Climate change

58 – The Hockey Stick again

I have referred previously to the very public debate about the so-called “hockey stick”, a central feature of the most recent report of the Intergovernmental Panel on Climate Change (IPCC) (see PD#6). There have been some interesting developments this year, and a particularly interesting one just recently.

Background: Mann et al. (1998) used a large set of proxy data (mainly measurements of tree ring widths) to estimate temperatures going back centuries. They found that temperatures have been pretty stable since 1400, until they suddenly increased, starting around 1900 (described as the “hockey stick” result). It’s a striking result, and the IPCC featured it very prominently in their last set of publications. (Interestingly, Mann himself was lead author of the chapter that featured his result.)

A geologist (McIntyre) and an economist (McKitrick) tried to reproduce the Mann et al. study, and in the process found errors in the basic data, and a major problem with the statistical method. With errors removed, the hockey stick shape is completely lost, and temperatures in the 1900s seem pretty unremarkable. M&M also found that the model has absolutely no statistical power or significance, with an R2 of 0. There are now three refereed publications by M&M describing their work on this, including one in the journal Geophysical Research Letters, where one of Mann’s papers was published.

Remarkably, Mann et al. have rejected all criticisms of their work. Worse than that, they have refused to provide the full data set and computer code that they used, so that the results could be reproduced independently. The behaviour of Mann and his team has been rather disturbing given that they are meant to be scientists.

Reading through all the material (and there is a lot of it), it is pretty clear that McIntire and McKitrick have thoroughly won the argument.

Mann’s refusal to lie down and concede defeat, even after he has been comprehensively maimed and dismembered, made me think of the wonderful movie, “Monty Python and the Holy Grail”. You have probably seen the scene where King Arthur (M&M) has a sword fight with the Black Knight (Mann). Arthur hacks off one of the Black Knight’s arms, then another, then one leg, then the other, but through it all the Black Knight refuses to concede, and keeps haranguing Arthur to continue fighting.

Arthur: What are you going to do? Bleed on me?

Black Knight: I’m invincible!

Arthur: You’re a looney!

This is such a wonderful description of the hockey stick “debate”. There is Mann, just a head and a bloody torso, madly clinging to the belief that he is still in the fight.

He had appeared determined to just bluff it out and hope to get away with it, but just recently the U.S. House of Representatives Energy and Commerce Committee has gotten involved. It has sent letters to Mann and his co-authors, requiring that their data and statistical code be released, as well as some other interesting related letters to the IPCC and the US National Science Foundation. The letters are publicly available here.

It looks to me like Mann might finally have met his match. I don’t think he will be able to get away with the usual bluff and bluster he uses, but I wouldn’t be surprised if he tries to. It will be fascinating to see whether he does.

In my view, the question of whether the hockey stick result is correct should not be that important in terms of influencing the sorts of actions we undertake. However, the whole saga is important for revealing the amazingly low quality of research processes used by some of the key scientists involved in the IPCC, and the IPCC’s failure to properly audit studies that they put forward as being important and which they hope will have an influence on the expenditure of billions of dollars.

David Pannell, The University of Western Australia

Postscript, 10 July 2005. Email from Glenn Fox: “Here is a modified hockey stick that I made to present to McKitrick and McIntyre at a recent policy workshop held by the Fraser Institute in Toronto. As I explained at the presentation (after McKitrick presented a summary of his work on the hockey stick), as a Canadian and a Hockey player, I was saddened to hear of the loss of the iconic hockey stick as a representation of long-term global temperature. So I went into my workshop and constructed the McKitrick/McIntyre hockey stick so that we could save the icon and be more consistent with the data.”

Further Reading

Web site that describes the debate and provides access to several of the papers:

Steve McIntyre’s blogg, for latest developments in the saga:

The web site of Mann’s defenders: