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Revisiting the Younger Dryas

Filed under: — group @ 17 July 2010

Guest Commentary by Chris Colose

One of the most intriguing and well-studied climatic events in the past is the Younger Dryas (YD), a rather abrupt climate change between ~12.9 and 11.6 thousand years ago. As the world was slowly warming and ice was retreating from the last glaciation, the YD effectively halted the transition to today’s relatively warm, interglacial conditions in many parts of the world. This event is associated with cold and dry conditions increasing with latitude in the North, temperature and precipitation influences on tropical and boreal wetlands, Siberian-like winters in much of the North Atlantic, weakening of monsoon intensity, and southward displacement of tropical rainfall patterns. RealClimate has previously discussed the YD (here and here) however there have been a number of developments in recent years which deserve further attention, particularly with respect to the spatial characteristics and causes of the YD.

The YD is often discussed in the same context as the ‘Dansgaard-Oeschger’ events seen in the ice cores during full glacial conditions, and the ‘Heinrich events’ of layers of ice-rafted debris in North Atlantic ocean sediments. Indeed, some people occasionally refer to the YD as Heinrich event 0, but this implies that the YD cooling was caused by an ice-rafting event (probably untrue) and should be avoided.  The YD occurred last of several prominent and abrupt deglacial events including Heinrich Event 1 (~17.5 to 16 ka) which is an event contained within the Older Dryas (18 to 14.7 ka), followed by the Bølling-Allerød warm period (~14.7 to 12.9 ka) whose end then marks the start of the YD. The end of the YD can be said to be the start of the Holocene. It has been proposed that the warmings before and after the YD can be viewed as Dansgaard-Oeschger events with the YD just a regular cold (i.e. stadial) phase in between (Rahmstorf 2002, 2003). In Antarctica (~15 to 13 ka), the most featured event is that as the Younger Dryas begins, warming is occurring in Antarctica.  The cold period in Antarctica that precedes the Younger Dryas is referred to as the Antarctic Cold Reversal (ACR) (see figure, from Shakun and Carlson, 2010) and was once thought to be in phase with the YD.  They are neither directly in phase nor anti-phased with one another (see e.g. Steig and Alley, 2002).










Fig. 1. Deglacial ice core time series and insolation. (a) GISP2 δ18O (black step plot) (Blunier and Brook, 2001). (b) Byrd δ18O (grey step plot) (Blunier and Brook, 2001). (c) Insolation (Incoming Solar Radiation) for 60ºN on June 21 (black line) and for 60ºS on December 21 (dashed black line) (Berger and Loutre, 1991). The timing of the Younger Dryas (YD), Bølling/Allerød (B/A), Heinrich Event 1 (H1), Oldest Dryas (OD) and Antarctic Cold Reversal
(ACR) are denoted.

Unlike changes in global temperature (such as modern day global warming) which can be understood as a result of perturbations to the planetary energy balance, the millennial-scale climate changes during the last glaciation are viewed primarily from the lens of internal dynamics, including ice retreat and re-organizations of ocean circulation. They are not dominated by changes in global mean temperature but rather changes in temperature distribution, explained by changes in oceanic or atmospheric heat transport. In particular, proxies of deepwater formation show large reductions in the Atlantic meridional overturning circulation (AMOC) coincident with the start of the YD. This suggested weakening of overturning circulation provides immense explanatory power for the onset of the YD although no consensus has emerged concerning the trigger of the AMOC reduction. There are some radiative changes associated with millennial-scale climate change induced by the ice-albedo effect, extra dust loading out of Asia during cold snaps, as well as greenhouse gas feedbacks– although they are relatively small. However, pinning down the exact sequence of causes and effects is rather difficult since precise chronologies and global-scale reconstructions are difficult to come by prior to the Holocene.

A new study though (Shakun and Carlson, 2010) has compiled over 100 high-resolution proxy records to characterize the timing and extent of the Last Glacial Maximum (LGM) and the deglacial evolution into the Holocene, including the shorter-lived Younger Dryas. Several of the key features of the study include:

  1. The global mean cooling of the LGM relative to the peak of our current interglacial is approximately 5ºC as a minimum value. It is likely larger than this since many of the records are from the ocean which are typically less sensitive to temperature change than landmasses, and further, adiabatic cooling of marine air advected over land masses would result from the ~120 m reduction in sea level. The cooling is global in scale and largest at high latitudes, as expected from polar amplification.
  2. In contrast, during the YD, there is much more spatial heterogeneity as the North became colder and drier (increasing with latitude) while the South became warmer and wetter in the opposite sense. The global mean cooling during the YD is only ~0.6ºC .  The tropics cooled by 2.5ºC (with an error of about a degree in either direction) at the LGM, yet exhibited very little temperature change during the YD. Thus, while the YD was a global scale climate change event with widespread signatures, it was not a widespread global cooling event.


    Fig. 2. Magnitude of the glacial-interglacial temperature change relative to absolute latitude. (Shakun and Carlson 2010)Fig. 3. Magnitude of the Younger Dryas temperature change. Map of the Younger Dryas temperature anomaly (a). Circle denotes the size of the temperature change. Blue is cooling, red warming (Shakun and Carlson 2010).

  3. The timing of the LGM and peak interglacial is synchronized between hemispheres on orbital timescales, which the authors attribute primarily to the global radiative forcing provided by CO2. As has been noted in the past, the CO2 lags the onset of deglaciation in most records, as this is paced by summer insolation changes. However the CO2 still acts as the dominant temperature-change influence throughout the deglacial period and provides an effective means to communicate temperature anomalies to the tropics. On the other hand, the YD exhibits the well-known bipolar see-saw effect which involves a reduction in northward heat transport, which warms the South. The see-saw is best expressed in the mid to high latitudes, although the see-saw model is a poor descriptor for the tropical variability.

