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The Younger Dryas comet-impact hypothesis: gem of an idea or fool’s gold?

There was a paper in Science last week that has gotten quite a bit of press. It reports further evidence in support of the idea that the Younger Dryas — a distinct period towards the end of the last ice age when the deglaciation in the Northern Hemisphere was interrupted for a period of about 1300 years — was caused by a barrage of comets hitting North America.

When the first papers on this came out last year, we expressed skepticism. We remain skeptical and our reasons remain unchanged. But we think it is worth saying a bit more on this, because the reporting on this issue has largely ignored just how big an idea this is, and therefore how much more work would need to be done before it could be taken very seriously.

For background, see the good article by Kenneth Chang in the New York Times, which, however, does not address our main concerns with the hypothesis.

The brief history is that in 2007, Firestone and others published an article in PNAS showing evidence of various materials that may be diagnostic of extraterrestrial origin (and hence an impact) in layers of sediment dating to 12.9 thousand years ago, just before the beginning of the Younger Dryas cold event. Now, in a Brevia piece in Science Kennett and others show further evidence: “abundant nanodiamonds in sediments dating to 12.9 ± 0.1 thousand calendar years before the present at multiple locations across North America.”

According to Richard Kerr’s news item that accompanies the article in Science, at least some experts are skeptical that Kennett and others have really found nanodiamonds, or that, even if they have, they are necessarily evidence of an impact. But we don’t claim enough expertise in nanodiamond detection or interpretation to have an opinion on this aspect, so let’s give them the benefit of the doubt. Suppose there really was an impact (or impacts) at the right time in the right place. We’d still be skeptical that this was a trigger for the Younger Dryas.

Among our reasons for skepticism (again, see our earlier post on this) there is a basic statistical problem. The problem is — and this context is missing from most if not all of the articles we’ve seen on this — that explaining the Younger Dryas in terms of an impact leaves all the other rapid climate change events (the so-called “Dansgaard-Oeschger events”) of the last glacial period unexplained.* One would have to either accept the conventional ideas for the causes of these events, or, alternatively, one would have to propose that there was an impact not only before the Younger Dryas, but before each of the earlier events.

We recognize that it isn’t entirely an either/or situation. Indeed, the suggestion appears to be that a cometary barrage causes various kinds of havoc, including the ice sheet collapse that led to ocean circulation change (the most well-evidenced proximal cause of rapid climate change). But the point is that if these events can happen as part of the inherent variability of the ocean-atmosphere-ice-sheet system, then there is no need to invoke the impact hypothesis in the first place. And indeed it would be virtually impossible to show it was other than mere chance that comet impacts occurred at the right time, especially given that it would still be necessary to show that the ice sheet would care about comets, which we also consider unlikely (see the good discussion — particularly Mauri Pelto’s comments — on this over at the Open Mind Blog). On the other hand, if abrupt climate changes don’t happen on their own — if they only happen due to extraterrestrial causes — then one would want to see evidence of impacts for at least a few more of them, not just one. That would be a truly exceptional paradigm-breaking discovery, going against just about everything we think we know about the system.

We emphasize that we are not saying “the Younger Dryas can’t have been caused by a comet, unless all the Dansgaard-Oeschger events were caused by comets”. We’re saying that we see no need to invoke such an hypothesis, so the level of proof required for this extraordinary idea will need to be extraordinarily strong. So far, it doesn’t appear that that is the case.

Think about it. If it turned out that rapid climate change events are caused by comets, it would imply the climate system is far more stable than we thought, that abrupt climate change events are not part of the inherent variability of climate during glacial periods. That would perhaps allay fears that we could be pushing the system towards an abrupt climate change in the future. On the other hand, it would also suggest that cometary impacts are far far more common than we thought. Now that would be news. Perhaps further research by Kennett, Firestone and others will indeed show that to be the case. We’re not, however, holding our breath.

———-
*Not to mention that there is an event similar to the Younger Dryas at the end of at least one other glacial period, “termination III” (see e.g. Carlson et al., 2008).


252 Responses to “The Younger Dryas comet-impact hypothesis: gem of an idea or fool’s gold?”

  1. 1
    Jim Bouldin says:

    Reminds me a little of the RC article titled something like “CO2 is not the only greenhouse gas and greenhouse effects are not the only CO2 problem” Here it would be “extraterrestrial rock impacts are not the only cause for a sudden cooling, and not all et impacts necessarily lead to a cooling” It seems that you’d have to have multiple, definite, rock layer evidence associated with definite, repeated cooling events for their hypothesis/conclusion to have much weight behind it.

  2. 2

    Evidence of perhaps 40 glacial Lake Missoulas and the Missoula flood events was first seen in the dry rock formations in Idaho, Montana and Washington States.

    It seems plausible that many such humongous glacial seas breached into other areas. Evidence long since eroded may be harder to detect. Although Wikipedia has a list
    http://en.wikipedia.org/wiki/List_of_prehistoric_lakes

    Diamond dust may have co-incidentally cast about at that time, but wouldn’t heavy dust impart a serious cooling effect?

  3. 3

    (( feel free to ignore my misinformed previous comment ))

  4. 4
    Brian Cockrell says:

    Hmmm, I think we could have a drastic climate change without it being grounds for someone declaring that every single drastic climate shift was caused by some extreme outside force and not natural, earth-bound processes.

    I think you’re missing the reason this is getting buzz outside of the climate science community, namely the extinctions and/or the impact of such an event on the Paleoindian population of North America.

  5. 5
    Ricki (Australia) says:

    I seems to me that evidence of impact would need to lead to assessment of the energy released. This might give the basis for any estimate of the effect on climate along with the air-borne particles, etc.

    However, if there is sufficiently widespread indication of impact, this should not be discounted as a potential trigger.

