Feb 08 2018

Did a Comet Kill the Mammoths

Between 12,800 to 11,500 bp (before present) there was a cold period in North America called the Younger Dryas – named after the dryas flower whose pollen is a good marker for such cold periods. During this time the megafauna of North America, including the Mammoth, largely died out. Along with them went the Clovis culture – a big game hunting culture with distinctive stone points.

What caused this period of climate change and mass extinction?

This is a genuine scientific controversy. One group of scientists believe that the melting glaciers dumped fresh water into the northern Atlantic, temporarily shutting down the ocean currents that bring warm water to North America. Another group think that a comet impact is to blame.

There is no smoking gun of a comet or meteor impact at this time, however. This is partly why the debate continues. However, there is significant inferential evidence, and as this evidence grows we may be getting to a tipping point where the comet theory becomes dominant.

There are two new published papers which the authors claim support the comet theory. I don’t know if this will be the tipping point, but the evidence is starting to seem pretty compelling. We’ll have to see how the other side reacts.

The Big Burn

So far the evidence presented for a comet impact is indirect but some scientists find it compelling. Mostly it consists of finding exotic materials in the geological layer that corresponds to the Younger Dryas boundary.

These exotic materials include spherules, nanodiamonds, melt glass, and a platinum spike. Comet impact proponents argue that these materials resulted from the impact. Critics claim that they are either erroneous or result from other causes, such as forest fires. And in any case – without an impact crater, the case for an impact is thin. Meanwhile the case for glacier melt is strong.

Now the comet proponents are firing another salvo – evidence from ice cores and other studies showing that right at the Younger Dryas boundary layer there is evidence for massive biomass burning. Further, there is a CO2 spike at the same time, enough to suggest that as much as 10% of the plant life on Earth at the time burned.

What this could mean is that there wasn’t a single impact, but that essentially the Earth passed through the cloud of a comet and was hit by a swarm of smaller pieces, igniting forest fires around the world.  Smoke from these fires could have then resulted in an impact winter – blocking out the sun reducing photosynthesis and reducing temperatures.

This conveniently explains away the absence of a large impact crater, while also explaining the presence of exotic impact-related materials, and also the products of massing burning, all at the Younger Dryas boundary layer.

This sounds like a reasonable case for the comet theory. Will it be compelling enough, however, to win over former critics?

Scientific Controversies

That is often how these scientific controversies work. Different factions make their case in the absence of definitive evidence, and also point to future evidence that will support or refute their hypothesis. The various sides duke it out, until eventually the evidence builds enough on one side that the majority of scientists come over.

There is no magic threshold, but at some fuzzy point we may have a consensus built on evidence that has been thoroughly debates by proponents of various hypotheses. A consensus does not mean 100%, no further evidence needed, the science is over. It just means that the evidence is strong enough in support of one side that the vast majority of scientists now accept that as the most likely answer.

There is always room for minority opinions, and sometimes surprising lines of evidence may disrupt the debate.

We see this process with many current scientific controversies. Is floresiensis (the Hobbit) a unique species or a diseased human? Did the dinosaur extinction result entirely from an impact, or did other sources of climate change play a role?

For a time there was serious debate about whether or not dark matter existed or were observations explainable by modifying Newtownian gravity. We have now reached a consensus that some form of dark matter probably exists – but until we identify exactly what it is, there is room for dissent.

There are countless such debates, big and small, in the world of science. That is how science works and progresses. For every question, there is a spectrum from unknown, to genuine controversy, to strong consensus, to rock solid. Science communicators should always strive to put any new study or bit of evidence into the context of where the scientific question is on this spectrum.

Further, there are practical implications for properly characterizing the state of a scientific controversy. What do we teach in classrooms? What research do we fund? Was medical treatments are acceptable, and should be reimburse with public funds? And what scientific conclusions are sufficient to base public policy on?

There is no formula. This takes judgment, a thorough understanding of the science, and an individual decision. Conclusions also have to remain open to revision as new evidence and theories come to light, but we also have to make decisions in the meantime.

