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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). The IPCC 3rd report (Climate Change 2001) then totally ignored the several thousand publications detailing the global climate changes during the MWP and the LIA and used the Mann et al. tree-ring study as the basis for the now famous assertion that “Our civilization has never experienced any environmental shift remotely similar to this. Today's climate pattern has existed throughout the entire history of human civilization.” (Gore, 2007). This claim was used as the main evidence that increasing atmospheric CO2 was causing global warming so, as revealed in the ‘Climategate’ scandal, advocates of the CO2 theory were very concerned about the strength of data that showed the Medieval Warm Period (MWP) was warmer than the 20th century and that global warming had occurred naturally, long before atmospheric CO2 began to increase. 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.
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FIGURE 28. Mann et al. (1998) “hockey stick” graph of temperature change over the past 1,000 years based on tree rings.
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.