One of the most interesting parts of the paleoclimate record over the last 100,000 years, is the series of abrupt climate changes prior to the Holocene that have occurred on very rapid timescales, ranging from years to decades (Alley et al, 2003). These changes were large, fast, and occurred when the climate was pushed across certain thresholds.
Of particular note, is the well over 20 Dansgaard-Oeschger events since the last interglacial. Typically, a rapid warming on timescales of decades was followed by slower cooling, rapid cooling, and then a brief period of little temperature change. A value near 1500 years between these events is common, although sometimes there are skips and so the spacing could be a scalar multiple of near 1500 years. Successive D-O oscillations become progressively cooler as the cold-based ice sheet grows in Hudson Bay, and when the base of the ice thaws, you get a Heinrich event surge that dumps large number of icebergs that calved from the Laurentide ice sheet into the North Atlantic, via the Hudson Straight (or perhaps other sources such as the Icelandic and British Isles ice sheet). This succession of progressively cooler D-O events, punctuated by a Heinrich event (until the next cycle begins or the climate becomes too warm for an ice sheet to grow) is a Bond cycle. These events are common before the Holocene which led to a climate punctuated by high-frequency variations and a much more variable situation than that which humans have enjoyed over the past 10,000 years.
Heinrich event 1 (which is actually the youngest Heinrich event, not the oldest) occurred approximately 17,000 years ago was followed by an abrupt warming around 14,500 years ago, something known as the Bolling-Allerod interval (an event interrupted by the Older Dryas and the Inter-Alleroid cold periods). This warming is seen in a wide variety of proxy records including the Greenland summit. This was then followed by the cooling into the Younger Dryas, and then finally a warming into our current Holocene interglacial.
The physical mechanisms responsible for the Heinrich event 1- Bolling-Allerod transitions have been controversial. In the recent issue of Science, a study by Liu et al., 2009 uses advanced modeling to tackle this particular question in more detail. One thing is clear: These abrupt changes are related to the Atlantic Meridional Ocean Circulation (AMOC)(loosely, the conveyor-belt or thermohaline circulation).
It has been known for some time that a reduction North Atlantic sinking would warm the south while cooling the north in a bipolar see-saw (Discussed previously at RealClimate), and so understanding how the AMOC, freshwater flux into the ocean, and other atmospheric changes are related is crucial. Attributing causes to various abrupt shifts is very important for our understanding of the physical climate system and possible tipping points in the future. Models of intermediate complexity (e.g., Ganopolski and Rahmstorf 2002) model D-O like events as a threshold process involving stochastic resonance. This is one possible mechanism in which “noise” and a very weak “signal” (a weak but true 1500 year periodicity in forcing) could combine (Alley et al., 2001) although dating issues are such that placing very high confidence on a true periodicity at 1500 years is difficult. Transitions like that of the Bolling-Allerod (which is in some ways similar to a D-O event) could involve surface warming of the North Atlantic or reduced melt water influx, but Liu et al. use the first synchronously coupled atmosphere-ocean general circulation model that goes from the Last Glacial Maximum to the Bolling-Allerod, in a rather unique way to investigate that topic- a transient modeling approach that prescribes the time evolution of external boundary condition changes. They force their model changes in insolation from Milankovitch, atmospheric greenhouse gas concentrations, continental ice sheets and coastlines, and meltwater flux over the North Atlantic and Gulf of Mexico.
The authors also get a bipolar seesaw response characterized by a cooling over the Northern Hemisphere and a warming over the Southern Hemisphere into Heinrich event 1 that is caused by decreased poleward heat transport from the AMOC. They demonstrate warming that is global in spatial extent from Heinrich event 1 to the Bolling-Allerod, showing signs of polar amplification and maximum warming in the Arctic and North Atlantic. Here is a global illustration of temperature anomalies of Heirich event 1 relative to the LGM, the Bolling-Allerod relative to Heinrich 1, and then the Bolling-Allerod relative to the LGM.
Liu et al 2009
At the Bolling-Allerod onset, Greenland warms tremendously, largely due to reduced melt water influx, and Antarctica continues to warm as a result of large increases in greenhouse gas concentration. The large increase in methane is mostly caused by an increase in wetland extent and temperature, as wetlands are the primary source of methane in the pre-industrial time period. Perhaps tropical wetlands were a major contributor since ice sheets covered the primary extratropical methane sources during this time (Chappellaz et al 1993). Changes in the position of the Intertropical Convergence Zone (ITCZ) cause rainfall suppression at Heinrich event 1 and enhancement at the Bolling-Allerod.
