Guest Post by Willis Eschenbach
Well, Nature Geoscience is on a roll. Their latest “scientific study” makes an old claim in a new way. After ascribing the temperature changes in Lake Tanganyika to human actions, in a new paper they are now ascribing the changes in the climate 12,000 years ago to the actions of humans in changing the methane levels …
Figure 1. The real reason for the ending of the Ice Age
No, that’s not from the Nature Geoscience article. We’ll get to that, but first , a short cruise through the historical methane data.
As usual, the NOAA Paleoclimatology site has the goods. The data shows an interesting thing. This is that, like CO2, the amount of methane in the air is a function of the temperature. Figure 2 shows the relationship.
Figure 2. Relationship between temperature and methane, Vostok ice core data, last half million years. Image Source
As you can see, temperature and methane are tightly coupled. The relationship is that when temperature raises by 1°C, the methane concentration in the atmosphere goes up by about 24 parts per billion by volume (ppbv). The reasons for this are not entirely clear, but the methane mostly comes from natural fermentation in wetlands. And as anyone knows who has made the prison liquor called “swipe” from potato peelings in a mason jar, fermentation increases with temperature … or that’s what I’ve read, at any rate, I wouldn’t know about that myself …
So what did the Nature Geosciences article say about methane? It is entitled “Methane emissions from extinct megafauna”, by Smith et al. (hereinafter S2010). You have to pay them $18 to have the privilege of reading it. My advice is, don’t waste your money.
Their claim is that the drop in temperature about 12,000 years ago known as the “Younger Dryas” is due in part to the loss of methane from the eeeevil humans killing off the large animals of North America. This reduced the amount of methane from the … well, let me call it “spontaneous release of large parcels of intestinal gases” of the extinct “megafauna”, the ground sloths and mastodons and wooly mammoths and the like. Here’s their graphic of the event:
Figure 3. Graphic from the S2010 paper.
Note how they clearly show that humans come to North America, and very quickly the methane concentration dropped. (As an aside, don’t they know that Jim Hansen said that American temperatures are meaningless because America is only a few percent of the planet’s surface area? Also, note that they claim that species loss could be responsible for “12.5 to 100%” of the methane decline. Now that’s what I call a robust confidence interval, a variation of eight to one. But I digress …)
I showed above that methane concentration is driven by temperature changes, and has been for a half-billion years. However, they say that this particular event is unique. Why? Not because suddenly the temperature/methane relationship broke down. After all, the methane concentration during the Younger Dryas event is totally predictable from the temperature, just like the during the rest of the half billion years.
Figure 4. Methane levels in the Younger Dryas, featuring the usual flatulent suspects. Methane data from NOAA, showing Greenland ice core methane levels. Note that the temperature changes correlate very well with the changes in methane. Temperature changes inferred from d18O levels. Difference in dating from Figure 3 is because this chart shows years BC.
So why blame megafaunal methane for the drop? Well, because the methane levels drop so fast. I kid you not. In their words:
Moreover, the changes in methane concentration at this time seem to be unique. A comparison with the five largest drops over the past 500,000 years shows that the Younger Dryas transition was characterized by a methane decrease that was two to four times more rapid than any other time interval (Supplementary Table T3, P < 0.01 to P < 0.001), which suggests that novel mechanisms may be responsible.
Now, they ignore the fact that among the historical drops in methane levels, one has to be the largest, so finding the largest one means nothing. And they ignore the well-known and aptly named “Noah Effect”, whereby the largest of a group of natural phenomena is often much, much larger than the second largest of the same phenomena. These together are more than enough to explain the rapidity of the methane drop at the start of the Younger Dryas.
Instead, following the Rahm Emanuael dictum, “Never let a serious crisis go to waste”, they have blamed the precipitous drop in methane at the start of the Younger Dryas on human meddling with the biosphere. We killed the mammoths, their argument goes, which stopped them from cutting loose with … large spontaneous emissions of biomethane … and that made the atmospheric methane levels plunge off of the proverbial cliff. QED.
Now, I suppose that their claim is theoretically possible, and they do a lot of plain and fancy tap dancing to show that it is so, but I’m just a cowboy, so that gives me the right to ask the dumb questions:
1. If missing mammoth methane was the cause of the extremely rapid drop in methane … then what was the cause of the following extremely rapid rise in methane? I mean, the megafauna didn’t suddenly become un-extinct and start passing gas again. So why did the methane suddenly rise again?