The see-saw effect during millennial-scale climate changes has been confirmed before (also discussed at RealClimate in the context of the somewhat similar Dansgaard-Oeshger events) and is consistent with modeling efforts of the climate evolution during the last deglaciation, including Liu et al., 2009 (discussed here) who show that current state-of-the-art models can simulate the magnitude of abrupt climate changes well.

So what caused the reduction in the AMOC?

The most prevalent concept for slowing AMOC involves a reduction in the surface water density at the ocean surface via adding freshwater into the ocean. The preferred location is primarily the North Atlantic, which is a key point for deep ocean convection. The original idea for this to cause a YD-event was proposed in 1976 by Johnson and McClure, and involved the opening of eastern Lake Agassiz outlet via northward retreat of the Laurentide Ice Sheet out of Lake Superior. This re-routed drainage from the Mississippi to the St. Lawrence River.

There is a difference between the diversion of continental runoff from the Mississippi River (routing) and the relatively fast pro-glacial lake drainage to a new level (flooding). In contrast to the Johnson and McClure paper, many recent studies have focused on short-lived floods, although the re-routing mechanism might be a necessary, and in fact primary ingredient (Carlson et al., 2007; Carlson and Clark, 2008) in accord with modeling studies which require a persistent forcing to substantially alter AMOC (Meissner and Clark, 2006).

Evidence of a specific flood water pathway at the right time has proven to be elusive. No clear evidence exists for a flood event into the Atlantic, though evidence discussed by Murton et al. (2010) for an Arctic pathway has recently emerged.

There has also been interest in the prospect of a comet impact during the YD triggering a flood (e.g., Firestone et al., 2007 discussed previously at RC) although subsequent work has suggested that their results are not robust (Surovell et al., 2009), and it is likely that the impacts a comet would have on atmospheric chemistry, particularly the formation of nitrate and ammonium, is inconsistent with observations in ice core records (Melott et al., 2010). Further, the problem with a comet impact still remains — how could it generate a continuous freshwater forcing? Because of dicey evidence and no predictive ability, the comet hypothesis has not gained much favour.

Recently another hypothesis has been put forward: The Younger Dryas, instead of being a freak occurrence, is instead a key (and normal) part of the deglaciation process. This was most clearly expressed in a new paper by Broecker et al (2010) ( including George Denton and Richard Alley). Their main point is that a catastrophic flood or comet would only serve as a trigger for an event that was already primed to happen. Evidence for this comes primarily for the existence of YD-like events during previous deglaciations, notably from Chinese stalagmite data (Cheng et al., 2009) who looked at monsoon patterns in the past. In particular, a YD-like event shows up during Termination III (~ 245 ka) and possibly Termination IV, which share similar characteristics to the YD. The finding of many events with characteristics like the YD further provides evidence against the necessity of comet-impact hypothesis. However, this concept doesn’t negate the need to understand the mechanisms for the YD or its potential predecessors. Whether it was primed to happen or not, what actually happened and how is still of great interest.

Broecker et al (2010) cite Lowell et al (2005) and Fisher et al (2008) to justify their reason for why the flood hypothesis is unappealing, but further work done by Carlson et al. (2007), Carlson and Clark (2008), and Carlson et al. (2009) provides newer support for the re-routing hypothesis. Furthermore, while Broecker et al. emphasize the lack of evidence for a catastrophic event, if the slower re-routing hypothesis is correct, then the lack of evidence for a sudden flood is irrelevant. This may very well be the mechanism that is common to previous deglacial events.

The existence of events similar to the YD in the more distant past has been proposed before (Carlson, 2008). By analyzing paleo-methane concentrations, Carlson (2008) also noted that events similar to the YD happened during T III and possibly earlier deglaciations (see figure, from Broecker et al 2010).






Fig. 4. Major events surrounding Termination III. (A) shows Vostok temperature deviation (purple) and CH4 (blue) records (Suwa and Bender, 2008). (B) shows EPICA/Dome C (EDC) δD (orange) and CH4 (blue) records (Loulergue et al., 2008). (C) is the Vostok CO2 record (Petit et al., 1999). (D) is the absolute-dated Asian Monsoon record from Sanbao Cave, China (Cheng et al., 2006). (E) and (F) show IRD and inferred seawater δ18O records from marine core ODP-980 (McManus et al., 1999). Both Vostok and EDC timescale were shifted in order to correlate the abrupt jump of the last portion of CH4 in ice cores to the abrupt monsoon jump in panel (D) (Cheng et al., 2009). The ODP-980 records are on original timescales. Two Weak Monsoon Intervals (WMI) are marked by yellow background. Termination III events, analogous to the YD, B/A, ACR and MI are labeled: YD III, B/A III, ACR III and MI-III. Figure is simplified from that in Cheng et al. (2009). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

As a conclusion, over the last couple of years, there has now been growing evidence that an event similar to the YD is not “unique” but instead is a common theme across various deglacial events; this provides evidence against the necessity for a “catastrophic trigger,” and while it may be the case that a comet or some other catastrophe occurs at each termination, that seems improbable.


