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How long does it take Antarctica to notice the Northern Hemisphere is warming?

Filed under: — eric @ 29 April 2015

Eric Steig

A series of large and abrupt climate changes occurred during the last ice age, most clearly expressed in ice cores from Greenland and other paleoclimate data from the circum-North-Atlantic region. Since the discovery of these events, we’ve been trying to pin down the timing of abrupt climate changes elsewhere on the globe. Were there corresponding events in the Southern Hemisphere? And did they occur at the same time? A new paper published this week in Nature (April 30th, 2015) provide a significantly updated answer to these questions. Many in the climate research community — both modern climate and paleoclimate — will find the results quite interesting.

The new paper is the culmination of a huge effort to develop the best-dated long ice core record from Antarctica, rivaling the GISP2 and GRIP ice cores obtained from central Greenland in the early 1990s, and the more recent NGRIP and NEEM cores from North Greenland (e.g. NEEM Community Members, 2014). The core was obtained at the West Antarctic Ice Sheet divide (WAIS Divide), led by the Desert Research Institute and the University of Nevada, with the University of New Hampshire. The new paper was written by a consortium of postdocs, faculty, and students at the Oregon State University and University of Washington: Christo Buizert, myself, and Joel Pedro (now at University of Copenhagen), with Brad Markle, Ed Brook, Jeff Severinghaus and Ken Taylor. We have more than 70 other co-authors — faculty colleagues, students, postdocs, logistics coordinators, and ice-drillers — who all made substantial contributions as well. These deep ice cores are a lot of work!

We published records from the WAIS Divide ice core in 2013, covering the last two millennia and the last 30 thousand years (Steig et al., 2013, WAIS Divide Project Members, 2013). Our new work, WAIS Divide Project Members, 2015, extends the record to the bottom of the core (nearly the bottom of the ice sheet at 3400 m depth), and an age of 68 thousand years. Details on the timescale for the core are given in the open-access paper in Climate of the Past (Buizert et al., 2015). The new paper in Nature provides a comparison of the timing of changes in Antarctic temperature with the timing of the abrupt warming and cooling events that characterize the Greenland ice core records. Note that the comparison is actually made between the records of oxygen isotope ratios (δ18O), but we have very strong evidence that these track temperature quite faithfully on the relevant timescales, so I’ll use “temperature” here for simplicity.

The abrupt events in Greenland, characterized by rapid transitions from cold “stadial” to warm “interstadial” conditions and back, and commonly known as Dansgaard-Oeschger (D-O) events, were felt across the Northern Hemisphere almost immediately, as far as we can tell. But the impact of D-O events in Antarctica has been ambiguous. We’ve known for some time that temperatures in Antarctica change more slowly, and with much smaller amplitude, than in the Northern Hemisphere. In general, the Antarctic temperatures begin to decline when Greenland warms abruptly, and to increase when Greenland cools abruptly (Blunier et al., 1998). This relationship is often called the “bipolar seesaw” and is commonly attributed to the redistribution of heat between the Northern and Southern Hemispheres via changes in the Atlantic meridional overturning circulation (AMOC). We’ve also been pretty sure that each of the D-O interstadials has a corresponding warm peak in Antarctica, referred to as the “Antarctic Isotope Maximum” (AIM) events (EPICA Community Members, 2006, and Stefan Rahmstorf’s write-up in an earlier RealClimate post). But the exact phase relationship has been unknown, making it problematic to validate model simulations with confidence (see e.g. Roe and Steig, 2004, Steig, 2006).

The first paper to really start to pin down the phase relationship was that of Pedro et al., 2011, who showed that the most recent of the major abrupt events in Greenland cores (the Bølling warming about 14,700 years ago and the Younger Dryas cooling about 12,880 years ago), the direction of Antarctic temperatures changed at almost exactly the same time. (Note that the Antarctic temperatures don’t change abruptly — their slow trends simply reverse sign, as shown in the figure). But the uncertainties estimated by Pedro et al. were about 200 years on either side of zero. That’s impressively good precision for something that happened more than 10,000 years ago, but not quite good enough. Our new work firms up these numbers a lot, and shows that Antarctic temperatures did not really change at the same time as the abrupt events in Greenland. Instead, when an abrupt D-O warming occurs in Greenland, it takes about 200 years until the concomitant cooling begins in Antarctica. Similarly, when an abrupt cooling occurs in Greenland, it takes about 200 years until Antarctic starts warming up. Our uncertainties are much smaller, +/-95 years*, and it is very unlikely that our numbers overlap with zero. Antarctica, in other words, almost certainly takes a century or two to notice what is happening in the Greenland.

The 200-year timescale is fascinating, because it is longer than suggested by a number of modeling studies, such as the simple “bipolar seesaw” model of Stocker and Johnson, 2003, as well as the fully-coupled transient run of a general circulation model by He et al., 2013: both show an essentially instantaneous response between the Northern Hemisphere and the Antarctic. Yet it’s also shorter than implied by many discussions of the relationship; for example, we (Steig and Alley, 2002) made the case for a 400-year lag between Greenland Antarctica, which Schmittner et al., 2003 reproduced in a climate model in which the AMOC is perturbed.

So what does the intermediate timescale of ~200 years tell us? It’s important to recognize that there are reasons why one might expect either a “fast” propagation or a “slow” propagation of the Greenland climate signal to the Antarctic. The very large sea ice changes in the North Atlantic associated with D-O events would have an impact on the atmosphere, and this should propagate global signals almost instantaneously. Indeed, this must have occurred, or we wouldn’t have the evidence that we do for abrupt changes in places as far flung as China, India, or the tropical Pacific that are in phase with Greenland temperature change. Methane variations, which are probably of tropical origin, are in phase with Greenland temperature within about 20 years (e.g., Rosen et al., 2014).

