As a climate scientist, I am quite familiar with the background facts that Prof Michael E. Mann (now at Penn State U) so shamelessly distorts in his new book The Hockey Stick and the Climate Wars: Dispatches from the Front Lines.
First, the scientific background:
Mann's claim to fame derives from his contentious (and now thoroughly discredited) "hockeystick" research papers (in Nature 1998 and Geophysical Research Letters 1999). His idiosyncratic analysis of proxy (non-thermometer) data from sources like tree rings, ice cores, ocean sediments, etc. did away with the well-documented Medieval Warm Period (MWP - 900-1200AD) and Little Ice Age (LIA - 1400-1800AD) -- documented by Prof. H.H. Lamb, the founding director of the Climate Research Unit of East Anglia U (CRU-EAU). Mann then asserted that the 20th century was the warmest in 1,000 years. His temperature graph, shaped like a hockeystick (on its side) immediately became the poster child of Al Gore and the IPCC, the U.N. science panel, to support their claim of anthropogenic global warming (AGW).
Two Canadian statisticians, Steven McIntyre and Ross McKitrick, soon found serious errors in Mann's analysis; they even showed that random data fed into Mann's algorithm would produce "hockeysticks." To add insult to injury, while the 2001 IPCC report used Mann's result to bolster its AGW claim, the most recent 2007 IPCC report no longer relies on it. Mann still defends it -- sort of -- and so do some of his uncritical supporters. We have had no word yet from Al Gore.