References
Broecker, W.S., Denton G.H., Edwards L.R., Cheng H., Alley R.B., Putnam A.E., 2010. Putting the Younger Dryas cold event into context. Quaternary Science Reviews , 29, 1078-1081
Carlson, A.E., 2008. Why there was not a Younger Dryas-like event during the Penultimate Deglaciation: Quaternary Science Reviews, v. 27, p. 882-887
Carlson, A.E., and Clark, P.U., 2008. Rapid climate change and Arctic Ocean freshening: Comment: Geology, v. 36, p. e177
Carlson, A.E., Clark, P.U., and Hostetler, S.W., 2009. Comment: Radiocarbon deglaciation chronology of the Thunder Bay, Ontario area and implications for ice sheet retreat patterns: Quaternary Science Reviews, v. 28
Cheng, H., Edwards, R.L., Broecker, W.S., Denton, G.H., Kong, X., Wang, Y., Zhang, R., and Wang, X., 2009. Ice Age Terminations: Science, v. 326, p. 248–252
Firestone, R.B., et al., 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling: Proceedings of the National Academy of Sciences of the United States of America, v. 104, p. 16016–16021

Fisher, T.G., Yansa, C.H., Lowell, T.V., Lepper, K., Hajdas, I., Ashworth, A., 2008. The chronology, climate, and confusion of the Moorehead Phase of glacial Lake Agassiz: new results from the Ojata Beach, North Dakota. Quaternary Science Reviews, 27, 1124–1135

Johnson, R.G., McClure, B.T., 1976. A model for Northern Hemisphere continental ice sheet variation. Quaternary Research 6, 325–353
Liu, Z., Otto-Bliesner, B., He, F., Brady, E., Thomas, R., Clark, P.U., Carlson, A.E., Lynch-Stieglitz, J., Curry, W., Brook, E., Erickson, D., Jacob, R., Kutzbach, J., and Chen, J., 2009. Transient Climate Simulation of Last Deglaciation with a New Mechanism for Bølling-Allerød Warming: Science, v. 325, p. 310-314
Lowell, T.V., Waterson, N., Fisher, T., Loope, H., Glover, K., Comer, G., Hajdas, I., Denton, G., Schaefer, J., Rinterknecht, V., Broecker, W., and Teller, J., 2005, Testing the Lake Agassiz meltwater trigger for the Younger Dryas: EOS (Transactions, American Geophysical Union), v. 86, p. 365–373
Meissner, K.J., and Clark, P.U., 2006. Impact of floods versus routing events on the thermohaline circulation: Geophysical Research Letters, v. 33, L15704
Melott, A.L., Thomas, B.C., Dreschhoff, G., and Johnson, C.K., 2010. Cometary airbursts and atmospheric chemistry: Tunguska and a candidate Younger Dryas event, Geology, v. 38, 355–358
Murton J.B., Bateman M.D., Dallimore S.R., Teller J.T., Yang Z., 2010. Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean, Nature, 464, 740-743
Shakun, J.D., and Carlson, A.E., 2010. A Global Perspective On Last Glacial Maximum to Holocene Climate Change: Quaternary Science Reviews.
Steig, E.J., and Alley, R.B. Phase relationships between Antarctic and Greenland climate records. Annals of Glaciology 35: 451-456 (2002).


92 Responses to “Revisiting the Younger Dryas”

  1. 1
    wili says:

    Perhaps I missed it, but I don’t understand how a reduction in AMOC would lead to the global cooling (however slight, or at least slow down of the warming) associated with YD. I can understand how it might cool the north Atlantic, but not how it would have this larger global effect.

    On the possible causes of the slowdown itself–given that something similar has happened at the end of previous glacial periods, isn’t it likely that there is something about the patterns of melt that is associated?

    Perhaps melt tends to happen on the surface of the ice sheet creating huge fresh water seas that then dump into the ocean at a great enough rate to slow or stop the AMOC. With the cooling that results, the process starts over until there is not enough glacier left to cause the slowdown.

    (OT, a bit. How much fresh water is necessary to stop the AMOC? Could such a figure be accurately calculated. It seems to me I have heard that it takes one Sverdrup. It this remotely accurate?)

  2. 2
    wili says:

    Not to hog the comment space, but this just popped up on another forum, and since it involves thermohalide circulation, I thought it might not be completely off topic:

    http://www.rinf.com/forum/showthread.php?t=16302

    “BP Oil Spill may cause an irreparable damage to the Gulf Stream global climate thermoregulation activity”

    Is this completely off base?

    (Sorry if this is too wildly off topic.)

    [Response: This is completely off base. - gavin]

  3. 3
    Alex Harvey says:

    Hi Chris, you said that “[u]nlike changes in global temperature (such as modern day global warming) which can be understood as a result of perturbations to the planetary energy balance, the millennial-scale climate changes during the last glaciation are viewed primarily from the lens of internal dynamics, including ice retreat and re-organizations of ocean circulation.”

    It is not clear to me, why? It has the appearance of an arbitrary choice. It looks like, “we prefer to explain millenial-scale changes by appealing to internal dynamics and we meanwhile prefer to explain the present global warming by appealing to perturbations of the energy balance.”

    Now the IPCC report says, “[i]t is ‘very likely’ that average Northern Hemisphere temperatures during the second half of the 20th century were higher than for any other 50-year period in the last 500 years. It is also ‘likely’ that this 50-year period was the warmest Northern Hemisphere period in the last 1.3 kyr, and that this warmth was more widespread than during any other 50-year period in the last 1.3 kyr.”

    Now after decoding the meanings of ‘likely’ and ‘very likely’ I am left with the conclusion that actually we just aren’t certain at all when it comes to global average temperatures and the last thousand years.

    So what can we possibly know of ‘global average temperatures’ from as far back as the Younger Dryas? Surely, the further back we go the less certain our knowledge of the global average ‘anything’ becomes.

    Where am I wrong here?

