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
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.