The unique aspect of this paper is that many studies in the past, using models of lesser complexity, show that abrupt warming from Heinrich event 1 to the Bolling-Allerod was caused by a sudden resumption of the AMOC in response to a gradual perturbation. However, Liu et al. simulates the Bolling-Allerod warming largely as a linear response to Melt water flux. When the discharge of meltwater from the retreating glacial ice sheets during Heinrich Event 1 stops suddenly, this is where there is a transition to a new state. As the Meltwater flux increases, the AMOC diminishes nearly linearly, in contrast to many intermediate climate models.
Much of the warming into the Bolling-Allerod is thus caused by the AMOC, and also by the increase of both methane and CO2 (about 40 ppmv for CO2) as well as an “overshoot” (by overshoot, they mean this is recovery beyond the glacial-state transport) of the AMOC due to convection in the Nordic sea. Whether this overshoot exists in observational records is actually unclear.
Graphic from the non-technical article in Science by Axel Timmermann and Laurie Menviel
As another note, this kind of modeling needs to be performed in the future by different groups to check the robustness of Liu et al and to provide further perspective on the mechanisms and spatio-temporal extent of abrupt climate change. Unfortunately, as pointed out in the accompanying perspective piece by Timmerman and Menviel , this is a very computationally demanding task, and already involved one and a half years of model number-crunching to get initial results. They close with an insightful line,
“Ultimately, breakthroughs in our understanding of Earth’s climate evolution will come from close interactions between paleoproxy experts, paleoclimate modelers, and climate dynamicists. It is time to train a new generation of scientists familiar with all these fields.”
Alley, R.B., S. Anandakrishnan, and P. Jung. 2001: Stochastic resonance in the North Atlantic. Paleoceanography, 16(2):190-198
Alley, R.B., et al, 2003: Abrupt climate change. Science, 299, 2005-2010
Chappellaz J., et al., 1993: Synchronous changes in atmospheric CH4 and Greenland climate between 40 and 8 kyr BP, Nature 366, 443-445
Ganopolski, A. and S. Rahmstorf, 2002: Abrupt glacial climate changes due to stochastic resonance, Phys. Rev. Let. 88(3), 038501
Liu Z., et al., 2009: Transient Simulation of Last Deglaciation with a New Mechanism for Bølling-Allerød Warming, Science 325: 310-314
Timmerman, A. and L. Menviel 2009: What Drives Climate Flip-Flops? Science 325: 273-274
I’d hoped to get to write about this as well, but now don’t really see the need. Nicely written. Feel free to use these links to the full pieces:
Click to access what-drives-climate-flip-flops.pdf
Click to access transient-simulation-of-last-deglaciation-with-a-new-mechanism-for-bc3b8lling-allerc3b8d-warming.pdf
thanks for the nice review and perspective!
it looks like you know a lot! perhaps a geology major…?
This is the most series effort so far to test general circulation models in simulating large abrupt climate changes. The two most important messages from this exercise are:
1) current state of art models are able to generate the magnitude of large climate changes well.
2) it is unclear if current models are able to generate the abruptness of climate change, this depends on observation validation.
Response– Thanks for dropping by Dr. Liu– chris
Can this modeling approach be run out looking at the future?
I wonder if the change in ocean pH as described here
http://ocean.mit.edu/~mick/Papers/Goodwin-etal-NatureGeoscience-2009.pdf has any place in this approach.
Response– CCSM3 is described in this paper (ftp://eos.atmos.washington.edu/pub/breth/papers/2006/CCSM3.pdf ) by the way. I can’t really tell you how they will apply it to future projections, but it’s right up there with other fully coupled AOGCM’s. The perspective piece offers some insight on this suggesting that this will be a demanding task, and may be some years before these transient simulations can be carried out more routinely– chris
What about these studies:
Appy Sluijs, Stefan Schouten, Mark Pagani, Martijn Woltering, Henk Brinkhuis, Jaap S. Sinninghe Damsté, Gerald R. Dickens, Matthew Huber, Gert-Jan Reichart, Ruediger Stein, Jens Matthiessen, Lucas J. Lourens, Nikolai Pedentchouk, Jan Backman, Kathryn Moran & the Expedition 302 Scientists: 2006, Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum, Nature 441, 610-613 doi:10.1038/nature04668.
Click to access sluijs_et_al_06.pdf
10 C is a pretty huge discrepancy I’m sure you’d agree. Surely, another possibility is just that models just aren’t right?