    The article implies the Younger Dryas is a Northern Hemisphere event only. Is this the case, as it would effect the potential for impact to cause it? I would have thought that an event large enough to cause a change in climate lasting 1000 years would have to be global in effect.

  6. 6
    jcbmack says:

    Eric,
    of course you could be right, but I for one am not so skeptical of this theory and it seems very plausible. A barrage of comets could most certainly change climate trends and during a different time period like the last ice age could certainly have a different degree of climate sensitivity. To clarify myself before my peers misunderstand me; I am not blaming all climate change or global warming on a bunch of comets or other phenomena alone and that GHG are not the main current culprit.

    Looking at the evidence and thought process over the possibility of these comets altering seems likely, but far more research is needed as you noted. No, extra terrestrial events alone are not affecting climate, but these and other similar events cannot be overlooked or ruled out at this time.

    Then again your mention of the statistical problems has strong merit; I suggest that the data will lead to somewhere in the middle of these polar extremes you mention, but you already allude to the possibility despite your skepticism of there being an influence of comets whether they were the actual trigger or not. I certainly do not think that comet showers are more common than we think or that the climate is so stable that only in the face of such a drastic change could affect sudden climate change, but we certainly cannot rule out a period of sensitivity to a barrage of comets during one or two time periods. Good post and I understand your skepticism and certainly I will be looking at more data and calling a friend of mine who is more qualified than I to speak about nano diamonds, but if more looking into this confirms these nano diamonds, and this could happen quite quickly, the validity of the hypothesis will increase and from there of course the burden of necessary evidence is still great as you mentioned. Then again such was the case for many other scientific discoveries. I would not make the claim of not holding your breathe, however, on this one, because the plausible factors are far higher than some of the other dubious claims you guys do not hold your breathe for, and for which are not published in reputable journals. Climate is more complex and dynamic than any of us can measure or realize even in light of the trends, other trends we cannot explain or really measure are still very elusive or enigmatic. Still I enjoyed your post and read all your references and I see your points. Time will tell on this one, no one can really say this has no merit or os wrong on scientific grounds as of yet.

  7. 7
    jcbmack says:

    Eric,
    of course you could be right, but I for one am not so skeptical of this theory and it seems very plausible. A barrage of comets could most certainly change climate trends and during a different time period like the last ice age could certainly have a different degree of climate sensitivity. To clarify myself before my peers misunderstand me; I am not blaming all climate change or global warming on a bunch of comets or other phenomena alone and that GHG are not the main current culprit.

    Looking at the evidence and thought process over the possibility of these comets altering seems likely, but far more research is needed as you noted. No, extra terrestrial events alone are not affecting climate, but these and other similar events cannot be overlooked or ruled out at this time.

    Then again your mention of the statistical problems has strong merit; I suggest that the data will lead to somewhere in the middle of these polar extremes you mention, but you already allude to the possibility despite your skepticism of there being an influence of comets whether they were the actual trigger or not. I certainly do not think that comet showers are more common than we think or that the climate is so stable that only in the face of such a drastic change could affect sudden climate change, but we certainly cannot rule out a period of sensitivity to a barrage of comets during one or two time periods. Good post and I understand your skepticism and certainly I will be looking at more data and calling a friend of mine who is more qualified than I to speak about nano diamonds, but if more looking into this confirms these nano diamonds, and this could happen quite quickly, the validity of the hypothesis will increase and from there of course the burden of necessary evidence is still great as you mentioned. Then again such was the case for many other scientific discoveries. I would not make the claim of not holding your breathe, however, on this one, because the plausible factors are far higher than some of the other dubious claims you guys do not hold your breathe for, and for which are not published in reputable journals. Climate is more complex and dynamic than any of us can measure or realize even in light of the trends, other trends we cannot explain or really measure are still very elusive or enigmatic. Still I read all your references and I enjoyed your post. I see your points. Still it is not in the realm of science as of yet to say it is wrong.

    [Response: Thanks for the positive comments on the post. I agree with you -- I do not mean to dismiss this entirely out of hand. I just wanted to put a more sober counterpoint to the breathless excitement in the popular press that the Younger Dryas 'problem' had been 'solved'. It is mostly a non-problem since much of the scientific community (though not necessarily me -- see here) think we understand it already--eric]]

  8. 8
    tamino says:

    Just how similar is the YD to D-O events? D-O events seem to be rapid warmings followed by more gradual cooling while the YD was a rapid cooling event. Also, the duration of the YD is nearly as long as the possible period of D-O events (if indeed they occur with any periodicity, which I understand is in doubt). Just curious.

    [Response: See my response to Stefan below. The YD event is not, in my view, unique. Many of the DO events look just like it (indeed, much of the oceanography literature that seeks to explain these events relies on that similarity. There are claims of periodicity (every 1500 years) but updated data from the N-GRIP ice core draws that into question. But the average spacing is about 1500 years.--eric]

  9. 9

    There is no reason to suppose that all climate change events have the same cause or even similar causes. Dansgaard-Oeschger events don’t have to have the same cause as the Younger Dryas. The earth, the solar system and this galaxy are a very complicated and chaotic system. There may be several or even many groups of causes for climate change events. The end-Cretaceous extinction event was caused by an impact. Impacts come in all sizes, but the larger the object, the lower the probability per unit time. Impactors also vary in structure and composition. Smaller and less solid ones don’t have to spread and iridium layer world wide. Bolides don’t have to contain iridium. The end-Permian extinction event was caused by a super-volcano. The closing of the “Panama straight” [I mean where Panama is now] had something to do with the “ice age” we have been in for 3 million years. Continents move around. The earth changes over billions of years. The sun heats up over billions of years. Planets perturb each others’ orbits. Evolving life changes the earth. Comets and asteroids pass by and sometimes collide. The sun orbits the galaxy and oscillates up and down through the galactic plane. Different stars have very different orbits around the galaxy. In 33000 years, Proxima Centauri will enter our Oort cloud, causing a period of heavy bombardment. Other stars occasionally and at irregular times enter our Oort cloud, causing heavy bombardments of new long period comets. In 25000 years, we will enter a giant molecular cloud, which is an object that astronomers have difficulty looking into. The earth is an active planet in a violent and poorly known universe. Check with your local paleontology, geology and astronomy departments. There is no reason to believe that we know all of the possible causes of climate changes yet.
    At this point, I neither believe nor disbelieve any of the several theories of why the Younger Dryas happened. Further discussion of the Younger Dryas would be interesting.