The obvious controversy to which all this applies is global warming, because it is such a hot political topic. The prevailing scientific opinion is that there is a strong-enough consensus that global warming is happening and is due to human activity to form a basis for policy. That does not mean the science is over, and there are always error bars on every scientific conclusion. But we have a practical consensus. It would be folly to ignore it, or to focus myopically on uncertainty in order to manufacture paralysis.

It is useful to examine non-political scientific debates and controversies, to see how they work, and how they are resolved. Not many people are going to have a strong ideological position on the Younger Dryas, and I don’t expect to see it in the platforms of major political parties. That is why it is a great way to see how science works. Those lessons can then be applied to more politically controversial topics.


23 responses so far

23 thoughts on “Did a Comet Kill the Mammoths”

  1. bachfiend says:

    Climate science is fascinating. And will continue to be funded despite a certain Internet Troll who not only wants it defunded but also climate scientists gaoled for a (non-existent) criminal conspiracy.

    The efflux of freshwater into the North Atlantic shutting down the Gulf Stream is supposed to have resulted from melting of an ice dam allowing the draining of Lake Agassiz, which was huge covering an area greater than that of all the present Great Lakes.

    The one problem with that hypothesis I’ve wondered about is why would shutting down the Gulf Stream cause cooling of North America? At the time of the Younger Dryas, the Milankovitch cycles would have meant that the Northern Hemisphere summer was occurring around perihelion, so summers in North America ought to have been warmer and shorter.

    What would be the local effects of draining a very large glacial lake? Instead of having a large body of liquid water absorbing around 90% of solar radiation, warming the surrounding air by outgoing infrared radiation, it would be replaced by bare earth reflecting around 50% of the incoming solar radiation.

    Would local changes in albedo be enough to explain the cooling in North America? I don’t know. Personally, I tend to discount bolide impact hypotheses almost as a reflex. They’re just too easy a hypothesis being proposed far too often, including for the end of Permian mass extinction.

  2. The Gulf stream significantly warms North America, and without it things we definitely be cooler here.

  3. bachfiend says:


    Does it? I thought it mainly warms northern Western Europe because the prevailing westerly winds carries its warmth to Europe not America. London is on much the same latitude as Newfoundland, which is much colder than London.

  4. bachfiend says:

    Anyway, I’ve found an article by Richard Seager from 2007 who argues that based on climate modelling the Gulf Stream doesn’t cause significant warming of Western Europe. Nor of North America.

    Western Europe is warmer in winter because the Northern Atlantic absorbs a large amount of heat during summer which it releases in winter, and the prevailing westerlies carry the heat to Europe.

    Eastern North America is colder because the Rockies create a standing atmospheric wave blocking the moderating influence of the Atlantic.

  5. BillyJoe7 says:


    The Gulf Stream warms both the eastern parts of North America from Florida to Newfoundland, especially Florida; and the western parts of Western and Northern Europe from Ireland, England, and Scotland to Norway, especially Norway. East winds move warm air inland along the east coast of North America, especially along the Florida coast. West winds move warm air inland along the west coast of Western and Northern Europe, especially along the Norwegian coast.

  6. BillyJoe7 says:


    I would be careful of relying on a single article by a single author. That author could be an outlier not representing the consensus (as seems to be the case here).

    From where I got the above information:


    “The Gulf Stream influences the climate of the east coast of North America from Florida to Newfoundland, and the west coast of Europe. Although there has been recent debate, there is consensus that the climate of Western Europe and Northern Europe is warmer than it would otherwise be due to the North Atlantic drift which is the northeastern section of the Gulf Stream”

  7. bachfiend says:


    I agree that one shouldn’t rely on a single article. I was noting that there is some contrary opinions concerning the warming effect of the Gulf Stream, which to me seems reasonable. It seems strange that the heat the Gulf Stream derives from the warming of the Caribbean should result in less warming of North America and more warming of Western Europe. Whereas the idea that heating of the entire North Atlantic in summer should allow comparative warming of Western Europe in winter.

    Your comments concerning a single article also applies to Wikipedia.

