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Revealed: Secrets of Abrupt Climate Shifts

Filed under: — stefan @ 8 November 2006

This story is the dream of every science writer. It features some of the most dramatic and rapid climate shifts in Earth’s history, as well as tenacious scientists braving the hostile ice and snows of Greenland and Antarctica for years on end to bring home that most precious material: kilometre-long cores of ancient ice, dating back over a hundred thousand years. Back in their labs, these women and men spend many months of seclusion on high-precision measurements, finding ingenious ways to unravel the secrets of abrupt climate change. Quite a bit has already been written on the ice core feat (including Richard Alley’s commendable inside story “The Two Mile Time Machine”), and no doubt much more will be.

It was the early, pioneering ice coring efforts in Greenland in the 1980s and 90s that first revealed the abrupt climate shifts called “Dansgaard-Oeschger events” (or simply DO events), which have fascinated and vexed climatologists ever since. Temperatures in Greenland jumped up by more than 10 ºC within a few decades at the beginning of DO events, typically remaining warm for several centuries after. This happened over twenty times during the last great Ice Age, between about 100,000 and 10,000 years before present.

The latest results of the EPICA team (the European Project for Ice Coring in Antarctica) are published in Nature today (see also the News & Views by RealClimate member Eric Steig). Their data from the other pole, from the Antarctic ice sheet, bring us an important step closer to nailing down the mechanism of the mysterious abrupt climate jumps in Greenland and their reverberations around the world, which can be identified in places as diverse as Chinese caves, Caribbean seafloor sediments and many others. So what are the new data telling us?

These data connect the Antarctic ups and downs of climate to the much greater ones of Greenland. This is hard, as dating an ice core is a difficult art (no pun intended). If one makes an error of only 5% in determining the age of an ice layer, for 40,000-year-old ice that’s an error of 2,000 years. But to understand the mechanisms of climatic changes, one needs to know the sequence of events – for example, one needs to know whether a particular warming in Antarctica happens before, after, or at the same time as a warming in Greenland.

To get around this problem, Thomas Blunier and colleagues nearly ten years ago pioneered an ingenious method to synchronise the ice cores of Greenland and Antarctica by analysing changes in the amount of methane in air bubbles in the ice. Changes in methane are recorded at both poles, and they should occur almost exactly in step as gases are quickly mixed through the whole atmosphere. After the ice cores are synchronised by aligning the methane variations, the relative timing of Greenland and Antarctic temperature changes can be seen.

While Blunier and colleagues were originally able to connect only a handful of large climate events, the results published today take this method to a new level by applying it to the new, high-resolution Dronning Maud Land ice core. The new data confirm with unprecedented precision what Blunier found: Antarctica gradually warms while Greenland is cold. But as soon as Greenland temperatures jump up in a DO event, Antarctic temperatures start to fall (see graph). This happens for every DO event, and it is a peculiar and tell-tale pattern that is also found in model simulations of these events (see graph).

Figure: The top two panels show idealised model DO events on an arbitrary time axis (in years), highlighting the phase relationship between Greenland and Antarctic temperatures: when a DO event hits Greenland, Antarctica switches from warming trend to cooling trend. The bottom panels show the “real thing”, the noisy data from ice cores. Note the expanded scale for Antarctica in both cases. Time here runs from left to right – normal for regular folks, but somewhat unusual for the ice core experts (my apologies to these).

It is (at least in the model) a result of a big change in northward heat transport in the Atlantic. If the heat transport by the Atlantic thermohaline circulation suddenly increases for some reason (we’ll come to that), Greenland suddenly gets warm (an effect amplified by receding sea ice cover of the seas near Greenland) and Antarctica starts to cool. Changes in Antarctica are much smaller and more gradual, as it is far from the centre of action and the vast reservoir of ocean around it acts as a heat store. The basic physics is illustrated very nicely in a simple “toy model” developed by Thomas Stocker and Sigfus Johnsen.

There is still debate over what kind of ocean circulation change causes the change in heat transport. Some argue that the Atlantic thermohaline circulation switches on and off over the cycle of DO events, or that it oscillates in strength. Personally, I am rather fond of another idea: a latitude shift of oceanic convection. This is what happens in our model events pictured above: during cold phases in Greenland, oceanic convection only occurs in latitudes well south of Greenland, but during a DO event convection shifts into the Greenland-Norwegian seas and warm and saline Atlantic waters push northward. But I am biased, of course: my very first Nature paper (1994) as a young postdoc demonstrated in an idealised model the latitude-shift mechanism. Other oceanic mechanisms may also agree with the phasing found in the data. In any case, these data provide a good and hard constraint to test models of abrupt climate events.

