NCPA - National Center for Policy Analysis


February 15, 2005

The latest debate over global warming focuses on a well-publicized graph depicting temperatures over the past 1,000 years, but some observers argue the graph, which is known as the "hockey stick," is inaccurate.

Created by Dr. Michael Mann of the University of Virginia, the graph indicates fairly stable temperatures up until the 20th century, and a sharp acceleration over the last century, graphically depicting the shape of a hockey stick.

However, Stephen McIntyre, a minerals consultant from Toronto, first disputed the calculations behind the graph, and his skepticism now has scientists questioning its accuracy as well. According to McIntyre and other skeptics:

  • Dr. Mann's mathematical technique in drawing the graph is prone to generating hockey-stick shapes even when applied to random data, therefore, McIntyre argues, it proves nothing.
  • Statistician Francis Zwiers of Environment Canada, a government agency, says he now agrees that Dr. Mann's statistical method "preferentially produces hockey sticks when there are none in the data."
  • The graph pays little emphasis to the "medieval warm period" around A.D. 1000 and a 15th century "little ice age," and it relies too heavily on tree ring data from a small number of trees.
  • According to scientist Hans von Storch of Germany's GKSS Center, the graph sharply underestimates previous variations in temperature, suggested that the 20th-century jump in temperature is not as drastic as depicted on the graph.

Other researchers, including Mann argue that even if the mathematical method used in producing the graph is a little off, the end results are the same: major global temperature increases are correlated with the 20th century.

However, each side accuses the other of playing politics. McIntyre argues that Kyoto supporters are pushing the hockey-stick graph to further their cause, despite the skepticism that abounds.

Source: Antonio Regalado, "In Climate Debate, the "Hockey Stick" Leads to a Face-Off," Wall Street Journal, February 14, 2005.

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