For this one, I have no answer other than the obvious one … both the drop and the rise in methane were caused by a drop and rise in temperature. The authors of S2010, however, show no interest in this important question … if the cause of the rapid drop in methane during the Younger Dryas is not temperature but a deficiency in ground sloth gas, then what is the cause of the rapid rise in methane?
2. Is the change in methane forcing significant enough to create such a large temperature change? The S2010 paper says:
Ice-core records from Greenland suggest that the methane concentration change associated with a 1 °C temperature shift ranges from 10 to 30 ppbv, with a long-term mean of about 20 ppbv (ref. 13).Thus, empirically, the 185 to 245 ppbv methane drop observed at the Younger Dryas stadial is associated with a temperature shift of 9 to 12 °C. The attribution and magnitude of the Younger Dryas temperature shift, however, remain unclear. Nevertheless, our calculations suggest that decreased methane emissions caused by the extinction of the New World megafauna could have played a role in the Younger Dryas cooling event.
Well, yeah … but the IPCC says that methane forcing varies linearly with concentration. It also says that a change in methane of 100 parts per billion by volume (ppbv) leads to a change in forcing of 0.05 Watts per square metre (W/m2). Given the methane change in the Younger Dryas of ~200 ppbv, this would result in a methane forcing change of a tenth of a watt per square metre (0.1 W/m2).
Now, the IPCC says that a forcing change of 3.7 W/m2 (from a doubling of CO2) would lead to a temperature change of 3°C. I think this is way too large, but we’ll let that be and use their figure. This means that the Younger Dryas change in methane forcing of 0.1 W/m2 would lead to a temperature change of 0.08°C …
Eight hundredths of a degree? These people are hyperventilating over eight hundredths of a degree? I spent eighteen buck to read their !@#$%^ paper for eight hundredths of a degree? That trivial change in forcing is supposed to have “played a role in the Younger Dryas cooling event”? … I weep for the death of science.
(And since you ask, yes, I do marvel that I was able to get through this without once saying the dreaded phrase “mammoth far…” … hey, wait a minute, whoa, that was close, you almost got me there …)
[UPDATE] There’s another oddity I just noticed about the paper. They use the following formula to calculate the methane emissions:
(4) DMIe = BMe^0.75 *[ (0.0119*NEma^2 + 0.1938)/NEma] where BMe = body mass in kg, and NEma = estimated dietary net energy concentration of diet in MJ/kg
Now, one of the rules of math that was endlessly drummed into our heads by my high schoo chemistry teacher (thank you, Mrs. Henniger) was that the units follow the same rules as the numbers. For example, here’s the formula relating distance (S), acceleration (A) and time (T)
S = 1/2 A * T^2
With S in metres, A in metres/second^2 and T in seconds, this is
metres = metres/second^2 * second^2
or
metres = metres
So far, so good. Now let’s look at the units in their formula:
kg = kg^0.75 * [ (MJ/kg)^2 / (MJ/kg) + 1/(MJ/kg) ]
Simplifying, we get
kg = kg^0.75 * [ (MJ/kg) + 1/(MJ/kg) ]
kg = kg^-0.25 *MJ + kg^1.75 /MJ
Well, that’s certainly a fascinating combination of units, but it is definitely not kilograms as advertised.
So I looked to see where they got the formula … and as I should have guessed, it is from the IPCC …
Mrs. Henniger would not approve, she used to wield her red pencil like Thor’s own hammer on this kind of nonsense.
jcrabb says:
May 27, 2010 at 8:06 pm (Edit)
That seems unlikely. If you get as much methane from rotting vegetation as you got from the megafauna, then variations in the numbers of megafauna would make no difference at all.
Willis said: ” Well, that’s certainly a fascinating combination of units, but it is definitely not kilograms as advertised.”
chris y says: “The constants 0.0119 and 0.1938 do not have to be unit-less. ”
Chris of course is right. This error on Willis’s parts shows something very important: he has no idea what he is talking about. It is such a basic issue, it is the smoking gun that shows he cannot possibly understand the documents he is reading.
http://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect
All of you who are so quick to follow your Piped Pipers need to think hard. Why are you blindly praising material you do not understand? Could it be that you want to believe AGW is false so bad that you lap up any bit of none-sense that seems to refute it? You are not being open minded. If AGW is wrong, it won’t be a Willis who shows this.
