A more serious accusation has come from two non-climate scientists from Canada, who claim to have found a flaw in Mann's statistical methodology. Stephen McIntyre, a mathematician and oil industry consultant, and Ross McKitrick, an economist at the University of Guelph, Ontario, base their criticism on the way Mann used a well-established technique called principal component analysis. This involves dividing "noisy" data into different sets and giving each set an appropriate weighting. McIntyre and McKitrick claim that the way Mann applied this method had the effect of damping down natural variability, straightening the shaft of the hockey stick and accentuating 20th century warming.
There is one sense in which Mann accepts that this is unarguably true. The point of his original work was to compare past and present temperatures, so he analysed temperatures in terms of their divergence from the 20th-century mean. This approach highlights differences from that period and will thus accentuate any hockey stick shape if - but only if, he insists - it is present in the data.
The charge from McIntyre and McKitrick, however, is that Mann's computer program does not merely accentuate this shape, but creates it. To make the point, they did their own analysis based on looking for differences from the mean over the past 1000 years instead of from the 20th-century mean. This produced a graph showing an apparent rise in temperatures in the 15th century as great as the warming occurring now. The shaft of the hockey stick had a big kink in it. When this analysis was published last year in Geophysical Research Letters it was hailed by some as a refutation of Mann's study.
McIntyre and McKitrick say that their work is intended to show only that there are problems with Mann's analysis; they do not claim their graph accurately represents past temperatures. "We have repeatedly made it clear that we offer no alternative reconstruction," McIntyre states on his Climate Audit blog.
The obscure statistical arguments were overshadowed in late 2005 when Mann refused to give Congressman Barton his computer code. Mann regarded the code as private property, but his opponents claimed he feared refutation of his findings. Mann did eventually publish the code, but the damage was done.
In the meantime, three groups had been scrutinising the claims of McIntyre and McKitrick. Hans von Storch of the GKSS Research Centre in Geesthacht, Germany, concluded that McIntyre and McKitrick were right that temperatures should be analysed relative to the 1000-year mean, not the 20th-century mean. But he also found that even
when this was done it did not have much effect on the result. Peter Huybers of the Woods Hole Oceanographic Institution in Massachusetts came to much the same conclusion.
The work of Eugene Wahl of Alfred University, New York, and Caspar Ammann of the National Center for Atmospheric Research in Boulder, Colorado, raised serious questions about the methodology of Mann's critics. They found the reason for the kink in the McIntyre and McKitrick graph was nothing to do with their alternative statistical method; instead, it was because they had left out certain proxies, in particular tree-ring studies based on bristlecone pines in the south-west of the US.
"Basically,
the McIntyre and McKitrick case boiled down to whether selected North American tree rings should have been included, and not that there was a mathematical flaw in Mann's analysis," Ammann says. The use of the bristlecone pine series has been questioned because of a growth spurt around the end of the 19th century that might reflect higher CO2 levels rather than higher temperatures, and which Mann corrected for.
What counts in science is not a single study, however. It is whether a finding can be replicated by other groups. Here
Mann is on a winning streak: upwards of a dozen studies, some using different statistical techniques or different combinations of proxy records (excluding the bristlecone record, for instance), have produced reconstructions more or less similar to the original hockey stick.
('Scuse huge C&P: I need the whole chunk because here Fred is being scrupulously fair, and the original is available only to subscribers, at http://www.newscientist.com/channel/earth/mg18925431.400.html)