The ocean itself can propagate signals very fast, via adjustment of the upper ocean by fast Kelvin waves propagating along the ocean boundaries (e.g. Johnson and Marshall, 2002). For example, Schmittner et al. (2003), and Rind et al. (2001) both found that the North Atlantic signal in their models propagates to the South Atlantic region very quickly, appearing in subsurface waters with a time lag of only about a few decades. However, they also find a century or multi-century delay in the further southward propagation from the South Atlantic to the Antarctic across the Antarctic Circumpolar Current (ACC). Furthermore, the propagation time varies with the strength of the ACC as imposed in the model.

There are, in short, multiple parts of the ocean and atmosphere system involved, and these have different timescales. It appears that our results are right about where the physics suggests they should be. A key factor in capturing this physics properly in models seems to be how (or how well) the ACC is simulated. That’s interesting, and highly relevant to modern climate studies. How the ACC is changing now, or may change in the future, is a topic of significant interest (Fyfe and Saeko, 2005; Böning et al., 2008). And the question of how long it will take the Antarctic to catch up with the rest of the globe is of critical importance to long term projections of the response of the ice sheet to climate change — and hence the response of sea level. The D-O events and the current anthropogenic global warming are of course very different beasts, but the long timescales indicated by our results are certainly in keeping with climate model projections of the future, showing that most of Antarctic should lag the rest of the planet (recent rapid warming on the Antarctic Peninsula and West Antarctica not withstanding).

Note that our results should not be taken as demonstrating anything very specific in terms of the cause of Dansgaard-O events. To be sure, the results demonstrate a clear north-to-south direction in the propagation of the climate signal associated with abrupt D-O warming and cooling events. But that doesn’t tell us what the driving “trigger” is. To use an analogy suggested by co-author Severinghaus, suppose we didn’t know anything about the physics relating lightning to thunder. Careful measurements of their relative timing would reveal that thunder always occurs very soon (or immediately) after lightning. But hearing thunder gives you only a general prediction of when the next lightning will be observed. One would correctly deduce that lightning causes thunder. That’s progress. But it would not tell you the cause of the lightning in the first place.

For my part, I’ve long been a skeptic about the old idea that the D-O variations are ultimately driven by meltwater and/or iceberg fluxes into the North Atlantic (the Day after Tomorrow scenario, if you will). There are only 6 clearly-identified Heinrich events (that is, layers of terrestrial sediment in ocean sediment cores from the North Atlantic, evidence for massive iceberg discharges from the Laurentide ice sheet) but there are at least 23 D-O events. It more likely that there is intrinsic variability in the coupled ocean-atmosphere system, as found for example in a long simulation with the climate model CCSM4 by Peltier and Vettoretti (2014 and 2015) (though there is debate about the validity of the very low values for ocean vertical mixing used in those simulations). Iceberg discharges are then just the consequence, not the cause, of changes in ocean circulation, as argued recently by Alvarez Solaz et al. (2013) and also suggested by Barker et al. (2015) who found that on average, evidence for icebergs in the North Atlantic follow, rather than precede, the abrupt coolings at the end of some D-O events. That doesn’t mean that the huge ice and meltwater fluxes associated with Heinrich events don’t have an impact; most modeling work suggests that they would. But it may be important in this context that our results show no dependence of the ~200-year lag on whether or not a Heinrich event has occurred: that is, there is no evidence that “Heinrich stadials” (the cold periods during which Heinrich events occur) are unusual with respect to ocean “seesaw” dynamics. The role of these events in millennial scale variability therefore remains an important, and open, research question (see e.g., Margari et al., 2010).

In the meantime our precise observations of the phasing of D-O and AIM events provide an important new constraint against which to validate model simulations designed to capture the dynamics of these interesting features of the climate system.

UPDATE: The News & Views article about our article, by Tas van Ommen is worth a read. Available here (subscription based only, I’m afraid).

Notes. *The ability to obtain such small uncertainties is owing to four main things. First, we have very high resolution measurements of methane in both the WAIS Divide and the Greenland cores; methane is globally well mixed, and so abrupt changes in methane must happen at the same time (within a year) in cores from both regions. This means that we can synchronize the age of the gas trapped in the bubbles within the cores very precisely. Second, we have very high resolution measurements of the nitrogen isotope ratio (15N/14N in atmospheric N2), also trapped in the bubbles in the cores. This isotope ratio provides information on the age difference between the gas and the ice, because gravitational settling increases the 15N/14N ratio; this depends on the thickness of the firn (the permeable ice between the surface and the impermeable ice at depth where bubbles are trapped). The deeper the firn, the longer it takes to trap gases, and the larger the age difference. It’s the age of the ice that we’re actually interested in, because this, not the gas trapped within the ice, is what the δ18O measurements are made on. Fourth, this age difference is much smaller at WAIS Divide than in any other long Antarctic record; it is at most ~500 years, compared with e.g., ~4000 years at Vostok. Finally, we also have unprecedentedly high resolution measurements of δ18O, and very high quality borehole temperature measurements, which together provide a very robust measure of the temperature variations through time.

Data: The data from the paper are all available in the Supplement to the paper. The timescale and the oxygen isotope data from our lab — what most people will be interested in — are available at the National Snow and Ice Data Center, at doi:10.7265/N5GT5K41.


Barker, S., J. Chen, X. Gong, L. Jonkers, G. Knorr, D. Thornalley. Icebergs not the trigger for North Atlantic cold events. Nature 520, 333–336, 2015.

Blunier, T., J. Chappellaz, J. Schwander, A. Dällenbach, B. Stauffer, T.F. Stocker, D. Raynaud, J. Jouzel, H.B. Clausen, C.U. Hammer, and S.J. Johnsen, “”, Nature, vol. 394, pp. 739-743, 1998.

Böning, C.W., A. Dispert, M. Visbeck, S. R. Rintoul, F. U. Schwarzkop. The response of the Antarctic Circumpolar Current to recent climate change. Nature Geoscience 1, 864-869 (2008).