Now, Linzden is worth listening to, but read what the mainstream scientists are saying as well. We need, in the next few weeks, to make real decisions: do we cross our figures and hope the majority of climate scientists are wrong, or do we take some measures to protect ourselves?
Mike;
We need, in the next few weeks, to make real decisions: do we cross our figures and hope the majority of climate scientists are wrong, or do we take some measures to protect ourselves?>>
IPCC AR4 scenario based on last 40 years trend = +3 degrees
IPCC AR4 mitigation scenario II = +2.8 degrees
Savings = 0.2 degrees
Requirement = 30% reduction in emissions by 2020.
I think not.
More information is always good … Here’s what the EPA says about natural (not anthropogenic) methane sources:
(Background picture is from the source document.)
Some immediate conclusions:
1. If the permafrost were to increase by a factor of ten, it wouldn’t make any difference.
2. Wild animals are only at 2%. Yes, there were many more wild animals back then, but even if every one of them stopped producing methane, it wouldn’t make the methane difference seen in the Younger Dryas. And remember, the megafauna dieoff was limited to the New World (38% of the land excluding Antarctica), and all of the animals didn’t die off by any means. In particular, bison were unaffected, as were a number of other herbivores including many species of deer, elk, and antelopes.
3. Wet areas, geological sources, vegetation, and bugs make up 95% of the natural sources.
4. All numbers are estimates … or as my Grandma used to say “You can believe half of what you read, a quarter of what you hear … and an eighth of what you say.”
Mike says:
May 27, 2010 at 8:47 pm
In the source document, the units work. In the IPCC information, copied from the source document, they don’t. They claim the final unit is kg, where the source document says the final unit is kg^0.75, which makes the units balance correctly.
Like I said, this makes me nervous. And like I said, doesn’t mean it’s wrong, just means they’re not paying attention.
Finally, in a scientific discussion, saying someone “has no idea what he is talking about” is meaningless. If you dispute my figures or my conclusions, provide your own. If you think the S2010 paper is valid, show us how. If I don’t understand the documents I’m reading, show me where. I protested sloppy math and poor copying from a source document. If you think sloppy math and poor copying are acceptable, fine … but don’t expect to get too much traction here with that attitude, this is a scientific blog (voted Best Science Blog on the Web two years ago).
Mike says:
May 27, 2010 at 8:47 pm
I know Dick Lindzen, and as you say he is very much worth listening to. He also thinks I am worth listening to … go figure.
As to whether we should “take some measures to protect ourselves”, I say yes. I outline some of those measures here. Come back if you think I’m wrong about what we should do, and tell me where.
PS – Why do we have to make these decisions in “the next few weeks”? What’s your urgency? If you are right, we won’t see deleterious effects for decades … so why the urgency? Twenty-five years ago James Hansen said we had to act immediately … and now there has been no statistically significant warming for the last fifteen years. You can cry “Wolf!” all you want, my friend … we’ve heard it before.
Willis Eschenbach;
and all of the animals didn’t die off by any means. In particular, bison were unaffected, as were a number of other herbivores including many species of deer, elk, and antelopes.>>
Yes, one wonders, once they wiped out the mammoths what those evil humans ate instead. You would have thought that they would have wiped out a few other species as well once they had no mammoths left. Then again, maybe they killed them all at once, dragged them up north and buried them in the ice like a gigantic freezer, and lived off their carcasses. They buried some of them standing upright for artistic merit.
Seriously, I thought there was a mega die off of mammoths in Europe and Asia about 10K years ago as well?
and you forgott the rise of CH4 from 750ppb to 1750ppb from 1750 to 2000.!
This should warm the earth by more than 30°C, if Smith et al. could be somewhere right….
Mike says:
May 27, 2010 at 8:47 pm “Could it be that you want to believe AGW is false so bad that you lap up any bit of none-sense that seems to refute it? You are not being open minded.”
Perhaps people just grow tired of the endless assault . The Younger Dryas saw a temperature drop of some of some 7C in less than 20 years. This rapid cooling returned us to near glacial conditions. A thousand years later temperature rebounds almost as quickly as it fell. The cause of this rapid cooling is thought to be related to the collapse of glacial Lake Agassiz but the floodway is still disputed . http://www.nature.com/nature/journal/v464/n7289/full/nature08954.html
The retreat of the glaciers caused dynamic changes in flora and fauna that continue to this date. Massive new perennial wetlands were created by the melting glaciers. Wetlands are the leading source of methane now and little reason to think they were not so at the time of the loss of the megafauna. Forests moved north. Complex interaction occur between forest and methane. Complex chemistires occur between emergent macrophytes and methane. There are complex interactions between not only methane generation but also methane oxidation- all being continually mediated by changes in temperature, flora, organic deposition and related changes in soil chemistry (including inhibitory processes of methane formation and oxidation).
