Not a whole lot. At least, not as portrayed in the IPCC's First Assessment Report.
A commenter writes,
Pardon me, but assuming Michael Mann and co-authors are correct...isn't Mann himself a contemporary example of a dogged contrarian overturning the long-held consensus regarding the so-called Medieval Warm Period postulated by Hubert Lamb and agreed to by like 97% or so(earlier) eminent climatologists? From the IPCC First Report (FAR) back into the history of science, the consensus seemed to be that the era about AD 1000 to AD 1300 was "warmer" (whatever that means) than the following "little ice age" between AD 1400 and AD 1800, and that "now" was again "warm". EVERYBODY tended to agree with Lamb and his acolytes....which is similar to what Mark Steyn said on a recent Powerline podcast:
"Dr. Michael Mann created this thing called the hockey stick, which purposted to show that late 20th century temperatures are warmer than they've been in a millennium. I don't think that's true -- a lot of people don't think that's true.... To get that result he had to eliminate....Is it hooey? I can't vouch for the quality of Steyn's science education, but that picture of the MWP is not what the IPCC wrote on their First Assessment Report in 1990. In the FAR Chapter 7, page 202, they gave the figure to the right.
"His big contribution was eliminating this thing called the Medieval Warm Period, when they had vineyards in Greenland, for example. The Medieval Warm Period is something I learned about at school. It was followed by the Little Ice Age where they were skating on the Thames, and this kind of thing, He eliminated all that and showed a flat line from the year 900 to the year 1900, and I think that's a lot of hooey."
Notice there isn't even a scale on the vertical axis (the IPCC calls it a "schematic diagram").
The text reads
"The period since the end of the last glaciation has been characterized by small changes in global average temperature with a range of probably less than 2°C (Figure 7.1), though it is still not clear whether all the fluctuations indicated were truly global....In other words, there wasn't a lot known then, and there were cold regions as well. They refer to only two papers, both of which pertain mostly to Europe.
The late tenth to early thirteenth centuries (about AD 950-1250) appear to have been exceptionally warm in western Europe, Iceland and Greenland (Alexandre 1987, Lamb, 1988) This period is known as the Medieval Climatic Optimum China was, however, cold at this time (mainly in winter) but South Japan was warm (Yoshino, 1978) This period of widespread warmth is notable in that there is no evidence that it was accompanied by an increase of greenhouse gases.
There just wasn't a lot known about the MCA (Medieval Climate Anomaly) before people started seriously collecting and understanding proxy data, and the theorists then started to analyze it.