But irrespective of the details: the new data from Antarctica clearly point to ocean heat transport changes as the explanation for the abrupt climate changes found in Greenland. We are thus not talking about changes primarily in global mean temperature (these are small in the model results shown above). We are talking about what I call a climate change of the second kind: a change in how heat is moved around the climate system.

As an analogy, think of your bath tub and the types of change to the water level you can get there. A change of the first kind would be a change in mean level, e.g. if you add water. A change of the second kind would be the changes you get by sloshing around the water in the tub.

There are very few possibilities to change the global mean temperature, a climate change of the first kind: you have to change the global heat budget, i.e. either the incoming solar radiation, the portion that is reflected (the Earth’s albedo), or the outgoing long-wave radiation (through the greenhouse effect). Temporarily, you can also store heat in the ocean or release it, but the scope for changes in global mean temperature through this mechanism is quite limited.

Changes of the second kind are due to changes in heat transport in the atmosphere or ocean, and these can occur very fast and cause large regional change. Think of your tub: if you want 10 cm higher water level at one end, you can achieve this by turning on the tap – but you can get there much faster by pushing some water over there with your hand, albeit temporarily and at the expense of the water level at the other end. That kind of “see-saw” (but with heat, not water) apparently happens during DO events, as the new data confirm.

The two kinds of climate change are sometimes confounded by non-experts – e.g., when it is claimed that DO events represent a much larger and more rapid climate change than anthropogenic global warming. This forgets that our best understanding of DO events suggests they are changes of the second kind. The same error is made by those who claim that the 1470-year cycle associated with the DO events could lead to an “unstoppable global warming”. A global warming of 3 or 5 ºC within a century, as we are likely causing in this century unless we change our ways, has so far not been documented in climate history.

One crucial point has been left unanswered thus far. If DO events are due to ocean circulation changes, what triggers these ocean circulation changes? Some have argued the ocean circulation may oscillate internally, needing no trigger to change. I am not convinced – the regularity of the underlying 1470-year cycle speaks against this, and especially the fact that sometimes no events occur for several cycles, but then the sequence is resumed with the same phase as if nothing happened. I’d put my money on some regularly varying external factor (perhaps the weak solar cycles, which by themselves cause only minor climate variations), which causes a critical oceanic threshold to be crossed and triggers events. Sometimes it doesn’t quite make the threshold (the system is noisy, after all), and that’s why some events are “missed” and it takes not 1,500, but 3,000 or 4,500 years for the next one to strike. But the field is wide open for other ideas – the cause of the 1470-year regularity is one mystery waiting to be solved.


Alley, R.B., 2002: The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future. Princeton University Press.

Blunier, T. and E. J. Brook, 2001: Timing of millennial-scale climate change in Antarctica and Greenland during the last glacial period. Science, 291, 109-112.

Blunier, T., J. Chappellaz, J. Schwander, A. Dällenbach, B. Stauffer, T. F. Stocker, D. Raynaud, J. Jouzel, H. B. Clausen, C. U. Hammer, and J. S. Johnsen, 1998: Asynchrony of Antarctic and Greenland climate climate change during the last glacial period. Nature, 394, 739-743.

Braun, H., M. Christl, S. Rahmstorf, A. Ganopolski, A. Mangini, C. Kubatzki, K. Roth, and B. Kromer, 2005: Solar forcing of abrupt glacial climate change in a coupled climate system model. Nature, 438, 208-211.

EPICA community members, 2006: One-to-one coupling of glacial climate variability in Greenland and Antarctica. Nature, 444, 195-198.

Ganopolski, A. and S. Rahmstorf, 2001: Rapid changes of glacial climate simulated in a coupled climate model. Nature, 409, 153-158.

Rahmstorf, S., 1994: Rapid climate transitions in a coupled ocean-atmosphere model. Nature, 372, 82-85.

Steig, E.J., 2006: Climate change: the south-north connection. Nature, 444, 152-153.

Stocker, T. F. and S. J. Johnsen, 2003: A minimum thermodynamic model for the bipolar seesaw. Paleoceanography, 18, art. no. 1087.

41 Responses to “Revealed: Secrets of Abrupt Climate Shifts”

  1. 1
    Jeffrey Davis says:

    That graph looks uncannily like an EKG.

  2. 2
    Nigel Williams says:

    I would imagine that +10C wouldnt do the Greenland icecap much good.

    Before I grab my life jacket, may I ask: Where are we in the 1470 year cycle today?

