The cat i' the adage

"The growth of knowledge depends entirely on disagreement" Karl Popper (1902 - 1994)

Friday, January 28, 2005

How the "hockey stick" came into being

In 1998 Mann et al. published an article in Nature which introduced the idea that the world's temperature history was shaped like a hockey stick and that current temperatures were the highest for a thousand years. This was enthusiastically adopted by the IPCC despite plenty of evidence that the Earth was considerably warmer in medieval times.

Now the methodology of the article is coming under attack in a paper by Stephen McIntyre and Ross McKitrick which has been accepted by Geophysical Research Letters:

McIntyre & McKitrick found that the Mann et al. methodology included a data pre-processing step, one which was not reported in the original study, that essentially guaranteed that a hockey stick curve would result from their analysis. They demonstrated this by applying the same methodology to many synthetic temperature records that were constructed with random noise. In almost every case, a hockey stick curve resulted. The claim of unprecedented warmth and the hockey stick shape appear to hinge on the treatment of one species of tree, the bristlecone pine, from North America in the 1400's. Further statistical tests showed that this critical signal in the early 15th century lacked statistical significance. This suggests that the results of Mann et al. were simply a statistical fluke, which greatly exaggerated a characteristic of the bristlecone pines, which may or may not be related to global temperatures.

In pseudo-science many results presented as empirical are actually included in the assumptions. Logicians used to call this "begging the question" or petitio principii. When the assumptions themselves are undeclared, fallacious reasoning becomes fraudulent. However, I wouldn't expect politicians, even as percipient as Stephen Byers, to be able to recognize that this affects the validity of any conclusions drawn.


At 28 January 2005 at 12:03, Blogger Jorgen said...

Tony Blair will ask himself: To believe or not believe in global warming. What will make me most popular?

I can't really see him fight newspapers, old labour and rabid environmentalists just before an election.


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