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.
Discover more from Watts Up With That?
Subscribe to get the latest posts sent to your email.
Sometimes you guys are very good with answers and sometimes you stink at it.
Like now… An Ice Age is designed to lower gas pressure in the atmosphere by killing off the plant growth an freeze the lakes and streams. The cycle is from pressure build-up to pressure build-up by explosive life growth giving off gases and the occasional volcano.
The plant and animal life was bigger due to the increased speed of planetary rotation (see Planetary Mechanics 101(oh ya, it’s not written)) which caused GREATER centrifugal forces.
Seems to be a classic chicken/egg problem here. Methane increase causes temperature increase but, as well, temperature increase causes methane increase. A positive feedback exists and that should make it impossible to determine which comes first.
This is another one of those things that, if true, would result in a runaway greenhouse. Yet the planet has never experienced a runaway greenhouse. It has however, more than once, experienced runaway glaciation. But even that comes to and end.
The positive feedbacks teh climate alarmists are proposing appear to be missing some negative feedbacks. The earth appears to have what can be characterized as a thermostat set at some ten or twenty degrees C above the freezing point of water with a hysteresis of about ten or twenty degrees.
The predominant and fastest acting negative feedback would appear to be clouds. As temperature rises the water cycle increases. Evaporation transports heat from the surface where it is released through condensation considerably higher in the atmosphere where it can more easily radiate into space. The higher albedo of the clouds adds more negative feedback by blocking more sunlight from reaching the surface.
On the cooling side the same thing happens. As the temperature drops the water cycle slows, there is less evaporative heat transport and fewer clouds which allow more sunlight to hit the surface. On the falling side the water cycle negative feedback appears to be insufficient to halt global glaciation in the short term. The snow and ice appears to be too good at reflecting sunlight back into space and without much water vapor in the frigid air the radiated heat has a clear shot out into space.
So the biggest question appears to be what eventually melts a snowball earth. I would propose that soot produced by volcanoes is what eventually does it. Soot floats on top of snow and ice accumulating over time until there’s a complete melt which allows it to join the soil or the ocean instead of floating on the snow & ice. This would be much slower-acting than the water cycle. Call it the soot cycle. After thousands and thousands of years of volcanic ash darkening up the surface of the snowball earth it eventually all melts away. This action is also aided by a positive feedback where as the snow disappears it exposes dark soil or water which acts to absorb even more solar radiation.
This explanation seems to fit pretty well with observations of alternating colder and warmer epochs during the earth’s history with upper and lower bounds which aren’t exceeded. The goofy GCM’s don’t have any mechanisms for bounding either heating or cooling so if those are right we should either be in a more or less permanent state of either frozen solid or boiling over.
When a computer model output doesn’t match what actually happens in historical fact or the unfolding future the model is broken. Reality always trumps computer models. The CAGW crowd appears incapable of accepting any reality which disagrees with their model outputs.
—
Louis Hissink makes mention of Thomas Gold’s The Hot, Deep Biosphere hypothesis and the possibility of methane released by anaerobic microorganisms metabolizing carbon compounds which were part of the earth’s original planetary aggregation (meaning that natural gas and liquid petrochemical “fossil fuels” might not, in truth, be fossil substances at all, and the squawking about “peak oil” might be about as well-grounded in fact as is my youngest grandchild’s fears of big boogey monsters crawling out of her closet to eat her in the night).
Moreover, more upon the “intense plasma interaction codified by the neolithic peoples” mentioned by Mr. Hissink would be welcome. Anybody got anything more on that?
—
“intense plasma interaction codified by the neolithic peoples” mentioned by Mr. Hissink…
Probably has to do with the discovery by nomadic hunters who followed the great herds of a rather peculiar mushroom growing in the dung of said beasts which allow their shamans (and anyone else bold enough) to actually see plasma field interactions which they then codified into their mythology.
Willis Eschenbach says:
May 27, 2010 at 8:26 pm
Oh good, more evidence that whatever I have to offer can be offered by others, thank you Chris.
I agree with both statements. Chris for showing where the missing units hide out, and Willis for being annoyed the authors dropped the ball.
The term (0.0119*NEma^2 + 0.1938)/NEma smells like a polynomial fit, in which case the coefficients have really bizarre units to let things balance.
I think I mentioned this recently in another comment, but physics instruction give dimensional analysis short shrift. I remember an article quoted by Science News a long time ago reported improved understanding when dimensional analysis was included in the work.