  3. 3
    gary says:

    The cycle has, or is getting ready to start now. Bond et al. 1997 Last one was dated 1410 years ago.

    [Response: There is no evidence of this “cycle’s” existence in the Holocene (last 11,000 years). What Bond showed is that there are quasi-cycles that Bond argued have the same cause as the ones during the glacial period. Personally, I’m very skeptical about this, but it is just a matter of conjecture on both parts. Unfortunately Gerard Bond died last year so he can’t defend himself. Certainly he had a more nuanced view than “the cycle is starting now”! –eric

  4. 4
    Nigel Williams says:

    Oh darn!

  5. 5
    Grant says:

    How confident is the number 1470? What’s the source of that figure, and the error range on that? I was looking at the GISP data, and the periodic behavior is undeniably present. But using GISP temperature from 12000 yBP to 50000 yBP, I get a “period” of 1461 +/- 4 yr. A minor point, admittedly — but I’m curious.

    [Response:Stefan wrote a very nice paper on this some years ago, showing that the +/- is remarkably good, much better than one expects from “noise”. However, there are serious questions about this, because this nearly pure “cycle” only shows up in GISP2. It is not demosntrably so precise in any other record. It is not a minor point, because we don’t expect such perfect cycles in sloppy geophysical systems like oceans. We’ll say more on this soon, I’m sure.–eric]

    [Response: I’m currently in Nairobi so a bit slow to respond, but the paper Eric mentioned is here. Indeed, more on this will come out some time, I just haven’t had the time to write it up yet. -stefan

  6. 6
    David B. Benson says:

    What about the North Pacific? Did the coastal climate there roughly track the DO warming in Greenland?

  7. 7
    Hank Roberts says:

    Has any correlation turned up between mud/sediment cores and the ice cores? I wonder, given the recent news about the various ways plankton actively affect the oceans, including churning the upper 100 meters, if any of the cycles could reflect big changes in which species predominate over time.

    Grin. To me those lines were reminiscent of the patterns in ecosystems of predator-prey-parasite interaction.

  8. 8
    Peter Pesola says:

    Could we actually be in the beginning of a new D/O event?

    There is a warm sea surface anomaly that has been persistent in the area between Greenland & Newfoundland for the past few years (even in winter) that may be part of the cause for loss of ice on the margins in the south of Greenland.

    While the Southern Hemisphere & Antarctic have been cooling for the past 3 years (Spencer & Christy, 2006) which also coincides with the drop in solar irradiance since 2002.

    Is this a coincidence or the start of a trend related to a new D/O event?

    [Response: This is a cute idea but there is absolutely no evidence for it. Let’s not translate climate from ice cores — more than 12,000 years ago — direclty to the present, shall we. The recent cooling over Antarctica is well understood and most certainly has nothing to do with ocean circulation. See. e.g. here. Note that it is essentially meaningless to talk about ‘trends’ that are only 3 years long, because the natural year-to-year variabilty, especially in Antartica, is simply too great.–eric]

    [Response: My answer to that is a clear “no”. There is no DO events in the Holocene – that’s an observational fact, if you look at the ice core data – the last one in my view is the end of the Younger Dryas. Since then we are in a permanently warm state, which already is like a permanent DO event. Convection is in the Nordic Seas in the Holocene, so if convection starting in the Nordic Seas is the mechanism of a DO event as our theory has it, then you can’t get a DO event any more. -stefan

  9. 9
    Charles Muller says:

    Very interesting. About your conclusion: What could be the physical link between a small variation in solar cycles (causing no major climate change of the “first kind”) and a treshold-crossing in oceanic circulation? That is: how such a solar variation could imply a second kind change without noticeable effects on first kind change?

  10. 10
    Grant says:

    Re: Response to #5

    I think I found the article you’re referring to (Rahmstorf 2003, GRL 30, 1510). Stefan concludes that the periodic phenomenon responsible for D/O events has an internal “clock error” <= 7% of the period, or just about 100 yr. The original estimate of 1470 yr. for the period apparently comes from Grootes et al. (1993, Nature, 366, 552). I haven’t got a copy, but if anybody does and wants to report the results of their period analysis, please do!

    Of course the underlying clock “period” can be nailed down more precisely than the clock stability. My analysis based on the CLEANEST Fourier spectrum (Foster 1995, Astron. J. 109, 1889) indicates a period of 1461 +/- 4 yr. However, the error calculation is predicated on a null hypothesis which we know to be false; under such circumstances a better estimate of the error range is the FWHM in a Fourier transform. This gives an estimated period of 1461 +/- 27 yr. So, the underlying “driving force” has a very precisely determined period, but it may exhibit random “cycle-to-cycle” fluctuations contributing to some of the irregularity in the timing of D/O events in the GISP2 data.