    Best, Alex

  4. 4
    Brian Lux says:

    I think this falls under the black diamond run category.

  5. 5
  6. 6
    Edward Greisch says:

    Figure 2: I see only blue dots on the maps and blue dots and grey dots on the keys. I am using Mac OS 10.6 and Safari 4.0.3

    Figure 4: I see black instead of blue. “timescales were shifted” is something for denialists to grab and make accusations.

  7. 7
    Martin Vermeer says:

    So what can we possibly know of ‘global average temperatures’ from as far back as the Younger Dryas? Surely, the further back we go the less certain our knowledge of the global average ‘anything’ becomes.

    Where am I wrong here?

  8. 8
    Martin Vermeer says:

    So what can we possibly know of ‘global average temperatures’ from as far back as the Younger Dryas? Surely, the further back we go the less certain our knowledge of the global average ‘anything’ becomes.

    Where am I wrong here?

    Alex, let me try.

    Yes, if you want to compute a well defined global average temperature to a precision of tenths of a degree, then having a good, global geographic distribution of sensitive temperature proxies is critical. And yes, this will get you into trouble already for the first millennium AD — for the current state of the proxy data. The IPCC statement you refer to tells more about the current state of the evidence (and remember, even “likely” is a lot better than not knowing at all — a military commander would kill for such knowledge!) than about reality back then… don’t place your hopes on the “God of the Gaps”, it won’t last :-)

    About YD, I see that in the article initially only temperature data (isotope proxy) from two continental ice sheet sites are used; global averaging isn’t even attempted (and would be of limited usefulness due to the asymmetry of the pattern).

    Then, in the glacial maximum / interglacial temperature contrast and transition study, again proxy data from many locations are used — but here we talk about temperature differences of several degrees; easier, even though much further back in time.

    This is my understanding. Hope this helps (and I got it all right).

  9. 9

    Fresh science! It’s about time…

    My guess at #1 (wili) is that northern cooling leads to extended ice cover, lower albedo, and hence lower total insolation. I’d also guess that the cooling affects both hemispheres, so it’s a double whammy. Both could in turn reduce CO2 levels (perhaps by covering over extensive plant growth without giving it a chance to decay and enter the atmosphere), or by affecting ocean CO2 levels by significantly cooling the oceans nearer the poles.

    Question: what would re-accelerate the AMOC? How did the YD end? And did an acceleration of the AMOC end the YD, or vice versa? I’d expect that that would also be a clue (or at least supporting evidence) as to how the YD started.

    Lastly, has anyone quantified the AMOC and its relationship to temperature (such as by setting the “fast” and “slow” AMOC rates in Sverdrups, and relating that to total change in annual hemispheric heat transport, e.g. W/m2/year)?

  10. 10

    Quick addendum: “Affecting ocean CO2 levels” meant (in case it wasn’t obvious) “affecting the relationship between atmospheric and ocean CO2 levels (i.e. more atmospheric CO2)”

  11. 11
    John Peter says:

    Denton, one of the first reference et al.s, recently published “The Last Glacial Termination” with a different set of et al.s. Denton seems to have been chasing termination triggers and couplings for quite some time now.

    If Science Direct isn’t free for you perhaps you’d find the current Science 328, 1652 (2010) Denton reference more to your taste and/or pocketbook.
    http://www.sciencemag.org/cgi/content/full/328/5986/1652 if it works..

    Otherwise, for your convenience

    “The Last Glacial Termination
    G. H. Denton,1 R. F. Anderson,2,3,* J. R. Toggweiler,4 R. L. Edwards,5 J. M. Schaefer,2,3 A. E. Putnam1

    A major puzzle of paleoclimatology is why, after a long interval of cooling climate, each late Quaternary ice age ended with a relatively short warming leg called a termination. We here offer a comprehensive hypothesis of how Earth emerged from the last global ice age. A prerequisite was the growth of very large Northern Hemisphere ice sheets, whose subsequent collapse created stadial conditions that disrupted global patterns of ocean and atmospheric circulation. The Southern Hemisphere westerlies shifted poleward during each northern stadial, producing pulses of ocean upwelling and warming that together accounted for much of the termination in the Southern Ocean and Antarctica. Rising atmospheric CO2 during southern upwelling pulses augmented warming during the last termination in both polar hemispheres…”

    Heinrich I and Younger Dryas stadials are connected to melt-water/iceberg outbursts and eventually to ocean released CO2. Interesting read…

  12. 12

    Okay, quick backtrack. I do need to remember to read every article 3 times before posting (and I’ve only read it twice now, so I’m already breaking that rule, again…)…

    My stupid comment #9 ignored Chris’s explicit statement in the original post that the North became colder and drier, while the South became warmer and wetter, and global temperatures only varied by ~0.6˚C, so my revised comment to wili (#1) becomes… the article didn’t say global cooling, it said regional cooling.

  13. 13
    Thomas says:

    Alex @3.
    If we think in terms of perturbations on a sphere. The simplest perturbation is L0, which is a uniform global warming/cooling. The next axisymmetric perturbation would be L1, cooling at one pole, and heating at the other. For a global driver, such as a change in solar intensity or a change in greenhouse gas concentrations the driver is primarily L0. If AMOC transfers heat from one hemisphere to another, and the strength of this transfer changes the driver would be primarily L1. The largest response mode is likely to be similar to that of the driver, so we would expect changes of global drivers to affect mainly L0, and changes in internal heat transfers to affect mainly L1. The reported changes during YD are primarly L1 (amplitude 2.5C), with some spillover to L0 (amplitude .6C). Primarily because the latitudinal distribution of land surface is very assymmetric (more land in the northern hemisphere), it is not unexpected that global temperatures would be affected as well, as northern hemisphere albedo is more temperature dependent than southern hemisphere albedo.

