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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).
WAIS_Divide_record

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.

WAIS_Divide_timing
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.

References:

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. http://dx.doi.org/10.1038/nature14330.

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. http://dx.doi.org/10.1038/29447.

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).
http://dx.doi.org/10.1038/ngeo362.

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. http://dx.doi.org/10.5194/cp-11-153-2015.

EPICA Community Members, “One-to-one coupling of glacial climate variability in Greenland and Antarctica”, Nature, vol. 444, pp. 195-198, 2006. http://dx.doi.org/10.1038/nature05301

Fyfe, J.C. and O. A. Saenko, 2005: Human-Induced Change in the Antarctic Circumpolar Current. J. Climate, 18, 3068–3073. http://dx.doi.org/10.1175/JCLI3447.1

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. http://dx.doi.org/10.1038/366552a0.

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. http://dx.doi.org/10.1038/nature11822.

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) http://dx.doi.org/10.1038/ngeo740.

NEEM Community Members, Eemian interglacial reconstructed from a Greenland folded ice core, Nature, vol. 493, pp. 489-494, 2013. http://dx.doi.org/10.1038/nature11789.

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. http://dx.doi.org/10.5194/cp-7-671-2011.

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. http://dx.doi.org/10.1002/2014GL061413.

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. http://dx.doi.org/10.1029/2001JD000954

Roe, G.H. and E.J. Steig. Characterization of Millennial-Scale Climate Variability. J. Climate, 17, 1929–1944. http://journals.ametsoc.org/doi/full/10.1175/1520-0442%282004%29017%3C1929%3ACOMCV%3E2.0.CO%3B2.

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. http://dx.doi.org/10.1038/ngeo2147.

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. http://dx.doi.org/10.1016/S0277-3791(02)00184-1.

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. http://dx.doi.org/10.1038/NGEO1778.

Steig, E.J., Climate change: The south–north connection, Nature, vol. 444, pp. 152-153, 2006. http://dx.doi.org/10.1038/444152a

Stocker, T.F. and S.J. Johnsen, A minimum thermodynamic model for the bipolar seesaw, Paleoceanography, vol. 18, pp. n/a-n/a, 2003. http://dx.doi.org/10.1029/2003PA000920.

Vettoretti, G. and W.R. Peltier (2015), Interhemispheric air temperature phase relationships in the nonlinear Dansgaard-Oeschger oscillation. Geophys. Res. Lett., 42: 1180–1189. http://dx.doi.org/10.1002/2014GL062898.

WAIS Divide Project Members. Onset of deglacial warming in West Antarctica driven by local orbital forcing. Nature, 500: 440-444, 2013. http://dx.doi.org/10.1038/nature12376.

WAIS Divide Project Members. Precise interpolar phasing of abrupt climate change during the last ice age. Nature http://dx.doi.org/10.1038/nature14401.

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

  1. 51

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

    So my point is, that the characteristic is different, even when Antarctica cools, it is effectively melting (during a period of warming in the Arctic).

  2. 52
    Hank Roberts says:

    > I’m not sure if they have found any yet.

    doi: 10.1073/pnas.1420232112
    Atmospheric composition 1 million years ago from blue ice in the Allan Hills, Antarctica

    This is not a comprehensive answer to your question; it’s the first hit I got from a 5 second ‘oogle scholar search.

    YMMV.

    [Response: Thanks for that! I knew about that paper, but I could not find it, so I wondered if it actually existed. –eric]

  3. 53
    Hank Roberts says:

    This search:
    https://scholar.google.com/scholar?as_ylo=2015&q=arctic+drilling+age+ice&hl=en&as_sdt=0,5

  4. 54
    Brian Dodge says:

    Does the Acc/Antarctic circumpolar vortex system change strength with the West Antarctic d18O temperature proxy? I read a paper where they modeled the Atlantic as a slab, with an inlet(source; Straits of Magellan) and outlet(sink) notch representing ACC flow, and showed that the resulting Eckmann transport would cause AMOC, which tropical evaporation & salt density increases, followed by North Atlantic cooling, would enhance. it seems to me that the AMOC would be more properly characterized as Eckmann-halo-thermal circulation. I’m thinking(well, wondering, based on a lack of knowledge) if changes in the ACC concomitant with AIM events could drive changes in AMOC which would trigger rainfall/ENSO/snow cover/vegetation changes leading to abrupt northern temperature and global CH4 changes. I also wonder if normalized methane 14C ages were plotted with the same resolution across the transition in your second figure, if release from fossil tundra deposits would cause detectable nonlinearities. I.e., no nonlinearities would imply low fossil tundra realeases and support tropical CH4 sources.

