A little relief from Mann source code (but don’t worry, I’m not finished with it.) In my post on IPCC 1990 arising out of the controversy about the WSJ editorial, I referred to the following quote from the Executive Summary to chapter 7 (p. 200), which stated (having the Younger Dryas in mind):
We conclude that despite great limitations in the quantity and quality of the available historical temperature data, the evidence points consistently to a real but irregular warming over the last century. A global warming of larger size has almost certainly occurred at least once since the end of the last glaciation without any appreciable increase in greenhouse gases. Because we do not understand the reasons for these past warming events, it is not yet possible to attribute a specific proportion of the recent, smaller warming to an increase of greenhouse gases. [my emphasis]
I attempted to trace the basis for overturning this view in later IPCC reports to see whether and how the changing views were based on updated science. I follow the trail a bit further here.
In my post on IPCC SAR , I noted that IPCC SAR  reversed the view of IPCC  that changes larger than those observed in the past century had occurred in the Holocene. For example, the chapter summary stated:
Based on the incomplete observations and paleoclimatic evidence available, it seems unlikely that global mean temperatures have increased by 1 deg C or more in a century at any time during the last 10,000 years.
The change in view was partly attributed to a paper by the omnipresent Wigley and Kelly  discussed here, and to Crowley and Kim  discussed here. The finding specifically attributed to Crowley and Kim  was the following:
Crowley and Kim (1995) estimate the variability of global mean temperature on century time scales over the past millennium as less than +- 0.5 deg C. (running text)
The finding attributed to Crowley and Kim  was not actually established there, but was attributed to the textbook, Crowley and North . Figure 3 in Crowley and Kim  stated:
"Error bars" represents a generous estimate of the range of natural variability based on records of the last 1000 years [Crowley and North, 1991]. [my bold - see the scale denoted +- 0.5 degree C in the bottom left hand corner of the figure.]
Elsewhere the running text in Crowley and Kim  stated:
The projected increase in global average temperatures for all scenarios also greatly exceeds the past record of climate variations over the last millennium [Crowley and North 1991] (Paleoclimatology, Oxford University Press, 339 pp). A generous estimate of that natural variability for that time is +- 0.5 degree C.
I then promised to look up Crowley and North . I’m not sure that I’ve closed the loop entirely, but here’s where I am. Despite Crowley’s fulminations against me elsewhere and his unreliable and fantastic accounts of our correspondence, I highly recommend Crowley and North, Paleoclimatology, to interested readers. It gives a readable and rational account of a variety of important paleoclimatological topics. Bradley’s text is also very good and covers a different vantage point. Both scientists seemed more pleasant before they were touched by IPCC glory.
I have diligently looked through Crowley and North 1991 (339 pages) and been unable to locate an exact source for this quote. I’m not saying that it isn’t in there somewhere, merely that I haven’t been able to locate it so far. (In passing, it is irritating to cite a textbook without a page reference.) The best that I can do is the following discussion in chpater 14, in the context of an interesting discussion of why warming to 1990 wasn’t as much as predicted by the models of Hansen et al. and Schlesinger et al. (I’ll try to remember to look up these predictions.) The excuse for the failure of warming to appear was that there were negative natural forcings and the observed shortfall to model predictions was within the range of natural variation. This excuse seems to have been superceded by the attribution of the shortfall to aerosols (but I haven’t followed the aerosol debate except at a remote distance.) Anyway, Crowley and North (page 255) say:
Despite the geologic evidence, some CO2 doubling studies suggest that the atmosphere should have already have warmed to levels greater than those of the present (e.g. Hansen et al 1984; Schlesinger 1986). Are the models wrong or are other processes operating that are obscuring the trend? One possibility involves sequestering of excess hear in the intermediate and deep layers of the ocean (Hansen et al 1984; Schlesinger 1986) a possibility for which there is some empirical evidence, as intermediate waters in the North Atlantic appear to have warmed significantly over the last 30 years (Roemmich and Wunsch 1984).
