Quoting from Hayword:
It has long been thought that over the last thousand years the earth experienced two significant natural climate cycles: the "medieval warm period" (MWP) centered around the year 1000 and the "little ice age" (LIA) from about 1500 to 1850 or so. The first report of the IPCC in 1992 displayed a stylized thousand-year temperature record showing that the MWP was warmer than current global temperatures, but this was mostly conjecture. Yet it was a huge problem for the climate campaigners: If the medieval warm period was as warm as today, as some scientists believe, it would mean that today's temperatures are arguably within the range of normal climate variability, and that we could not yet confirm greenhouse gas emissions as the sole cause of recent increases or rely on computer climate models for predictions of future climate apocalypse. There had long been rumors that leading figures in the climate community believed they needed to make the medieval warm period go away, but until the CRU leak there was no evidence besides hearsay that scientists might be cooking the books.So what happened between 1992 and 1998? Mann's "hockey stick" chart in 1998 seemed to disregard both the MWP and the LIA. Based on Hayword's article, Mann dug in his heals over the matter and wouldn't even listen to those in his inner circle.
The CRU emails reveal internal doubts about this entire enterprise both before and after the hockey stick made its debut. In a 1996 email to a large number of scientists in the CRU circle, Tom Wigley, a top climatologist working at the National Center for Atmospheric Research in Colorado, cautioned: "I support the continued collection of such data, but I am disturbed by how some people in the paleo community try to oversell their product." Mann and his colleagues made use of some of the CRU data, but some of the CRU scientists weren't comfortable with the way Mann represented it and also seemed to find Mann more than a bit insufferable.Be sure to read the ENTIRE ARTICLE as it provides an "inside baseball" look into the culture within this group. It also puts the emails in a very logical and hard-to-argue-with context.
One of the best articles on this scandal I've seen.