Good find on the NAP equations. I think you have enough data to send a letter to Nature Geoscience and, point out the misuse of the NAP equation and ask for clarification. $18 for a defective product? My local grocery store offers double your money back if they sell you something unsatisfactory.
I suspect that there are a lot of papers out there with obvious flaws in dimensional analysis, just be very, very careful with the dimensions of various coefficients, especially if it’s “1”.
re; hot deep biosphere
This is more science fiction than fact. At least it’s hard (works under known principles) science fiction. In point of fact I just read a very recently written short story where hot deep life was the major plot element. In the story there was the carbon based life discussed in comments here but at an even deeper level silicon life which could only exist in exceedingly hot temperatures and could only survive very brief forays into cooler temperatures where carbon based life can dwell. The very limited overlap region is where all the action takes place.
In reality it seems like we’ve done enough deep drilling and had enough volcanic eruptions so we could have reasonably expected to have observed a hot deep massive layer of microscopic life by now.
I evaluate hypotheses on a rough scale as follows:
1) impossible
2) possible
3) plausible
4) likely
5) confirmed
For the hot deep life (carbon/DNA based) hypothesis I’d rank it between plausible and likely. Call it a 3.3 on the Springer Hypothetical Credibility Scale (SHCS). For the further speculation in the hard science fiction story of silicon-based life which requires temperatures higher than carbon-based life I’d rank it at 2.0 which is pretty near the minimum required for hard science fiction.
So, either
Megafauna eat plants, by-product methane;
or
Uneaten plants eventually die and rot, by-product methane.
Seems like the Herbivore-genic Global Warming hypothesis is as much [snip] as the Anthropogenic Global Warming hypothesis.
What’s really interesting about the Younger Dryas is here was this period of deglaciation and then some big happened, perhaps cosmic or volcanic, that had absolutely nothing to do with greenhouse gases and Wham! Within a few years the planet was tossed back into the bowels of an ice age.
This shows:
1. that rapid climate change can happen 100 times faster than today. Putting a lie to the myth that we are experiencing unprecedentedly rapid climate change.
2. That really rapid climate change occurs mostly in and out of glaciation. The Akkadian Collapse is another example of rapid (within a few years) cooling. It’s as if, at least in the Holocene, the climate’s strange attractor lies towards cooling. Temperatures are trending up as they do in an interglacial, but with any slight perturbation, Wham! Back to rapid cooling overnight. No recent warming periods of the same speed and amplitude are known. Right?? MWP, Minoan Warming, Roman warming were all more gently inclined events. (and also not correlated to CO2 atm concentrations.)
3. That catastrophic climate change is never towards warming…those are days of milk and honey, civilizational expansion…but towards cooling. If a new Younger Dryas were to begin tomorrow, we wouldn’t be talking about catastrophic climate change by 2090, but by 2013. It would mean the end of civilization.
So, why we are wasting time and money on “carbon pollution” what we should be thinking about is what caused the Younger Dryas and the Akkadian Collapse? And then try to avoid a similar fate.
Willis Eschenbach says:
May 27, 2010 at 2:57 pm
Johnny D says:
May 27, 2010 at 1:52 pm
Perhaps you should submit your thoughts as a comment to Nature Geoscience, where it will be printed, if it truly does have merit, along with the authors’ response. (I won’t be holding my breath, however.)
Clearly, the authors, editors, and peer reviewers, likely experts in paleoclimatology, thought it was a worthwhile analysis. I’m going to trust their judgment more than I trust some blogger’s
Trust climate scientists? That’s charmingly naive, Johnny. Me, I don’t trust anyone … including myself. If you think I’m wrong, I invite you to show me where.
Finally, if you think “authors, editors, and peer reviewers” are the judge of scientific validity, you don’t understand the scientific method. A large percentage of papers printed in peer-reviewed journals, which have been approved by “authors, editors, and peer reviewers”, are shown to be false within a few years of publication.
========
Willlis, regarding your analysis of the Nature Geoscience article “Methane emissions from extinct megafauna”, by Smith et al, I agree with Johnny D’s suggestion that “you should submit your thoughts as a comment to Nature Geoscience, where it will be printed, if it truly does have merit, along with the authors’ response.”
The authors of the methane article probably know more about their work than the readers posting here, so it would be interesting to find out what the authors think about your comments and report it here as a follow-up.
