The "hockey stick" describes a reconstruction of temperatures over the past 1000 to 2000 years using tree-rings, ice cores, coral and other records that act as proxies for temperature (Mann 1999). The reconstruction found that global temperature gradually cooled over the last 1000 years with a sharp upturn in the 20th Century. The principal result from the hockey stick is that global temperatures over the last few decades are the warmest in the last 1000 years. A central and critical plank of the alarmist global warming case is that the current phase of warming that started in the late 19th century is unprecedented. This claim is important because a similar or greater warming phase has occurred in the very recent past, before human CO2 emissions had caused CO2 levels to rise, then clearly any such recent warming must have been natural and was not caused by CO2. And if any recent similar warming phase was natural then clearly the current phase of warming could also be a natural phenomenon. If the current phase of warming could be natural then those arguing that it was primarily caused by human CO2 emissions would have to prove their hypothesis. And this is something they cannot do. The only "proof" that CO2 currently forcing up global temperatures is the claim that the current warming is somehow unusual, unique and unnatural. That's the total argument for CO2 forcing. Something unprecedented is happening to the climate and CO2 is the only candidate for what is causing this unique phenomenon.
Michael Mann of the Department of Geosciences, University of Massachusetts, who was the primary author of the paper, had in one scientific coup overturned the whole of climate history. Using tree rings as a basis for assessing past temperature changes back to the year 1,000 AD, supplemented by other proxies from more recent centuries, Mann completely redrew climate history, in every other science when such a drastic revision of previously accepted knowledge is promulgated, there is considerable debate and initial skepticism, the new theory.