    BTW, Stefan’s analysis of sources of the irregularity in event timings was, in my opinion, very nicely done. It reminds me of “O-C” analysis as is regularly applied to variable stars in astronomy, but Stefan treats the sources of variation correctly (as is, sadly, too often not the case in astronomy); my compliments.

  11. 11
    andrew worth says:

    I wonder if GH forcing can be a substitute for solar forcing in initiating a DO event (if it is caused by solar forcing).

  12. 12
    L. David Cooke says:

    RE: #8 Eric’s Comment

    Dr. Steig;

    I don’t know that that I would feel comfortable saying there is no evidence of warming entering the area. There does appear to be a warming in the NA at the Labrador Sea interface.

    Looking at the Rossby Wave effect on the Jet Stream it appears to be affecting the Bermuda High whaich has moved from the normal 60 Deg. W to around 30 Deg. W which would be sufficient for an extension of warmer tropical waters moving towards this region and appears to be causing the higher temperature subtropical SSTs to pool in the this area.

    (Based on the 2005 data there had appeared to be a notherly wind reducing the ice pack in the Bering Straits and now we appear to have the opposite effect this Fall in both the Norwegian Sea and Labrador Sea areas.)

    When looking at the ice pack data this Fall the extension of the Arctic ice pack towards the Bering Strait seems greater and the icepack build up aroung the islands at the western edge of the Labrador sea is not as great.

    To be certain I still need to get a link to insure that there is no chance of a Gulf Stream Meander or ring from setting up shop near by as well. In the meantime, here are a few links that seem to support this possibility:

    Dave Cooke

    [Response: See Stefan’s repsonse following mine, above. Neither of us are saying there is no evidence of warming in the North Atlantic. We’re saying that jumping to the conclusion that we’re seeing the beginning of a DO event is pure conjecture, and contradicts pretty much everything we know about such events in the past. –eric]

  13. 13
    Nereo Preto says:

    I know about a physical phenomenon called “stochastic resonance”: Some noisy systems may greatly amplify weak signals, expecially if thresholds are involved.

    May this be the case? Some unknown natural forcing exists with a period of 1470 yrs, and the climate system is subject to stochastic resonance during glacials. During interglacials, it is not as sensitive as in glacials, or thresholds are shifted to other values, which are no more critical (I know all this is very generic, but I’m not phisicist or a climatologist, sorry!).

    A good understanding of this may be very important: does global warming keep us out of danger (of D-O cycles: I’m not suggesting that global warming is good!), or it makes the whole system more sensitive? We don’t know, of course. But I’d like to know if studies are ongoing on this topic.

    This was a great post. As usual, but this time the argument is among my favourites!

    [Response: Well, good idea! Our papers on DO events triggered by stochastic resonance are here and here. -stefan

    [Response: On the other hand please see our paper showing where the stochastic resonance idea may be wrong. Don’t get me wrong here; I think the papers Stefan cites are important, and largely correct, but the statistics supporting “stochastic resonance” as a good model are simply not convincing, at least to me.– eric]

  14. 14
    Brad Arnold says:

    To reiterate, according to the above article there are two types of warming, one of heat distribution, and the other of total heat accumulation.

    The comment I would like to make is that an abrupt climate change that affects global heat distribution would also affect regional deforestation, devegetation, precipitation, and desertification. In other words, abrupt climate change would increase rapid climate change.

    For instance, changes in ocean convection over the north Atlantic would affect the jet stream, which would have a dramatic effect upon regional precipitation, probably leading to acute drought in the northern hemisphere, which will weaken and push the northern jet stream toward the Arctic.

    The point I am trying to make is “when it is claimed that DO events represent a much larger and more rapid climate change than anthropogenic global warming,” perhaps DO events do cause rapid regional climate change larger and more rapid than anthropogenic global warming generally.

    In other words, a DO event (brought on this time by anthropogenic global warming) should be seen as larger and more rapid climate change than anthropogenic global warming.

    I think that temperature extremes brought on by global warming lend themselves more to rapid climate change than higher average temperature. The extremes are more a matter of regional heat distribution than global heat accumulation.

  15. 15
    Paul Biggs says:

    A 1470 year solar cycle? Sounds like a plug for Singer’s new book.

    [Response: The 1470 year solar cycle is purely conjecture. There is some evidence for millennial-scale variability in the sun, but it is not very strong evidence, and it is not periodic. –eric ]

  16. 16
    pete best says:

    Great article RC, just goes to show how complicated the climate is and why this site is necessary for all of us interested quasi scientific types. Fantastic stuff, better than buying any book (and I have bought a few of em).

