NCPA - National Center for Policy Analysis


October 6, 2009

Most of the proxy data for the "Hockey Stick" graph -- a statistical compilation of tree ring data supposedly proving that air temperatures had been stable for 900 years, then soared off the charts in the 20th century -- does not show anything unusual about the 20th century.  But two data series have reappeared over and over that do have a hockey stick shape.  One was the flawed bristlecone data that the National Academy of Sciences panel said should not be used, so the studies using it can be set aside.  The second was a tree ring curve from the Yamal Peninsula in Siberia, compiled by UK scientist Keith Briffa.

It turns out that many of the samples in the Yamal composite were taken from dead (partially fossilized) trees and they have no particular trend, says Ross McKitrick, a professor of environmental economics at the University of Guelph:

  • The sharp uptrend in the late 20th century came from cores of 10 living trees alive as of 1990, and five living trees alive as of 1995.
  • Based on scientific standards, this is too small a sample on which to produce a publication-grade proxy composite.
  • The 18th and 19th century portion of the sample, for instance, contains at least 30 trees per year, but that portion doesn't show a warming spike.
  • The only segment that does is the late 20th century, where the sample size collapses.

Once again a dramatic hockey stick shape turns out to depend on the least reliable portion of a dataset, says McKitrick.

But an even more disquieting discovery soon came to light.  Climate researcher Stephen McIntyre searched a paleoclimate data archive to see if there were other tree ring cores from at or near the Yamal site that could have been used to increase the sample size:

  • He quickly found a large set of 34 up-to-date core samples, taken from living trees in Yamal by F. H. Schweingruber, a colleague of Briffa's.
  • Had these been added to Briffa's small group the 20th century would simply be flat and It would appear completely unexceptional compared to the rest of the millennium.

Combining data from different samples would not have been an unusual step.  Briffa added data from another Schweingruber site to a different composite, from the Taimyr Peninsula.  The additional data were gathered more than 400 km away from the primary site.  And in that case the primary site had three or four times as many cores to begin with as the Yamal site.  Why did he not fill out the Yamal data with the readily-available data from his own coauthor?  Why did Briffa seek out additional data for the already well-represented Taimyr site and not for the inadequate Yamal site?

Thus the key ingredient in most of the studies that have been invoked to support the Hockey Stick, namely the Briffa Yamal series, depends on the influence of a woefully thin subsample of trees and the exclusion of readily-available data for the same area.  Whatever is going on here, it is not science, says McKitrick.

Source: Ross McKitrick, "Defects in key climate data are uncovered," National Post, October 1, 2009.


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