In Part 2 of our tour of the new Clintel Analysis of the IPCC AR6 Report we summarize their look at the IPCC’s latest hockey stick. CDN readers know that in 2001 the IPCC 3rd Assessment Report became notorious for highlighting the Michael Mann hockey stick. As we showed in our Hide the Decline video (which has nearly 400,000 views so if you haven’t seen it yet, you’re missing the fun) the IPCC was determined to erase the Medieval Warm Period and present a “nice tidy consensus” even if it required fiddling the data and rushing an unproven chart into print. After the hockey stick was busted the IPCC backed off and didn’t put another one into their next two reports. Now 20 years later they’re back at it, only this time the handle is twice as long. But as the Clintel team shows, it’s just as dubious and required just as much subversion of the review process as before.
The IPCC can’t seem to stop high sticking. The problem, explains Clintel, is that they have gone on record as attributing all (not just some) of the 20th century warming to greenhouse gases. Which means there is no natural source of warming and cooling in the climate system. So when people talk about the Medieval Warm Period and the Little Ice Age, it creates a contradiction. If they exist, there can be natural warming and cooling periods. But if so, then some or all of the current warming could be natural. And we can’t have that.
The new hockey stick, like the old one, solves the problem by flattening out the past, this time all the way back to 0 AD, even more radical than the Mann hockey stick which only flattened out back to 1000 AD. This new version is called the PAGES2k reconstruction:
And naturally it was given pride of place in the Summary for Policymakers.
The first problem, notes the Clintel Team, is that the graph wasn’t in the peer-reviewed literature when the first draft of the IPCC report went out to review. It was in revision for submission to a journal, which means the IPCC set aside its rules about only using material published in peer-reviewed journals.
Another problem, just as in the 2001 case, was that there have been multiple proxy reconstructions in the literature that show a lot more variability than the PAGES2k version, as the Clintel team shows in this comparison chart:
The red and green lines are from other research groups and they were left out. The one line used by the IPCC was the white one encased in gray. If the other authors had wanted the fame and glory of having their work highlighted by the IPCC, all they had to do was learn to hide the variations.
The Clintel team also document how even the terms “Medieval Warm Period” and “Little Ice Age” were expunged from the IPCC report after the close of peer review. Down the same memory hole where many a politically incorrect Soviet Commissar went.
As for the statistical details of the PAGES2k chart, as in 2001 the problem is it is brand new in the literature and people haven’t yet had a chance to review how it was put together or what some of the problems might be. Steve McIntyre did some postings at his Climate Audit website, a few math buffs have put together Twitter threads exposing the odd methods used by the PAGES2k group, and there are unpublished drafts starting to circulate detailing other problems. But as the old saying goes, a hockey stick can be around the rink many times before the truth gets its skates on. Or something along those lines.