  17. 17
    jhm says:

    As impressive as it is, how sure are we that our growing understanding of the climate isn’t on too small a time-scale? Having read the recent NYT article about evidence on a paleontological time scale makes me think that we might get close to underastanding climate until panama falls into the sea, or some more likely, but equally momentous thing happens.

    [Response:That NYT article you quote is extremely misleading, and in some places simply wrong. We’re putting up a post on that today. -eric]

  18. 18
    L. David Cooke says:

    RE: #12

    Dr. Steig;

    Here is a great link for looking at the SSTs near the Labrador Sea interface and it is clear that there is no indication of a GS deviation, meander, eddie or “ring” in the area.*/217/*/19

    I suspect that the possible warmer temper intrusion above the NA into the Arctic appears related to the surface fetch and pooling related to the anti-cyclonic zone near 30 Deg. W (“Bermuda High”). Now if you back up and go look at Greenland Sea – Norwegian Sea interface you can see a clear intrusion between Svalbard Is. and the east coast of Greenland here:*/215/*/19

    Whether this is a abnormally high intrusion of warmer temperatures remains an opportunity for additional study. As to whether this is due to winds or GS also remains. (I suspect it is purely due to the force behind the GS as the increased Arctic Easterlies should push most of the warmth westward. However, with the “Bermuda High” east and slightly north it may be sufficient to drive the Gulf Stream with enough force to intrude deeper into the Arctic.)

    The important point is that this really is not news, anyone with a PC and the interest should be able to see this data. The tough part would be to define how or why this is occurring. So far this seems to be eluding us all… (Please note that during most of the years in which there appeared to be significant support for GW, that the Bermuda High appeared in it’s “Classic” position of approximately 60 Deg. W and 30-35 Deg. N during the peak of summer, and at times would retrograde towards 25 Deg, N and 75 Deg. W.) Is it possible the Rossby Wave effect on the Jet Stream is the driving force behind the NAO and PDO? If so what drives the deviation of the Jet Stream?

    Dave Cooke

  19. 19


    In addition to Alley’s Two Mile Time Machine, another excellent book that describes the Greenland ice cores and what they might mean in terms of abrupt climate change is
    Climate Crash: Abrupt Climate Change and What It Means for Our Future
    by John D. Cox. Cox is a science journalist, albeit a good one, and the book is written at about the Scientific American level. Perfect for someone with a technical background coming to this topic for the first time. I’ve read it twice in succession because I find it so helpful for understanding and supplying context for the articles here in RealClimate.

  20. 20
    JimR says:

    If the theory is that DO events are driven by some “regularly varying external factor” why would these events not be seen during the Holocene? And if this is externally driven shouldn’t the driver of DO events still be in play today?

    [Response: In our model they only occur under glacial conditions, not under Holocene conditions. The reason is that the Holocene has a different pattern of ocean circulation with vigorous convection in the Greenland-Norwegian Seas. It’s already in something like a permanent DO event state, the basin is well-flushed, and small variations in freshwater input, which accumulate under glacial conditions, cannot accumulate. Hence we don’t get events there from the same forcing that triggers events in the glacial, when the circulation is close to a threshold so a small forcing can trigger big changes. -stefan]

  21. 21
    Jim Cross says:

    At the risk of being one of the non-experts confounding first and second kinds of climate change, I assume it is possible both could happen at once. Using the bathtub analogy, we could be running more water into the tub at the same time we are sloshing the water around.

    Is there any evidence that something like this might be involved in the transitions between glacial and interglacial periods? In other words, there is general warming occurring at the same time ocean circulation changes to accelerate the general warming. Or, a general cooling accompanied by a change in flow in the opposite direction?

    [Response: Absolutely. During the transition from glacial to interglacial climate, the evidence is that the MOC became more vigorous. So in Greenland it got warmer both because of higher CO2, more sunlight at high latitudes during summer, AND because of increased poleward heat flow. It is still debatable just how much the increased poleward heat flow really increased, but it almost certainly did change.–eric]

  22. 22
    Lynn Vincentnathan says:

    I brought up GW to my anthro students, and one claimed she read in SCIENCE recently that the Antarctic is cooling, thus disproving GW. The other claimed her father in Utah (name??) is the leading GHG scientist, and he says GW is not happening.
    Are they tapped into some reality that most others are missing?