    My computer is too old to read the Sciencemag articles or the New York Times or PNAS.

  10. 10
    Jim O'Donnell says:

    From the perspective of an archaeologist, this is very interesting. Why? Because right at the time of this supposed comet strike, we find that the Clovis culture abruptly disappeared from north and central america. Boom. Gone. At the same time was a mass extinction of mammalian mega fauna such as the mammoth, giant sloth, saber tooth, etc. 35 species abruptly went extinct. Why? The archaeological record indicates that something big happened right then. Interesting.

  11. 11
    Robin Johnson says:

    I’m with Jim O’Donnell. The Comet Impact Theory would nicely explain the demise of BOTH the megafauna and the Clovis peoples from the paleontological record.

    From a Climate Change perspective, it seems much less interesting. Sure, an impact, in the right place would likely have hastened the demise of an ice dammed lake that in theory would have reduced the ice sheets faster and flooded the Atlantic with fresh water that might have helped produce the Younger Dryas. But the lake would have collapsed eventually anyway – and so maybe the supposed impact simply affected the timing of the Younger Dryas? Which is a much different supposition than being the ’cause’ of the Younger Dryas.

    And the way I read both papers, the impact theory was sought to explain the paleontology not climate change. In a way, it actually refutes the theory that the Younger Dryas (ie climate change) drove the megafauna to extinction. Similar megafauna survived in Asia until human populations eliminated them much later. And so, at the end of the day, it would explain the ‘timing’ of the Younger Dryas AND the extinctions of the Clovis people and the megafauna.

    [Response: This is a very good point. My concern here is the climate impact of the purported comets. I am less skeptical about the significance of these results from an archaeological point of view.--eric]

  12. 12
    Anders L. says:

    So the extinction of the dinosaurs and the extinction of the megafauna both were caused by celestial objects crashing into or very near the USA? That is a cosmic coincidence of some magnitude.

  13. 13

    I love ‘crazy’ ideas like this. Thanks for highlighting the issues.

    I agree that far more research is needed, but it is worth considering this hypothesis. So we better start thinking :-).

  14. 14

    “One would have to either accept the conventional ideas for the causes of these events, or, alternatively, one would have to propose that there was an impact not only before the Younger Dryas, but before each of the earlier events.”

    Eric, I am also very skeptical about this hypothesis. But concerning your above argument: DO events were abrupt warming events, while the YD was a cold event. Although it ended by something similar to a DO warming, the YD is somewhat special and could have a unique explanation different from the regular DO events. However, I think it is more likely that this explanation has to do with the deglaciation going on at the time and the associated ice melt and freshwater fluxes, rather than with comets. – Stefan

    [Response: Stefan. Yes, of course, as I have often pointed out, the defining character of the D-O events is rapid warming, not rapid cooling. But as you well know, many of the DO events end in rapid cooling, rather like the YD. I think the case for saying the YD is special is extremely weak.--eric]

  15. 15
    mauri pelto says:

    We have mountains of data on the retreat of the Laurentide ice sheet and the vegetational development as the ice sheet retreated from pollen records. Much of the glacial geologic data is stratigraphic in nature. When one completes such a stratigraphy the key is finding some organic matter including burned material for dating. Thus, it seems odd that this recently noted layer has not been evident. We have lots of evidence that does not support the unrealistic notion of a an impact shattering an ice sheet substantially or melting it. This does not suggest the impact did not happen, just puts into perspective the potential impact. Consider carefully existing data in evaluating the current impact idea. The authors have made some observations of a horizon that suggests an impact at multiple locations at the start of the YD. That is all. They offer no evidence to tie it into ice sheet behavior. Take a look at a couple of examples of the level of detail just from a couple of studies of the Laurentide Ice Sheet. In this case the former Lake Warren, (ancient Lake Erie) and its relative surface level. Affected by ice advances and retreats. http://www.eeescience.utoledo.edu/faculty/stierman/OakOpen/GSA/P05.htm or for further chronology of the basin http://www.geo.msu.edu/geogmich/glacial.html and for lake Agassiz http://adsabs.harvard.edu/abs/2001AGUFMPP21C..03T This latter abstract indicates a release at 12.9, but it of the same magnitude of several other smaller releases and much smaller than the largest release. The point is does the hypothesis offered on the affect of the impact test out against the existing data. From what I know, and the links offered here are less than the tip of the iceberg of what is available, the hypothesis with respect to ice sheet impact does not.

  16. 16
    Ray Ladbury says:

    As John Mashey pointed out in the Open Mind discussion, it’s nice to see a real scientific controversy unfolding. Laymen should note that the debate is not about whether the evidence is “real,” but about which evidence is most important and should be emphasized. This is in contrast to pseudo-debate about climate change where the presence of all the evidence on one side of the debate leaves the denialist side with no strategy but to…, well, deny the evidence.

    [Response: Well said. And I do wish to emphasize that I do not mean to imply with my title about fool's gold that the authors of this work are fools. They are not. This is good work they are doing and I hope they keep doing it. We by no means fully understand the Younger Dryas, and all such ideas ought to be entertained.--eric]

  17. 17

    One other thing to point out about the Younger Dryas/Impact theory… They call upon the rapid break up of a portion of the Laurentide Ice Sheet culminating with the original freshwater routing hypothesis to explain the 1.3 kyr reduction in Atlantic overturning circulation. However, such a break up of the Laurentide would raise sea level and sea level rise slowed down at 12.9 kyr BP and during the Younger Dryas.

