Conclusion: no one knows how high the sea will rise, and for a risk assessment you are on your own.

  39. 39
    Hank Roberts says:

    > prokaryotes says:
    > MacAyeal … August 2011: Iceberg-capsize tsunamigenesis.
    > Annals of Glaciology, 52(58), 51-56
    > … fjords

    confined, yes, a serious problem; fjords/lakes make huge waves

    > … open-water tsunami height … upper limit of 0.01H
    > where H is the initial vertical dimension of the iceberg

    I wonder– is that H above water line, or H total ice thickness?
    Well, 0.01 is a somewhat reassuring modifier for those of us not right under the edge of the ice.

    I confess I do have the nightmare sometimes of one of these huge ice sheets, having gotten floated, perforated, and lacerated, riddled with meltholes and cracks, suddenly going pop-pop-pop, doing a fast crumble-and-rush-to-the-sea as a huge mass of icecubes. But I hope it’s only a bad rate of change dream.

  40. 40
    prokaryotes says:

    “H is the initial vertical dimension of the iceberg.”

    So what i read from that is (see quote above) a 100 m Iceberg can generate 1 meter high tsunami. ANd again which might be modulated further, based on local topography.

    Again about WAIS collapse
    Here at this mark 35 mins in R. Alley explains why Ice Sheet retreat is based on thresholds (local topography). And because of these features SLR will be not gradual but rapidly.

    The question is how fast we approach the threshold. And when we look into the past we have examples of abrupt SLR – rapid Ice Sheet disintegration. Which might have been more suitable during MP1a but the rate of CO2 today and because of a different Ozone layer state, different SAM might generate a similar outcome.

  41. 41
    sidd says:

    Mr. Hank Roberts writes on the 19th of April, 2013 at 4:47 PM:

    “I confess I do have the nightmare sometimes of one of these huge ice sheets, having gotten floated, … ”

    once it gets floatitude, we dont care …

    ” … perforated, and lacerated, riddled with meltholes and cracks, suddenly going pop-pop-pop, doing a fast crumble-and-rush-to-the-sea as a huge mass of icecubes. But I hope it’s only a bad rate of change dream.”

    Your nightmare might work better if you confine the perforated, lacerated, riddled icecube rush to the sea bit to grounded icesheets. They will float after all the volume above flotation has rushed to the sea. Thats the bit that counts.

    (Not to knock your nightmare, it’s a very nice mightmare as they go, just trying to make it more physically … grounded … as it were …)


  42. 42
    Killian says:

    [Response: That’s Meltwater Pulse 1a (see e.g. paper by Weaver et al., 2007) that you’re referring to. This is the same event that gives Hansen his example of super-fast sea level rise. It occurred when Canada and most of Europe were covered by ice sheets, and when the Antarctic ice sheet extended to the edge of the shelf. If Antarctica tried to dump that much ice out now, all the icebergs would ground on the continental shelf. It, like Greenland, is in a far more stable configuration now than it was then. The conditions that produced Meltwater Pulse 1a ares simply not a good analogy for the present.–eric]

    Yet, no era in Earth’s history correlates to the speed with which we are adding GHGs to the atmosphere, nor to the myriad different forcings happening all at once, yet we keep drawing parallels.

    A number of the problems we face come from the same source, GHG’s, many others are the direct result of other behaviors. When has that ever been true? We can presume those associated directly with GHG’s might have been in play at any point in history where CO2 rose, to some extent, but we have no way of knowing if the rapidity of the rise drives those issues in strange ways. Most systems pushed too hard and too fast tend to get pretty wiggy, no? Why not the overall Earth system? Does the speed of the forcing make up for the fact the ice sheet was sitting on the continental shelf? I’d have to guess yes since the majority of melt comes from the ocean and CO2 is going into the ocean faster than ever before. But, ok, maybe not. But the sanguine nature of the response is rather frightening.

    Let’s look at some complicating issues:

    Ocean acidification comes from GHGs, but not eutrophication which is driven primarily by chemical and soil runoff which comes from ignorance of natural systems design and the dysfunctional food system. Thus, the oceans are being degraded concurrently by two different processes. Has that been the case before? If not, might not we want to assume, for the sake of risk assessment, that this will drive faster break down in the system and faster heating of the planet as the ability of the ocean to support life is diminished, and likely it’s ability to continue to act as a carbon sink? In fact, wouldn’t eutrophication speed up the time line in terms of no longer being a carbon sink as the oceans are depleted of oxygen from dying plankton and dead zones?

    Then there’s other forms of pollution of the world’s waters, the destruction of ecosystems by simple over-fishing/population reductions from feeding people and farmed biota, as well as built environment destroying supporting ecosystems. Merely removing apex (large fishes) or food chain bases (phytoplankton) from ecosystems can result in significant changes to ecosystems, so all these things happening at the same time? How can we *not* expect the sum of these effects to not likely be worse than the natural processes we measure from the past? The system is massively overstressed and any sanguine response is worrisome to me.

    So, while the specific size of the ice sheet was different, the overall conditions are degraded in ways not likely to have existed in the past all at the same time. I figured SLR based on the 10 year doubling rate from the current 3 mm/yr and got @16 meters.

    Holy bejeezus, Batman!

    And if it’s accelerating now, what, exactly, is supposed to slow it going forward other than drawing down carbon?

    One of the hosts here also said the chance of 5 to 10 meter rise this century was a non-possibility. And we haven’t even dealt with seabed clathrates or permafrost… (looking at you Prof. Archer. ;-) ) But, again, what’s supposed to slow the current doubling. (Where did I see, in the last few days, comment about a paper showing a 5 year doubling rate in the last five years?!)