The reason for the infamous, rapid “re-rise” in methane (and other gases of this type) is genetic. Once our glorious forefathers ate all the big mammoths and sabretooth tigers and giant sloths, etc., etc., they gained a lot of weight and lost a lot of hair; the first picture, above, was actually taken AFTER the “Great Feast” and the genetic change took place, not before (there was actually less snow and more flowers before anyone really noticed anything different about this original AGW event). Previously, humans were very much smaller, very thin, and very hairy. This “Great Gene Shift” occured because of viral infestation and intestinal flora/fauna changes resulting from the meat of the animals consumed. This is the reason that the “Great Methane Bounce” is clearly seen on the above graphs.
It should be noted that recent AGW data is grossly in error of actual levels. With the number of humans on the planet today being in excess of 45,326 biomass units greater than the amount of units that were ever on the planet just prior to the Younger Dryass, and these numbers are expected to double every 20 years, AGW should continue unabatted for the next 7 months.
The AGW Hurricane Predictor for the next 6 months is for a moderately high season (wet and dry) with extreme events (hot and cold) expected for Northern Europe. Do NOT plant Palm Trees in Northern Canada before 6 June 2010.
Willis:
Another small units error–the first non-italicized paragraph under point #2:
Given the methane change in the Younger Dryas of ~200 ppmv, this would result in a methane forcing change of a tenth of a watt per square metre (0.1 W/m2).
Should be ~200 ppbv, right?
Great Job Willis! Now what these Klowns are saying is my Native ancestors, who came here from Asia, had to have such an organized hunting method that you could bring down a Woolly Mammoth without say, yer .458 win. spear. Let alone skin it, pack and cook it. Why then there were Bison in the millions? and the Native Americans seem to have no problem in killing them but not taking too many.
BTW I’m using “Native American” as I prefer, Indian is too confusing, to some, but I
do resent these “Scientists” blaming the Red Man for Global Cooling. I call “Racist”!!
-Just kidding. Next- my Celtic Ancestors get blamed for the demise of the Irish Elk
and Dire Wolves….
Willis Eschenbach says:
May 27, 2010 at 8:32 pm(edit)
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.
That’s the point, while the Mega fauna is grazing on vegetation there is little vegetation left to rot to release Methane.
The amount of vegetation converted into methane by Megafauna is roughly the same that grows back when the mega fauna is no longer around to eat it, hence Methane levels return to pre-extinction levels when the un-consumed vegetation runs riot producing massive amounts of foliage that falls to the ground to create Methane.
This regrowth takes a while as temperatures have cooled, as the reconstruction shows.
This graph, from the PNAS paper you linked, shows a large drop in Megafauna species around the time of the Clovis settling of the Americas.
http://www.pnas.org/content/105/suppl.1/11543/F2.large.jpg
This is scientific. Not. A post-normal scientist contends that if CO2 were not colorless, we would be able to detect the increasing ppm visually. Hmmm. This scientist must not have taken 5th grade math (the year where you learn about %). The change in the % of CO2, were it blue, green, or purple with green strips, would not be detectable as a color change in our air. Hell, we can’t even see it as a line, whether it changes or not, on a % graph of all the stuff in our air. Just looks like the x axis line.
http://www.cnn.com/2010/OPINION/05/28/reddy.oil.carbon/index.html?hpt=C2
—
Douglas Dc writes about how the pre-Columbian inhabitants of North America “…had to have such an organized hunting method that you could bring down a Woolly Mammoth without say, yer .458 win. spear. Let alone skin it, pack and cook it. Why then there were Bison in the millions? and the Native Americans seem to have no problem in killing them but not taking too many.”
I seem to recall that foot-mobile hunters (remember, Equus ferus caballus was not reintroduced to the western hemisphere until the Spanish brought them to the New World as military mounts and pack animals) went after big herd-clumping herbivores by devising pitfalls and by engineering stampede battues.
By ambush (hazardous!) a hunter riding Shank’s mare might get close enough to a Bison occidentalis or B. bison to wound it with a spear or with a projectile from a self bow. On horseback, however, both hunting range and the ability to close with the prey animal improve.
As for the Plains Indians “not taking too many” of B. bison, it must be remembered that these hunter-gatherer tribes and clans competed – quite viciously, developing a warrior culture in the process – with each other to gain and defend hunting territories. The limits imposed upon these native Americans’ taking of bison derived from the native Americans’ conscious decisions to fend off rival hunting parties and the vicissitudes of life on the Neolithic technical level (meaning death by disease and starvation from time to time suppressing the hunter-gatherer populations).
The native Americans of the high plains – where the bison was their principal economic resource – had only a very poor appreciation of the sorts of maximal resource extraction facilitated by the ownership of capital assets.
It always works down to praxeology.
—
One thing to note is that the temperatures dropped precipitously within a matter of decades. At the end of the Younger Dryas the temperatures rebounded even more quickly (perhaps within a decade).
