Guest contribution by Willis Eschenbach
I have to admit that Michael Mann’s new (and unfortunately financially strong) Opus Magnum on the Atlantic Multidecadal Oscillation (AMO), sometimes also called Atlantic Multidecadal Variability (AMV), amused me a lot. Here are a few quotes from our boy on the subject, emphasis mine:
Man 2011:
“The AMO, defined as a 40-60-year time scale oscillation that arises from coupled processes in the atmosphere in the North Atlantic, is almost certainly real“
and
“This is an important finding from Knight et al. (2005) (which I co-authored) and Delworth and Mann (2000). [the origin of the term ‘Atlantic Multidecadal Oscillation’ (AMO) which I coined in a 2000 interview about Delworth and Mann w/ Dick Kerr of Science]. ”
followed by Mann in 2021
“Two decades ago, in an interview with science journalist Richard Kerr for Science magazine, I coined the term“ Atlantic Multidecadal Oscillation ”(AMO) to describe an internal oscillation in the climate system that results from interactions between North Atlantic currents and wind patterns . … Today, in a research article published in the same journal Science, my colleagues and I provided what we consider to be the most definitive evidence of the AMO to date actually doesn’t exist.”
I enjoy Mann’s implication that he was the discoverer of the AMO phenomenon, although it was fully described by Schlesinger and Ramankutty in 1994, six years before its publication by Delworth and Mann. In the linked Kerr article in Science that Mann refers to above, despite the extensive discussion of the name “Atlantic Multidecadal Oscillation,” Kerr never says that Mann named the phenomenon … but I digress.
What is the AMO? It is a slow temperature change in the Atlantic that is most visible in the North Atlantic. Here is a graph of the vibration.
Figure 1. Long AMO from NOAA. This shows a period of about 65 years. There are several instrumental versions of the AMO data. This is the longest instrumental version of the AMO held by NOAA from 1856 onwards.
Since the AMO was first described in 1994, the phenomenon has been studied in depth by any number of scientists. A search in Google Scholar shows 31,300 websites discussing the AMO. Why does Michael Mann now claim that it is not a natural variation of the Atlantic?
Because “state-of-the-art climate models” say so … his study begins as follows:
An analysis of the most modern climate model simulations from the last millennium provides no evidence of an internally generated, multidecadal oscillating Atlantic Multidecadal Oscillation (AMO) signal in the climate system and instead indicates the presence of a 50- to 70-year “AMO-like” signal, the from Episodes of explosive volcanism with high amplitude and multidecadal stimulation is driven
(Protip – every time someone starts talking about “state-of-the-art climate models” you can ignore their claims … but I digress again.)
Mann’s claim in his new work, “Multidecadal Climatic Variations During the Last Millennium Due to Volcanic Forces” (paywalled) is that in pre-industrial times what people referred to as “AMO” was actually a stable Atlantic, forced by sporadic volcanic eruptions that happen to be the same frequency as the AMO. But then that volcanic forcing died out in modern times, and just in time the volcanic forcing was replaced by anthropogenic forcing … funny how it works. In the world of M. Mann, people are always to blame.
In any case, I thought I’d see what I could learn from the data in both the instrumental and proxy AMO records, along with the Vulcan records discussed by Mann. First of all, Amman et al. Dataset that Mann et al. used from 61 tropical eruptions that they say drove the AMO before modern times. I’ve shown the eruptions as vertical lines. Over these volcanic lines I superimposed some of the empirical modes of a CEEMD (Complete Ensemble Empirical Mode Decomposition) analysis of the eruptions showing the various longer term cycles in the data.
Figure 2. Tropical volcanic eruptions and different CEEMD modes.
Here’s the thing with signals. As the brilliant mathematician Joe Fourier showed centuries ago, any signal can be decomposed as the sum of the underlying signals of different periods. CEEMD is like Fourier analysis except that it doesn’t break a signal into regular sine waves. It breaks down a signal into underlying signals that can change over time, as you can see above.
Now is there a cycle in the eruption data that is similar to the ~ 65 year period of the AMO? Well … kind of. However, since each individual signal can be broken down into underlying signals, this can only be random. The underlying signals must have a period of time, and it can only be fifty to sixty years, like with volcanoes.
