Two new attacks on hockey stick by paleoclimatologists

by William Yeatman on April 13, 2004

in Science

Following the questions raised by Stephen McIntyre and Ross McKitrick over the quality of the data employed by Dr. Michael Mann of the University of Virginia in compiling his now infamous hockey stick graph, Manns interpretations of proxy temperature data are now coming under fire from within the community of paleoclimatologists.

In 2002, Esper et al. published in Science magazine a temperature record for the Northern Hemisphere over the past 1000 years that looked quite unlike the hockey stick.  Both the Medieval Climate Optimum and the Little Ice Age were clearly evident.  In the March 23 edition of Eos, Esper and other colleagues examine why this should be so.  According to the Greening Earth Society, Esper basically eliminates all the possibilities except the technique used to process tree-ring data sets the primary information relied on to construct early portions of the temperature reconstructions.

The problem with tree rings appears to be that their variations reflect more than year-to-year climate differences (temperature and/or precipitation). As the trees age, tree-ring production changes and introduces a spurious trend in the tree-ring series. This aging effect differs among tree species, as well as within species, depending on the trees growing conditions (soil type, elevation, slope aspect, etc.). It becomes difficult to separate trends due to aging from those due to climate.

Although various research groups use different techniques to account for this problem, the absence of ground truth (true temperature) makes it impossible to ascertain whose technique is best. Esper uses a method aimed at retaining long-period (greater than a century or so) variations in the tree-ring records, whereas Mann uses a method that virtually eliminates all long-term variation.  Esper concludes, Higher-frequency [decadal] climate variations are generally better understood than lower-frequency variations.

Meanwhile, David S. Chapman, Marshall G. Bartlett, and Robert N. Harris of the University of Utah, published in the April 7 edition of Geophysical Research Letters an examination of how Manns imputation of temperatures from boreholes contradicts their work.  Mann argues that borehole records of ground surface temperature (GST) do not accurately reflect surface air temperature (SAT) because of the effects of snowfall.  Chapman et al., however, have found that (1) GST tracks SAT extremely well at time scales that are appropriate for climate change considerations.  (2) Snow cover can either warm or cool the ground relative to a no snow case and need not lead to any bias. (3) Finally, our observations have not revealed any physical process that would explain the supposed preconditioning of GST by a prior season SAT.

In describing the differences between their work and Manns, Chapman et al. use surprisingly strong language for a scientific paper.  They describe three of Manns conclusions as misleading, and his end-point analysis as erroneous and just bad science.

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