Climategate: McIntyre and the 'Divergence Problem'
With the Climategate data, however, McIntyre was able to reconstruct the complete Briffa series, which turns out to look like this:
The green line is the version we saw above as part of the IPCC report, and the red line is the full series -- which goes down rather dramatically, instead of up as the story suggested. If the full Briffa series had been included, the figure would look rather different. The hook upward, the blade of the hockey stick, would have been much less dramatic, the implied global warming much less significant. By truncating the data as they did, the global warming looks much worse.
And as the Climategate emails show, this was the result of a long discussion of how to best deal with "pressure to present a nice tidy story." A story that fit the IPCC's political goals, whether it suited the science or not.