  14. 14
    pete best says:

    I thought it was an event due to the ice plugged lake abruptly collapsing and releasing it contents into the north atlantic via a natural warming which causes an abruot reversal in the due to the weakening of the THC.

    However I am presuming that the actual idea of the trigger of a YD eventis presently not understood and not likely a comet so some other mechanism?

  15. 15
    Hank Roberts says:

    Pete, a lot of people thought “North Atlantic” — until recently.
    http://www.google.com/search?q=Younger+dryas+flood+arctic

  16. 16
    Aaron Lewis says:

    Just as the ice under an ice skater’s blade melts under the pressure, ice under a head of ~ 6 meters melts under the pressure of the water above it. Thus, an ice dam cannot form a lake of (liquid) water deeper than ~ 20 feet. If we are looking for a lot of fresh water retained by an ice dam, the lake would have to be broad rather than deep. The unpleasant possibility is that a shallow lake fell through the ice sheet and rapidly mobilized a relatively large volume of ice.

    Could the wave terraces at Lake Missoula have been formed by a shallow lake sitting on a thick wedge of ice?

  17. 17
    Christopher Hogan says:

    If I understand it, what you’re saying is this:

    A consistent indirect effect of the initial warming at the end of recent ice ages appears to be a meltwater-mediated northern hemisphere hiatus in the warming.

    That much ice, melting at that rate, and running off North America, appears to have the consistent and predictable effect of slowing the AMOC (which I would have naively termed the gulf stream), and thereby generating a pause in the warming for the northern hemisphere.

    And if you just look at the big sonofagun of a blue dot above, and read the text, we’re talking about drainage of the Canadian ice shield, via the Great Lakes and the St. Lawrence river.

    So, no dramatic event is needed, it’s just a routine consequence of storing then releasing that much water off the land surface of Canada.

    Knowing none of the detail, that all seems completely reasonable to me.

    Now I’m back to comment #1. Can you use this new information to help triangulate our current situation. We don’t have a Canadian ice shield now. All we have is Greenland. Is there any reasonable back-of-the-envelope calculation that rules out such an effect from a much more rapid melting of a much smaller amount of ice?

    So now I’m trying to trade off area versus time, from the charts above. (Mercator projections are unhelpful). And my response is, basically, hmmm.

    Unless I’m reading this wrong, the land area of Canada is 4.8x the land are of Greenland.

    http://en.wikipedia.org/wiki/List_of_countries_and_outlying_territories_by_total_area

    So, if Greenland melts 4.8x as fast as the Canadian ice shield did at the end of the last ice age, then … we ought to get about the same effect, … eh?

    Not knowing the depth of the ice, and so on and so on, I’m brought back to what James Hansen has said about sea level change. Yeah, sure, the average change is slow. But, historically, you’ve seen periods of rapid change, and, hey, that just might happen to your grandkids. So don’t take the average change as indicative of your (immediate progeny’s) risk from this.

    Secondarily, maybe the east coast of NA will be spared the worst of it, for a century or two, as the Greenland ice sheet melts?

  18. 18
    Hank Roberts says:

    > ice under a head of ~ 6 meters melts under the
    > pressure of the water above it

    Wait, if so, icecaps on land couldn’t be more than 6 meters thick? Can you cite a better explanation for what you mean?

  19. 19

    Christopher Monckton and other deniers get far more press coverage than they deserve. Journalistic false balance has caused the public to be confused on climate change – the greatest threat to humanity this century. Worse, these deniers have used mainstream media to attack climate science and the scientists who pursue the truth. Let us now turn the tables.

    Monckton has been exposed by Dr. John Abraham and instead of hiding his tail and whimpering away, Monckton has gone on the offensive by attacking Dr. Abraham and asking his followers to essentially “email bomb” Dr. Abraham’s university president. We need to alert the media to this story.

    I have assembled a list of 57 media contacts in the hopes that my readers will follow my lead and send letters asking for an investigation of Monckton and his attack on Abraham. I have placed mailto links that will make it easy to send letters to several contacts at once with a single click.

    In the thread comments, please suggest other contacts in the US and from abroad. This blog thread can then be used in the future to alert the media to denialist activity.

    http://profmandia.wordpress.com/2010/07/18/turn-the-tables-on-monckton/

  20. 20

    19–

    Great idea! (I’m presuming you’re managing this in such a way as not to “bombard” journalists.)

    On a parallel thread here, Geoff Wexler makes some good points about “defensive writing,” but we can’t always be on the defensive. You’re playing some offense here, by being proactive in disseminating accurate information about denialist tactics.

    It will of course be called “politics”–which it is, politics being how human affairs are managed. But it will be written/said sneeringly! Par for the course.

  21. 21
    jyyh says:

    #4 Brian Lux: Definitely a hard blue, if not black one…
    introduction to black, perhaps – rewiew on recent very technical papers on paleoclimate, it looks like. I’ve never actually made clear to myself what the names of different periods refer to but this post (I think) makes it easier to go look for possible explanations and actual records.

  22. 22
    Edward Greisch says:

    Thank You Scott Mandia.

  23. 23
    Mike says:

    There was an article arguing that hunting of megafauna in North America reduced methane emissions which caused the YD. Anything to that?

    http://content.usatoday.com/communities/sciencefair/post/2010/05/mammoth-extinction-triggered-climate-cooling/1
    http://www.nature.com/ngeo/journal/v3/n6/full/ngeo877.html

  24. 24
    Chris Colose says:

    Mike (#23)

    The articles on the Megafauna extinction was interesting but received far too much attention in secondary sources with respect to causing the Younger Dryas, or trying to extend the “Anthropocene” for that matter. Even in the primary article the authors seemed quite unclear about distinguishing between global vs. Greenland impacts, and the methane changes at the YD are too small to have a significant global effect.

