The chronologies for Taymir in Esper et al [2002] and Osborn and Briffa [2006] differ – not a whole lot but enough to be noticeable. Esper provided measurement data for Taymir, while Osborn and Briffa have refused. The letter from Science said:
The other three series contain some non-identical tree-ring series derived from the same sites; thus the series they used can not be reproduced using the Esper et al. data; there are fewer tree cores in the Esper et al. data. The source for these three series is Briffa (2000).
Both Esper and OB cite Naurzbaev as the source of the data. (Naurzbaev’s articles are ones that I’ve recommended as good ones.) Naurzbaev et al [Holocene 2002] have a graphic showing the count of ring width measurements by year; so I plotted up the corresponding information from Esper’s measurement file with curious results.
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Top : Taymyr core count from Naurzbaev et al; above – core count from Esper measurement file (March 2006). Image prettied up by Mike Carney below.
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The visual match is exact except at the beginning and end of the series. At the beginning, Esper has not used some of the earlier data. It’s the end that interests me. Here Esper has apparently not used 6 cores, which was not apparent from the report in the chronology file. These 6 cores probably account for the difference between Esper and Osborn-Briffa. It’s not the use of completely different files. So the question is: why did Esper exclude these 6 cores? If Esper excluded these 6 cores, should Briffa have excluded them as well?
An observation made in Esper et al [2003]:
When using RCS, one has to decide whether differences between subsamples reflect (i) climatic signals and should be preserved or (ii) non-climatic signals and should be eliminated. If significant differences occur between various subsamples, some discussion of the impact these differences will have on the resulting chronologies would be useful.
Quite so. Another observation from Esper et al [2003] which I’ve mentioned before, but is worth repeating:
Before venturing into the subject of sample depth and chronology quality, we state from the beginning, “more is always better”. However as we mentioned earlier on the subject of biological growth populations, this does not mean that one could not improve a chronology by reducing the number of series used if the purpose of removing samples is to enhance a desired signal. The ability to pick and choose which samples to use is an advantage unique to dendroclimatology. That said, it begs the question: how low can we go?
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