    [Response:[Some alternate reality perhaps, but not this planet. Antarctica cooled in the 1980s to 2000. But it was warming from the 1960s to the 1980s. If you look at the overall trends 1957-2005, for most of the continent, it didn’t change at all (on average). Oh, and the Antarctic Peninsula is the fastest warming place on the planet, and has been for some 50 years or more. None of this either proves nor disproves anything about global warming. We’ve had several posts on this, beginning with the very first week of RealClimate. Have your students read these articles here and here then come back to you. As for the famous Utah professor, um.. not anyone we know of. — eric

  23. 23
    Brad Arnold says:

    I would like to elaborate on my previous posting (i.e. abrupt climate change vs. rapid climate change, and heat distribution vs. total heat accumulation).

    Dr Lovelock is known for a unifying theory of earth’s eco-systems. In other words, the gestalt of earth’s eco-systems. He came away from a visit to the Hadley Climate Centre with a belief that earth’s eco-system is going to crash sooner than was conventionally believed by other scientists.

    This is directly relevant to abrupt climate change, because heat distribution unbalancing eco-systems means earth will soon catch a “fever” (I apologize for the anthropomorphic imagery). In other words, the total heat accumulation won’t abruptly rise (which is impossible per the above article), but abrupt climate change will affect heat distribution, which will unbalance eco-systems, which will led to rapid global warming (i.e. rapid heat accumulation).

    When it is written that within a decade or two abrupt climate change will occur and the earth will switch from the mild climate of the last ten thousand years (the Holocene), to a hotter dryer climate that has been responsible for mass extinctions in the past, what is meant is not that total heat accumulation will instantaneously jump.

    Instead, what is meant is that heat distribution will change, consequencially leading to a higher rate of total heat accumulation. Abrupt climate change shouldn’t be dismissed as regional, and not be implicitly tied to an abrupt rise in total heat accumulation, but instead should be seen as a big step toward an increased rate of total heat accumulation (i.e. a fever).

    Let me go farther: the problem with many global climate models is they don’t properly evaluate events of imbalanced heat distribution. Under the right circumstances glaciers melt abruptly, regional forests and soil become carbon emitters abruptly, and oceanic/atmospheric currents shift abruptly, leading to rapid global warming.

    In my opinion, these events only work one way (i.e. to make global warming more rapid). In other words, we may warm up quicker than consensis scientific opinion predicts, but we are going to cool down much slower after the “fever” peaks.

  24. 24
    susan maclove says:

    Changes in the atmosphere, ie; increased moisture levels at higher elevations, are contributing in driving the climate change, so I believe the ocean models need to include atmospheric data and how those combine to drive wind currents.

    Additionally, the solar energy that is now being absorbed rather than reflected in the Artic must be a major contributor to weather patterns.

    The composition of the troposphere has been altered through manmade contributions, such as: exhaust from combustion engines accumulating over a more than a century, and the destruction of forests (which act as sinks and replenish oxygen, as well as keep surface temps cooler..)

    The comparison of ice cores and postulating effects of gases and temperatures appears intriguing, but one must admit that what is occurring NOW is a first in a Global lifetime.

    Just explain how anything in the past history of the Earth can compare to the changes that have occurred in the last century…

    [Response: You are right on your first point – the models we use of course are coupled climate models, not ocean models, so they account also for changes in the atmosphere. Also, we do not use the past as an analogue, comparing the present with ice core data to draw conclusions. Rather, we build a physical understanding of past changes, and assume that the laws of physics stay the same. -stefan]

  25. 25
    Joseph O'Sullivan says:

    This was a very informative post. I understand Dansgaard-Oeschger events better now.

    There is a reuters article about climate change and the climate talks in Nairobi. It quotes Stefen, mostly about climate change and the oceans.

    [Response: The webcast of my talk in Nairobi is here:
    Look for the WBGU event on 9 November. Also contains a link to the WBGU report on the oceans. -stefan]

  26. 26
    Wacki says:

    Good post stefan. Just curious, when you say:

    It’s already in something like a permanent DO event state, the basin is well-flushed, and small variations in freshwater input, which accumulate under glacial conditions, cannot accumulate.

    Can you clarify this? I am under the impression that freshwater only affects the thermohaline belt if a massive amount of water is released at once. For instance, a giant lake on top of a glacier in canada bursts free and changes the salinity of the water. You seem to imply there are other ways freshwater can affect the circulation.