  8. bachfiend says:

    BTW, the link to Richard Seager on Wikipedia is broken. I found the article elsewhere by a google search. I found the article interesting because it illustrates just how interesting and complex climate science is.

  9. BillyJoe7 says:

    Except that the Wikipedia article refers to the consensus, whereas the Richard Seager article is his own opinion.

  10. bachfiend says:


    Then why is Newfoundland so cold and England so warm?

  11. Kabbor says:

    This strikes me as an argument of: Is it X or Y? It is probably X and Y. The gulf stream impacts the temperature, and so do the prevailing winds. The water travels north and also heats from the summer sun.

  12. bachfiend says:


    Richard Seager argues that it’s consensus opinion that the Gulf Stream warms Western Europe just because it’s accepted wisdom ever since 1855. All papers discussing the Gulf Stream start with words similar in effect to ‘it’s well known…’, and then don’t bother to examine the claim.

    He argues, based on one climate model, that if the Gulf Stream disappeared, the climate of Western Europe and North America wouldn’t change much at all, the average temperature wouldn’t decrease.

    That’s not to say that the Gulf Stream, as part of the global thermohaline circulation, isn’t very important for the health of the oceans.

    If he’s right, then a shutting down of the Gulf Stream wasn’t the cause of the Younger Dryas cooling (a hypothesis that has the problem as to the mechanism by which an influx of freshwater into the North Atlantic from melting glaciers or drainage of a large glacial sea would cause it to shut down).

    And I find the hypothesis that a comet impact, with widespread forest fires, was the cause of the Younger Dryas rather unconvincing. The arguments against an all out exchange of nuclear weapons not resulting in a nuclear winter apply with much greater force to widespread forest fires having a similar effect. Forest fires are just not hot enough to loft soot high enough in the atmosphere to persist for long – the soot would be washed out quickly, leaving the extra CO2 in the atmosphere, which should result in global warming. Not to mention that snow and ice covered with soot would melt very quickly causing glaciers to recede not advance.

    I favour the hypothesis that the cause of the Younger Dryas cooling was a series of increased numbers of volcanic eruptions (not just one or two large ones) similar to those that caused the Little Ice Age. And we know that volcanic eruptions can cause very significant and profound global cooling as shown by the 1815 eruption of Tambora (and the earlier 1809 Krakatoa-sized eruption of volcano ‘unknown’).

    Many of the findings of the linked studies in this article can also be explained more economically by volcanic eruptions. Forest fires could have been ignited by volcanic eruptions. And many of the products of bolide impacts are also produced in volcanic eruptions.

    I must admit I’m a volcano ‘fan’. I worry just as much about the supervolcano under Yellowstone erupting (it’s overdue) as I do about AGW. And there’s nothing we can do to prevent volcanic eruptions, unlike AGW, so it’s not worth worrying about.

  13. nhults says:


    In response to the “why is Newfoundland colder than England” question, I don’t think atmospheric scientists would suggest that we can say we understand all of the mechanics at play, but I think I can summarize the scientific consensus, which is appropriate given the larger scope of discussion concerning this post. I will note: I may be completely misunderstanding this, as I am not one of the aforementioned atmospheric scientists, so if someone knows/understands this better than I, I welcome any corrections.

    The primary contributing factors to the northern-latitude climate seem to be the now-infamous Gulf Stream and the Coriolis effect. I think we, on this thread, understand the Gulf Stream at this point, and recall that the Coriolis Effect is the roughly counter-clockwise motion of weather and atmospheric fronts in the northern hemisphere, and clockwise motion of such in the southern hemisphere, resulting from the rotation of the Earth (the direction of spin observed in hurricanes being the most obvious and dramatic example of the Coriolis Effect). Basically, the current model goes that the warm water from ocean currents like the Gulf Stream warms the air along the East coast. This air rises, creating a low-pressure area which is filled by air from the north moving counter-clockwise (i.e. roughly south and south-west), which in this case is colder polar air, causing the colder mid-latitude climates observed on the east coast and midwest of North America. In short, thanks to the Coriolis Effect, the warmer water of the Gulf Stream, somewhat paradoxically, causes the observed cooling of the east coast and midwest of North America.