Buizert, C., Cuffey, K. M., Severinghaus, J. P., Baggenstos, D., Fudge, T. J., Steig, E. J., Markle, B. R., Winstrup, M., Rhodes, R. H., Brook, E. J., Sowers, T. A., Clow, G. D., Cheng, H., Edwards, R. L., Sigl, M., McConnell, J. R., and Taylor, K. C.: The WAIS Divide deep ice core WD2014 chronology – Part 1: Methane synchronization (68–31 ka BP) and the gas age–ice age difference, Clim. Past, 11, 153-173, 2015.

EPICA Community Members, “One-to-one coupling of glacial climate variability in Greenland and Antarctica”, Nature, vol. 444, pp. 195-198, 2006.

Fyfe, J.C. and O. A. Saenko, 2005: Human-Induced Change in the Antarctic Circumpolar Current. J. Climate, 18, 3068–3073.

Grootes, P.M., M. Stuiver, J.W.C. White, S. Johnsen, and J. Jouzel, “Comparison of oxygen isotope records from the GISP2 and GRIP Greenland ice cores”, Nature, vol. 366, pp. 552-554, 1993.

He, F., J.D. Shakun, P.U. Clark, A.E. Carlson, Z. Liu, B.L. Otto-Bliesner, and J.E. Kutzbach, “Northern Hemisphere forcing of Southern Hemisphere climate during the last deglaciation”, Nature, vol. 494, pp. 81-85, 2013.

Margari, V., L.C. Skinner, P.C. Tzedakis, A. Ganopolski, M. Vautravers, N.J. Shackleton. The nature of millennial-scale climate variability during the past two glacial periods. Nature Geoscience 3, 127 – 131 (2010)

NEEM Community Members, Eemian interglacial reconstructed from a Greenland folded ice core, Nature, vol. 493, pp. 489-494, 2013.

Pedro, J.B., T.D. van Ommen, S.O. Rasmussen, V.I. Morgan, J. Chappellaz, A.D. Moy, V. Masson-Delmotte, and M. Delmotte, The last deglaciation: timing the bipolar seesaw, Climate of the Past, vol. 7, pp. 671-683, 2011.

Peltier, W. R., and G. Vettoretti, Dansgaard-Oeschger oscillations predicted in a comprehensive model of glacial climate: A “kicked” salt oscillator in the Atlantic, Geophys. Res. Lett.,41, 7306–7313, 2014.

Rind, D., G. Russell, G. Schmidt, S. Sheth, D. Collins, P. deMemocal, J. Teller. Effects of glacial meltwater in the GISS coupled atmosphere–ocean model, 2. A bipolar seesaw in Atlantic Deep Water production. Journal Geophysical Research, 106, pp. 27,355–27,365, 2001.

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Rosen, J.L., E.J. Brook, J.P. Severinghaus, T. Blunier, L.E. Mitchell, J.E. Lee, J.S. Edwards, and V. Gkinis, An ice core record of near-synchronous global climate changes at the Bølling transition, Nature Geosci, vol. 7, pp. 459-463, 2014.

Schmittner, A., O. Saenko, and A. Weaver, Coupling of the hemispheres in observations and simulations of glacial climate change, Quaternary Science Reviews, vol. 22, pp. 659-671, 2003.

Steig, E.J. et al., Q. Ding, J.W.C. White, M. Küttel, S.B. Rupper, T.A. Neumann, P.D. Neff, A.J.E. Gallant, P.A. Mayewski, K.C. Taylor, G. Hoffmann, D.A. Dixon, S.W. Schoenemann, B.R. Markle, T.J. Fudge, D.P. Schneider, A.J. Schauer, R.P. Teel, B.H. Vaughn, L. Burgener, J. Williams, and E. Korotkikh, “Recent climate and ice-sheet changes in West Antarctica compared with the past 2,000 years”, Nature Geosci, vol. 6, pp. 372-375, 2013.

Steig, E.J., Climate change: The south–north connection, Nature, vol. 444, pp. 152-153, 2006.

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65 Responses to “How long does it take Antarctica to notice the Northern Hemisphere is warming?”

  1. 1

    Thanks for making the study open to read. Even when it might take longer for the signal to impact the Antarctic continent, short time impacts may include regional extreme sea level rise. Also it is unclear to me how an abrupt northern hemisphere event could potentially accelerate ice breakup in Antarctica, because of the grounding line of ice sheets, besides assumed stronger Westerlies, or how atmospheric reconfiguration possibly impact distribution of icebergs and sea level height.

    Extreme sea level rise event linked to AMOC downturn

  2. 2
    sidd says:

    Nice. Is there any physically intuitive reason for the century timescale for propagation of the signal north to south across the ACC ?

    [Response: I don’t know about “intuitive”. At least it is not intuitive to me. You might check out the literature on this (a couple of citations above e.g., Schmittner et al is especially clear, I think)). –eric]

  3. 3
    Killian says:

    I would only offer the very general observation oft repeated before, and supported, at least generally, by the comments of Prof. White at the 2014 AGU conference, that the overall disruption of the Earth system makes comparisons with paleoclimate findings problematic. We have seen this WRT the speed of melt, rate of warming, permafrost thaw… you name it, it generally is happening either faster than expected or at the high end of expectations.

    I posited some years back this had to necessarily be expected as an Earth system that was intact would have built in hysteresis via multiple routes, but a planet on which all systems were being simultaneously degraded would be much more fragile and more prone to the effects of forcings.

    Simply, don’t be surprised if your 200-year finding doesn’t stand the test of real time. At some point, modelers ought to consider this in their modeling.

    [Response: I agree — I doubt that the 200-year finding won’t really apply to Antarctica under global warming. The forcing is totally different. Even so, the mechanisms (ACC perhaps, but maybe something else) need to be understood, and are relevant. –eric]

  4. 4
    wili says:

    Good points, Killian. How much soot/black carbon makes its way down to the Antarctic Ice Sheets? I here a lot about this wrt GIS, but I haven’t heard much about it for WAIS or EAIS. Is albedo shift from this or other factors playing much of a role at that end of the world?

  5. 5
    Doug says:

    Does this mean that we can expect Antarctica to actually cool, while the Arctic warms? What is the chance that the current warming is anomalous and the “bipolar seesaw” will be decoupled, or differently coupled, so both poles warm simultaneously on relatively short timescales?

