One rather amazing characteristic of the hockey stick is the so-called "divergence problem": the strong warming in the late 20th century is not even indicated in the tree ring data that were used to reconstruct the last 1,000 years of supposed temperature variations. Much of the 20th century warming (the blade of the hockey stick) represents real temperature measurements, not tree ring reconstructions, since they don't show the warming. This raises a natural question, which the panel shrugged off: If tree rings do not show the strong warming of the late 20th century, how do we know there wasn't a similar temperature spike 1,000 years ago?
The temperature readings of the 20th century are from thermometers, while the temperature readings from hundreds of years ago are from proxies of various sorts. The particular proxies (bristlecones) have been called into question. But even if the proxies were very good, how close to thermometer readings would they be? The Hockey Stick might be a Hockey Puck. If the earth was coming out of "the Little Ice Age" within the last 400 years, what does that do to the NAS conclusions of the 20th century and late 20th century being the warmest period over the last 400 years?