    [Response: I'm not sure where the "sea level rise slowed" from. Reference? But in any case this isn't a problem for the comet-impact hypothesis per se. If it is a problem, then it is a problem for the meltwater-caused-the YD hypothesis.--eric]

  18. 18
    Rod B says:

    I find this interesting, but, even as a skeptic looking for justification, I fail to see the relevant effect on the current climate debate. Or is there none implied? If the skeptics are saying (are they?) we don’t have to worry about AGW (if true) because (again even if true) a comet shower will come along and mitigate the problem, that seems to be putting a heavy bet on a horse that might or might not even ever show up for the race.

    [Response: It isn't relevant! Not everything we write about on RealClimate has direct relevance to the "debate", except insofar as research on paleoclimate increases our understanding of the system in general.--eric]

  19. 19

    Anders, don’t forget that the “coincidental” events in question were separated by ca. 65 million years, which may render the coincidence less (wait for the pun) striking. For more on hypothesized impacts and alternatives, see this summary:

    http://www.sciam.com/article.cfm?articleID=00037A5D-A938-150E-A93883414B7F0000&sc=I100322

    (I’m not sure how up-to-date or well-accepted these ideas are, but they are certainly interesting.)

  20. 20

    Rod, I’d say this is pretty much a *paleo*-climate story–as you imply.

  21. 21
    SteveF says:

    Robin said:

    I’m with Jim O’Donnell. The Comet Impact Theory would nicely explain the demise of BOTH the megafauna and the Clovis peoples from the paleontological record.

    I don’t think it’s massively more successful than any other other “natural” explanation for megafaunal extinction. If this impact did cause extinction, it had remarkable powers of selectivity.

    IMO, the overkill hypothesis is by far the most parsimonious. John Alroy published an excellent paper a few years back in Science which demonstrates quite clearly the plausibility of overkill. You can read it at his website:

    “A Multispecies Overkill Simulation of the End-Pleistocene Megafaunal Mass Extinction”

    http://www.nceas.ucsb.edu/~alroy/pdfs/2001-Science-292-1893.pdf

    This isn’t to say that a comet wouldn’t have been an influence, but I doubt it was the primary mechanism.

    Cheers

    Steve

  22. 22
    David R. Hickey says:

    “….. If it turned out that rapid climate change events are caused by comets, it would imply the climate system is far more stable than we thought, that abrupt climate change events are not part of the inherent variability of climate during glacial periods. That would perhaps allay fears that we could be pushing the system towards an abrupt climate change in the future. On the other hand, it would also suggest that cometary impacts are far far more common than we thought. …”

    As a retired invert paleontologist, I’ve seen this controversy over comet/asteroid impact induced climate disruption several times before. My first reaction in 1979 to the K-T asteroid impact event was the same as that of most paleontologists–disbelief and denial that such a thing could ever happen so late in Earth history. Students were asked to counter the hypothesis. It was quite an easy task to accomplish, but our ‘evidence’ was never disproof–merely ‘objections’.* Especially when some of that evidence was disbelief that such events could be more common than we’d thoguht-a denial that turned out to be false if only on statistical grounds. Another was the suggestion that if the K-T extinction were impact induced then wasn’t it likely that all other extinctions may also have had an extraterrestrial forcing factor? 25 yr later we can say that some were impact-induced, others not and some others are still undecided.
    Both of these arguments are easy to make, just as you have done. But neither argument follows logically as any sort of dis-proof. All you’re really saying is that if due to an extraterrestrial impact, the YD is (feared to be) taken out of your sphere of study and influence. This isn’t a well-founded fear as both paleontologists and geologists have come to realize.
    As you said…you’re not experts on nano-diamonds, so let’s wait until the verdict is in on that one. Just as the debate on shocked quartz and the K-T went on for needless unproductive years, this one could too. However, our instrumentation and methods are much improved over those of 25 yr ago, so this matter should soon be decide.
    And please, let’s not complain if we don’t soon (or ever) find a crater. Remember ice cover was much greater at the time and there was still nearly 75% of the surface covered by oceans. No, lack of evidence isn’t evidence on the pro-comet/asteroid side either…However, a nanon-diamond-bearing layer in the right place is rather good evidence itself unless one can come up w/a way for gradual, common sedimentological process to concentrate nano-diamonds in certain layers by chance immediately at an extinction & abrupt climate-change horizon.
    It’s sad to see that you’re leaning toward the ‘denialist’ side just because the Younger Dryas event might be snatched (partially) from your realm of study by an extraterrestrial event. We’re all too familiar w/GW denialist claims and their reasoning already. Don’t fall into the trap. Let the experts do their work, and we’ll go from there.

    *To my knowledge the single most vexing aspect of the K-T extinction has never been clearly explicated. Why the large animals and all dinosaurs, many birds, many shallow water marine shelled organisms among others, to the exclusion of insects, small mammals, lagoonal, swamp and estruarine animals and plants such as alligators, frogs, turtles, etc? Perhaps the following 1993 hypothesis (perhaps made by others, but personally propoed to & rejected by “The New Scientist”) may find some following. So please indulge me for a pararaph…
    I find the following realization–based on damage to ground-nesting birds’ eggs by acid rain–quite convincing. Dinosaurs were ground-nesters and could utilize either ‘upland’s', deserts and forests or fluvial, lacustrine or onshore swampy environments for nests. Thus indicating no particular adaptation to typically stressful environments where all habitat variables-especially water chemistry vary wiidely and inpredictably (e.g., alligators and croc’s). The same was/is true of most shallow off-shore marine invertabrates. But these stressful habitat conditions could easily be tolerated by small burrowing mammals reptiles and insects. This also explains the selective survival of ground-nesting Cretaceous water-birds over others. Acid rain fall-out from the impact is thus the likely culprit causing the Selective extinction.
    It’s not that acid rain hadn’t been considered an important killing mechanism before this time (& thus the superficial reason for the paper’s rejection). What hadn’t been recognized earlier or by the editors was acid rain’s probable greater/est importance and it’s central role in explaining the Selective Extinctions.

