Much time has been spent investigating Mann's 'hockey stick', and a marvelous book has been written about its bogosity and the tangled web of Climategate scientists behind it. Two separate investigations by U.S. authorities eventually discredited this science to the point where the IPCC was forced to omit it from its 2007 report.
As a reminder, the infamous 'hockey stick' was an attempt by Climategate scientists to replace a well established, consensus temperature record with a new one that would show modern temperatures as being "unprecedented," even when compared to the Medieval Warming Period. (Note: Image source here. Notice how the above hockey stick graph depicts pre-modern temperatures as being of insignificant variation, with both the Medieval Warming and the Little Ice Age being nothing more than blips in temperatures.)
This clownish version of "science" included doing the following to accomplish this objective for the IPCC and its reports:
- 1. Utilized a discredited statistical technique that selected tree ring records that exhibited a sharp 20th century increase (the hockey stick "blade") - plus excluding those tree ring records that had a lack of 20th century warming
- 2. Included tree species that exhibited a sharp 20th century increase, which all tree ring experts knew were not a response to higher temperatures but to other factors (mainly to the increase of CO2 fertilization)
- 3. Utilized a bogus statistical technique that over-weighted tree-ring records that exhibited a sharp 20th century increase, by at least 390 times
- 4. The rapid 20th century "blade" temperature increase mostly disappears when the above three points are properly corrected
- 5. Michael Mann, and authors of subsequent hockey stick look-alike studies, withheld the key r2 statistical validation measurement of their temperature reconstructions for a critical reason - the extremely low r2 of their studies statistically proved their proxy temperature reconstructions were worthless (statistically insignificant)
- 6. Michael Mann, and authors of subsequent hockey stick look-alike studies, withheld the key data, algorithms and software used in their proxy reconstructions to eliminate the possibility that other scientists would ever challenge their findings
- 7. Michael Mann and other paleo-climate scientists appear to misrepresent/hide other proxy temperature reconstructions that did not exhibit a sharp 20th century increase - commonly referred to as "Mike's trick" and the "hide the decline" tactic.
Point #7 can be seen in the below graphs (click on images to enlarge).