Another explanation for the CO2 “delay” involves modulation of atmospheric signals by volcanism and solar variability i.e. by “natural” climate fluctuations. There have been significant decadal and centennial-scale climate fluctuations over the last 1000 years (e.g. Figure 5.6). The characteristic temperature range of these fluctuations is 1.0-1.5 deg C, i.e. enough to modulate any uniform CO2 warming trend by the amount observed in the instrumental record. [my emphasis]
I guess that an estimate of +- 0.5 deg C can be construed as being equivalent to a range of 1.0 deg C., but 0.5 deg C seems to have been dropped pretty casually. The argument itself then depends on Figure 5.6, which is shown below.
Original Caption. Figure 5.6. Examples of decadal and centennial scale climate variability. (a) The phenological temperature in China (after Zhu, 1973). This index is based on timing of recurrent weather-dependent phenomena, such as dates of lowering of shrubs or arrivals of migrant birds or distribution of climatically sensitive organisms. (b) the growth rings of a juniper tree from western China (after Wang et al 1983). (c) The winter temperature index in the lower reaches of the Yangtze River (after Zhang 1980) (d) The number of Chinese counties affected by drought (after State Meteorological Association 1981). (e) The frequency curve of dust rains in China (after Zhang 1984), (f) the àÅ½àⳏ18 record from Camp Century, Greenland (after Johnsen et al 1970); (g) the àÅ½àⳏ18 record from Quelccaya ice cap, Peru (Mosley-Thompson et al 1990); (h) the àÅ½àⳏ18 record from the South Pole (Mosley Thompson et al 1990). Shading equals cool intervals. [From E. Mosley-Thompson; modified from Zhang and Crowley, 1989 and Ren, 1987]. Courtesy. E. Mosley-Thompson.
This is pretty sketchy evidence for overturning a finding of IPCC . It is also not new scientific evidence: all of this was available to IPCC  and presumably considered by them. The choice of proxies is interesting, as I’m familiar with many of them from later multiproxy studies. The Zhu  phenological dates were resoundingly criticized in Zhang [Climatic Change 1994], who argued that Zhu  had misinterpreted the medieval Chinese lunar calendar and wrongly calculated medieval phenological dates (which were actually earlier than Zhu’s calculations). This had a big impact on the supposed medieval coldness of Zhu .
Zhang  reported that tax records showed that medieval citrus and boehmeria crops ranged to the north of modern locations. Despite this resounding contradiction of Zhu , Crowley and Lowery  unaccountably used Zhu  as one of the series supposedly showing wildly differing medieval regional patterns. I asked Crowley why he used this series – a hardly unreasonable request. Crowley refused to answer and has of course fulminated against me recently in EOS.
The growth rings of "a" juniper tree in west China is pretty weak evidence of past temperature change to say the least. Juniper trees in the US are precipitation proxies. I saw a picture of a juniper tree on the internet about 18 months ago (from around Dulan, I think) and it was in a desert location, but I can’t locate the picture now. This juniper tree gets used from time to time – Bradley and Jones , as I recall. I have not read the original reports of the next 3 Chinese articles and I’m not even sure that they are in English. The number of Chinese counties affected by drought and the frequency of dust rains in China do not appear to me to be obvious indicators limiting the temperature range to +- 0.5 deg C. I’ll try to locate Zhang 1980 (but for present purposes, this article was presumably within the purview of IPCC 1990).
So we’re left with the three ice core àÅ½àⳏ18 records: Camp Century, Greenland; Quelccaya and the South Pole – all of which were available to IPCC 1990 (and even discussed by it). The Quelccaya àÅ½àⳏ18 series is a mainstay of subsequent multiproxy studies, but the Camp Century and South Pole àÅ½àⳏ18 series are dropped from the later studies – I wonder why. Figure 5.6 does not translate the àÅ½àⳏ18 fluctuations into deg C – I wonder what the basis for the translation was. It’s a pretty frail foundation for overturning the equivalent of a Supreme Court decision. I have seen no new scientific information involved in the overturning.