  23. 23
    mauri pelto says:

    In evaluating how widespread the affect of the potential impact let us not forget the data that has been meticulously put together and represents more than 50 years of paleoclimate research efforts in the area or pollen. Eric Grimm worked hard to create this database, so let us not forget about it. If you go to the following site you can view pollen diagrams from specific sites. The scientists are not only noting pollen counts but also are looking for any burn horizons etc. For Minnesota alone there are 120 records, shows the emphasis at the U of Minnesota. For the US there are 510 and for Canada 335. http://www.ncdc.noaa.gov/paleo/ftp-pollen.html It is too bad we do not have a similar database for glacial geologic stratigraphy

  24. 24
    Jim says:

    “ocean circulation change (the most well-evidenced proximal cause of rapid climate change)”

    Are there others?

    What if the comet stuck in such a way that most of the water precipitated over Greenland, the Arctic, and Siberia?

  25. 25
    Phil. Felton says:

    SteveF Says:
    8 January 2009 at 11:20 AM
    Robin said:

    I’m with Jim O’Donnell. The Comet Impact Theory would nicely explain the demise of BOTH the megafauna and the Clovis peoples from the paleontological record.

    I don’t think it’s massively more successful than any other other “natural” explanation for megafaunal extinction. If this impact did cause extinction, it had remarkable powers of selectivity.

    IMO, the overkill hypothesis is by far the most parsimonious.

    It’s not necessarily the most parsimonious if it is established that a comet did hit in that location at that time and that massive fires/floods occurred. Then any explanation would have to take that into account and such assumptions as ‘overkill’ would not necessarily be the most parsimonious.
    There’s huge difference between saying ‘maybe a comet did it’ i.e. introducing a new ‘entity’, and saying ‘maybe the comet impact that we know happened at that time and place did it’ where no new ‘entity’ is introduced.

  26. 26
    tamino says:

    It seems to me (although it’s not my field) that the evidence for an impact is very strong. Whether or not it was the “trigger” for the YD is quite another question.

    I’m not persuaded by the argument based on D-O events. It appears to be an open question whether or not the YD is really like D-O events, and even if it is there’s no reason the YD couldn’t be triggered by an impact while D-O events have some other trigger.

    In my post on the subject, I didn’t say the evidence was a “done deal” but I certainly argued its plausibility more than I would today. The main reason I’ve revised my opinion is Mauri Pelto’s reminder that there’s a lot more evidence to consider than just the evidence presented for an impact event. There’s lots more stratigraphy, plenty of pollen data, and since that’s nowhere near my field I have to accept the fact that I don’t just lack thorough knowledge of it, I lack even the passing acquaintance required to combine old data with new and get a reliable perspective.

    But the single comment I agree with most is Ray Ladbury’s paraphrase of John Mashey. It’s good to discuss some actual controversial science! As important as it is to debunk climate junk, this is way more fun.

  27. 27
    SteveF says:

    Phil. Felton said:

    It’s not necessarily the most parsimonious if it is established that a comet did hit in that location at that time and that massive fires/floods occurred. Then any explanation would have to take that into account and such assumptions as ‘overkill’ would not necessarily be the most parsimonious.

    Sure. However, it would still have to account for the faunal structure of the event. Which a purely natural mechanism struggles to do, IMO, given the selectivity involved. I think there’s a fundamental issue of parsimony that natural mechanisms will struggle with in this regard.

    Cheers

    Steve

  28. 28
    Jerry says:

    Does the shape of the temperature curve during YD not rather argue against a single event cause? The graphs I’ve seen show a square-wave shape, with a rapid fall, then about 800 years of sustained low temperatures followed by an equally rapid rise to the trend line. From a single event cause I’d expect a sawtooth shape, rapid fall followed by a steady(ish) climb back up.

    [Response: Not necessarily. The shape is widely believe to reflect hysteresis in the way that the ocean responds -- the circulation gets suppressed, and then it take a while for it to "re-start". See Stefan's various writings on this (starting here]-eric

  29. 29
    Hank Roberts says:

    > more fun
    Amen. And I’d love to see the various scientists who specialize in different bits of this puzzle suggest to one another what sort of thing they might look for in their own areas. If there are enough sediment cores (I recall Gavin’s comment that those folks rarely cross-correlate their work, they tend to go deep in one spot!) maybe a map of the impact(s?) or airburst(s?) could be made, to get some idea if it was one big whack or flash, or a myriad of smaller ones, and whether the result was one continent-sized firestorm or many “smaller” ones.

    Did anyone ever look for example downwind of Tokyo or Dresden to see how far the soot was distributed and how well it could be documented? I suppose that sort of thing was done for the big volcanos.

  30. 30
    John A says:

    This is a really interesting topic from the perspective that it, at least temporally, links failure of human populations with rapid climate change and biodiversity loss. However, I am interested in any comments folks on this site may have on a related issue.

    At the same AGU conference, there was a presentation, that has also gotten some headlines and publicity, by researchers who were reporting that the 65 Ma K-T extinction was not caused by ET impact. The evidence, as I understand, was that their finer dating technique showed that the impact and extinctions were offset by something less than a million years, but did not coincide. The massive volcanism of the Deccan Traps, however, occurred during the course of the extinction and drove the global changes. (Which if I recall from my old geology books was a favored explanation before the impact theory.)
    Anyways, I wanted to hear any opinions from the folks here (esp anyone who may have attended the talks) how this ranks up and is being received. Is the timing difference they cite within the error associated with dating things from 65 m.y.a?

