Dansgaard-Oeschger events

Yesterday’s post had more impact than I was expecting. I probably should avoid calling other people jerks, even if I imply that I am one by doing so. I should also try and stick more to my resolution of not talking about hockey sticks. It is quite interesting and I’ve certainly learned a lot, but it does have a nasty habit of not being particularly constructive, nor particularly pleasant (although, it actually turned out reasonably well – so far, at least). To get back onto a more sensible track, I though I might quickly post something about a recent talk I attended. I should stress that this is not a topic about which I’m particularly familiar, so some of my explanations and terminology may be wrong. I’m also doing this from memory, so some of that might also be wrong.

The talk was about Dansgaard-Oeschger events. These are rapid warming (and then cooling) events that are seen in the Greenland ice core data. They occur during the glacial periods, and what makes them interesting is that they are thought to be internally forced (i.e., they are events that are not triggered by some external influence). The figure below shows a series of such events in the Greenland ice core data; although you should bear in mind that the x-axis is in years before present, so it shows rapid warming events followed by slower cooling.
1024px-Grip-ngrip-do18-closeup
The main point of the talk I went to was to try to understand what was happening in the Antarctic at the same time as Greenland appeared to be undergoing these rapid warming events. This was done using the West Antarctic Ice Sheet (WAIS) Divide Ice Core, a new ice core that is expected to rival the Greenland Ice core in terms of depth and resolution.

As I said, this is mostly from memory (although I did find this RealClimate post that discusses some of this). My understanding is that one of the debates is about whether or not the warming signal from the Northern Hemisphere travels to the Southern Hemisphere via the atmosphere (fast) or via the oceans (slow). From this new WAIS ice core, they were able to identify a signal in the Antarctic that lagged the Greenland warming by about 200 years and that showed cooling. My understanding is that this is consistent with these events being internally forced, since if the energy in an ocean current is being used to warm the northern hemisphere, then that should produce cooling elsewhere (in the Southern hemisphere). However, these changes can be quite slow and so there is a 200 year lag between the Northern and Southern hemisphere.

Maybe the more interesting thing about the talk was that even though the time delay appeared to be suggestive of an important ocean influence, one would still expect some fast atmospheric response. If memory serves me right, the idea was that a warmer northern hemisphere should move some of the climate zones in the southern hemisphere northwards. The consequence of this is that precipitation in the WAIS would come from a more northern – and hence warmer – part of the Pacific (I think). This was then found produce some signature in another isotope (which I can’t quite remember), and so the analysis of the WAIS Divide ice core produced a signature of this change in precipitation, that was coincident with the northern hemisphere warming.

So, if I understood the talk properly, they identified a cooling in the Antarctic that lagged the Greenland warming by about 200 years and indicates a primary role for ocean currents, and then found a signature of a change in precipitation coincident with the Greenland warming, indicating an effect from a change in the atmosphere. I found it quite fascinating what they could do with these ice cores. What I’m still unsure of is what this really means about how these events are actually triggered. That there is warming in the north followed by cooling in the south seems consistent with an internal process (although the reduction in ice sheets in the north should have some impact on albedo and hence overall warming), but it still wasn’t clear if there was a convincing explanation for the actual trigger.

The other thing I’m not sure of is what the relevance is for today. Showing that internal variability can produce rapid climate shifts is clearly interesting, but this was an era with very extensive ice sheets and the rapid shifts are probably partly associated with changes in albedo. It also doesn’t suddenly mean that what we’re experiencing now is likely internal variability, since basic physics tells us that it is mostly anthropogenic. There’s of course the value in understanding our climate and how it changes in more detail, and there may be a more direct relevance than I’m not quite getting. I’m also not sure if I’ve really explained this as properly, or as clearly as I could have, so if anyone knows better, feel free to point it out.

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14 Responses to Dansgaard-Oeschger events

  1. > they are thought to be internally forced

    I believe that Rahmstorf thinks they are externally forced: e.g. http://www.pik-potsdam.de/~stefan/Publications/Journals/rahmstorf_grl_2003.pdf (I neither endorse this nor not)

  2. William,
    Interesting. I did notice that there was some suggestion of them being externally forced, but – as far as I know – none of the external forcings have a period that matches. On the other hand, what Stefan says in his abstract seems sensible. It seems difficult to explain an internal cycle that could be so regular over such a long period. Admittedly, that isn’t a particularly strong argument if one can’t actually find something that does match.

  3. The externally forced feedback mechanism was my take on this too, a combination of fresh water from the continents and ice shelf collapse and sea ice feedbacks, then pumping into the hemispherical see-saw, since the Younger Dryas was more or less the mother of all Heinrich events, and Heinrich events were extreme examples of D-O events, which then transitioned into the modern final Heinrich events as the interglacial approached and more continental fresh water became available. Or vice versa. Or not. Who knows. It seems to be the right playground in which to sort out the various internal and external forcings and feedbacks from all the natural variability.

  4. captdallas2 0.8 +/- 0.2 says:

    Interesting post. Is there a power point or transcript available? Toggweiler, J.R. with the GFLD had estimated an ~150 year lag with THC circulation. 200 years is in the same ballpark, but a little longer than I would suspect. As for internal or external forcing, it would seem likely that initially it would be external, with ice sheet or ice dam collapse likely caused by tidal fluctuation and the lag of so many years with some likelihood of pseudo-oscillation during settling, would be internal and easily confused with internal or unforced variability.

