Ah, you fell for the Hockey Stick too, did you? This is precisely the reason why I don't trust the Sierra Club, or indeed any other environmentalist organization with an agenda.
Quite simply, the hockey stick was
made up by Mann, by using inaccurate indicators, and analyzing those inaccurate indicators with inappropriate statistical methods. The bottom line is that Mann spent a whole bunch of time trying to find the right parameters to make exactly such a plot "pop" from his data. He decided on the conclusion he wanted to present, then adjusted his method until he made a plot that supported it.
And people think this kind of crap constitutes
scientific investigation...
http://en.wikipedia.org/wiki/Hockey_Stick_graph
- Warren