  25. 25
    Anonymous says:

    Can I just throw it out there that although there is a lack of IRD spikes during the YD, I don’t think calving instabilites can be ruled out. Just look at how sensitive marine terminating glaciers are: the PIG is loose, SE Greenland recently doubled its speed and Alaskan tidewater glaciers collapsed out of the little ice age. Drainage re-routing and megafloods are certainly important, but Terminations occur by Milankovich driven calving episodes.

  26. 26
    wili says:

    Bob at 12, thanks for the thoughtful responses. Note that, parenthetically at least, I noted that the warming was slight and that even a slow down or temporary pause in the global temperature increase needs to be explained.

    It looks as though the most we can ‘hope’ for, as far as a similar pause to our current warming goes, is a temporary cooling, or a slowing of the warming, mostly in the north, but at the cost of greatly and quickly increased sea levels.

    But even such a slow down may help slow or postone the crucial methane feed backs from tundra and Arctic continental shelves.

    At this point, I find myself grasping at any possible unknown (or barely known) unkowns that might step in at the last minute and mitigate what is looking more and more like a biblical or worse catastrophe facing the planet. Responses of most individuals, institutions and nations to the crisis seem tepid at best given the gravity of the situation.

    And I join in thanking Mandia for his attempt at organizing a response to the disinformation and smear campaign.

  27. 27
    Nick Davis says:

    Aaron Lewis,

    Back-of-the-envelope calculations are a good way to ensure that claims like this are reasonable or not.

    Consider an ice sheet 3000m thick, and for the sake of simplifying calculations, has a density of 1000 Kg/m^3. The pressure on the surface below the ice sheet due to the ice sheet’s mass is (density)*(gravity)*(height) = (1000 Kg/m^3) * (9.8 m/s^2) * (3000m) = 2.94 x 10^7 Pa, or 29.4 MPa.

    Wikipedia has a simple plot of the melting point of ice in K vs. pressure at http://upload.wikimedia.org/wikipedia/commons/c/c6/Melting_curve_of_water.jpg

    As you can see, a pressure of 29.4 MPa equates to a ~3 degree reduction in melting temperature – not very much!

  28. 28

    Chris, very exciting article. It leads to even more exciting questions about the mechanism for the termination phase. It will be fun to see what is explored from these new directions, and of course new information and articles yet to come.

    OT Besides the new video on natural Cycle :)
    http://www.youtube.com/watch?v=VlzQ1i2caj4

    I have updated the Lindzen page with some dialogue from Ben Santer and Mike Mann from the Marketplace forum held on June 9th.

    I have transcribed the Q&A from the Marketplace web site video of the session. My main goal with the question was to get something in the public that addressed the ‘Iris Hypothesis’. Both Mike & Ben contributed to the answer.

    http://www.ossfoundation.us/projects/environment/global-warming/myths/richard-lindzen


    A Climate Minute The Natural CycleThe Greenhouse EffectHistory of Climate ScienceArctic Ice Melt

    ‘Fee & Dividend’ Our best chance for a better future – climatelobby.com
    Learn the Issue & Sign the Petition

  29. 29
    Thomas says:

    18:
    I think what was meant was that at a differential pressure (deviatoric stress) of approximately one bar (about 10meters of water) that ice can be considered to flow plastically. If you place a lake of water on a ice sheet, after its depth reaches roughly 100M it has enough pressure to push its way through. This places a constraint on icedam height.

  30. 30
    David Horton says:

    #23 There is no firm evidence about humans “causing” megafauna extinction anywhere in the world. It is much more likely that climate change caused the extinctions. In the great scheme of things, the “reduction in methane from megafauna” is unlikely to be more than a very minor feedback, particularly since the megafauna absence would be balanced by increases in numbers of other herbivore species.

    There was a similar kind of hypothesis in Australia, but in that case it was that the lack of megafauna caused more grass to not be eaten and therefore more human fires were needed (see http://davidhortonsblog.com/history/). Absolute nonsense, again, not least, because the absence of megafauna would be compensated for by increasing kangaroo numbers.

    These kinds of theories present a curiously static view of the way ecosystems work.

  31. 31
    Ike Solem says:

    “the methane changes at the YD are too small to have a significant global effect.” – Chris Colose

    Hmmm… that’s not at all what I would have said. In fact, the graphs of methane and CO2 at the Younger Dryas should have been incorporated in this discussion. Furthermore, those increase took place over a 1,000 year period – while modern climate models barely project 100 years into the future, a result of the lack of biogeochemical modeling efforts.

    http://www.realclimate.org/?comments_popup=637#comment-108830

    http://www.sciencedaily.com/releases/2009/04/090423142457.htm

    ScienceDaily (Apr. 24, 2009) — An expansion of wetlands and not a large-scale melting of frozen methane deposits is the likely cause of a spike in atmospheric methane gas that took place some 11,600 years ago, according to an international research team led by Scripps Institution of Oceanography at UC San Diego.

    From previous work:

    During these events, atmospheric methane concentrations increased by 200-300 ppb over time periods of 100-300 years, significantly more slowly than associated temperature and snow accumulation changes recorded in the ice core record. We suggest that the slower rise in methane concentration may reflect the timescale of terrestrial ecosystem response to rapid climate change. We find no evidence for rapid, massive methane emissions that might be associated with large-scale decomposition of methane hydrates in sediments.