    I also found this –
    http://onlinelibrary.wiley.com/doi/10.1029/2000PA000513/full
    Phase relationships between millennial-scale events 64,000–24,000 years ago; Nicholas J. Shackleton, Michael A. Hall, Edith Vincent December 2000; DOI: 10.1029/2000PA000513
    “A core recovered on the Iberian margin off southern Portugal can be correlated with Greenland ice cores using oxygen isotope variability in planktonic foraminifera which closely matches the ice core records of temperature over Greenland. Our age model identifies the base of every interstadial between 64,000 and 24,000 years ago and uses the Greenland Ice Core Project (GRIP) timescale. The oxygen isotope signal in benthic foraminifera (on this GRIP-based timescale) is quite different from the planktonic record and resembles the temperature record over Antarctica when this is synchronized with Greenland using the record of methane in the atmospheric air in the polar ice cores. We interpret the benthic record as indicating significant fluctuations in ice volume during millennial events, and we suggest that Antarctic temperature changed as a function of ice volume.”
    Maybe the trigger was a transition from seasonally ice covered N Atlantic(with Heinrich event icebergs in it, fresher surface water and snowier weather), to a perennially ice free state(with rain, more vegetation, stronger monsoons, and CH4 emissions) driven by strengthening AMOC; and with the rapid rate in the N. hemisphere being controlled by the freezing/thawing threshold. Passing the freeze/thaw threshold caused the rapid and ongoing collapse of the Larsen & Wilkins ice shelves, as well as Arctic sea ice, and the Glacial Lake Missoula outbreak floods.

  5. 55
    Mike Flynn says:

    eric,

    “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).”

    This, if true, may have at least some of the Antarctic ice free, at least 2 to 3 million years ago. It would seem to fit in with the oldest ice found to date being no older than this.

    It just seems interesting to me, because it doesn’t seem to fit very well with some things which are taken for granted. What do you think? Might the continent have been mainly ice free 2 or 3 million years ago?

    [Response: We know how much global ice there is from oxygen isotopes in deep sea sediment cores. Getting rid of all Antarctic ice 2-3 million years ago is not compatible with that evidence. Certainly, there may have been substantial deglaciation 2-3 million years ago, but not “mainly ice free”. As for the age of oldest ice, the age of the ice in Antarctica has very little to do with for how long Antarctica has had an ice cover. The ice flows, compresses, thins, and melts at the bottom (and gets dropped into the sea as icebergs). Think of a river, like the Amazon. The water in that river has not been sitting there for thousands of years, but the river has been there all that time. –eric]

  6. 56
    Hank Roberts says:

    http://onlinelibrary.wiley.com/doi/10.1002/grl.50797/abstract
    (model)

    But there’s new information coming: where the ice sheets have collapsed, that exposes the seabed — drilling into sediments is now easier to do and much is being learned about the paleo record there:

    http://www.sciencedirect.com/science/article/pii/S0921818114000642
    Global and Planetary Change
    Volume 118, July 2014, Pages 25–41

    Variability in Cenozoic sedimentation and paleo-water depths of the Weddell Sea basin related to pre-glacial and glacial conditions of Antarctica
    doi:10.1016/j.gloplacha.2014.03.010

  7. 57
    Mike Flynn says:

    The ice cap can be measured by counting its layers. It’s at least 800,000 years old.

    The Amazon, though, has no layers. Neither you or I know how old it is.

    Ah well, off to the bore hole with me. I think you know I’m right, (more or less), otherwise you wouldn’t throw the irrelevant analogy into the discussion.

    I’m still none the wiser, as far as reasonable assumptions are concerned.

    I’ll leave you alone, unless you have some better ideas. Sorry.

  8. 58
    Hank Roberts says:

    Same Mike Flynn as at JC’s who asserts that the Antarctic ice is 1.5 million years old? Facts vary?

  9. 59
  10. 60
    Marco says:

    Mike Flynn, the analogy is not irrelevant at all. This press release may help you understand that there is a limit to how old ice can get on Antarctica:
    https://www.egu.eu/news/77/the-oldest-ice-core-finding-a-15-million-year-record-of-earths-climate/

    Also, there is a rough idea of how old the Amazon is, thanks to “layers”:
    http://www.sciencedaily.com/releases/2009/07/090707155827.htm

  11. 61
    Rick Brown says:

    “The Amazon, though, has no layers. Neither you or I know how old it is”

    Late Miocene onset of the Amazon River and the Amazon deep-sea fan

    Layers, even.

  12. 62

    Mike Flynn said:
    ” I think you know I’m right”

    That confirms that it is the same Mike Flynn who drips with condescension on Curry’s blog.

    @WHUT

  13. 63
    Mike Flynn says:

    Part of a comment relating to my initial question –

    “The Antarctic ice seems to be less than 1.5 million years old.”

