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.
Read the whole thing. It's helpful for understanding some of the issues relating to the "hockey stick" graph which suggested sudden, drastic global warming.