    Also, I can see this gaining (or at least staying in the news) in the same way this theory has (without specific debate to merits) because it’s about a pop-culture relevant topic and apparently History Channel is already making a program on “what really killed the dinosaurs” much like they did last year on the Y-D impact to coincide with the “10,000 BC” movie. So the potential for further public misunderstanding of scientific developments is there too.

  31. 31
    Rod B says:

    Eric (18), thanks; makes sense.

  32. 32
    David B. Benson says:

    That some extraterrestrial body or swarm of bodies struck much of North America about 12,900 years ago seems, to me at least, well established. Naturally, I’d like to see more evidence of fires or the lack thereof at more locations. I’d especially like one or more impact craters. But for what I take to be good reasons, neither additional bits of evidence may ever be forthcoming. What we do have is

    (1) The end of Clovis culture without any continuation, AFAIK.

    (2) The extinction of many large mammal species. I attribute this to a combination of (i) climate change stress, (ii) human predation somewhat reducing population sizes, but not that much, (iii) extraterrestial body impact taking the population below viability.

    (3) Variious indicators of extratrestrial impact, but no crater(s).

    So a reasonable hypothesis is that a comet hit the Laurentide Ice Sheet along the southern portion somewhere. This is hypothesized to occur so that an ice damed proglacial lake, going to release sometimes in the next few hundred years anyway, was suddenly released. It carried an unusual quantity of shattered ice with it; this gave rise to the long cool period until all this ice melted.

    Now I found a program here

    http://www.lpl.arizona.edu/impacteffects/

    which calculates effects of small impacts quite nicely, but after some experimentatiion, does not seem quite so good for comet impacts on ice. Noetheless, assuming a comet similar to Shoemaker–Levy 9 in size and density

    http://www.nature.com/nature/journal/v370/n6488/abs/370349a0.html

    impacting ice of very great thickness, the program rapidly calculates

    Your Inputs:
    Projectile Diameter: 1800.00 m = 5904.00 ft = 1.12 miles
    Projectile Density: 500 kg/m^3
    Impact Velocity: 51.00 km/s = 31.67 miles/s
    Impact Angle: 35 degrees
    Target Density: 1000 kg/m3
    Target Type: Ice

    Energy:
    Energy before atmospheric entry: 1.99 x 1021 Joules = 4.74 x 10^5 MegaTons TNT
    The average interval between impacts of this size somewhere on Earth during the last 4 billion years is 2.6 x 10^6 years

    Crater Dimensions:
    Transient Crater Diameter: 18.6 km = 11.6 miles
    Transient Crater Depth: 6.58 km = 4.09 miles

    Final Crater Diameter: 27.4 km = 17 miles
    Final Crater Depth: 0.802 km = 0.498 miles

    The crater formed is a complex crater.
    The volume of the target melted or vaporized is 9.46 km^3 = 2.27 miles^3
    Roughly half the melt remains in the crater , where its average thickness is 34.8 meters = 114 feet

    I interpret this to mean that all ice in the transient crater would be removed and some of the bedrock below would be shattered. Either such shattered rock lies at the bottom of one of the Great Lakes or else was scraped off by subsequent reformation of the local portion of the ice sheet.

    Whether a crater of this size would be sufficient to promote Younger Dryas release I must leave to others; I am not an expert on the cryosphere. What matters to me is that no surviving crater remains; indeed there is no crater of the right age which matches the rest of the evidence (which might well include many of the meteorites picked up in the Great Plains in the 1930s).

    We certainly do not use extraterrestrial impacts to explain

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

    nor do extraterrestrial bodies of this magnitude strike often enough to explain both Younger Dryas and the similar termination 3 event.

    We know that proglacial lakes drain through their ice dams; indeed it is not necessary to suppose otherwise for Younger Dryas except for two matters. (i) Younger Dryas is much the most massive during the destruction of the LGM ice sheets, much more so than the 8.2 kya event for example. If the corresponding event during termination 3 is of about the same size, then it would seem that such can occur without extraterrestrial intervention. (ii) Some extraterrestrial body (or bodies) did impact, leaving traces from Alberta to Belgium, but without a crater. Which is the point I made before; while not necessary, such might well have been sufficient.

  33. 33
    DBrown says:

    Most of you who object to the ‘comet selectivity’ do not understand the argument that the scientist made. Simply speaking, a swam of comet pieces (many hundreds or thousands entered the atmosphere just over North America. These hyper-velocity fragments would heat the upper atmosphere to a few hundred centigrade. Any animal unable to take cover would be burned alive or badly injured by burns and have trouble lasting. Small animals and animals that tend to be shielded (live in water/mud/underground) will fully escape. Birds can cover huge distances so the lost of nearly all of these creatures in North America would not appear on the geo record since they would be replaced in just a few years. This does not mean the idea is correct, just that its selective nature is easy to explain. Of course, forest MUST be destoryed and leave a heavy soot footprint or else the idea is nonsense. Of course, the breakup of a comet that has vast number of fragments will leave few (if any) and very small (shallow) impacts that may not leave any long term evidence.

  34. 34
    Ron Taylor says:

    Please do not put too much confidence on the History Channel. I just watched its program on “Dispensationalism” (a theological topic, prophecy) and it was pathetic.

  35. 35
    John Mashey says:

    re: #16, #26
    It’s not just way more fun, it’s way more instructive for people learning about science:

    1) It’s a paleo problem, requiring ferreting out of partial, difficult data sources. i.e., it’s detective work done millenia after the crime :-)

    2) It’s a highly multi-disciplinary problem, with a variety of data sources, not all of which are necessarily visible to any given investigator, and some of which may be or seem contradictory.