    [Response: I don’t think one think about it this way. Both areas *are* warming in response to the radiative forcing of the planet. To the extent that the seesaw is relevant on century timescales, the relative amount they warm might be affected somewhere. But at least some work suggest that AMOC will slow down, even while the North Atlantic warms. That would distribute more, not less, heat to the Antarctic. –eric]

  6. 6
    sidd says:

    Thanks for the pointer, that helped a lot: I quote from Schmittner:

    “The cooling appears almost simultaneously between 20S and 40S. Its further southward propagation, however, is delayed. At the surface the signal reaches 60S about 400 yr after the warming in the North Atlantic and at greater depth this time lag is even larger. The slow southward propagation between 40S and 60S is due to the presence of the ACC at those latitudes. This is illustrated by the results of experiment W in which the Southern Ocean is not driven by winds. In this experiment, the ACC has only 1/3 the strength of that in the standard experiment (see Fig. 4 third panel), leading to a faster meridional propagation of the temperature signal in the Southern Ocean (dashed lines in Fig. 6) and hence a much shorter time lag of Antarctic air temperatures with those in Greenland (Fig. 5). Thus, the comparison of our two experiments indicates that the time lag of high latitude air temperatures is mainly determined by the strength of the ACC.”

    The context of this discussion are the temperature signals from DO events propagating south. Today we see signs of AMOC slowdown, but in the context of a global forcing and not a bipolar seesaw. Nevertheless, food for thought, indeed.

    Thanx again for the nice writeup.


  7. 7
    fp says:

    What is going on with the slowdown of the current now? We are getting mixed signals…

    “Our finding that abrupt climate change began in the north and came some 200 years later in the south is good news for the future,” said Severinghaus. “This finding confirms the view that the abrupt changes were due to changes in North Atlantic currents, and we know that these currents are unlikely to shift in coming centuries.”

    [Response: I have no idea what Jeff is talking about here (assuming the quote is accurate). I don’t think our paper provides any “confirmation” about “North Atlantic currents”. Certainly, our results are entirely consistent with the idea that the AMOC is a major part of the “seesaw” picture. But as I said, our results don’t tell us about the cause. I also don’t see how this tell us anything about the future, or the likelihood of current or future changes in AMOC.–eric]

  8. 8
    Bob Bingham says:

    can anyone tell me how long it takes for water from the Gulf Stream that sinks in the Arctic to surface again in the Southern ocean?

  9. 9
    Jim Eager says:

    Doug at 5, the current warming _is_ anomalous compared to the warming coming out of a glaciation.

    Interglacial warming is initiated by a slight orbital-driven increase in regional solar insulation at high northern latitude, followed later by albedo and greenhouse gas feedbacks, which then induce a global-scale warming.

    The current warming is being caused by an initial global-scale increase in greenhouse gas forcing, which will be followed by those same global-wide feedbacks. Thus the initial (and feedback) forcing is acting on both polar regions simultaneously instead of only on the northern polar region.

  10. 10
    Lawrence Coleman says:

    Another interesting correlation would be the timing of the D.O.interstadials to increased seismic and tectonic plate hyperactivity. As billions of tonnes of ice is melting off Greenland, Ellesmere and the polar cap the tilt of the earth’s axis changes as has been documented already. Correct me if I’m wrong but isn’t that analogous to a weight coming off one of your car’s tyres rim and causing an imbalance and shaking with the rotation of the entire wheel. That imbalance surely would place additional stresses on the earth’s tectonic plates and changes to underlying magma reservoirs. Nepal earthquake, then PNG etc. Am I putting 2 and 2 together and getting five or is there somre credibility to what I sense is happening.

  11. 11
    Chuck Hughes says:

    So for all us Climate dummies out there, is Antarctica in the process of collapsing? Or as Eric Rignot put it, “The fuse is blown on Antarctica.” I assume that is still the case.

    It seems to me that the entire process of warming is completely different today than in the past due to human activity. How do we know how fast the ice will melt or move based on past events when we’re the ones causing it?


    [Response: I agree that the process is very very different today. It is not “totally” different, in the sense that the same ocean and atmosphere circulation dynamics still are relevant. It is thought that the changes in the ocean circulation that we believe explain the data we have from the ice cores are associated with changes in the salinity balance of the North Atlantic. Since current and future changes in salinity balance can occur, the same types of ocean circulation changes could occur to. Perhaps.]

    As for whether the ice sheet is currently collapsing though, I don’t think that’s proven yet. I’m quite conservative on this subject though, and certainly Eric Rignot knows what he’s talking about. I am working on a post on this for RealClimate, so stay tuned. –eric]

  12. 12
    Killian says:

    [Response: I agree — I doubt that the 200-year finding won’t really apply to Antarctica under global warming. The forcing is totally different. Even so, the mechanisms (ACC perhaps, but maybe something else) need to be understood, and are relevant. –eric]

    Agree they are relevant. I’m always glad the science is being done, always looking at the systemics not covered by climate science specifically.

    Reading through the comments, it occurred to me if this process remains intact and is not disrupted or overridden by this non-natural forcing of GHG’s, then at some future point this effect will *add* to warming, i.e. be yet another forcing added to current forcings.

    [Response: Yep, ‘cept that in my opinion how sign of develops is unknown (that is, I don’t buy the “AMOC will slow down under global warming” yet, even though that is certainly a common feature of model simulations, and possibly even observations. –eric]

    Am I reading this right?

  13. 13
    Hank Roberts says:

    fp says:
    30 Apr 2015 at 10:38 PM

    That’s a quote found in a press release:

    Press release quotes, um, need verification by the scientist to whom they’re attributed. Press officers seem prone to leave out some important words that didn’t fit the press release format, changing the sense.

  14. 14
    Tas says:

    re Eric’s response to Doug (#5 above), the propensity of any contemporary slowdown in the AMOC to exacerbate heating in the south was something I was alluding to at the end of my News and Views comment
    Better understanding and modelling of inter hemispheric coupling, prompted by work such as this latest study will help to determine how the overall warming signal is expressed at high latitudes, and importantly on what timescales.

