Why are you even discussing Dr. Mann? There's way more going on in that tempestuous teapot than you can use here to suggest climate change doesn't really exist. In his own field Mann has been largely discredited (though the AAAS seems to like his outreach attempts); he's right up there with the British doctor who suggested (with no evidence) that vaccines cause autism. He has a tight reign on his own publicity and clearly monitors the Wikipedia page about himself. He seems to be protecting his brand, he isn't contributing to science.

Now try looking at the 97% of scientists who acknowledge that climate change has sped up due to human activity.

Easterbrook:
Oxygen isotope studies in Greenland, Ireland, Germany, Switzerland, Tibet, China, New Zealand, and elsewhere, plus tree-ring data from many sites around the world all confirm the presence of a global Medieval Warm Period.

The Hockey Stick Trick
Over a period of many decades, several thousand papers were published establishing the Medieval Warm Period (MWP) from about 900 A.D. to 1300 A.D. and the Little Ice Age (LIA) from about 1300 A.D. to 1915 A.D. as global climate changes. Thus, it came as quite a surprise when Mann et al. (1998) (Fig. 28) concluded that neither the MWP nor the Little Ice Age actually happened on the basis of a tree-ring study and that became the official position of the 2001 Intergovernmental Panel on Climate Change (IPCC).

Mann appears to have tried for a paradigm shift, but it backfired and scientists resumed doing real science and paid attention to studies that show actual facts.

The contrived elimination of the MWP and Little Ice Age by Mann et al. became known as “the hockey stick” of climate change where the handle of the hockey stick was supposed to represent constant climate until increasing CO2 levels caused global warming, the sharp bend in the lower hockey stick.

The Mann et al. “hockey stick” temperature curve was at so at odds with thousands of published papers, including the Greenland GRIP ice core isotope data, sea surface temperatures in the Sargasso Sea sediments (Fig. 29) (Keigwin, 1996), paleo-temperature data other than tree rings (Fig. 30) (Loehle, 2007), and sea surface temperatures near Iceland (Fig. 31) (Sicre et al., 2008) one can only wonder how a single tree-ring study could purport to prevail over such a huge amount of data. At best, if the tree-ring study did not accord with so much other data, it should simply mean that the tree rings were not sensitive to climate change, not that all the other data were wrong. McIntyre and McKitrick (2003, 2005) evaluated the data in the Mann paper and concluded that the Mann curve was invalid “due to collation errors, unjustifiable truncation or extrapolation of source data, obsolete data, geographical location errors, incorrect calculation of principal components and other quality control defects”. Thus, the “hockey stick” concept of global climate change is now widely considered totally invalid and an embarrassment to the IPCC.

Why, then, did Mann's hockey stick persuade so many non-scientists and gain such widespread circulation? The answer is apparent in revelations from e-mails disclosed in the Climategate scandal (Mosher and Fuller, 2010; Montford, 2010). These e-mails describe how they tried to “hide the decline” in temperatures, using various “tricks” in order to perpetuate a dogmatic view of anthropogenic global warming.

With this kind of friend, Mann or Easterbrook, climate scientists don't need enemies. Scientists didn't need to deny previous cool spells in order for climate change to be accepted as real. We understand that there are lots of factors that have warmed and cooled the planet over billions of years, but we all understand that human pollution is as bad or worse than the explosion of a supervolcano for super-charging the atmosphere.