Then there is the Nature Geoscience article that’s just been published that everyone’s talking about:
Richard E. Zeebe, James C. Zachos & Gerald R. Dickens 2009: Carbon dioxide forcing alone insufficient to explain Palaeocene–Eocene Thermal Maximum warming, Nature Geoscience, doi:10.1038/ngeo578
Here is the Rice University Press Release and we can see that one of the authors has gone quite a bit further than this:
Response– These papers show that more things are involved then just CO2. They also show that the PETM is a case-study incompatible with a very low sensitivity, since paleoclimate models only account for a fraction of the observed warming. Unfortunately the PETM is a time with a much different base climate and unknown feedbacks (one hypothesis is increased production and levels of trace greenhouse gases like CH4 as a response), not much of an ice-albedo feedback, and so the analogue to the present day climate is limited. But it does show strong GHG-induced warming over periods of a few thousand years and strong ocean acificiation which together lasted a long time.
Anyone somewhat familiar with the deep-time paleoclimate literature and equable climates knows that CO2 is not the only thing going on, and for a while the pole-to-equator temperature gradient issue was a big one. While the more recent estimates of warmer tropical temperatures have reduced the disagreement with models, some discrepancy remains, and various ideas (such as Emanuel’s paper on increased ocean meridional heat flux, polar clouds, etc) have been invoked but no one serious claims to know everything happening.– chris
The whole thing has an air of arbitrariness about it to me. Dr. Liu has stated above that the models are getting the magnitude of temperature change about right but failing to get the rate of change right. I can’t, myself, see how that is a scientific statement. If the prediction was that the change would happen both in a certain timeframe and at a certain magnitude, getting one bit right doesn’t validate your model. The prediction was falsified.
It seems that Gerald Dickens, at any rate, has decided that the models are getting the CO2 sensitivity wrong.
And repeating what he said again, he said there seems to be something fundamentally wrong with the way temperature and carbon are
linked in climate models.
Do you think perhaps you could contact Dr. Dickens and ask if he would be willing to present his own views here?
Response– That isn’t what Dr. Liu said. He said that model validation was difficult in this particular regard because observations may not be clear on what actually happened. For instance, if proxy records cannot show us with very high confidence whether an “AMOC overshoot” occured or not, or if dating errors are such that relatively rapid climate changes across different regions of the globe cannot be identified as occurring synchronously, asynchronously, lagged, etc then it is hard to tell whether the model is deficient. Furthermore, if forcings are unknown (such as a volcanic eruption) then it is not reasonable to expect the model to perform correctly, since models don’t generate their own volcanic eruptions, asteroid impacts, etc. It may come as a surprise to some that situations exist where we don’t even expect a perfect model to match reality.
You also don’t expect a model to perform well under situations of slow feedback like massive methane release from the ocean floor since they don’t do this on their own (or are prescribed rather crudely, a paleo-oceanographer might have better insight). The model does capture features of the observed carbon isotope and deep-sea dissolution records well (so they feel they can constrain the total carbon input), but it undersubscribes the magnitude of warming, which they attribute to unknown forcings and/or feedbacks, possibly deep sea methane release. It is not clear what else was going on, and it’s also not clear if those processes have relevancy to modern global change, but it is worrysome since it suggests possible surprises lurking in the climate system which could substantially amplify global warming…probably something to do with carbon feedbacks. I posted before about one possible mechanism in the Cretaceous warm interval that may be relevant but is difficult to test. And for the record, I think the statement about a fundamental problem with the way models handle carbon-temperature relationship was irresponsible and not really following from the studies focus. Then again, many of these scientists are actually busy and are probably unaware of the pseudo-debate or those groups with a vested interest at nitpicking everything they say to suit an agenda.– chris
[quote]”proxy records cannot show us with very high confidence whether an “AMOC overshoot” occured or not, or if dating errors are such that relatively rapid climate changes across different regions of the globe cannot be identified as occurring synchronously, asynchronously, lagged, etc then it is hard to tell whether the model is deficient.”[/quote]
Just to make sure you are really saying that this model is significant even though there is no current way scientifically validate it. In the following sentence you go further to say that disagreements between observations here in reality, if they differ from the model, do not invalidate the model. Is that right?
Zeebe put out his own press release a few days later, which looks to me like an attempt at damage control. Looking at the paper’s conclusions, I don’t see anything that disturbs prior thinking that clathrates were probably responsible for the large temperature spike.
“Some feedback loop or other processes that aren’t accounted for in these models…”
The PETM study is a bit disconcerting. It could suggest that some positive climate feedback has been substantially underestimated or completely unaccounted for. Pushing the boundaries of our current climate could trigger one of these feedbacks. But as Chris notes, this was a long-ago period with a very different base climate.
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