    3) It’s not a problem of measuring/modeling some behavior, say like temperature records, where people generate reconstructions with error bars, and reconstructions may differ, but one can analyze the data and look at the graphs to see if they make any sense together. (I.e., like Figure 6.10 in IPCC WG I, which do. Anyone sensible knows that different lines whose error bars generally overlap are more in agreement or not.

    It’s an *attribution* problem: the major effect is known, but it’s unclear *why*, and specifically, whether or not a particular event happened to trigger the effect, and how. What makes it fun is that it’s a murder mystery where the butler doesn’t have “murderer” written on his forehead.

    4) We’re still in a state where reasonable people can disgree.

    5) Of course, as usual, it’s not a problem you can do in a high school lab… In particular, replicating this experiment is not recommended…

  36. 36
    Jim Cross says:

    For a comet impact animation, look at this:

    http://www.sandia.gov/video/CometSim.htm

    It is not clear to me that a comet impact directly into the Laurentide ice sheet would leave either an impact crater or evidence of soot and fires. Depending upon the size the comet, it might seemly vaporize a big chunk of ice and eject it high into the atmosphere. From there, much of the water vapor might condense and fall in the direction opposite from the comet. If that direction was towards the north, much might fall as snow or ice or water that rapidly freezes leaving relatively small amounts of fresh water injected into the North Atlantic. Significant but smaller amounts would go higher and possible circle the world. This could cause massive floods in some areas but might also block sunlight sufficiently to trigger major cooling along with the longer term effect of enhanced albedo of the expanded snow/ice area in the Northern Hemisphere.

    This would not be inconsistent with DO events occurring independently from this and either before or after it.

  37. 37
    Thomas says:

    I think most modeling should wait until the evidence of impact(s) has been better evaluated, and estimates of the mass of extraterrestrial material are available. One question for the glaciologists, if we postulate that an impact might have triggered the release of a glacially dammed lake, just what sort of time window are we talking about? I.E. how long is such a lake vulnerable to say an impact generated tsunami, versus when it would have failed without any help.

    Comets, and some asteroids may well be extremely fragile objects. At a low impact angle, it is possible that the energy release might be spread over a large area, perhaps without the formation of a large crater. At any high impact angle, once the density per unit area of the impactor exceeds the area density of the atmosphere (roughly 3KG/cm**2) the mass of the impactor should be able to ram through the atmosphere, regardless of its cohesive characteristcs.

  38. 38
    PHE says:

    Latest wisdom from the BBC
    “Heat may spark world food crisis”
    http://news.bbc.co.uk/2/hi/science/nature/7817684.stm

  39. 39
    Mark Stewart says:

    The craters exist. There is a band of lakes across North America. The traditional explanation is that the lakes were caused by the ice sheet. Another possible explanation is that the lakes are the results of a Shoemaker-Levy type comet impact. The linear relationship between the Great Slave, the Great Bear and the Great Lakes seems a little improbable. The other lakes seems to form a band. I would think that the lakes formed by the ice sheet would be more randomly scattered.

  40. 40
    Chris Colose says:

    Eric, I agree with much of what you said but I do not really agree with your line of reasoning. I have not gone through the comments here so I apologize if this is repetitious.

    I am also skeptical of the impact idea, but that’s because (like you) I have not seen any convincing evidence for a mechanism on how it produces a YD-like climate change. The skepticism does not arise because it contradicts the other abrupt changes (which obviously makes no sense).

    For one thing, the YD was more like a seventh Heinrich event (H0) rather than a D-O event (which is marked by relatively rapid warming, not cooling). The YD was not at all like a D-O event and probably had causes much more similar to Heinrich events, of which we have a good story for. This involves the sudden delivery of freshwater influx into the North Atlantic after successive but progressively cooler D-O events (or a Bond Cycle).

    You’re certainly right in that there is no need to throw in another forcing mechanism. There’s not much room for big forcings like cosmic rays in modern times either, and they don’t do any good for predictive ability, but they should not be excluded because we “don’t need them.” The study of the abrupt changes is in its infancy, but I agree the impact idea is unlikely to make significant progress. RC’s own Ray Bradley has proposed that paleocryptic ice from an isolated arctic ocean (thick multi-year sea ice overlain by ice and firn) forced a reduction of the THC and was largely responsible for the YD, with the termination of the paleocryptic ice possibly resulting from warm water inflow from a relatively warm Atlantic and Pacific after sea levels rose, the Bering Strait was flooded over, and the end of arctic isolation.

  41. 41
    Lab Lemming says:

    How would icesheet dynamics be effected by a small to medium impact? My guess is that
    A. You’d melt a bit of ice.
    B. You’d shatter the underlying ice below the melt zone.
    C. That would allow the water to drain down to the icesheet-bedrock interface.

    However, The reasoning expressed above makes no sense. Nobody is arguing that impact is the ONLY way to cause kiloyear duration North Atlantic cooling events. So there is no need to re-examine D-O events.

    Even if you think you understood the YD before the impact hypothesis was raised, if you accept an impact, it means that your assumptions about the initial conditions for the YD were incorrect. So you need to demonstrate that the primary and secondary effects of impact won’t alter the pre-existing climactic state before you can discount it as a potential cause.

  42. 42
    SteveF says:

    DBrown

    Most of you who object to the ‘comet selectivity’ do not understand the argument that the scientist made.

    I do understand the argument made. Unfortunately it doesn’t fly IMO. There strikes me as a fair bit of special pleading in your argument. Moreover, the distinction between small and large can’t explain the selectivity of the impact precisely because the small and large distinction does not neatly exist. Not all large megafauna went extinct, only around 3/4.

    It wasn’t simply a neat divide between big beasties and little ones; the faunal structure of the event was much more complex. Therefore a blanket natural event will struggle to explain this. It is much easier to explain the structure with an overkill framework. The natural changes of the time would certainly have added greater stress to animal populations, but humans were required to complete the picture.

