I’m not sure there’s any argument to say Michael Mann’s work isn’t fraudulent. I tend not to do it myself because I don’t think it’s necessary, but it’s certainly a defensible position. Consider:
1) When Michael Mann published his 1998 temperature reconstruction, he said:
the long-term trend in NH is relatively robust to the inclusion of dendroclimatic indicators in the network, suggesting that potential tree growth trend biases are not influential in the multiproxy climate reconstructions.
This statement was untrue. Mann’s results were entirely dependent upon a relatively small amount of tree ring data. Not only do we know this now, we know Michael Mann became aware of it shortly after publishing his paper. On page 51 of his book, Mann tells us after MBH98 was published, he performed tests that:
revealed that not all of the records were playing an equal role in our reconstructions. Certain proxy data appeared to be of critical importance in establishing the reliability of the reconstruction–in particular, one set of tree ring records spanning the boreal tree line of North America published by dendroclimatologists Gordon Jacoby and Rosanne D’Arrigo.
It’s not clear how Mann managed to make the false claim in his original paper, but he was obligated to inform people of its falsity. We could perhaps forgive not correcting an earlier mistake except Mann went on to publish his 1999 paper, extending the 1998 reconstruction back another 400 years. By building upon his 1998 paper while knowing it made false claims about his results, Mann committed fraud.
Anyone reading MBH99 would be directed to MBH98 for information about the reconstruction. MBH98 contained important claims about his results Mann knew to be false (at the time he published MBH99). It is perfectly reasonable to say by directing people to information about his results he knew to be false, Mann committed fraud.
2) Michael Mann was responsible for text in the IPCC TAR which said:
Mann et al. (1998) reconstructed global patterns of annual surface temperature several centuries back in time. They calibrated a combined terrestrial (tree ring, ice core and historical documentary indicator) and marine (coral) multi-proxy climate network against dominant patterns of 20th century global surface temperature. Averaging the reconstructed temperature patterns over the far more data-rich Northern Hemisphere half of the global domain, they estimated the Northern Hemisphere mean temperature back to AD 1400, a reconstruction which had significant skill in independent cross-validation tests.
Mann knew perfectly well his reconstruction failed r2 verification, a test he used multiple times in MBH98. By saying his reconstruction “had significant skill in independent cross-validation tests” while hiding the fact his reconstruction failed one of his own cross-validation tests, Mann committed fraud. That was fraud even if we ignore the fact Mann went on to lie about it and repeatedly try to cover up the fact he did calculate those scores.
We don’t have to call what Michael Mann did fraud if we don’t feel like focusing on the word, but if we are going to focus on the word, what Mann did was fraud.
I originally wrote this as comment at Judith Curry's blog. I thought it made for a good overview so I've decided to post it here as well. It's worth noting the numbered points are both links to previous posts which give more detail.