    [Response: Thanks for stopping by! I should have linked to your (excellent) News and Views article, and will do so. –eric]

  15. 15
    t marvell says:

    I wish someone could answer the question posed in #8 above:
    “can anyone tell me how long it takes for water from the Gulf Stream that sinks in the Arctic to surface again in the Southern ocean?”

    [Response: It’s about 1000 years. This isn’t a precise number, because of course it’s not quite like a “river”, but on average this is about the time it takes. The way we know this is rather neat: the radiocarbon (14-C) content of the atmosphere and ocean are pretty much in equilibrium over the North Atlantic region, so the radiocarbon “age” of the atmosphere and the surface ocean is about the same there: zero. Radiocarbon in the atmosphere is constantly being produced by cosmic rays, and is also decaying of course, with a half life of about 5700 years. The concentration in the atmosphere is roughly constant. But once the ocean water sinks, the carbon within is no longer in communication with the atmosphere, so no new radiocarbon is produced, yet it is still decaying. The amount of radiocarbon left (or, precisely, the ratio of radiocarbon to normal carbon-12) tells you how much time it has been since that water was last in contact with the atmosphere. The radiocarbon age of water upwelling off the Antarctic coast is about 1000 years. Neat, eh?]

    Note, however, that the time it takes the water itself to travel north-to-south is not the same as the time it takes for wave adjustment to occur, which is thought (and calculated) to be much faster. But of course, the transit time of the water itself may also be a factor, hence one might expect an “advective” timescale of 1000 years, rather than the 200 years we found. That’s why the 200 year result is so interesting. –eric]

  16. 16
    Hank Roberts says:

    Poking at those odd quotes in the Scripps press release asked about above

    a previously unknown salt-driven stabilization of the sinking of dense water in the North Atlantic

    That, I’d guess, may refer to discoveries made in the past 20 years rather than to something brand new.

    I’m guessing the interviewer took bits from a longer conversation with Severinghaus about past work other than the current paper and perhaps speculation, and edited those into text intended to grab headlines and attention for Scripps.

    I’m guessing, out of suspicion of the creative ability of press officers to craft “newsworthy” text to get their institution’s name into headlines.

    They often don’t show the scientist the final text.

  17. 17
    Chris Colose says:

    Hi Eric,

    Nice paper. We had a PhD student from another department give a talk here the other day arguing that sea ice could provide a fresh-water mechanism in a glacial-like climate that wouldn’t leave behind as much (any?) physical evidence as icebergs. The model results he showed were somewhat idealized and I think a bit too large for the discharge that you’d get, but how do you feel about the idea?

    [Response: I think all such ideas are on the table. Heaven knows we need fresh ideas on this, because actually, this part of the science isn’t settled! See for example RC’s own Ray Bradley’s paper on sea ice as an explanation for the YD, in QR, some years back. doi:10.1016/j.yqres.2008.03.002. Not many people cite this paper but I’ve yet to see a cogent critique of it. –eric]

  18. 18
    Mark A. York says:

    Thanks for this. I asked for it on the last thread but had no idea such research was pending at this level. My concern stemmed from an impromptu argument with author and naturalist Doug Peacock, who wished the WAIS and ROSS would collapse in 2015 via an earthquake so that it would get people’s attention. An odd wish for sure but in any even not likely. My survey of research returned 200 years at the very least. I reported back with this and he said, “think decades.” I suggested he come here to confer but no such luck. When you hold the answer you want, why bother. I have to go with the science over emotion. Everyone should.

    [Response: Note, however, that this work has little to say on WAIS collapse. Of course, it certainly does take hundreds (or thousands) of years for WAIS collapse to occur, even if it has already started as some think). –eric]

  19. 19
    Mark A. York says:

    Thanks, Eric!

  20. 20

    Forgive my off-topicness, but my 2nd article in a peer-reviewed science journal will appear this month:

    Levenson, B.P. 2015. Why Hart Found Narrow Ecospheres–A Minor Science Mystery Solved. Astrobiology 15, 327-330.

  21. 21
    Chuck Hughes says:

    What are the chances that the ice sheets follow a melting pattern that has never been seen before? Since human activity is to blame for our current situation and things are warming up much faster with CO2 levels skyrocketing at an unprecedented rate. Could we be in for some real surprises in regards to Antarctica and other areas, like maybe Greenland? Call it one of those Unknown, unknowns.

    I ask this because so many processes and timescales have been underestimated already. We have CO2 levels at 403ppm now. What happens when we hit 450ppm or 500ppm? I would think that would have to speed things up dramatically. Are we planning for that sort of scenario?


  22. 22

    Congratulations, Barton!

  23. 23
    Mike Flynn says:

    How hot was the Northern Hemisphere when the Antarctic supported a rich semi tropical flora and fauna?

    When is this likely to happen again? Will it get this hot in the Antarctic again?

  24. 24
    Hank Roberts says:

    Mike Flynn knows how to ask this misleading question; he’s been practicing:
    About 2,120 results for "Mike Flynn" antarctica subtropical

    The answer, he knows, but for any youngster first seeing the question:
    — before Antarctica drifted away from South Africa, South America, and Australia, it was warmer:

  25. 25

    #23–Dunno about the NH specifically, but on a related topic, when the Arctic (not, as in your question, the Antarctic*) was semitropical, the globe was ~5 C warmer:

    *That I don’t know about–what fossil record is retrievable from Antarctica, I wonder?

  26. 26

    #23, and my responding comment:

    Finding out stuff like this is one of the things I love about this site:

    When Robert Falcon Scott’s party was found dead in its tent on the way back from the South Pole, specimens of Glossopteris from the Beardmore Glacier region were found with them. While they had jettisoned almost everything else on the way back, they recognised the immense scientific importance of Glossopteris and refused to leave the specimens behind. These are some 280–300 million years old.


