ding dong the stick is dead « Watts Up With That?

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1. Phoenician in a time of Romans | September 29, 2009 at 11:45 pm

   To repeat the discussion from here:

   Flaw in methodology

   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.

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2. hairybeast | September 30, 2009 at 12:16 am

   Your pet propaganda site says:

   “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.

   Which is a very incomplete and biased recount of the debate. McKintyre stated the issue himself (not in the weasely words attributed to him by your pet propaganda site):

   Also keep in mind the implausibly small size of the current portion of the Yamal archive. It would be one thing if they had only sampled 10 trees and this is what they got. But they selected 10 trees out of a larger population. Because the selection yields such different results from a nearby population sample, there is a compelling prima facie argument that they’ve made biased picks. This is rebuttable. I would welcome hearing the argument on the other side. I’ve notified one dendro of the issue and requested him to assist in the interpretation of the new data (but am not very hopeful that he will speak up.)

   Let’s hear the rebuttal – it ought to be interesting in the usual fallacious “Appeal To Authority” genre.

   And there’s the fact that Mann’s Hockey stick is “The most discredited study in the history of Science”. Witness this gem in the U.K. Spectator:

   This shoddy research was subsequently torn apart so comprehensively that it has been called the most discredited study in the history of science (and has been quietly dropped by the IPCC, leaving man-made global warming theory with no more substance than the grin on the face of the Cheshire Cat. Go here, here and here for a history of the titanic battle that ensued over its unmasking).

   Now that his data has been pried loose, Mann’s in a bind.

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3. gurusteve | September 30, 2009 at 11:03 am

   For a layman’s description of the data issue concerning the “hockey stick” click here.

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4. hairybeast | September 30, 2009 at 11:50 am

   Good link Steve. The Beast loved this paragraph:

   Why did Briffa only have half the number of cores covering the Medieval Warm Period that the Russian had reported? And why were there so few cores in Briffa’s twentieth century? By 1988 there were only 12 cores used, an amazingly small number in what should have been the part of the record when it was easiest to obtain data. By 1990 the count was only ten, dropping still further to just five in 1995.

   Maybe someone will notice, but that seems unlikely.

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5. Phoenician in a time of Roman | September 30, 2009 at 5:50 pm

   Let’s hear the rebuttal – it ought to be interesting in the usual fallacious “Appeal To Authority” genre.</i.

   Uh-huh. So, in other words, if I point out that the US National Academy of Sciences endorsed Mann’s work (with reservations), you’re going to dismiss it – in favour of work from an economist and a mining engineer that wasn’t acceptable to “Nature”…

   While ignoring corroboration via other evidence.

   Yesterday, panel members said there was isolated evidence of a warmer earth about 1,000 years ago, but it appeared regional. They suggested that it did not reflect the overall global temperature at the time. They also said that while there were certain biases in the report, they got roughly the same results as the initial study when they used different methodologies to put together the data. The report said that Mann’s study had been supported by “an array of evidence” in other studies and that his and his collaborators’ findings were not even a primary piece of evidence of human-induced global warming.

   What you have is a conspiracy theory, not science.

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