So these are the volcanoes. How about the AMO proxy records? The most important one discussed by Mann is the study by Wang et al. “Internal and external drive of multidecadal Atlantic climate variability over the past 1,200 years”. You can find the data here. It is based on “a network of annually resolved terrestrial proxy data sets from the region around the North Atlantic”. In this study, Wang et al. made a difference between what they called “AMV”, Atlantic Multidecadal Variability “and the AMO. They said that about 30% of the variability in AMV comes from volcanoes, and if that is taken out we stick with AMO. I doubt that because modern volcanoes have little impact on the AMO, I also wanted to see how well the eruptions match their data, so I used their raw AMV data.
First, I examined how well the Wang proxy records matched the instrument records shown in Figure 1. I also added the 50-60 year empirical mode of CEEMD analysis to the Amman eruption records shown in Figure 2 above.
Figure 3. Two AMO records and one eruption record from 1856 to the present day.
In Figure 3 we see a few things. First, the Wang Paleo proxy AMV (red) is very close to the modern instrumental AMO (blue).
However, the Amman eruption dates are pretty poorly in tune with the modern AMO dates. That’s no surprise. Take a look at Figure 1. If you don’t know what year the great Pinatubo outbreak occurred, you won’t be able to tell from Figure 1.
Next, I looked at the longer term view of the same data. Figure 4 shows this result.
Figure 4. Two AMO records and one eruption record, 800 to date.
Again, some interesting things in Figure 4. First, the average length of cycles in the Wang Paleo AMV is 65 years, which is in line with modern data.
However, as in modern times, there is a very poor match between the Amman eruption dates and the Wang Paleo dates. Among other things, the period of the eruption data averages 55 years, not the 65 years of the Wang Paleo data or the modern instrument data. Although it sometimes coincides with the Wang data, it is synchronized and not synchronized with both the instrumental AMO and Wang AMV data.
So … how did Mann et al. come to their conclusions? As mentioned above, computer models …
The CMIP5 load multimodel experiments of the past millennium provide a pseudo-ensemble of N = 16 simulations powered by estimated natural forcing (volcanic and solar, with minor additional contributions from astronomy, greenhouse gases, and land use changes) over the pre-industrial period (interval 1000) until 1835 CE is common to all simulations). We estimate the only forced component of the temperature fluctuation by averaging over the ensemble, based on the principle that independent noise realizations are canceled out in an ensemble mean.
(As an aside, let me note that it is certainly not always true that averaging a series of model outputs means that the “noise realizations cancel” each other. But I digress again …)
However, I rather liked the idea of a “pseudo-ensemble”. Is that a bunch of random computer models hanging on a street corner, smoking cigarettes and posing as an ensemble? But I digress …
And what were their conclusions (emphasis mine)?
The collectively available evidence from instrumental and proxy observations and controls, as well as forced historical and last millennium climate model simulations, indicate the existence of externally forced multidecadal oscillations that are a consequence of competing anthropogenic forces during the historical era and the accidental multidecadal stimulation of more explosive tropical ones Volcanoes have been activity for centuries past. There is no compelling evidence of a purely internal multidecadal AMO-like cycle.
His claim is that for about eleven centuries, because of the “explosive tropical volcanic activity” it looked like there was an AMO. And coincidentally, a competition between CO2 and sulfate forcing, just as volcanic forces ceased, caused the AMO fluctuations.
You will forgive me if, given the illustrations above, I don’t find this argument even a little convincing.
After all, that’s what I love about studying climate. The science is far from clear, and that gives me the opportunity to learn something new from every paper that comes out.
Here on our dry coastal slope in Northern California, rain is forecast from tomorrow morning and will last for two days. Around here, rain forecasts that are even twelve hours later are sometimes wrong, and this is generally true of rain forecasts that are three or four days later. Funny thing about chaotic systems. They tend to be … well … messy.
[NOTE: It’s now “tomorrow morning” when the rain was supposed to start … bright sunlight and not a cloud in the sky. Gotta love chaotic systems.]
It seems like the modellers out here in the real world haven’t fully worked out the whole deal on undoing noise realizations … but I’m digressing.
My best regards to everyone, skeptics and mainstream people alike,
w.
PS: I sign everything I write with my initial “w.” And for the same reason I choose my words very carefully. Because I wrote it, I take responsibility for it and know that it is always possible that it will be me who will be asked to defend it. However, I cannot defend your interpretation of my words. So if you leave a comment, please quote the exact words that you are discussing. This avoids endless misunderstandings.
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