    The same effect can be observed in the northern Pacific ocean, as well: the east coast and central steppes of Asia have cooler climates than comparable latitudes of the Pacific Northwestern coast of North America. The effects would theoretically be reversed in the southern hemisphere, but I don’t think as dramatic an effect is observed due to the relative absence of higher-latitude landmass in the southern hemisphere, other than Antarctica, where it’s cold as f*** regardless (“cold as f***” being, I believe, the preferred scientific parlance).

    Lots of sources consulted, but https://www.livescience.com/13573-east-coast-colder-europe-west-coast.html has a good summary of this argument. If anyone who knows better than me cares to refine/refute this post, please do so. The only thing I like more than being right is being wrong and learning something!

  14. bachfiend says:


    ‘In short, thanks to the Coriolis Effect, the warmer water of the Gulf Stream, somewhat paradoxically, causes the observed cooling of the east coast and Midwest of North America.’

    I think that goes along with my impression. I don’t think that the Gulf Stream causes any warming of North America, so that shutting down the Gulf Stream wouldn’t cause cooling and couldn’t be the cause of the Younger Dryas cooling.

  15. Nitpicking says:

    Bachfiend, you seem to think that North America starts at New York latitudes. The Gulf Stream warms Florida, then *crosses the Atlantic* to warm England. This is actually in no way controversial. It doesn’t warm Nova Scotia for the same reason it doesn’t warm Japan: it doesn’t actually get very close to it.

  16. bachfiend says:


    Then again, Florida isn’t North America either. Shutting down the Gulf Stream wouldn’t cause a significant cooling of Northern America (because the Gulf Stream ‘doesn’t actually get very close’ to most of the mass of the North American continent), the Younger Dryas and the extinction of the mammoth and the Clovis culture as claimed.

  17. Pete A says:

    The North Atlantic Gyre, located in the Atlantic Ocean, is one of the five major oceanic gyres. It is a circular system of ocean currents that stretches across the North Atlantic from near the equator almost to Iceland, and from the east coast of North America to the west coasts of Europe and Africa.

    The currents that compose the North Atlantic Gyre include the Gulf Stream in the west, the North Atlantic Current in the north, the Canary Current in the east, and the Atlantic North Equatorial Current in the south. This gyre is particularly important for the central role it plays in the thermohaline circulation, bringing salty water west from the Mediterranean Sea and then north to form the North Atlantic Deep Water.

    END of QUOTE

    I recommend reading the section Seasonal variability, the article Ocean gyre, and the article Gulf Stream from which I quote below:

    “… As a consequence, the resulting Gulf Stream is a strong ocean current. It transports water at a rate of 30 million cubic meters per second (30 sverdrups)** through the Florida Straits. As it passes south of Newfoundland, this rate increases to 150 million cubic metres per second.[18] The volume of the Gulf Stream dwarfs all rivers that empty into the Atlantic combined, which barely total 0.6 million cubic metres per second.”

    ** The specific heat capacity of water is ~4.2 J/(g·K); ~4 kJ/(litre·K); ~4 MJ/(m³·K). Therefore, 30 sverdrups transports heat at the rate of 120 terawatts per kelvin temperature difference!

  18. BillyJoe7 says:


    Again, we have to be careful to distinguish between the consensus view and an outlier. The outlier might be true but the odds are against it. Of course, it depends on the strength of the consensus, and on what that consensus is based. I’m not sure how strong the consensus is, but it is obviously in the interests of the outlier to trash it.

    And, sorry, when I said Newfoundland, I meant Nantucket. Along the east coast of North America from Florida to Nantucket, the prevailing trade winds blow warm air inland. Further north, the trade winds give way to the Westerlies, so this warming influence is no longer relevant as far north as Newfoundland. At Newfoundland, the Gulf Stream turns towards Northern Europe as The North Atlantic Drift and the Westerlies blow warm air over Northen Europe as far north as Norway.