    [Response: You’re talking of cold events that are a response to massive freshwater influx to the Atlantic and a subsequent shut-down of deep water formation. That holds for Heinrich events (surges of continental ice into the Atlantic) or for meltwater influx (e.g., the 8k event). Now this post was on DO events, and these are warm events. There are no signs of massive forcing of these events, and my feeling (based on our model runs) is that they don’t require that, because the ocean was very close to a threshold, so some subtle changes to the freshwater budget were all it took to trigger DO events. -stefan]
    Changes in Antarctica are much smaller and more gradual, as it is far from the centre of action and the vast reservoir of ocean around it acts as a heat store

    Is it just the sheer amount of water that protects Antarctica? Or does the circumpolar circulation act as an “water door” much like commercial “air doors” operate?

  27. 27
    alex says:

    I vaguely rember a Carl Winsch talk where he noted that if you take two signals with 365 and 365.25 day periods, the resulting beat frequency would have a 1460year period! – someone analysing ice cores mucking up the number of days in a year :)!

  28. 28
    Hank Roberts says:

    “To prevent this drift between the calendar year and the astronomical (seasonal) year, we add one extra day every four years. Thus, over the four year period, we have 1461 days, not 1460, for an average of 365.25 days per year. …. This innovation was imposed in the year 709 AUC (ab urbe condita, after the founding of the city), when Julius Caesar regulated the calendar. …”

  29. 29
    Grant says:

    Re: #27

    Good one!

    But if we really want to apply this theory, maybe we should use the difference between the “calendar year” (365.2425d in the Gregorian calendar) and the actual tropical year (365.2422d). This gives us a “beat period” of 1,217,475 years.

    Which of course proves that Nostradamus was right. ;)

  30. 30
    Anders Lundqvist says:

    I am not a scientist by occupation, so you can stop reading here if you are only interested in comments by scientists.

    What strikes me in most discussions regarding rapid climate change is that everyone seems to be looking for “causes” and “effects”. If X happens, the Y and Z will follow. However, from what I have read about chaotic systems it would seem that it is futile to search for simple causes and effects. A chaotic system simply moves in a trajectory in its phase space and it is impossible (impossible not because of lack of data, but because it is – well – impossible) to forecast precisely where it will be at a given moment in the future. If we forecast that the system will be at point A and it in fact turns up at point B, we can not conclude that something specific has “happened” to the system. Well, we can do it, by all means, but it will be a quite arbitrary conclusion. In the case of the Earth’s climate, there are obviously billions of things “happening” all the time, and it is the sum of all these events that brings the system to a certain state at a certain point in time. We will effectively never find out what the human impact on the climate system actually is – whether we are heating the planet or cooling it or having no impact whatsoever. The only sensible thing to do, therefore, is to minimize our ecological footprint. Then, when disaster finally strikes, it will at least not be our fault!

  31. 31
    Brad says:

    I have a curiosity. According to the EPICA press release:

    “the European Project for Ice Coring in Antarctica (EPICA) established a precise link between climate records from Greenland and Antarctica using data on global changes in methane concentrations derived from trapped air bubbles in the ice.”

    while your graph notes oxygen.

    I’m curious about how one gets temperature from methane or oxygen concentration.

    [Response: The methane in the bubbles in the ice is used to line up the records on the same time scale (since changes in methane are recorded in both cores at pretty much the same time). The proxy used for temperature is the isotopic composition of the ice (the ratio of 18O to 16O in the H2O (ice)) which is measured independently. The linking of the time scales through methane then lets you see the leads and lags of the system and in particular demonstrates that there is a partial see saw in the temperatures at both poles. This would be impossible to derive without the matching because the other ways of dating the cores this far back have enough uncertainty that you wouldn’t be able to tell whether the South was leading or following or behaving exactly synchronously. – gavin]

  32. 32
    Grant says:

    Re: #31

    I’m no expert, so please correct me if I’m wrong.

    They used methane concentrations to synchronize the ice core records from Greenland and Antarctica.

    But it’s oxygen that gives us clues about temperature. Most oxygen is “normal” 16O, but a small fraction is the heavier isotope 18O. The fraction of 18O in the ice is an indicator of temperature.

  33. 33
    Joseph O'Sullivan says:

    The Pew Trust has some interesting papers regarding the climate talks in Nairobi, mostly on the regulatory side of things.

  34. 34
    Matt says:

    OK, I will guess about the 1470 year cycle. Some portion of the earth mantle or core is rotating at that speed and we should ecpect to see 1470 year cycles in volcanic eruptions.

  35. 35
    Coby says:

    Re 30, Anders,

    Why do you characterize the climate system as chaotic? It may have some chaotic behaviours on some time scales, but I think it might be best described as broadly deterministic with some chaotic aspects.

