One of the primary drivers of concern regarding AGW was Dr Michael Mann’s publication of his famous ‘hockey stick’ graph demonstrating the rapid warming of the planet over recent history.
For reasons that aren’t obvious, the graph showed no warming in the (very warm) 1890s and 1930s. The Medieval Warm Period and the Little Ice Age disappeared, as did the Roman Warm Period. Due to concerns regarding the accuracy of the graph there have been many attempts to validate the data. Dr Mann has refused to provide the raw data, so the graph remains as a reference point.
Richard Muller (2004) was one of the first to question the data, and his critique remains valid.
This improper normalization procedure tends to emphasize any data that do have the hockey stick shape, and to suppress all data that do not. To demonstrate this effect, McIntyre and McKitrick created some meaningless test data that had, on average, no trends. This method of generating random data is called Monte Carlo analysis, after the famous casino, and it is widely used in statistical analysis to test procedures. When McIntyre and McKitrick fed these random data into the Mann procedure, out popped a hockey stick shape!
https://www.technologyreview.com/s/403256/global-warming-bombshell/