Guest Post by Willis Eschenbach
I have to learn to keep my blood pressure down … this new paper, “Abrupt onset of the Little Ice Age triggered by volcanism and sustained by sea-ice/ocean feedbacks“, hereinafter M2012, has me shaking my head. It has gotten favorable reports in the scientific blogs … I don’t see it at all. Anthony provides an overview of the paper here.
First, the authors say:
Here we present precisely dated records of ice-cap growth from Arctic Canada and Iceland showing that LIA summer cold and ice growth began abruptly between 1275 and 1300 AD, followed by a substantial intensification 1430– 1455 AD. Intervals of sudden ice growth coincide with two of the most volcanically perturbed half centuries of the past millennium.
OK, precisely dated records show that big volcanoes equals Little Ice Ages. Peaks in ice growth coincide with volcanoes. Got it. Then in the paper they discuss the later interval of “sudden ice growth”. They start by saying …
The PDF peak [in ice growth] between 1430 and 1455 AD corresponds with a large eruption in 1452 AD …
What’s the problem with these claims? Figure 1, which is from their study with a couple of my annotations, shows the problem …
Figure 1. Original Caption Figure 2. … (b) Global stratospheric sulfate aerosol loadings [Gao et al., 2008]. (c) Ice cap expansion dates based on a composite of 94 Arctic Canada calibrated 14C PDFs (probability distribution functions). I added the vertical red line down from the top of the “D” panel that shows the full size of the sulfates from the eruption in 1258 (258 teragrams [megatonnes]). The vertical blue line, also added, indicates the timing of the following large eruption in 1455.
I get nervous when people cut off important data in a graph, it’s a bad sign regarding their transparency … but I digress …
I always look for alternative ways to verify what the authors are showing. In this case, the GAO et al 2008 aerosol loadings shown in figure 1(B) are calculated loadings using a record of the volcanoes and a climate model. Me, I always prefer actual data. Fortunately, we have very accurate data thanks to the ice core record from a place with the lovely name of Dronning Maud Land. You may not recognize it by its Norwegian name, but when I say “Queen Maud Land”, everyone knows where that is … well, everyone but me, I had to look it up …
Ice cores record how much sulfate has fallen on the ice during past years. Sulfate comes from volcanoes, and is ejected high into the stratosphere. From there it is mixed worldwide, and eventually it settles out on the ice. The sulfate record from two different ice cores in Dronning Maud Land agree to within a couple of years, so we can have confidence that they are accurate.
Next, before I go further, what is the “probability density function (PDF)” that the paper uses? It is a function that gives the probability of an event occurring in a certain year. For example, carbon-14 dating of some dead moss might give the date it died as say 1135. Are we sure it died in exactly 1135? No way, that’s just the most probable value. It might have died in 1134, or 1136. It might also have died in 1130 or 1140, but the probability of it being either of those years is much lower than the probability that the date is actually 1135. The probability density function is the function that gives us the probability of the event actually occurring in each years. Typically it looks like the famous “bell curve” or Gaussian curve, peaked in the middle and fading to zero on either side. It may be asymmetrical, with different probabilities that the event is before or after the most probable date. It is a good way to aggregate data
With that as prologue, here is the overview of the two records. One is the ice expansion record from the M2012 paper. The other is the volcanic sulfate record from the Maud Dronning Land ice cores.
Figure 3. Volcanic sulfate records from Maud Dronning Land (blue and green) and the ice cap expansion records from Baffin Island (purple line). The PDF values are the probability percentages multiplied by 100, so for example if the scale reads “400″ that means 40% (0.40).
Right away you can see some curious things. There is a large expansion of the ice cap (increasing purple line) in the century from 900 to 1000, but nary a volcano in sight. They say in the paper that “cold summers can be maintained by sea-ice/ocean feedbacks long after volcanic aerosols are removed …”, but what started and maintained the cold summers from 900 to 1000?
Then there’s the claim that the intervals of sudden ice growth in 1280 and 1435 occur during “two of the most volcanically perturbed half centuries of the past millennium” … I’ll buy that for the year 1280, but 1435? One lousy volcano in the half century around 1435, it wasn’t even as “volcanically perturbed” as the last half of the 20th century or the first half of the 19th century.
Intrigued by these problems with their claims, I looked closer. Figure 4 shows a closeup of the time in question:
More oddities. First, the expansion of the ice cap started in 1215, about 45 years before the eruption in 1258. Then in 1250, the rate of ice cap expansion increased, almost a decade before the eruption. And while you would expect an immediate increase in the rate of ice cap expansion, the increase doesn’t begin until about .
But that’s nothing compared to the other end of the period. The peak ice cap expansion occurs in 1435, a full two decades before the eruption in . Nor does the eruption speed up the ice cap expansion. In fact, the expansion slows markedly after the 1455 eruption.
Now, you may recall that I quoted the start of a sentence above, which said:
The PDF peak between 1430 and 1455 AD corresponds with a large eruption in 1452 AD …
Um … well … they are being most expansive with their claim that the 1435 peak and the eruption “correspond”. The volcano is well after the expansion in ice area. How do they explain this?
Well, the sentence goes on to say:
… although the ages of the three largest 5-year bins appear to precede the eruption date. In contrast to the earlier 13th Century peak, the second PDF peak occurs at the end of a 150-year interval of variable but falling snowline (Figure 2c), raising the possibility that the PDF peak plausibly reflects a brief natural episode of summer cold that preceded the large 1452 AD eruption. Alternatively, the apparent lead of kill dates with respect to the 1452 eruption may be a consequence of combined measurement and calibration uncertainties.
To me, that’s special pleading. Not only that, but it destroys their entire case. Here’s why:
If the 1435 peak “plausibly reflects a brief natural episode”, then why should we believe the much smaller 1280 peak is not just another “brief natural episode”?
Alternatively, if the timing of their “precisely dated” 1435 record is really off by twenty years due to “combined measurement and calibration uncertainties”, then why on earth should we believe the timing of the “precisely dated” 1280 peak?
I’m sorry, but I just don’t see the evidence that volcanoes had anything to do with the changes in the Baffin Island ice cap. And their whole sea/ice feedback claim? I note that the claim is supported by … well … I fear all it is supported by is models all the way down.
PS—An oddity. The 1258 volcanic eruption was the largest in the last 2,000 years … and as far as I can determine, nobody knows where it occurred.
DATA: All data used in this post is available here as a comma-separated (CSV) file.