The megafauna extinction at 13000 BP was fundamentally an American extinction event. While Asia and Africa have not seen a megafauna extinction in the last 50K years.
So when anyone says that the methane decline could be as much as 100% result of the North American megafauna extinction and that this methane decline is associated with the YD temperature decline– well its just too much. At 100% it means 0 role for anything else. And please tell me the mechanism by which the loss of the megafauna give a 7C drop in 20 years and what fauna replaced them in 1000 years to cause the temperature to go back up. As the megafauna were going extinct the “smaller” grazers the bison and elk were filling the niches- so we need this offset. The range of the Pacific and Atlantic salmon was also increasing during this period with the massive conveyance of marine derived nutrients and carbon from the ocean to freshwater environments. Most trees found along Alaskan stream are the nutrient transfer from salmon to bear and the bear scat to trees. (Although we can expect these too declined with the YD). Also look at Figure 1 we also need to believe that terrestrial megafauna were the leading source of atmospheric methane.
The simplification of the complex and the associated hubris is just too much at times. It is not those on this site that are making the extraordinary claims made by this paper and as such we can throw as many stones as we like.
What a way to make a Friday fun! We’ve had discussions of this all week at work (blaming the lack of mammoth flatus for the return of winter so late in spring). I think the movie behind figure 4 was the main source of their research (information is much easier to get from talking animals).
But now I’m worried.
It occurs to me that when we reduce our nasty CO2 the temperature will drop, but we no longer have the benefit of megafauna emissions to prevent a drastic plunge into a new ice age (obviously good mammoth flatus worked better than rotten, gas-mask wearing cattle emissions). We must do a study to find the proper balance of exhaling and far—- um, intestinal methane release. I’m not a climatologist, but then again neither is Algore. WoHoo! I’m off to write a grant proposal!!$$$$$$
Just hit me that they are claiming that a drop in greenhouse gas on one continent dropped the whole planet into an ice age. Hmmm. Sounds to me like reducing greenhouse gases is a bad idea. We should put in a mitigation strategy that requires everyone to do their part. Produce a minimum amount of GHG or be fined. Countries with low carbon economies will be given deadlines to get their act together, or else they will be taxed and the money sent to high carbon economies as an off set.
Well, always more to learn. I found a fascinating NAS publication entitled “Megafauna biomass tradeoff as a driver of Quaternary and future extinctions“. It contains data on the timing of the loss of the megafauna over the entire globe. I was surprised to find out that the the major loss didn’t occur during the Younger Dryas. Here’s the figures:
As you can see, during the Younger Dryas the loss in total body weight of megafauna (which they define as animals that weigh over 44 kg = ~ 100 pounds) was fairly small compared to the overall loss. Another nail in the coffin of the hypothesis of mammoth far… excuse me, I mean the megafaunal methane hypothesis.
Mike, this is how we do science here, go out, find things, think about them, cite them, discuss them … your move. If you think the info I cited is bunk or that my conclusions from that information are wrong, here’s your chance, that’s how science works. I make claims and draw conclusions and cite information to support them, you try to tear down my data and my math and my logic … science as a blood sport, wouldn’t have it any other way.
I’m sure no one will take me seriously – but I can tell you what suddenly killed all the wmammoths 12000 years ago. An Asteroid – that split into 3 large pieces, the largets of which destroyed an island in the atlantic submerging it near the bermuda triangle. See Otto Muck for further details
Willis Eschenbach says:
May 27, 2010 at 9:21 pm
“More information is always good…
“the megafauna dieoff was limited to the New World (38% of the land excluding Antarctica), and all of the animals didn’t die off by any means. In particular, bison were unaffected, as were a number of other herbivores including many species of deer, elk, and antelopes.”
(First, pardon me but I just can’t figure out how to italicize font in these posts…)
Actually all these species were affected, though forms survived. In the case of bison, a large form, Bison antiquus was replaced by a smaller form, B. occidentalis. The latter was a cold-adapted species which arrived in North America from eastern Siberia probably during the Younger Dryas, and it was adapted to surviving predation pressure from gray wolves and human hunters.