    And, more to the present point:

    Seymour Island, just off the Antarctic Peninsula, is one of the most important fossil sites on earth. It contains a bewildering array of different fossils, representing a great array of different environments, and also includes one of the very few uninterrupted sections across the Cretaceous/Tertiary boundary, the time when dinosaurs and many other life forms died out, probably due to meteorite or asteroid impact.

    Abundant remains of the southern beech, Nothofagus (N. beardmorensis) have been found at 86°S, only 400 km from the South Pole. The age is contentious but may be as young as 2–3 million years, suggesting that the modern environment of Antarctica, with the large icesheet, may be a more recent development than was thought a few years ago. The leaves occur abundantly in lake sediments which are in turn under- and over-lain by glacial sediments. The leaves represent an autumn leaf-fall and are very similar, though larger than, the Tasmanian species Nothofagus gunnii also known as the Deciduous Beech or Tanglefoot. Study of the wood suggests that the plant grew as a straggly shrub similar in growth form to the Arctic willow (Salix arctica).

    I don’t see anything that goes directly to Mike’s question, though. Obviously, the ice core record doesn’t go far enough back, so temperature reconstructions would have to rely on oxygen isotopes or some other proxy.

  27. 27
  28. 28
  29. 29
    Mike Flynn says:

    Hank Roberts,

    Just goes to show how many people named Mike Flynn are apparently not me.

    The Antarctic ice seems to be less than 1.5 million years old. Are you saying that Antarctica moved very rapidly from the sub tropics to its present location within a million years or so? Did the other continents move similarly vast distances at around the same time?

    [Response: You are mistaken. The oldest actual ice on Antarctica is perhaps 1.5 million years old, but ice has been there since the Oligocene some 30 million years ago, associated with the opening of the Drakes Passage and likely long term draw down of CO2. – gavin]

    It doesn’t appear likely to me, but you may have evidence to the contrary.

    My question stands. Have you any answers? For example, is it possible that Antarctica will return to the location you claim it came from?

  30. 30
    Chuck Hughes says:

    Yes! Congratulations Barton Paul Levenson. I look forward to reading your paper.

  31. 31
    Jeff L says:

    I haven’t been following RealClimate as actively as I used to, but this is the most interesting article I have read in a while.

  32. 32
    Hank Roberts says:

    Thank you Gavin.

    The Oligocene … extends from about 33.9 million to 23 million years before the present…. During this period, the continents continued to drift toward their present positions. Antarctica became more isolated and finally developed an ice cap.

  33. 33

    Thanks, Chuck! Unfortunately, it’s behind a paywall. But Eli has posted a brief summary at his blog, Rabett Run.

  34. 34

    Some thoughts came up…

    Wouldn’t it be possible to simulate past ice sheets and their behavior, based on GRACE data, i.e. weight induced geomorphological features which are driven by the amount of ice thickness-weight. You could take the current state measured of decadal time scale as a control then model-calculate past motions based on geomorphological features. Thus, if the control suggest a linear motion and then look how the landscape is distributed, factor in age, you might find something that way?

    The related science and with Grace — Particular: East Antarctic deglaciation and the link to global cooling during the Quaternary: evidence from glacial geomorphology and 10Be surface exposure dating of the Sør Rondane Mountains, Dronning Maud Land

    Reconstructing past variability of the Antarctic ice sheets is essential to understand their stability and to anticipate their contribution to sea level change as a result of future climate change. Recent studies have reported a significant decrease in thickness of the East Antarctic Ice Sheet (EAIS) during the last several million years. However, the geographical extent of this decrease and subsequent isostatic rebound remain uncertain. In this study, we reconstruct the magnitude and timing of ice sheet retreat at the Sør Rondane Mountains in Dronning Maud Land, East Antarctica, based on detailed geomorphological survey, cosmogenic exposure dating, and glacial isostatic adjustment modeling. Three distinct deglaciation phases are identified for this sector during the Quaternary, based on rock weathering and 10Be surface exposure data. We estimate that the ice sheet thinned by at least 500 m during the Pleistocene.

  35. 35
    pete says:

    I do not understand why Greenland first then the Antarctic.
    Could it not also be the other way round?

  36. 36
    Hank Roberts says:

    > pete … do not understand … could it
    > not also be the other way round?

    Which “it” and which “other way” could you be thinking of changing?

    … delay in the further southward propagation from the South Atlantic to the Antarctic across the Antarctic Circumpolar Current (ACC). …. varies with the strength of the ACC as imposed in the model.
    There are, in short, multiple parts of the ocean and atmosphere system involved, and these have different timescales….

    Which of the many parts of the model are you imagining doing what other way?

  37. 37
    Hank Roberts says:

    This may help pete — a thin seasonal skin of ice over water (Arctic) changes faster, than a miles-thick ice cap (Antarctic) during warming. Hard to imagine a way that “it” could also be “the other way round” without a rather different planet for “it” to happen on.


    Rises in surface air temperature (SAT) in response to increasing concentrations of greenhouse gases (GHGs) are expected to be amplified in northern high latitudes, with warming most pronounced over the Arctic Ocean owing to the loss of sea ice….
    … near-future simulations document a preconditioning phase of Arctic amplification, characterized by the initial retreat and thinning of sea ice, with imprints of low-frequency variability. Observations show these same basic features, but with SATs over the Arctic Ocean still largely constrained by the insulating effects of the ice cover and thermal inertia of the upper ocean. Given the general consistency with model projections, we are likely near the threshold when absorption of solar radiation during summer limits ice growth the following autumn and winter, initiating a feedback leading to a substantial increase in Arctic Ocean

  38. 38

    I do not understand why Greenland first then the Antarctic. Could it not also be the other way round?

    This might be the culprit:

    In today’s climate, the annually averaged surface air temperature in the Northern Hemisphere (NH) is 1°–2°C higher than in the Southern Hemisphere (SH). Historically, this interhemispheric temperature difference has been attributed to a number of factors, including seasonal differences in insolation, the larger area of (tropical) land in the NH, the particularities of the Antarctic in terms of albedo and temperature, and northward heat transport by ocean circulation.

