Seems that these scientists took core samples from 254 trees and then used only 12 that proved the point they wanted to make. They made such statements as
"Late 20th century warmth is unprecedented for at least roughly the past two millennia for the Northern Hemisphere," wrote the two authors of Global Surface Temperatures over the Past Two Millennia published in Geophysical Research Letters in 2003 - Mann, and Phil Jones of CRU.Fortunately we are assured by the science fetishists that science is self correcting and the peer review process is used to weed out frauds like this. Sure glad that worked! Oh, wait, thats not how it worked. Seems that this got thru the peer review process several times. No worries still, because science is all about reproducing expirements and double checking data. Hmmm, seems that didn't work either...
Briffa's 2008 paper concludes that: "The extent of recent widespread warming across northwest Eurasia, with respect to 100- to 200-year trends, is unprecedented in the last 2000 years."
Scientists have ensured much of the measurement data used in the reconstructions remains a secret - failing to fulfill procedures to archive the raw data. Without the raw data, other scientists could not reproduce the results. The most prestigious peer reviewed journals, including Nature and Science, were reluctant to demand the data from contributors.Seems that Mann's research was accidentally discovered on his FTP server in a folder titled "BACKTO_1400-CENSORED" Oops! (I bet the person who "accidentally" found it was employed by Big Oil!) So from these 12 trees we have determined that the Medieval Warm Period AKA the Medieval Climate Optimum never happened and that we are now killing the planet due to greenhouse gasses.
Mann too used dendrochronology to chill temperatures, and rebuffed attempts to publish his measurement data. Initially he said he had forgotten where he put it, then declined to disclosed it.
Still, I predict that AGW will emerge unscathed from this as they do from every other fraud that is exposed. AGW is more about a worldview than it is about science and as such science could thoroughly debunk it and the AGWers would still believe it. Even if we went into a deep ice age it would be man's fault in their minds.