The Clovis people hunted B. antiquus. It was gone by about 10,000 years ago, replaced by B. occidentalis which eventually evolved to become the current B. bison.
This information is published in a variety of papers but there is an excellent summary of this whole bison story in the popular book ‘Buffalo Nation: History and Legend of the North American Bison’ by Dr. Valerius Geist (Fifth House Publishers 1996).
Don’t let the popular book format fool you. Google the author.
Another excellent paper which you might find very interesting is:
Martin, P.S. 2002. Prehistoric Extinctions: In the Shadow of Man in Kay, C.E., and R.T. Simmons (eds.) 2002. Wilderness & Political Ecology: Aboriginal Influences & the Original State of Nature, The University of Utah Press.
Great global review. Megafauna extinctions wherever/whenever modern humans showed up, not just in the New World.
Lots more outstanding and rather revolutionary papers in that book. Unfortunately, since its publication the Conservation Biology gang has done its best to bury it – as you might guess from the provocative title. CB begins with the premise of a “pristine wilderness” where humans had no impact.
Pat Moffitt says:
May 27, 2010 at 10:02 pm
“The range of the Pacific and Atlantic salmon was also increasing during this period with the massive conveyance of marine derived nutrients and carbon from the ocean to freshwater environments.”
Except for some small coastal rivers, that happened much later. Salmon didn’t start going up the Fraser River system in British Columbia until about 5-6,000 years ago because of all the glacial meltwaters created too much flow and turbidity. Salmon need clear water to spawn in, once they can actually get upstream.
“Most trees found along Alaskan stream are the nutrient transfer from salmon to bear and the bear scat to trees.”
Sorry. Green fairy tale based only on recent history. If you look into the archaeological record you will find that almost all those salmon stream where bears gather in abundance now were occupied by human settlements and humans from the time the salmon got to them until those indigenous people were removed by smallpox, etc., and/or concentrated in Euro-defined settlements.
All those cultures used deadfall traps to kill bears, as well as hunting them. Just can’t live with bears when your house is full of dried fish.
Willis – re 10:32 pm
That weight loss graph is more consistent with predation by humans than anything else. Optimal foraging theory – kill/eat the biggest ones first.
Wilis
When the basic information is dissected by such as yourself it merely demonstrates how little we actually definitively know (and can prove) about the earth and its climate.
For our ‘leaders’ to make profound decisions on our future economic well being based on profound collective ignorance on the subject by science and politicians alike is the height of arrogance and stupidity.
Tonyb
So, can we now assume that the warmists are coming to the conclusion that CO2 isn’t the bad boy of AGW and have begun to officially remove it from the frame? Or is this just another stinkeroo rising from the slurry formally known as climate science?
Excellent article – I’m glad you wrote about this! When I read the news release I just had to sigh and think about how incredible it is that the “greenhouse gas paradigm” has made it possible that articles like this one appear in once respected journals. It’s so obviously far-fetched that you would think that only some weird pseudo science magazine would be willing to put this on print. I bet that even ufologists can get their fantasies printed now if only they’re able to make a greenhouse gas twist to them 🙁
Well done Willis, but I am worried that you might have given Gavin and Michael too much solid scientific clues in the cartoon sketch at the beginning of this article. The observation is clear, primitive “Mann” takes two steps out of the cave, relieves himself, on the frozen tundra, concludes with a satisfied methane emission, and perhaps a dump or two. This is science!! there is a definite correlation to be shown between the action of Primitive Mann and his fellows and rise in spot temperatures (the Hot spot?) a rise in methane all combining to a tipping point that ended the icy ages and ushered in the new dawn of Climate change, not hard to factor in a Hockey stick or two, some fir trees or conifers the odd flower or two, or perhaps it was the lemon tree that received the drenching and yes you then have run-a-way climate change of Mann made Global warming.
This could be the reverse engineered result if these scientific facts and truths are fed into the IPCC modelling machine. Willis, Al Gore will rub his hands!! What have you done!!
—
I regret that I have no citation to offer on this, but I throw it out as a potential line of inquiry for those with access to (and familiarity with) the literature in ecological research.
In any given biosphere, the so-called “charismatic megafauna” represent a relatively small percentage of the animate critters eating, reproducing, and excreting. In kingdom Animalia, most of the biomass is in the form of little things.