    Clearly, methane was a key player in the warming associated with the Younger Dryas, regardless of the claims made in this post. See the following:

    http://www.realclimate.org/?comments_popup=637#comment-108927

    Here’s the graph that really should have been included in this article – spend some time looking at this, and wondering what it means:

    http://www.ncdc.noaa.gov/paleo/pubs/monnin2001/fig1-lg.jpg

    [Response: I'm with Chris on this one. The ice core evidence is clear that methane was strongly affected by the YD (and indeed by other D-O events), but the radiative forcing involved in those changes are too small to have any significant effect on the global temperatures. Look at the graph, the change in CH4 is around 200 ppbv (about a fifth of the anthropogenic change since 1750), and has a forcing of ~0.14 W/m2, equivalent to a 0.1 deg C temperature change at equilibrium - far smaller than the actual temperature changes. - gavin]

  32. 32
    david says:

    One more comment on aaron (16) – the idea that the pressure of an ice skaters blade causes the ice to melt and thus makes ice slippery is not correct. The pressure of a skaters blade only reduces the melting point by a few degrees, but ice is slippery down to about -40 celsius. Exactly why ice is slippery is still a research question, it seems that there is a thin layer of liquid water on the surface of ice even at temperatures well below zero.
    see, e.g. http://people.virginia.edu/~lz2n/mse305/ice-skating-PhysicsToday05.pdf

  33. 33

    #32–Fun reference; thanks!

    Interesting mention of Faraday, working on the problem of regelation. John Tyndall, the first to measure the greenhouse properties of various gases, including CO2, was interested in regelation, believing it to be an important mechanism in glacial movement.

  34. 34
    John Pollack says:

    I’m uncomfortable with the interpretation of EOF2 as a “real” mode of climate variability. Since EOF1 represents much of the variance, and is in the same phase globally, and since EOF2 must be orthogonal to EOF1, and there is more variance at the poles than near the equator, EOF2 will be a hemispheric pattern with the axis of variation forced to line up with the poles. This will be robust to statistical jacknife procedures, but it has limited physical reality. In other words, the EOF process does not confirm the physical reality of the bipolar see-saw, but is forced by the concentration of variance at polar latitudes to look like a see-saw, regardless of what is really happening in the climate system in the way of “non see-saw” behavior.
    To quote from the abstract “Empirical Orthogonal Functions: The Medium is the Message” by A. Monahan, et. al., Jour. of Climate, 15 Dec. 2009, 6501-14 : “…in general individual EOF modes (i) will not correspond to individual dynamical modes, (ii) will not correspond to individual kinematic degrees of freedom, (iii) will not be statistically independent of other EOF modes, and (iv) will be strongly influenced by the nonlocal requirement that modes maximize variance over the entire domain.”
    I am not saying that the see-saw does not exist. I am saying that the act of running EOFs on the data and treating EOF2 as “real” will tend to disguise other aspects of the climate dynamics that do not resemble EOF2.
    That doesn’t strike me as helpful in trying to understand how the climate system was operating during this period.

  35. 35
    Hank Roberts says:

    > . If you place a lake of water on a ice sheet, after its depth
    > reaches roughly 100M it has enough pressure to push its way through.
    Reasonable
    > This places a constraint on icedam height.
    http://www.google.com/search?q=outburst+flood+jokulhaups

    Also look up ‘drumlins’ — up til recently these were much debated, how did they form, did they form rapidly or slowly. Try the comments starting here for some recent references:
    http://scienceblogs.com/stoat/2007/07/meanwhile_back_at_tggws.php#comment-499354

  36. 36
    Brian Dodge says:

    > . If you place a lake of water on a ice sheet, after its depth
    > reaches roughly 100M it has enough pressure to push its way through.
    Reasonable
    > This places a constraint on icedam height.

    Don’t forget that the weight of 100 meters of ice creates about 90% of the pressure of the water, and that the plastic flow of the ice will create a side pressure opposing the water pressure at the bottom of the ice dam. A 1 km thick ice sheet could retain about a 900 meter deep lake, at least until it flowed downhill enough to fail

  37. 37

    Does anyone know the truth on this? WOWT and a dozen other anti-science blogs are reporting that the U.S. Department of Energy is suspending their funding of CRU, which would be both a crime against humanity, silliness (considering all of the official reports clearing them), and lastly a black mark on the administration (did G.W. get back in office?).

    The only “credible” source (say that with muffled laughter) that they link to is a story by good old stalwart Jonathan Leake at the Sunday Times, but the story is behind a pay wall.

    Does anyone know the facts/substance behind the story? Is this as simple as him twisting a standard procedural step, or pulling out old news (like a suspension while things were under investigation) and acting as if it’s new, or just plain making it up?

  38. 38
    Alan of Oz says:

    RE #37 – “Does anyone know the truth on this? WOWT and a dozen other anti-science blogs are reporting that the U.S. Department of Energy is suspending their funding of CRU”.

    The most effective propoganda uses the same phycological tool as magicians do, they lead their audience into accepting unstated assumptions. A quick google confirms the CRU is funded by the Natural Environment Research Council, which as one would expect is a British government agency doing it’s job funding a British university.

  39. 39

    Whatever the final explanation turns out to be, whether hydrogeological or cosmological, I’m still fairly well convinced the locus tis thing is just south of Lake Nipigon. The proxy work that has purportedly refuted the comet impact hypothesis has been sparse and located far from the proposed impact site, and only refutes the widespread comet shower with continental wildfires scenario, and not anything more localized and ablative. Continental geography is set up such that if this deglaciation scenario has repeated itself through multiple glaciations, then the locus would be roughly right where we find this highly indicative set of unusual geological features as well. So it truly is an unresolved question, and clearly further work is coming.