    It is found that for the preindustrial climate the temperature difference is predominantly due to meridional heat transport in the oceans, with an additional contribution from the albedo differences between the polar regions. The combination of these factors (that are to some extent coupled) governs the evolution of the temperature difference over the past millennium. Since the beginning of industrialization the interhemispheric temperature difference has increased due to melting of sea ice and snow in the NH. Furthermore, the predicted higher rate of warming over land as compared to the oceans contributes to this increase. Simulations for the twenty-first century show that the interhemispheric temperature difference continues to grow for the highest greenhouse gas emission scenarios due to the land–ocean warming contrast and the strong loss of Arctic sea ice, whereas the decrease in overturning strength dominates for the more moderate scenarios.

  39. 39
    Steve Fish says:

    Re- Comment by pete — 9 May 2015 @ 7:42 PM, ~#35

    Antarctic ice is 61% of the earth’s fresh water and 71% of the earth’s freshwater ice.

    The northern hemisphere tends to warm (surface temperature) more than the southern hemisphere because there is much more land area in the north.

    Antarctica warms more slowly because of isolation by the circumpolar winds.

    Antarctica warms more slowly because of the Antarctic ozone hole. Ozone is a greenhouse gas.

    Much of the surface of Antarctica is at an altitude that supports year round ice, even in the tropics.


  40. 40
    Mal Adapted says:


    I do not understand why Greenland first then the Antarctic.
    Could it not also be the other way round?

    Multiple reasons, as others have suggested, all contingent on the Earth’s orbital mechanics and tectonic history. Here’s another contingency: the emergence of the Isthmus of Panama, starting about 15 million years ago, blocked the North Equatorial Current and created the Gulf Stream.

  41. 41

    #39–Sensible points, Steve.

    But one question: aren’t stratospheric GHGs cooling rather than warming? Negative lapse rate up there…

    SOD quotes Ramanathan on this:

    As we mentioned earlier, in our explanation of the greenhouse effect, OLR reduces (with an increase in CO2) because of the decrease in temperature with altitude.

    In the stratosphere, however, temperature increases with altitude and as a result the cooling to space is larger than the absorption from layers below. This is the fundamental reason for the CO2 induced cooling.

    [Response: This is not true (but in Ramanathan’s defense, I thought something similar for a long time). The counter example that shows this doesn’t work is the mesosphere – also cooling, but with no inversion and no ozone. The domniant reason is the spectral nature of the absorption. High CO2 in the troposphere reduces upwelling in the CO2 bands into the stratosphere, and so stratospheric CO2 absorbs less radiation from below and yet radiates increasingly effectively. Even with constant CO2 in the stratosphere you’d still cool if trop CO2 was increasing. – gavin]

    But that’s GHGs in the atmosphere generally, not specifically in the stratosphere. Earlier in the same post, we find:

    By contrast, the stratosphere is warmer at the top because of the effect of solar absorption by O2 and O3. If there was no absorption by O2 or O3 the stratosphere would be cooler at the top (as it would only be heated from underneath by the troposphere).

    Just about everyone has heard about ozone depletion in the stratosphere due to CFCs (and other chemicals). Less ozone must also cause cooling in the stratosphere… Less ozone means less ability to absorb solar radiation. If less energy is absorbed, then the equilibrium stratospheric temperature must be lower.

    Which does support your point.

    [Response: This is true. Strat ozone depletion cools the lower stratosphere and the surface. – gavin]

    This whole topic seems to be conceptually messy, with competing effects that need to be carefully specified–and the SOD article’s conclusion seems to be that not everything is disentangled yet–but then again, that piece is from 2010, so there could be more helpful work that’s come out since.

    SOD piece:

  42. 42
    Jim Steele says:

    The interpretation that Antarctic temperatures lag the Arctic by 200 years is not so obvious from your last graph. From -500 years to +250, Antarctic temperatures are steadily rising while the Arctic remains flat. The Arctic exhibits little response until year 0 when a D-O event occurs revealed by a rapid increase in Arctic temperatures. That suggests the Arctic lags the Antarctic. The rapid Arctic warming suggests that Arctic waters had been gradually warming but due to insulating ice cover the trend is not detected until there is a sudden loss of ice allowing the sudden ventilation of heat.

    [Response: I agree with you! This interpretation is indeed where I think the science is heading (see e.g. Dokken et al., 2013 (pdf, here). But that isn’t what our paper is about. What’s clear is that Antarctic temperatures do not start falling until ~200 years after the abrupt warming events in Greenland. As I said in the post, our paper shows that thunder (Antarctic cooling) closely follows lightning (Greenland abrupt warming). But that doesn’t tell us what leads to the lightning (Greenland abrupt warming) in the first place. As you point out, it happens only after centuries of Antarctic warming, but that doesn’t tell us the full mechanism. More work to do on this. –eric]

  43. 43

    Replying to Kevin McKinney’s comment (#41), in 2013 I got around to doing the radiative transfer calculations for myself (using the HITRAN database in a Matlab model) to explore the details of a number of cases – e.g., at what altitude the radiation to space takes place from (by wavelength).

    The results for the stratosphere under doubled CO2 are shown in Visualizing Atmospheric Radiation – Part Eleven – Stratospheric Cooling.

    (These calculations didn’t include the Voigt line shape but Pekka Pirilä did include it in a better version of the Matlab model and commented on many of the results shown in the series).

    Hopefully the detailed results show why it is that the stratosphere cools.

  44. 44
    steve Fish says:

    Re- Comment by Kevin McKinney — 11 May 2015 @ 8:21 AM, ~#41

    Your response to my “sensible points” greatly augments my latent ability to understand atmospheric heat.


  45. 45

    #44–I’m glad, Steve. I learn a lot from these conversations and (given that, like most of us, I don’t deal with these concepts in my daily life) find that it’s useful to revisit and review from time to time. Not to mention try to catch up a bit!

  46. 46
    Mike Flynn says:


    I believed Nature Reports Climate Change about the oldest ice on Antarctica being less than a million years old.