(I recall Farley Mowatt’s popularization of the understanding that the lupine “charismatic megafauna” were far from exclusively predators upon herbivorous “charismatic megafauna” in their habitats but derived much – perhaps the greatest part – of their caloric intake by way of pouncing upon Rodentiae and the like. Kinda fits with what any dog owner can tell you about how Fido not only goes nuts in pursuit of but will also pounce upon and successfully kill a squirrel, chipmunk, mole, or rabbit whenever he gets a chance. The Fissipediae [almost entirely predator species] are “built” to hunt and kill things that are little and quick; big guys like Panthera leo and Canis dirus are exceptions, not the rule – though naturally those of us in species Homo sapiens pay a helluva lot more attention to these whacking big meat-eaters; after all, we’re among their prey species. Among the herbivores, I strongly suspect that the unimpressive little things process a great deal more cellulose – and pass a great deal more methane into the atmosphere – than do the big, bulky, more striking animals.)
Any thoughts along this line – to the effect that the megafauna (as an absolute percentage of methane-flatulating life in any ecosphere) really don’t matter all that much?
—
Never heard of “swipe”. Is it the same as Poitín?
Willis, another brilliant exploration and demonstration of a truism an old friend, sadly long deceased, would quote when catching the scent of BS, saying “Why do those ignoramii who see themselves as authorities on anything not realise that pooling ignorance just gives us a bigger pool of the stuff”.
The proposition is so completely hideous as to beg credulity!
If it got really cold such that the foods which the mammoths et, al., fed upon, became scarce, then it follows that the animals would have ‘broke wind’ to a lesser degree.
Heck, they might have even starved to death, or became easier prey to whatever humans and other predators which roamed at the time owing to their diminished strength because of lack of food.
But I’m not buying any of their nonsense. More it is that the swampy areas of the time produced less gas as a result of lesser atmospheric heat, i.e., sunshine.
For the authors to proclaim that mammoths were sufficient in number to have produced that much gas? THAT is beyond the pale of the completely absurd.
Willis,
Excellent presentation of the data but there is more!
1. At about this time the neolithic peoples were carving millions of petroglyphs all over the Earth documenting a long period of a persistent auroral plasma phenomena (various papers here http://plasmauniverse.info/NearEarth.html ) immersing the Earth. The timing of this event is “about” 10,000 years ago but essentially the same time as the methane drop shown above.
2. The idea that the Clovis people killed the North American pachyderms is a nonsense – even today killing an elephant with wooden spears is not plausible, and in any case why did the Clovis people massacre their pachyderms and the Africans, Indiands and Chinese not?
3. One of the problems reconstructing paleaoclimates is due to the Lyell factor – the arbitrary stretching of geological time to, almost, incomprehensible extents so that obvious causal relationships become implausible because of the uniformitrian time stretching methodology.
I’m basically a diamond geologist and the science is about figuring out the hows and wherefores of kimberlite and lamproite eruptions, and how to find these rocks (and I am one of the world’s best in that area given my track record in finding those rocks, or so my peers tell others).
What I have noticed is that kimberlite eruptions are also associated with biosphere mass extinctions, but under the present model of geochronology, not permissble since a kimberlite eruption millions of years before a biosphere mass extinction cannot be invoked as a cause for that extinction.
But it can be if we have geochronology wrong.
This is not to say that I side with the religious fundamentalists and support their belief that the earth was formed recently, though I do agree that the basis for that belief, an historical physical event, documented by badly interpreted accounts, did happen, and was most likely the Younger Dryas event etc. (Time becomes a bit of a furphy if one accepts that the Universe always existed).
And there is another connection as well – Tommy Gold’s deep hot biosphere hypothesis – in which it is proposed that the Earth’s interior hosts a massive bacterial domain feeding on the mantle derived hydrocarbons continually releasing methane to the surface.
This leads to suggest another reason for the observed methane drop – not from the extinction of some megafauna in the US at the time, but from a catastrophic interaction between the Earth and some external body, comet, planet, whatever, that caused an intense plasma interaction codified by the neolithic peoples, and which zapped, electrically, the Earth, possibly affecting (killing) large populations of the subterranean biosphere, and hence shutting off, temporarily, global methane production.
But neolithic hunters exterminating mammoths and mastodons, while letting the contemporary Musk Ox to live is a bizarre concept.
Dare I say it, the mindset that proposed the Clovis Extinction hypothesis then, would have also proposed the AGW nonsense of today.