Thanks to J who alerts me to this little matter. So, this is all mostly summarised in [[Description of the Medieval Warm Period and Little Ice Age in IPCC reports]]. From which I reproduce:
A schematic (non-quantitative) curve was used to represent temperature variations over the last 1000 years in chapter 7. The vertical temperature scale was labelled as “Temperature change (°C)” but no numerical labels were given; it could be taken to imply that temperature variations of the MWP and LIA were each of the order of 0.5 °C from the temperature around 1900. The section specifically states recent climate changes were in a range of probably less than 2 °C. The 1990 report noted that it was not clear whether all the fluctuations indicated were truly global (p 202). The graph had no clear source (it resembles figure A9(d) from the 1975 U.S. National Academy of Sciences report, which is sourced to Lamb, 1966), and disappeared from the 1992 supplementary report. Within the 1990 report, the LIA is taken to be global in extent but the MWP is not. Climate over the last 1000 years is mentioned very briefly in the SPM of the 1990 report. The MWP is not mentioned at all, and the LIA described by …probably fluctuated by little more than 1°C. Some fluctuations lasted several centuries, including the LIA which ended in the [19th century] and which appears to have been global in extent. The MWP is mentioned in the executive summary to chapter 7, as MWP around 1000 AD (which may not have been global).
So: just in case it isn’t clear from the above: fig 7.1.c isn’t useful anymore. It was vaguely useful then because there was nothing better available. It was a hurridly drawn sourceless schematic that no-one uses nowadays; and if anyone *did* use it they would be roundly criticised.
[Update: JM made a nice comparison of the two graphs, available here]
Here is the graph:
Obviously the *exciting* thing about that graph (if you’re a bozo septic denialist) is that it shows that the MWP was warmer than it is now. Woot! Obviously what you think happened (if you’re a fool) is that IPCC in 1990 was pure; it released this graph by mistake; but subsequent IPCC reports have had to supress it. Bring on the black helicopters! (I’ve said this before).
Which brings me to the Wegman report. Here is their picture (fig 4.5, their page 34):
The pic is similar enough that this must be the graph from IPCC ’90 that they mean (though being academically careless they don’t trouble themselves to say which fig they mean). But the digitisation has been done poorly – there is noise added, most obviously at the height of the “MWP” and around 1850. Perhaps this has been done deliberately to avoid making it obvious just how smooth the original was.
Notice how they have silently added the temperature labels to the Y-axis: these are not in the original. And they have labelled them “newy” – why. The point about the original was that it was a schematic – which explains the lack of axis labels, otherwise unforgiveable. This expalins why Wegman has added the labels, since he wants to hide its schematic nature.
Less excitingly, Wegman has supressed some of the time labels, but has added a label for 2000.
More excitingly, Wegman has completely mangled the “now” end of the graph – his version ends up going upwards, whereas the IPCC version ends up going downwards. This isn’t “hide the incline” – this is “fake the incline”. But why? I think the reason is that Wegman wants you to think that the endpoint is representative of “now”. Obviously, if you see a graph with the T flat from ~1910 you’re going to think “oops that isn’t right” so he has faked in an increase at the end.
Update: even odder, look at Wegman fig 4.7. What they claim as the “original curve”, the blue line, isn’t. But neither is the red curve. What are these bozos up to?
Update: OK, the game seems to be: look at the “modern” end and observe that it doesn’t look right: the end of it has been truncated. I’m told that if you view this on a slow machine under zoom, you can see the blocks being put in place: if you do that, you can see the “full” curve and then see the end blocked out (presumably if you have PDF writer/editor you can see this too). Also, I can think of a reason *why* Wegman needed to “add the incline”: in figs 46 / 47 he is playing some games to prove that MBH’s method producing hockey sticks when “properly centered PCA” doesn’t; but I rather suspect that if you do that on the actual IPCC series, it doesn’t work: I suspect Wegman found he needed to bump up the modern end a bit.
Update: SS has more graph fun at http://www.skepticalscience.com/Common-graphical-tricks-and-the-Medieval-Warm-Period.html
Update: AR says
The appendix in Jones et al. 2009 (“High-resolution palaeoclimatology of the last millennium: a review of current status and future prospects” The Holocene, 19, 3-49) tries to describe where the IPCC 1990 Fig. 7c comes from.
They conclude that it was compiled from a series of publictions by H. H. Lamb and was only based on temperature records associated with Central England, so not global.
Further, Jones et al. point out that “At no place in any of the Lamb publications is there any discussion of an explicit calibration against instrumental data, just Lamb’s qualitative judgement and interpretation of what he refers to as the ‘evidence'”.