Wilson on millennial temperature reconstructions
Oct 21, 2013
Bishop Hill in Climate: MWP, Climate: Mann

Last week I attended a lecture given by Rob Wilson at the University of St Andrews. This was a two-hour marathon, a format that is excellent if your lecturer is good enough to carry an audience, as it enables issues to be addressed in much more depth than is the norm. In the event, the time shot by, and if you read on you will see why.

Rob was doing a review of the millennial temperature reconstructions, following the story from the First Assessment Report through to AR5. As readers here know, Rob is no kind of a sceptic (a point he repeated over lunch), but on the northern hemisphere paleo studies his position is not a million miles away from mine. In places our positions are identical, as you will see.

Because of the prominence of Michael Mann's work in the area, some of the lecture was devoted to the Hockey Stick, to the 2008 paper (the "upside down Tiljander" study to the initiated) and to Mann's most recent area of focus, the influence of volcanoes on tree ring growth. Students learned that the Hockey Stick included a whole lot of inappropriate proxies and heard something of the issues with its verification statistics. The wallpapering of the Third Assessment with Mann's magnum opus and John Houghton's claims about unprecedented warmth based on this single study were described as "ridiculous". "Ultimately a flawed study" was the conclusion, with a gory list of problems set out: inappropriate data, infilling of gaps, use of poorly replicated chronologies, flawed PC analysis, data and code withheld until prised from the grasp of the principals. In the paper's defence, it was noted that it was an early attempt at a millennial reconstruction and that it did at least attempt to discern spatial variability, something that had not previously been done.

We also heard about Mann's  minumus parvum opus, The Hockey Stick and the Climate Wars, which Rob, like so many others, had given up on in fairly short order, and then saw an excerpt from Iain Stewart's Climate Wars TV programme. Having seen Mann's paper criticised so forcefully, I assume that Stewart's unquestioning faith in the graph will have left the audience with a pretty low opinion of his abilities.

That was the gentle beginning. When we got onto Mann et al 2008, we learned about the silliness of the screening process, and students were invited to try screening a set of random generated timeseries in the way Mann had gone about this study. Tiljander didn't get a mention, but I guess there are only so many flaws one can take on board, even in a two-hour lecture.

The real fireworks came when Mann's latest papers, which hypothesise that tree ring proxies have large numbers of missing rings after major volcanic eruptions, were described as "a crock of xxxx".

Away from the Mann stuff, this was, as I have suggested a very fair representation of the science of millennial temperature reconstructions, with the overwhelming impression being of a field that is still trying to work out if is possible to constrain the answers to the point where they are useful. The students were undoubtedly hearing the truth, warts and all, about the field they were studying. If only policymakers could hear the truth too.


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