Previous IPCC-reports had a separate chapter for paleoclimate (historical climate) whereas AR6 WG1 did not. That does not mean the topic was totally disregarded, among other things a now debated temperature graph over the last 2000 years was included.
One of the fiercest fights in the climate ”debate” have been a reconstructed temperature graph from the northern hemisphere (Mann et al 1999) over the last millenia, presented i IPCC TAR 2001. Beacuse of it’s shape it was nick-named the hockeystick:
The authors acknowledges the uncertainties (shown in grey) from the early centuries: Though expanded uncertainties prevent decisive conclusions for the period prior to AD 1400, our results suggest that the latter 20th century is anomalous in the context of at least the past millennium.
It was scrutinized by other scientists like new studies supposed to be in the scientific process. For instance, the statistical methodology was heavily critized. The study stirred controversy reaching all the way up into the US Congress and Mann et al were accused of fraud.
To cut a long story short: NRC conducted a investigation on the matter, aquited Mann et al from misconduct, and showed that even when more robust methodology was applied, the conclusions held. Other scientists replicated the study. Raw data and code have been shared (contrary to claims). None of this is however mentioned in the criticism of AR6 Clintel just posted (what a surprise).
Mann and his colleagues accepted the fair part of the criticism and repeated their study according to recommendations (and with updated and new proxydata). It was published in 2008:
Today there is a long list of these kind of studies showing that today’s temperatures are very likely warmer than in any time in the last 2000 years (although the ”shaft” of the hockeystick is not as straight as in the original).
This is how science always progesses. Contrarians are however still stuck in the original graph-controversy – over twenty years later. The accuracy of GPS are much better today than 20 years ago. Do you therefore think the scientists and their models were fraudulent back then? (Yes GPS requires mathematical models to work properly).
Fast forward to IPCC AR6 WG1 from 2021. They showed a reconstructed global temperature (the above are from the northern hemisphere). A new hockeystick!
The graph was taken from a study by Page 2k published in 2019. They used five different statictical methods ending up with this graph:
Just recently, Clintel published a long report with criticism of the AR6 synthesis report. And wrote this:
”It cannot be ruled out that the new hockey stick was particularly commissioned for the 6th IPCC report. Five of the 19 authors of the new field hockey stick curve are from Bern (PAGES 2k Consorrtium, 2019).
Evidence suggests that a significant part of the original PAGES 2k researchers could not technically support the new hockey stick and seem to have left the group in dispute. Meanwhile, the dropouts published a competing temperature curve with significant pre-industrial temperature variability (Büntgen et al., 2020)12 (EA and EA+ in Fig. 5). On the basis of thoroughly verified tree rings, the specialists were able to prove that summer temperatures had already reached today’s levels several times in the pre-industrial past.”
They also showed this graph from Büntgen et al 2020:
So I contacted Fredrik Charpentier Ljungqvist (FCL), co-author of both the Page 2k (2019) and Büntgen (2020)-papers. What did he say?
First of all, the IPCC did NOT in any way commission the Page 2k-study as Clintel implies (FCL used the capital letters).
FCL did however confirm that some leading paleonologists do not think the Page2k graph was properly used in the IPCC AR6-report. They don’t think there is sufficient data to even reconstruct a global mean temperature with precision 2000 years back (most data comes from northern hemipshere). FCL somewhat regrets putting his name on the Page2k-study.
The criticism is for instance outlined in a scientfic paper by Anchukaitis, Smerdon 2022. Below you can see the Common Era-temperatures from the last four IPCC-reports, note that the last one is global, wheraes the others are from the northern hemisphere (it was visually diffent in the IPCC-reports but includes the same studies):
Despite the criticism it should be noted that the paper still concluded: ”As new reconstructions have been developed and estimates of past climate have been refined, results continue to show that by the late 20th century temperatures very likely exceeded those of any time in at least the last millennium.” Clintel does not mention this with a word.
And, as FCL points out, Büntgen et al 2020 does not show that pre-industrial summertime temperature reached today’s level as Clintel claim. ”Just that summer temperatures have occurred at the level of the early 2000s (which is 10-20 years ago). The reconstruction also lacks data for recent years.”
And whoever argues for a warm medieval warming period, also argues that there was a relatively large temperature change back then, which in turn means that the climate is sensitive to variations in the natural parameters that caused this change (mainly increased solar activity and decreased volcanic activity) .
It also means that the climate is sensitive to an increased effect from greenhouse gases, which should make us even more concerned about how the climate will develop.
So yes, scientists obviously disagree on the global temperature reconstruction. If there had been a separate chapter on paleoclimate in AR6, it would likely had been described differently (?). But it still does not change the conclusions from the AR6 in any way.
Clintel want to frame this as huge scandal, an alarmistic conspiracy. Their report is full of these accusations. Marcotts ”doctored graph” from 2013 and so on.
When IPCC AR6 WG1 was released I checked part of Clintel’s report with criticism. It was full of flat out lies. Like:
”Other historical reconstructions by Tamo, Fortin and Gajewski found that Arctic temperatures were 1-2°C warmer during most of the first millennium and in particular during the Medieval Warming Period, see: https://www.tandfonline.com/doi/pdf/10.1080/15230430.2019.1640527. ”
Everyone who can read can tell that the study did not show that:
Clintel also claims that IPCC has ignored Lindzen and Choi’s paper from 2011: “The Earth’s energy budget, climate feedbacks, and climate sensitivity” makes no reference to Lindzen and Choi (2011; hereafter LC11), the most important paper on climate feedbacks and climate sensitivity written in recent decades.”
It’s also a flat out lie. The scientific community has responded to Lindzen’s iris hypothesis i many studies. IPCC has also commented specifically on the LC11 study (which PNAS by the way refused, four different referees agreed on that – of which two were chosen by Lindzen himself. L&C did not bother to update, but sent it to another journal instead).
But that was in IPCC AR5 2013, ten years ago (zombies don’t die). LC11 was claiming that the climate sensitivity should be way below 1.0 °C (i.e. temperature change after a doubling of CO2 in the atmosphere). We’ve just recently passed a 50% CO2-increase but the temperature already increased more than 1.0°C.
Scientific conclusions are not static. They might be adjusted when new data and analysies appear. Sometimes mistakes are made. People do different judgments. Some shout conspiracy again and again.
A fair debate must be able to take criticism, that goes also for the IPCC. But you can communicate criticism in different ways. Above you can see one way showing a lot of background and full motivation in scientific paper, and one way that takes some justified criticism and mashes it with cherry picked data and accusations of fraud.
One is constructive, taking the scientific debate forward. The other is contagioius leading nowhere. Yes, Clintel is poisoning the scientific and political debate. They can only blame themselves for being irrelevant.
By the way, here’s a comment on the Soon, Connolly, Connolly-paper they’re refering to: RealClimate: Serious mistakes found in recent paper by Connolly et al.