"For example, some suggest that the ‘medieval warm period’, the 350-year era that started around 1000, when red wine grapes flourished in southern England and the Vikings tilled now-frozen farms in Greenland, was considerably warmer than even 1998.
"Of course, this is inconvenient to climate change believers because there were no cars or factories pumping out greenhouse gases in 1000AD - yet the Earth still warmed.
"Some tree-ring data eliminates the medieval warmth altogether, while others reflect it. In September 1999, Jones’s IPCC colleague Michael Mann of Penn State University in America - who is now also the subject of an official investigation --was working with Jones on the hockey stick. As they debated which data to use, they discussed a long tree-ring analysis carried out by Keith Briffa.
"Briffa knew exactly why they wanted it, writing in an email on September 22: ‘I know there is pressure to present a nice tidy story as regards “apparent unprecedented warming in a thousand years or more”.’ But his conscience was troubled. ‘In reality the situation is not quite so simple - I believe that the recent warmth was probably matched about 1,000 years ago.’"
"Over the next few days, Briffa, Jones, Folland and Mann emailed each other furiously. Mann was fearful that if Briffa’s trees made the IPCC diagram, ‘the sceptics [would] have a field day casting doubt on our ability to understand the factors that influence these estimates and, thus, can undermine faith [in them] - I don’t think that doubt is scientifically justified, and I’d hate to be the one to have to give it fodder!’
"Finally, Briffa changed the way he computed his data and submitted a revised version. This brought his work into line for earlier centuries, and ‘cooled’ them significantly. But alas, it created another, potentially even more serious, problem.
"According to his tree rings, the period since 1960 had not seen a steep rise in temperature, as actual temperature readings showed - but a large and steady decline, so calling into question the accuracy of the earlier data derived from tree rings.
"This is the context in which, seven weeks later, Jones presented his ‘trick’ - as simple as it was deceptive.
"All he had to do was cut off Briffa’s inconvenient data at the point where the decline started, in 1961, and replace it with actual temperature readings, which showed an increase.
"On the hockey stick graph, his line is abruptly terminated - but the end of the line is obscured by the other lines.
"‘Any scientist ought to know that you just can’t mix and match proxy and actual data,’ said Philip Stott, emeritus professor of biogeography at London’s School of Oriental and African Studies.
"‘They’re apples and oranges. Yet that’s exactly what he did.’
"Since Warmergate-broke, some of the CRU’s supporters have claimed that Jones and his colleagues made a ‘full disclosure’ of what they did to Briffa’s data in order to produce the hockey stick.
"But as McIntyre points out, ‘contrary to claims by various climate scientists, the IPCC Third Assessment Report did not disclose the deletion of the post-1960 values’.
"On the final diagram, the cut off was simply concealed by the other lines.
This is just the "tip of the iceberg", so to speak. The original article has plenty more.