  36. 36

    Re ” We will effectively never find out what the human impact on the climate system actually is – whether we are heating the planet or cooling it or having no impact whatsoever.”

    This is just plain wrong. We know what effect humans are having. We may not understand all the details yet, but there’s no question that a) the world is warming, and b) human activity is most of the cause. You can’t just babble about chaos theory and assume it somehow overturns thousands of empirical observations. It doesn’t.

  37. 37

    Re “OK, I will guess about the 1470 year cycle. Some portion of the earth mantle or core is rotating at that speed and we should ecpect to see 1470 year cycles in volcanic eruptions.”

    Probably not. They recently found out the core rotates slightly quicker than the rest of the planet, but pretty much every layer of the Earth rotates once every 24 hours, give or take an hour.

  38. 38
    Joseph O'Sullivan says:

    Pew has come up with a summary of the climate talks at Nairobi. Its a good description of whats happening in the international policy and action arenas.

  39. 39
    Jim Cleland says:

    Collapse as Adaptation to Abrupt Climate Change in Ancient West Asia and the Eastern Mediterranean
    Harvey Weiss

    Although Holocene climate events are relatively minor on a glacial/interglacial perspective, the small Holocene changes in the polar vortex and atmospheric storminess documented by O’Brien et al. (1995) would probably cause widespread disruption to human society if they were to occur in the future (Keigwin and Boyle 2000:1343).”

    There is evidence to suggest that strengthening and more ‘stable’ polar vortices can cause rapid climate change.

    Proton storms can cause substantial loss of ozone.

    Our recent very large proton storm, occuring during a solar minima shows that they do not require maxima to occur.

    Stronger vortices convect warmth and moisture poleward.

    As the upper-latitudes warm, the danger of glacial collapse increases. And, as Dr. Alley notes, collapse can be almost ‘instantaneous’.

    A very quick Heinrich Event.

    Rapid glacial collapse results in substantial glacial rebound.

    Which may the reason increased volcanism seems to correlate with climate-change.

    And it may discharge the Clathrate Gun.

    The �increased storminess� noted by Dr. Weiss may be the �amazing� snowstorm that froze Dr. Thompson�s little Andean plants during the 5200 YBP event.

    �In 2002, Thompsons team made a surprising find along the margin of the Quelccaya ice cap a remarkably preserved wetland plant that had been remarkably preserved under the ice. Later testing yielded viable DNA from the plant and dated it back 5,200 years ago.

    This is a soft-bodied plant, he said. It had to be captured by a very large snowfall at the time, a snowfall and climate change that began very abruptly fast enough to capture a plant but not kill it. That is astounding.�

    Thus, I believe it possible that there is a 1470-year solar cycle (perhaps 7 of the provable 210-year cycles �overlaid� over some other cycle) that can force rapid change.

  40. 40
    William Astley says:

    Re: GCR role in Abrupt Climate Change & the Polar See-Saw

    The following are excerpts from Svensmark’s Dec. 14, 2006 Paper, see attached link to the paper for details, that provides an explanation for the polar see-saw.

    What are your thoughts concerning the data provided in the paper and the hypothesis?

    “Borehole temperatures in the ice sheets spanning the last 6000 years show Antarctica repeatedly warming when Greenland cooled, and vice versa… The phenomena has been called the polar see-saw… Attempts to account for it have included the hypothesis of a south-flowing warm ocean current with a built in time lag… There is (however) no significant delay in the Anarctica climate anomaly…

    While mechanisms involving ocean currents might help to intensify or reverse the effect of climate changes, they are to slow to explain the almost instantaneous operation of the Antarctic climate anomaly…”

    Svensmark’s explanation for the polar see-saw is GCR effects on cloud formation. He provides calculatons and data to support the hypothesis that an increase in low level clouds above 75 degree South, can produce warming. This is opposite to lower latitudes, where an increase in low level clouds, results in cooling.

    “In polar regions the clouds can have a warming effect if their re-radiation of long-wave radiation downward dominates over the loss of short wave solar energy blocked by the clouds.”

  41. 41
    Jim Cleland says:

    I’m clearly not articlulating well.

    My good friends need to, argued, now begin to speculate that current truncation is the result – and not the cause – of rapid change.

    The climatic record is replete with ‘spikes’ – sudden warmings followed by sudden coolings.

    To attribute this to ocean currents is to try to ‘drive a round peg into a square hole’.

    We are clearly seeing a ‘spike’ in upper-latitude tropospheric temperatures.

    And a drop in stratospheric temperatures.

    Stefan needs to talk to Lonnie.


    And then both need to ‘make a phone call’ to Cambridge.