  40. 40
    Graham Saunders says:

    In reference to Thomas (#39) and the statements “Evidence of a specific flood water pathway at the right
    time has proven to be elusive. No clear evidence exists for a flood event into the Atlantic. . .
    Many erractics (large boulders different from surrounding rock and landscape) exist in present day water courses into Lake Nipigon. The timing of these is 11,000 years before present is very close to the Younger Dryas.

  41. 41
    Hank Roberts says:

    > a quick google

    A perhaps slightly less quick Google:
    http://www.cru.uea.ac.uk/cru/about/history/

    “Although EU funding is very important, we also endeavour to maintain the diverse pattern of funding reflected by the research described in this “history of CRU” and in the list of Acknowledgements ….”

    at the bottom of the page just after the words

    “This list is not fully exhaustive, but we would like to acknowledge the support of the following funders (in alphabetical order) ….”

  42. 42
  43. 43
    Chris Colose says:

    Tom,

    Very sad news. Schneider will be among those such as Hansen who stood out and spoke about climate change loudly in an insane world.

  44. 44
    jo abbess says:

    Off-Topic : I’m truly sorry to learn that Professor Stephen Schneider has died.

    Here is a recent interview on the Anderegg et al. paper on expert credibility which he co-authored :-

    http://climateprogress.org/2010/07/14/stephen-schneider-pnas-climate-science-expert-study/

  45. 45
    flxible says:

    re US DOE and CRU – I would be very suprised if the funding is/was as stated here quoting Leake continuosly since 1990.

    Casting about did return this list and nothing else from the ” Biological Systems Science Division”, somewhat less than quoted in the article.

    Also note also that at the US DoE site, “CRU” stands for Chernobyl Research Unit, their funding may have been terminated. :)

  46. 46
    jo abbess says:

    @ChrisColose

    Thank you so much for your post on the Younger Dryas. I picked up some time ago from various reading that a sudden emptying of Lake Agassiz was likely to be involved in the reduction of the AMOC, and that lodged in my mind, and I repeated it to others on several occasions. However, the recent research you summarise indicates otherwise, and so clearly I will need to adjust my internal narrative and external communication about this.

    Most useful.

  47. 47
    Neal J. King says:

    Funding of CRU/UEA: If you look at their own site,
    http://www.cru.uea.ac.uk/cru/about/history/ ,
    the list of funders begins with “British Council, British Petroleum…”, goes on to “United Nations Environment Plan (UNEP), United States Department of Energy, United States Environmental Protection Agency…”, and ends with “and the World Wildlife Fund for Nature (WWF).”

    So the story is definitely not impossible.

  48. 48
    Lorne Booth says:

    Since we are looking at historical climate change here, how should we view Prof. Robert B. Laughlin’s “don’t concern yourself about climate change, it’s not something we can control” article titled “What the Earth Knows”, published recently in The American Scholar? Agree or not, it’s an interesting read. BTW, he is a Stanford University physicist and a co-recipient of the Nobel Prize for physics 1998.

    What the Earth Knows

    [Response: Hmm... on geologic time scales nothing that we care about will matter, and the planet will still be here. But I have a fondness for the here and now, and so whether the CO2 has been absorbed by the deep ocean in 20,000 years time is less important than what it does to the temperatures and the ice sheets in the meantime. - gavin]

  49. 49
    GlenFergus says:

    Brian Dodge says:

    Don’t forget that the weight of 100 meters of ice creates about 90% of the pressure of the water, and that the plastic flow of the ice will create a side pressure opposing the water pressure at the bottom of the ice dam. A 1 km thick ice sheet could retain about a 900 meter deep lake, at least until it flowed downhill enough to fail

    Yes, in theory. Two problems:
    1. It presupposes that the water level stays 10% below the ice level. There would be special geometries where that would be so, but in general one expects the thing to fill to the lowest level on the crest of the ice dam. At that point basal water pressure exceeds the basal ice pressure, and crack propagation should commence. It may be slow, because ice is stiff and can transfer load laterally. Maybe on-going ice flow could resist it somehow.
    2. It presupposes an excellent basal seal. As one who has designed dams to resist high heads, I know how tough that is to achieve. At multi-hundred metre heads the slightest leak can grow rapidly by (physical) internal erosion. With ice, add melting due to latent heat transfer and from flow friction. One can imagine a stable leak, but it will be a long and slow one. Surely the physics of that is done and published long ago; maybe even ground-truthed, given recent intense interest?

    G.

  50. 50
    PaulW says:

    Just noting that I think some of the Greenland dO18 isotope data has been miscalibrated to temperature. It changes something equivalent to Antarctica in the ice ages (a little more in Greenland during the YD) so the temperature change should be roughly equivalent as well given the similarity in latitude and conditions.

    The temperature change should be more like -8.5C in the ice ages and -7.0C or so in the Older and Younger Dryas cool-down. One chart above has Greenland at -18C in the ice ages which is twice too much given the high latitude. This error has appeared in many papers and should be corrected.

    [Response: Actually, no. The standard paleo-thermometer calibration was shown to be wrong for glacial-interglacial temperature changes via the borehole temperature reconstructions, and for the more rapid changes via the nitrogen isotope thermal fractionation results pioneered by Sevringhaus and Brook. The explanation is likely to be the shift in seasonality in colder periods (so that most of the snow arrives in summer, rather then all year round as at present). Werner et al (2001) had a very nice model study that provides a convincing demonstration of the effect. - gavin]


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