Read here and here. This image, which became the favored icon of global-warmists, has taken on a life-of-its-own during the past decade. The hockey-stick icon has already cost taxpayers billions of dollars and has the potential to cost taxpayers and consumers trillions more. When Michael Mann created the temperature dataset that the hockey-stick graph is based on, little did he realize that trillions were at stake, or that he would unleash a science-fraud tsunami that now threatens to legally engulf him. (click on images to enlarge)
The Mann icon was relentlessly published by the IPCC (until being mathematically and statistically discredited) because it showed modern temperatures to be significantly higher than past temperatures - in the vernacular of the "hockey stick", its 20th century "blade" was way above the "handle" of the stick. Prior to Mann's research, the IPCC had no unprecedented "blade" to proselytize with, so to speak.
So, how did Mann manage to find a modern, huge warming "blade" that others had missed in the historical temperature proxies, such as tree rings? Ahhhh, the beauty of using unique, but questionable mathematical/statistical techniques, that can often produce very artful science.
Mann's methodology, as outlined by one of the experts who tore it asunder, was relatively simple. First, one designs software code to search through thousands of tree ring data records. The objective is to identify those tree ring series that display exceptional growth during the 20th century (see top panel for Sheep Mountain tree rings). Second, once those exceptional tree ring records are identified, design your programming to over-weight those exceptional records by 390 times. Third, ignore the majority of tree ring records (see bottom panel for Mayberry Slough tree ring example) that reveal non-exceptional growth for the 20th century. Fourth, attribute 100% of exceptional tree ring growth to warming temperatures, not to other critical tree growth factors such as water, CO2 and soil nutrients.
And, voila, one can magically produce a very pronounced "blade." And like a true magician (unlike a true scientist), one never tells how the trick was done. As it turns out, it is the last sentence that has really been Michael Mann's undoing. Instead of openly sharing all the data and software coding he used to produce his results, so that other scientists could attempt to replicate his work, he chose not to cooperate with experts. To this day, he still has not released all the data/software evidence that he based his research on. As a result, many are now concluding that the original hockey-stick chart was based on scientific fraud, and that similar subsequent research by others is highly suspect also.
Because the science institutions, the peer-review journals, the universities and, most importantly, the IPCC climate scientists all failed in their identifying and correcting Michael Mann's methodologies from the beginning, he now finds himself in the legal, shark infested waters. When others miserably fail to perform their responsibility, the legal community will gladly do so for them.
For a good basic review of Ross McKitrick and Steve McInytre's effort to analyze Mann's hockey-stick research, read this. I can highly recommend A.W. Monford's, The Hockey Stick Illusion: Climategate and the Corruption of Science. It provides an insight to climate science that is incredibly damning and fascinating at the same time.