Well, there's a few problems.  Gore didn't actually use the Mann hockey stick that is presented, although the hockey stick in 'An Inconvenient Truth' is very simliar (in that it omits the Medieval Warming Period)<p>Those commenting on the start date fail to realise that the start date of the data set he uses is 1929.  You can't blame the author for that.<p>The fact that it's part of the 'head' or 'blade' of the stick is irrelevant, all of the 'handle' is based on multi-proxy reconstruction, which has major data problems, as the endless to-and-fro on this issue says.  One of the major problems highlighted has been the use of tree-rings as a proxy, and the tree rings do not agree with the instrumental record.  This is the famous 'hide the decline', where the authors sought out statistical methods (the trick) in order to splice the tree ring proxies with the instrumental records, which they really shouldn't have done.<p>The field of coming up with global averages from temperature datasets is so complex I won't even try and make a comment on that.  Plus I don't know any clojure.