    Gavin’s response –

    “[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]”

    I apologised, and indicated the Nature source. I acknowledged it may be wrong. Eric then took me to task –

    “[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”

    I am now confused. He suspects I’m wrong, but uses his time to refer to something else, rather than read the article in question, and correct me if I misread it

    “[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: http://eesc.columbia.edu/courses/w4937/Readings/Ruddiman_2010_enigma.pdf. –eric]”

    At this time, I am starting to regret that I said that the Antarctic ice seems to be less than 1.5 million years old. Gavin says “perhaps 1.5 million years old”, which was my rough doubling of actual age established to date.

    At #55 Eric takes me to task again, I guess –

    “[Response: We know how much global ice there is from oxygen isotopes in deep sea sediment cores. Getting rid of all Antarctic ice 2-3 million years ago is not compatible with that evidence. Certainly, there may have been substantial deglaciation 2-3 million years ago, but not “mainly ice free”. As for the age of oldest ice, the age of the ice in Antarctica has very little to do with for how long Antarctica has had an ice cover. The ice flows, compresses, thins, and melts at the bottom (and gets dropped into the sea as icebergs). Think of a river, like the Amazon. The water in that river has not been sitting there for thousands of years, but the river has been there all that time. –eric]”

    I wasn’t aware that the ice cap itself “ flows, compresses, thins, and melts at the bottom (and gets dropped into the sea as icebergs).”. I thought that applied to glaciers, but that the kilometres thick ice caps were more or less immobile. I assumed that if they flowed, thinned, and melted, it would be difficult to make any reasonable assumptions about their age, as they would be continuously losing bottom layers. As far as I know, the ice cap temperature profile would seem to preclude this, in the main.

    Now Marco#60 suggests that I read a press release that explains why the oldest ice cannot be older than a certain period. Full circle. I have been trying to point out all along that it is certainly possible that the ice record on Antarctica may go back 1.5 million years (or maybe older, I don’t know).

    All I’ve got for my efforts to find out if I am right, is a good bollocking from all and sundry. So yes, Antarctica may have been covered with ice for longer than 1.5 million years. It may have some permanent ice for a much longer period. On the other hand, the plant remains found indicate that the climate was a lot warmer when there was extensive vegetation. How much is unknown, as much of the continent is overlain by kilometres of ice.

    I get criticised for pointing out that a river is not like an icecap. I suppose I could have said that the liquid, solid, and vapour phases of water behave differently, but I assumed that was common knowledge. People respond with sediment layers deposited by the Amazon. This can be done with glacial movements as well, but is quite inexact compared with counting layers in ice.

    As to WHT’s final comment, I’m not sure what accusing me of condescension achieves. I’m usually on the receiving end of condescending and patronising remarks, so maybe I inadvertently respond in kind.

    My interaction with scientists over the years has shown that the ones that know their stuff can explain their reasoning, and back it up with facts, or logic, and don’t seem to have any problem saying “I don’t know.” Some things are just unknown, and I cheerfully admit to lack of knowledge in many cases.

    Anyway, sorry if I have upset anyone. I know I’ll probably be accused of arrogance, ignorance, trolling, false humility, and many other things. All I wanted to know was –

    “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?”

    A simple “Nobody really knows” would have sufficed. I’m still none the wiser. I realise that my question was not completely on topic. Sorry.

    I’ll leave it go. Thanks.

  14. 64
    Hank Roberts says:

    http://www.nap.edu/catalog/12168/antarctica-a-keystone-in-a-changing-world

    Antarctica is the center from which all surrounding continental bodies separated millions of years ago. Antarctica: A Keystone in a Changing World, reinforces the importance of continual changes in the country’s history and the impact of these changes on global systems. The book also places emphasis on deciphering the climate records in ice cores, geologic cores, rock outcrops and those inferred from climate models. New technologies for the coming decades of geoscience data collection are also highlighted. Antarctica: A Keystone in a Changing World is a collection of papers that were presented by keynote speakers at the 10th International Symposium on Antarctic Earth Sciences.

    Not the latest information; a start at reading for those interested.

  15. 65
    Hank Roberts says:

    PS, far more info out there for anyone interested — the ocean drilling work is fascinating and there’s more coming out every time I look.
    http://iodp.org.au/wp-content/uploads/SWPacificDetailedReportv1-400.pdf

    … studies instead attributed the extreme warmth of the Antarctic region during Eocene times solely to high levels of radiative forcing related to elevated greenhouse gas concentrations. However this hypothesis itself has since been challenged because of conflicting paleotemperature estimates at key locations.

    These data are instead consistent with the Tasmanian Gateway Hypothesis invoking tectonically driven circulation changes influencing heat distribution, long-term shifts in the carbon cycle and Cenozoic climatic evolution. … Related to this, the evolution of the Southern Ocean gateways is also important in the context of the deep overturning circulation, with the long-term cooling observed in the δ18O benthic isotope records largely being a direct consequence of sea surface cooling in regions of active bottom water formation at high southern latitudes.