It seems more likely to me that widespread extinction of the megafauna was a consequence of the sudden drop in temps in NA rather than the other way around.
You are correct sir. Fermentation is significantly impacted by temperature. It takes about 2 weeks in the summer for me to ferment it but 3 – 4 in the winter. In both cases it is done in an unheated / air conditioned spare bedroom. There is nothing like real world experience.
The farting animal kill connection could, I think, be cross checked with the decimation of Buffalo herds in the previous two centuries. There are good estimates of numbers at their peak and the rapidity of the decline. I would also imagine methane levels can be proxied as well.
My null hypothesis: Not a damned bit of correlation.
According to Steven Mithen in his book “After the Ice” p.252-253 the Mammoth population had survived previous interglacials multiple times by migrating to refugia where they survived the warm times when vegetation was unfavorable, only to repopulate wider areas when the ice sheets reformed. He cites Wrangel Island as one of the last refugia after the last ice sheet retreat by recalling that Russian scientists found miniature Mammoth bones there in 1993 which they dated to around 4000 years ago. He says that the end of these mini-mammoths is coincident with the arrival of humans on Wrangel Island.
He later theorizes that humans may have also killed off the the mammoths elsewhere because it requires only a small percentage of kills of breeding females to send a population of slow maturing and breeding animals into an irreversible decline. He also doesn’t rule out climate change as helping in the extinction.
—
On the subject of abiotic petrochemicals and The Hot, Deep Biosphere, Dave Springer writes:
“In reality it seems like we’ve done enough deep drilling and had enough volcanic eruptions so we could have reasonably expected to have observed a hot deep massive layer of microscopic life by now.”
First, it would seem that the magma conducted to the surface by volcanic eruptions must have first sustained such heating as to make it certain not only that organic (DNA-based) life could not survive the transit, but that whatever remained thereof – including methane and petrochemical stuff – would have been denatured, volitilized, incinerated, and/or vented away before the solid, semisolid, and liquid discharges of the volcanoes present themselves.
Second, deep drilling in oil exploration predicated upon the reasoning of the abiotic methane and petrochemicals hypothesis seems to be discovering productive reserves.
Short stories from the pages of Analog aside, there seems to be one of those “if it’s stupid and it works, it ain’t stupid” aspects to this notion.
—
If the Clovis people killed off the N. American mammoths, who did in the Russian ones at about the same time?
http://www.guardian.co.uk/world/2009/oct/11/mammoth-find-herders-climate-change
This latest Greenpeace-organised expedition to study the effects of climate change did not turn up any fresh mammoth remains. But on previous trips Romanenko has stumbled across skulls, molars and tusks left behind by the 5 million mammoths that once roamed across the icy steppes of northern Eurasia, co-existing with early humans.
But the estimated five tonnes of mammoth tusks unearthed across Russia every year are not merely objects of scientific curiosity. They are also big business.
Woolly mammoths lived in northern Europe, Siberia and north America up to 10,000 years ago. Climate change, hunting by early humans, and even a meteorite are among theories for their abrupt disappearance. A population of 3 metre-tall dwarf mammoths survived on Russia’s Wrangel Island until 3,700 years ago.
Thanks for the EPA graphic, Willis! I tried to find it & couldn’t locate that one.
Don’t forget, Clovis man and his chums on other continents hadn’t yet invented the modern animal confinement technology, so ruminant manure & enteric methane production back then was probably a fraction of today.
Definition of Junk Science =
Rich Matarese says:
May 28, 2010 at 4:09 am
I believe (lol) the abiotic argument goes something like….
The liquid is below the usual “fossil boundary”, as it were, and when liberated rises upwards dissolving some organic matter on its way to the pool where from we tap t. If we (mostly the Russians) dig deeper for it it has no organic signature. It self replenishes due to the nature of its, presumed continuous, creation.
The other matter – electromagnetic haloes – is documented in caves by over 50 nation’s prehistoric members. The representation is front elevation normally and the halo is represented by two dots at waist level as the observer would see most manifestations in this manner.
Bill S says:
May 28, 2010 at 6:13 am
Good catch, thanks, fixed.
w.
D.S. Overcast makes an interesting point but doesn’t take it far enough. Before Europeans came to the American West there were over 40 million bison. Within 150 years their numbers had dropped to less than 1000. Now the megafauna were larger but as a total biomass the bison probably where larger. So why wasn’t there a drop in methane with the associated cooling similar to the Younger Dryas in the mid to late 1800s?