    That is the present consensus. The outlier view goes back as far as 2002 as far as I can tell. Apparently the consensus hasn’t yet moved in that direction, so either the outlier doesn’t hold up, or the outlier needs to do a lot more convincing.

  19. bachfiend says:


    Look at the link provided by nhults. Seager isn’t an outlier. The article states that the Gulf Stream causes cooling of the eastern coast of North America. And only contributes 10% of the warming to North America.

    I think it’s a consensus opinion that the Gulf Stream warms because it’s a consensus that hasn’t been examined.

  20. nhults says:

    I think both sides of the debate in this thread are right. It has been assumed for decades that the Gulf Stream, carrying warm water from the tropics to the northern latitudes, warms said latitudes. That would be bachfiend’s referred to “consensus that hasn’t been examined.” But recent observation of temperature trends and atmospheric motion has been examined, and has suggested that the Gulf Stream may only be responsible for warming Western and Northern Europe, and have the contrary effect on North America. Certainly, the warming of Europe by the Gulf Stream is now, based on current science, the majority opinion.

    Seager, I think it’s safe to say, remains an outlier by suggesting that the cooling in Central and Eastern North America is 50% due to the atmospheric wave off of the Rockies, the most notable hole in which, it seems to me, being that it doesn’t explain the warmth of the Pacific Northwest compared to similar latitudes in Central and Eastern Asia, while the motion of ocean currents does offer an explanation for that. Seager says, when he talks about his idea of heat transfer over the oceans, that “a similar process occurs across the Pacific Ocean,” but gives no further detail, which seems necessary given that lack of continuous continent-spanning mountain ranges in Asia comparable topographically to the Rockies, plus the vast size of the Pacific Ocean, which (I think) would result in more ambient heat loss through his proposed mechanism.

    It seems to me that the most parsimonious explanation is that climate is complex, and a combination of effects, such as the Gulf Stream, the Coriolis Effect, atmospheric movement off the Rockies, and probably some other mechanism we don’t yet grok, all contribute to the northern-latitude climate as we understand it. And since the evidence suggests AGW might be slowing down the Gulf Stream, we should probably be worried about that, because whether or not it caused the Younger Dryas extinction, it’s a part of the global climatic equilibrium we had been enjoying for several millennia, and if it stops, whatever the specific effects are, it is probably better to assume they are bad and try to stop it than to wait for it to happen and say, “Well, what now?”

    By the way, here is the link to the Seager article, since it’s generated so much discussion:


  21. stevegingto says:

    In reading the comments here, it is disappointing that the commenters have hijacked the story to be about global warming.

  22. Kabbor says:


    I certainly hope you didn’t mean that seriously. I don’t know you so I have no idea if you are.

  23. bachfiend says:


    I certainly wasn’t trying to highjack this thread into global warming. The thread was about the cause of the Younger Dryas cooling, for which there were two hypotheses proffered; it was a comet impact or it was due to a shutting down of the Gulf Stream as a result of an influx of freshwater into the North Atlantic (which was the hypothesis I’d accepted until I read this thread).

    I found the Seager article (there’s actually a much longer one available on the Internet with equations – I don’t provide links because it’s not possible to provide links using a iPad in commenting) in response to Steve’s assertion that the Gulf Stream warms North America – which wasn’t my understanding.

    So I looked for references and found Seager who claimed that the Gulf Stream warms neither North America nor Western Europe significantly.

    I agree with nhults – ‘climate is complex.’ AGW deniers make the error in assuming that if any other factor other than greenhouse gases caused past climate change, then current CO2 increase can’t cause global arming.

    If I had to bet my dog’s life (or better still my neighbour’s dog’s life) on the cause of the Younger Dryas cooling I’d pick the hypothesis was that it was due to a series of increased numbers of volcanic eruptions over a period of several centuries similar to the Little Ice Age. Not one big eruption, like Tambora – which caused its cooling for only a few years (one of the reasons I don’t like the hypothesis that widespread forest fires would cause cooling lasting centuries).

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