    I’d like to suggest this time is different. Sanguine feels good, sure, but is it getting to be dangerous? (Yes, I know policy is the duty of We, the People, but we seem to be abdicating…)

  43. 43
    Lennart van der Linde says:

    So what may happen to WAIS (and GIS and EAIS) under BAU-emissions, in view of these new findings Eric has presented above? Should we take into account a risk of about 10m of SLR by 2300?

    At least Meehl et al (2012) seem to explicitly consider this risk in their BAU-scenario:

    Goelzer et al (2012) is interesting too, when considering the risk of even longer-term and very large SLR:

    In their figure 7 they seem to view about 25m of SLR by the year 3000 as a worst-case scenario under BAU and with high climate sensitivity.

    Foster & Rohling (2013) find a likely chance of 9-31m of SLR for a CO2-level of about 400 ppm, and of 7-21m for about 350 ppm:

    They can’t know how fast the rate of SLR could be, but can’t exclude meters per century either. And their conclusion is that EAIS is unstable enough to contribute up to about 10m of SLR before becoming (maybe/probably) more resistant to further melting/disintegration.

    So for now, my conclusion is it will be very hard to still prevent 10 meters of SLR in the long term, even under strong mitigation, and there seems to be a real risk of 10 meters of SLR by 2300 and 25 meters by 3000 under BAU.

    Now should we take such long views in considering mitigation and adaptation options and risks? As stated by the IPCC-workshop report on sea level rise and ice sheet instabilities from June 2010: “[T]he millennial timescale is relevant to the mitigative policy deliberations, including ethical considerations.”

    My impression is we do not stress these risks (clearly) enough, so we may be ‘erring on the side of least drama’ (ESLD).

    For more on ESLD by Brysse, Oreskes et al (2012):

    Their conclusion, also based on the history of WAIS-risk assessment in AR4:

    “Our hypothesis of ESLD is not meant as a criticism of scientists. The culture of science has in most respects served humanity very well. Rather, ESLD provides a context for interpreting scientists’ assessments of risk-laden situations, a challenge faced by the public and policy-makers. In attempting to avoid drama, the scientific community may be biasing its own work — a bias that needs to be appreciated because it could prevent the full recognition, articulation, and acknowledgment of dramatic natural phenomena that may, in fact, be occurring. After all, some phenomena in nature are dramatic. If the drama arises primarily from social, political, or economic impacts, then it is crucial that the associated risk be understood fully, and not discounted.”

    If I’m misinterpreting the references above, or taking them too seriously, let me know.

  44. 44
    Hank Roberts says:

    > Thus, the oceans are being degraded concurrently by two different
    > processes. Has that been the case before?

    Yes. You’ll find this is well known and much written about.

    I’ve mentioned one paper that I found interesting in responses here several times since it came out; the number of papers citing this one keeps going up, so I’d guess it’s an interesting area for those studying this sort of thing. This isn’t -the- answer, it’s an example of what you’ll find.

    doi: 10.1130/G23261A.1 v. 35 no. 3 p. 215-218
    Abrupt increase in seasonal extreme precipitation at the Paleocene-Eocene boundary
    B Schmitz, V Pujalte – Geology, 2007 –
    A prominent increase in atmospheric CO2 at the Paleocene-Eocene boundary, ca. 55 Ma, led to the warmest Earth of the Cenozoic for∼ 100 ky High-resolution studies of continental flood-plain sediment records across this boundary …. The megafan formed over a few thousand years to ∼10 k.y. directly after the Paleocene-Eocene boundary. Only repeated severe floods and rainstorms could have contributed the water energy required to transport the enormous amounts of large boulders and gravel of the megafan during this short time span. The findings represent evidence for considerable changes in regional hydrological cycles following greenhouse gas emissions.

    Cited by 72

  45. 45
    patrick says:

    Hank, it seems to me the question has to do with the eutrophic side of the double-degrade. That’s how I read the comment.

  46. 46
    michael sweet says:

    I find the informed discussion of sea level rise very interesting. Perhaps you could think of doing a post to summarize your thoughts? The general internet has so much unsupported blather that it is difficult to separate the wheat from the chaff. A new contribution from Real Climate would freshen the discussion.

    [Response: Thanks. We’ll try to do that some time in the next month or two. –eric]

  47. 47
    Hank Roberts says:

    Patrick, yes, I pointed to one paper, cited by 72 more recent papers.

    Click the Cited by 72 link. When that opens, click in the little square labeled “search within citing articles” and type “eutrophication” into the search box there.

  48. 48
    Chris Dudley says:

    eric (#28),

    Since the volume of ice at risk under BAU is within a factor of two of the volume of ice at risk during a deglaciation under orbital forcing, while the forcing is much more rapidly applied under BAU, looking at sea level rise rates in the paleo-record might actually be considered a search for lower limits on what to expect if reticence did not run so strongly in our approach.

    No, when Hansen speculates he is clear that that is what he is doing. When he explains how a 5 m rise this century could be consistent with the current data on sea level rise, he points out that he is speculating. When he says that rapid sea level rise has occurred in the past and can’t be ruled out by ice sheet models as a response to BAU, that is evidence supported argument and not speculation.

  49. 49
    Chris Dudley says:


    “If Antarctica tried to dump that much ice out now, all the icebergs would ground on the continental shelf.”

    Isn’t that argument a little circular? If that happened the ice sheet would then be in the configuration you consider unstable since it would be on the shelf.