    I accept that they are mistaken. I thought their information was reliable. Sorry.

    [Response: Where does Nature Reports Climate Change — or an article therein — say this? I suspect this isn’t quite what it said. The oldest ice core bottoms out at about 850,000 years. But we know there is older ice than that; we just don’t know the exact age yet, but it is very likely to be older than 1 million. See e.g. Fischer et al., Climate of the Past, 2013. –eric

  47. 47
    Hank Roberts says:
    Timeline : Nature Reports Climate Change … Earth’s oldest ice: a history of polar ice-core research.

    The search for a million-year core
    2009 Spurred by International Polar Year, the hunt is on for the oldest ice. Researchers seek the holy grail of polar paleoclimatology — a frozen trace of the major transition 1 million years ago when Earth’s glacial cycles went into a stately 100,000-year rhythm, slowing down from a previous 40,000-year cycle. A Chinese team begins setting up a research station at Dome A, one promising site for drilling in the next several years. (See China builds inland Antarctic base)
    More than 40 years after the first deep core, important secrets still lie buried under ice, and drilling projects at the poles are expected to yield up new climatic clues well into the future.

  48. 48
    Hank Roberts says:

    P.S., opinions vary; a Mike Flynn at JC’s last week asserts that the Antarctic ice is 1.5 million years old, and based on that claims what he wants. Odd.

  49. 49
    Mike Flynn says:

    Hank Roberts,

    Thanks. I thought the RC team would have read the article.

    This is what I read –

    “The deepest ice in the Dome C core from EPICA — 3,270 metres down and aged at about 800,000 years — is analysed, yielding the oldest air samples yet. Modern-day greenhouse gas concentrations remain unparalleled through those many millennia, with CO2 levels now 28 per cent higher, and methane 134 per cent higher, than at any other point in the record. Abrupt climate changes punctuated previous glacial periods, much like the last one. (See Jouzel et al. Science 2007, Lüthi et al. Nature 2008, Loulergue et al. Nature 2008 and Ice cores reveal climate secrets)”

    Followed by the one you quoted, of course.

    I doubled the age of the oldest known ice, just in case someone wanted to quibble. Sorry for the exaggeration, but I didn’t think anyone would mind, because younger ice supports my contention better.

    You can see that Gavin said to me “You are mistaken. The oldest actual ice on Antarctica is perhaps 1.5 million years old, but ice has been there since the Oligocene some 30 million years ago, associated with the opening of the Drakes Passage and likely long term draw down of CO2. – gavin] – See more at:

    Maybe I should have said perhaps, rather than younger.

    Eric said “Where does Nature Reports Climate Change — or an article therein — say this? I suspect this isn’t quite what it said. The oldest ice core bottoms out at about 850,000 years. But we know there is older ice than that; we just don’t know the exact age yet, but it is very likely to be older than 1 million. See e.g. Fischer et al., Climate of the Past, 2013. –eric – See more at:

    I’m not sure who is right. Nature World News says that scientists are looking for 1.5 million year old ice, but I’m not sure if they have found any yet. You might know.

    It’s all a bit confusing, obviously, but I’m obviously not trying to spin things in my direction. It just seems that claims of very old ice haven’t been supported yet, so my original question about temperatures in Antarctica when it was ice free in the past remain. It has obviously cooled quite a lot since then. I’m just wondering why.

    [Response: You have raised several different rings here. First, of course we haven’t read ever single article out there, and in particular “newsy” articles like that tend not to be a priority. Second, no we have not yet found 1.5 Ma old ice. Third, there is very good reason to believe that ice that old is there; this isn’t guess work but serious glacier flow dynamics calculations — but there is still uncertainty. Fourth, as to why it has been cooling on averaged for the last 50 million years – that’s a geological question that a lot of people would love to know the answer to. It’s not that well pinned down, but it probably has to do with gradual sequestration of CO2 by the lithosphere. See e.g. this commentary by Bill Ruddiman: –eric]

  50. 50

    How long does it take Antarctica to notice the Northern Hemisphere is warming?

    For context, it must be pointed out that today’s warming is certainly about a century and less, and this topic scope are abrupt changes.

    For instance this study highlights ocean processes which melt today on decal scale:

    Totten Glacier, the primary outlet of the Aurora Subglacial Basin, has the largest thinning rate in East Antarctica1, 2. Thinning may be driven by enhanced basal melting due to ocean processes3, modulated by polynya activity4, 5. Warm modified Circumpolar Deep Water, which has been linked to glacier retreat in West Antarctica6, has been observed in summer and winter on the nearby continental shelf beneath 400 to 500 m of cool Antarctic Surface Water7, 8. Here we derive the bathymetry of the sea floor in the region from gravity9 and magnetics10 data as well as ice-thickness measurements11.

    We identify entrances to the ice-shelf cavity below depths of 400 to 500 m that could allow intrusions of warm water if the vertical structure of inflow is similar to nearby observations. Radar sounding reveals a previously unknown inland trough that connects the main ice-shelf cavity to the ocean. If thinning trends continue, a larger water body over the trough could potentially allow more warm water into the cavity, which may, eventually, lead to destabilization of the low-lying region between Totten Glacier and the similarly deep glacier flowing into the Reynolds Trough. We estimate that at least 3.5 m of eustatic sea level potential drains through Totten Glacier, so coastal processes in this area could have global consequences. Link with video

    From above

    In general, the Antarctic temperatures begin to decline when Greenland warms abruptly, and to increase when Greenland cools abruptly (Blunier et al., 1998). This relationship is often called the “bipolar seesaw” and is commonly attributed to the redistribution of heat between the Northern and Southern Hemispheres via changes in the Atlantic meridional overturning circulation (AMOC).

    Ok, this conclusion from Blunier is from 1998, but does it take into account the induced cooling of the SH, due to warm water intrusion, which leads to increase of basalt melt etc.(see melt pattern as outlined in above quote on Totten Glacier) and possible disintegration events, which cool the ocean surface layer, thus cool the air above too, further contributing to the reinforced Westerlies cooling.