    Cheers

    Steve

  43. 43
    Nick O. says:

    Hi folks,

    Slightly off topic, I know, but are there any predictions yet of the expected minimum Arctic ice cover for this year? Last year’s minimum areal extent was a bit higher than the record set in 2007, but I have read that this was probably offset to some degree by the thinning of the ice, so that the volume was a minimum, or near minimum, record. Also, high pressure over much of Europe (and other parts of the near Arctic) seems to have encouraged unusual wind and temps, leading to a reduced freezing (formation of sea ice) over the last couple of weeks. Could we be seeing the setting up of conditions for even greater ice loss this year? Any thoughts, anyone?

    Nick O.

  44. 44
    mauri pelto says:

    I still see an irrational tendency to want to invoke widespread catastrophic floods and fires to this potential event. Each of these affects would leave telltale and obvious evidence in the stratigraphy that would not have been missed in the literally thousands of stratigraphy’s completed. This does not mean there was no impact or some fires, as the authors found. To date no evidence of flooding has been cited. People including the scientists got way ahead of their evidence, which was black mats and nanodiamonds. Their evidence says nothing about climate, glaciers, or megafauna directly. We have lots of evidence that does speak directly to these topics, that can be trusted. If a scientific idea does not fit the existing data all that well, then there is something wrong with it. In this case it is the scale of the supposed affects. In looking back through the pollen diagrams and glacial geology in the Lat Quaternary Einvronments of the United States book and in The Last Great Ice Sheets, I am not seeing anything suggestive of a catastrophic change. A glacier dammed lake outburst leaves some pretty tell tale signs as well, particularly in water level drops. There is shoreline data as I noted yesterday available for the lakes. If there is a sudden lake level change it is apparent. Such events do not occur without leaving evidence that survives a few thousand years. If you buy into hugely catastrophic impact, then you have to ignore all the commentary on glacier recession and pollen changes that do not indicate anything that abrupt.

  45. 45
    SteveF says:

    Mauri Pelto said:

    In looking back through the pollen diagrams and glacial geology in the Lat Quaternary Einvronments of the United States book and in The Last Great Ice Sheets, I am not seeing anything suggestive of a catastrophic change

    There are potentially problems with cause and effect. The transition into the YD in terms of vegetation was pretty substantial (if not necessarily catastrophic). It’s a sufficiently significant shift and this could be overprinting the very temporally precise signal of the impact in the pollen, if it were the impact that “caused” the YD.

    So, to elaborate somewhat, the scenario could be:

    1) An impact. Impact causes catastrophic vegetation change immediately. A high resolution event.

    2) The impact results in a rapid climate change. The ecosystem responds to this resulting in a cold climate flora. The brief period of time of total vegetation catastrophe has therefore not been recorded. The YD has, effectively, overprinted it.

    Hopefully what I’m trying to say is clear. I’m not familiar with North American YD pollen diagrams, which doesn’t help. That said, your wider point is well taken. There should be good evidence from within the old fashioned, tried and tested late quaternary palaeoarchives for this impact.

  46. 46
    Sekerob says:

    SteveF/mauri pelto, how many/little years of transition from previous to “cold climate” flora is needed to “overprint” such an event?

  47. 47
    Hank Roberts says:

    Mark Stewart, have you read something suggesting those particular lakes are craters, or is this idea your own work? How do you distinguish a crater from a lake besides the fact that they line up?

  48. 48

    Ron — I hear ya about the Discovery Channel. They also run UFO stuff. “Discover how to misinterpret evidence so it’s cooler!”

    Interestingly, the CAPTCHA words are “claims Mackay.” Does anyone remember a famous book by an author with that name?

  49. 49
    Hank Roberts says:

    Jim: 30 years is a conclusion about the particular data you’re trying to understand. It’s not arbitrary; it’s done to know what you can understand from the data, so you can collect useful facts.

    For climate:
    http://moregrumbinescience.blogspot.com/2009/01/results-on-deciding-trends.html

    You can learn:

    “… to discuss climate trends in global mean temperature, you need to use 20-30 years of data centered on the date of interest.”

    It’s worked out there for students. He will convince you, if you work through the steps shown.

  50. 50
    SteveF says:

    Sekerob,

    SteveF/mauri pelto, how many/little years of transition from previous to “cold climate” flora is needed to “overprint” such an event?

    That was my suggestion and I should say that it isn’t gospel. I was just batting around ideas as to why the impact might not show up in pollen diagrams (if indeed it did occur). Thinking aloud as much as anything.

    That said, we have an idea of roughly how long the transitions took and not just from ice cores. I’m most familiar with European work – there was an excellent multidisciplinary project at a site in Norway called Krakenes (and also Gerzensee in Switzerland). In particular see this paper:

    Birks, H.H et al. (2008) The development of the aquatic ecosystem at Krakenes Lake, western Norway, during the late glacial and early Holocene – a synthesis. Journal of Palaeolimnology, 23, 91-114.

    there’s also a shorter summary here:

    Birks, H.H. and Ammann, B. (2000) Two terrestrial records of rapid climatic change during the glacial-Holocene transition (14,000-9,000 calendar years BP) from Europe. PNAS, 97,1390-1394.

    This showed extremely rapid response from the vegetation to the Younger Dryas event. It’s hard to be entirely precise; the chronologies are never going to be as good as we want them to be. Nonetheless, as the latter paper says:

    Both studies demonstrate the sensitivity of terrestrial and aquatic organisms to rapid temperature changes and their value for quantitative reconstruction of the magnitudes and rates of the climatic changes. The rates in these two terrestrial records are comparable to those in Greenland ice cores, but the actual temperatures inferred apply to the terrestrial environments of the two regions.

    Wether all this is good enough to support the explanation I suggested, well I wouldn’t bet my house on it. It’s something to think about though.


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