    That was what I was trying to explain last time I was here, ocean energy imbalances between hemispheres take time to settle out.

  5. Eli Rabett says:

    Not that it really applies directly, but there are huge changes in Arctic albedo going on in the Spring/early summer when it counts.

  6. cartoonmick says:

    We can’t keep pumping all that carbon into the atmosphere without something eventually happening.

    We may disagree about exactly what will happen (heating Vs cooling), but for sure, something is going to happen.

    And if we don’t know exactly how bad this “something” is going to be, doesn’t it make sense to stop pumping all that carbon into our air?

    It’s a bit like kicking a dog. You know it’s going to do something in response to your unwelcome kicking, but you have absolutely no idea what it will be. So why do it.

    Politicians and those controlling the mega wealth are the only ones who can implement appropriate action, but they won’t. Their agendas and other interests lure them away from listening to advice from the experts.

    This cartoon looks at that aspect . . . .

    http://cartoonmick.wordpress.com/editorial-political/#jp-carousel-775

    Cheers
    Mick

  7. BBD says:

    ATTP

    The other thing I’m not sure of is what the relevance is for today.

    Essentially none. I don’t think anyone disputes that D-O events are an artefact of glacial climates only.

  8. BBD says:

    ATTP

    It seems difficult to explain an internal cycle that could be so regular over such a long period.

    Although the ‘binge-purge’ hypothesis does attempt this MacAyeal (1993) by invoking ice sheet dynamics (thickness increasing basal melt driving cyclical regional collapse etc).

  9. BBD,
    There was a question after the talk about the relevance to today, and I can’t quite remember the response. I think it was partly, as you say, that this was during the glacial periods when the ice sheets were much more extensive and hence the impact could be greater. Additionally, I guess, most of the energy doing the melting today is a consequence of increasing anthropogenic forcings. Then, I suspect that even if the trigger was external (and it may not have been) the energy was probably still internal and hence energy conservation would imply cooling somewhere else.

    I imagine, though, that if we continue to add extensive quantities of cold, fresh water to the oceans it will have an impact on ocean currents and on regional warming/cooling.

    Capt,
    I’m not aware of a PPT presentation or a transcript, but if I find one, I’ll post a link.

  10. BBD says:

    ATTP

    I imagine, though, that if we continue to add extensive quantities of cold, fresh water to the oceans it will have an impact on ocean currents and on regional warming/cooling.

    Yes, perhaps eventually, although IIRC AR4 and AR5 suggest that significant MOC suppression is unlikely this century (I need to check this again).

    What I was trying to say was that those who suggest that the mechanism that drove D-O type variability is also responsible for modern warming have no real argument (see eg. Fred Singer). Not least because the timing is wrong 😉

  11. These topics, the Younger Dryas, the Heinrich events and the D-O oscillations while not directly relevant to the current situation of dramatic human induced atmospheric carbon dioxide spiking over short geological time span, are still relevant, because while the continental ice sheets are long gone, and the mountain glaciers are on the way out, there is still a great deal of ice left to melt and disappear. Floating arctic sea ice, the Greenland ice sheet and West and East Antarctica come to mind. I would be nice to know what the effects are going to be as they disappear over the next 100 to 1000 years, Although it’s not hard to guess what those effects might be, the specific details of atmospheric and oceanic response to this would be nice to know. And of course it would be a nice academic exercise to know just why they occurred in the past.

  12. Joihn Mashey says:

    Actually, people tried to bring D-O into more modern times.

    Search Strange Scholarship for Dansgaard.
    For some weird reason, Wegman+Said made Wunsch(2006) on D-O Events an Important Paper, and their summary was mostly plagiarized, plus the usual sorts of errors introduced in making trivial edits.

    Given the general level of incompetence, one wonders what or who led them to this, given how totally irrelevant it was, and nto in the mesh of relevant references.

  13. BBD says:

    Ah yes, Wunsch (2006). That would be the study with the abstract that concludes:

    The disappearance of D–O events in the Holocene coincides with the disappearance also of the Laurentide and Fennoscandian ice sheets. It is thus suggested that D–O events are a consequence of interactions of the windfield with the continental ice sheets and that better understanding of the wind field in the glacial periods is the highest priority. […] Connection of D–O events to the possibility of modern abrupt climate change rests on a very weak chain of assumptions.

  14. Since this is nominally a physics blog, I thought I would throw this out there. This recently published paper in honor of Tom Crowley who passed away last spring, synthesizes what is known about the relationship to Heinrich events, D-O events, continental ice sheet evolution, sea level, ocean currents and orbital variability, among other things, and should be very helpful to those who may be interested in such matters.

    http://www.sciencedirect.com/science/article/pii/S0012821X14005494

    Climate variability and ice-sheet dynamics during the last three glaciations, Stephen P. Obrochta, Thomas J. Crowley, James E.T. Channell, David A. Hodell, Paul A. Baker, Arisa Seki, Yusuke Yokoyama, Earth and Planetary Science Letters, Volume 406, Pages 198–212 (15 November 2014)

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