The trouble is that there's no reason to believe Mann, or his "hockey stick" graph of global temperature changes. Subsequent studies have shown Mann's analysis to be less than definitive:
* In 2003, Ross McKitrick and Steven McIntyre published an article in a peer-reviewed journal showing that Mann's methodology could produce hockey sticks from even random, trendless data.
* Furthermore, in a soon to be released report by the House Energy and Commerce Committee, the three researchers -- Edward J. Wegman of George Mason University, David W. Scott of Rice University and Yasmin H. Said of Johns Hopkins University -- find that Mann's methodology is biased toward producing "hockey stick" shaped graphs.
In addition to debunking the hockey stick, Wegman goes a step further in his report, attempting to answer why Mann's mistakes were not exposed by his fellow climatologists. His conclusion is that the coterie of most frequently published climatologists is so insular and close-knit that no effective independent review of the work of Mann is likely.
Source. An extended summary of the report can be found here. Note this quote from the Wegman report:
"It is important to note the isolation of the paleoclimate community; even though they rely heavily on statistical methods they do not seem to be interacting with the statistical community. Additionally, we judge that the sharing of research materials, data and results was haphazardly and grudgingly done. In this case we judge that there was too much reliance on peer review, which was not necessarily independent. Moreover, the work has been sufficiently politicized that this community can hardly reassess their public positions without losing credibility. Overall, our committee believes that Dr. Mann's assessments that the decade of the 1990s was the hottest decade of the millennium and that 1998 was the hottest year of the millennium cannot be supported by his analysis.
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