In spite of suffering a serious illness (which I understand to be a kidney problem), Keith Briffa has taken the time to comment on the Yamal situation. The comment should be read by interested readers. If Briffa or any of his associates wishes to post a thread here without any editorial control on my part, they are welcome to do so.
Briffa’s comment leads off with the accusation that I had implied that the recent data had in this chronology had been “purposely selected” by Briffa “specifically because they exhibited recent growth increases”. I want to dispense with this up front. While I expressed surprise that there were so few cores, not only did I not imply that Briffa did any sub-selecting, but I specifically said the opposite. While the precise relationship of the CRU archive to the Hantemirov and Shiyatov subset is not entirely clear, I had speculated that H and S had created a subset that was relevant for their purposes (corridor standardization), but that it was not of an adequate size in the modern period for Briffa’s RCS standardization, stating clearly that it was not my belief that Briffa had crudely selected cores.
Since Briffa provides no quotation from any of my threads or comments to support his allegation I will review what I actually said.
Here is Briffa’s accusation:
The substantive implication of McIntyre’s comment (made explicitly in subsequent postings by others) is that the recent data that make up this chronology (i.e. the ring-width measurements from living trees) were purposely selected by me from among a larger available data set, specifically because they exhibited recent growth increases.
This is not the case. The Yamal tree-ring chronology (see also Briffa and Osborn 2002, Briffa et al. 2008) was based on the application of a tree-ring processing method applied to the same set of composite sub-fossil and living-tree ring-width measurements provided to me by Rashit Hantemirov and Stepan Shiyatov which forms the basis of a chronology they published (Hantemirov and Shiyatov 2002). In their work they traditionally applied a data processing method (corridor standardisation) that does not preserve evidence of long timescale growth changes. My application of the Regional Curve Standardisation method to these same data was intended to better represent the multi-decadal to centennial growth variations necessary to infer the longer-term variability in average summer temperatures in the Yamal region: to provide a direct comparison with the chronology produced by Hantemirov and Shiyatov.
Dealing with the second paragraph first, in my first post on the topic, I clearly distinguished between H and S corridor standardization and Briffa’s RCS standardization, noting that corridor standardization was known not to preserve centennial-scale vairability, as follows:
There is one other version of these series that readers may encounter: Hantemirov and Shiyatov archived a Yamal reconstruction at NCDC that has no hockey stick blade whatever. This version was promoted by a commenter (Lucy Skywalker) at Jeff Id’s as being a priori more valid than Briffa’s. Although the Hantemirov and Briffa chronologies have a very different visual appearance (especially the non_HSness of the Hantemirov version), there is an extremely high correlation between the very different looking Hantemirov-Shiyatov and Briffa Yamal chronologies. (If you regress the Briffa recon against the Hantemirov recon for the pre-1800 version, you get a huge r^2 of 0.81). The two series clearly have the same raw material.
However, in my opinion, the issue is considerably more nuanced than simply preferring the Hantemirov chronology. The Hantemirov and Shiyatov chronology adjusts for age (“standardization”) through a “corridor” method, whereas the Briffa chronology uses a “RCS” method to standardize for age. In other studies involving relatively short-lived trees (such as as Yamal), the corridor method has been found to yield very similar results to “conventional” standardization; such methods are also known to remove any centennial-scale variability from the reconstruction. As a result, no conclusions should be drawn with respect to centennial-scale variability from the Hantemirov chronology. No adverse conclusions should be found against the Briffa chronology merely because it differs from the Hantemirov chronology. There are other reasons to be concerned about the Briffa chronology, but these have to be presented and supported.
Briffa’s third paragraph states:
These authors [H and S] state that their data (derived mainly from measurements of relic wood dating back over more than 2,000 years) included 17 ring-width series derived from living trees that were between 200-400 years old. These recent data included measurements from at least 3 different locations in the Yamal region.
In the main post on this topic, I stated:
It is highly possible and even probable that the CRU selection is derived from a prior selection of old trees described in Hantemirov and Shiyatov 2002 as follows:
“In one approach to constructing a mean chronology, 224 individual series of subfossil larches were selected. These were the longest and most sensitive series, where sensitivity is measured by the magnitude of interannual variability. These data were supplemented by the addition of 17 ring-width series, from 200–400 year old living larches.”
In a comment to the same post, I clearly stated my view that there was no crude cherrypicking of the type that Briffa accuses me of implying. I stated :
bender, I agree with your point. I’ve tried to steer a careful line here. If you think otherwise, can you give me particulars as I don’t wish to unintentionally feed views that I don’t hold. It is not my belief that Briffa crudely cherry picked. My guess is that the Russians selected a limited number of 200-400 year trees – that’s what they say – a number that might well have been appropriate for their purpose and that Briffa inherited their selection – a selection which proved to be far from random and which, as you and I agree, falls vastly short of standards in the field for RCS chronology (as opposed to corridor or spline chronologies).
The substantive issue is whether the selection (that Briffa now confirms to have been inherited from the Russians) was appropriate for the RCS standardization method that Briffa applied. I brought this up, stating:
The subfossil collection does not have the same bias towards older trees. Perhaps the biased selection of older trees [results in] an unintentional bias, when combined with the RCS method. This bias would not have similarly affected the “corridor method” used by Hantemirov and Shiyatov themselves, since this method which did not preserve centennial-scale variability and Hantemirov and Shiyatov would not have been concerned about potential bias introduced by how their cores were selected on a RCS chronology method that they themselves were not using.
Briffa’s own caveats on RCS methodology warn against inhomogeneities, but, notwithstanding these warnings, his initial use of this subset in Briffa 2000 may well have been done without fully thinking through the very limited size and potential unrepresentativeness of the 12 cores. Briffa 2000 presented this chronology in passing and it was never properly published in any journal article. However, as CA readers know, the resulting Yamal chronology with its enormous HS blade was like crack cocaine for paleoclimatologists and got used in virtually every subsequent study, including, most recently, Kaufman et al 2009.
In his piece, McIntyre replaces a number (12) of these original measurement series with more data (34 series) from a single location (not one of the above) within the Yamal region, at which the trees apparently do not show the same overall growth increase registered in our data.
The basis for McIntyre’s selection of which of our (i.e. Hantemirov and Shiyatov’s) data to exclude and which to use in replacement is not clear but his version of the chronology shows lower relative growth in recent decades than is displayed in my original chronology. He offers no justification for excluding the original data; and in one version of the chronology where he retains them, he appears to give them inappropriate low weights.
The basis for replacing the CRU 12 with the Schweingruber 34 was, I think, quite clear: to see how the use of a larger and readily-available sample from the same area would affect the chronology. I think that I described my selection and exclusion procedures for this sensitivity study far more clearly than Briffa et al 2008 described its selection and procedures for, say, the Avam-Taimyr site. What was the basis for including the Avam site with Taimyr and not other sites in the area? What was the basis for including the Schweingruber Balschaya Kamenka with the Taimyr site and why wasn’t its inclusion mentioned in Briffa et al 2008? Why was Balschaya Kamenka included, but not Schweingruber’s Aykali River, Novaja Rieja, or Kotuyka River? Why was Balschaya Kamenka included with Taimyr, while Schweingruber’s Khadyta River, Yamal wasn’t included with Yamal? And what effect did all these changes have on the resulting chronologies?
While Briffa, in a peer-reviewed publication, omitted these relevant details, I provided a much clearer description of my methodology in the sensitivity study. The Avam-Taimyr example showed that Briffa was not opposed in principle to using Schweingruber data. There was reason to believe that the CRU data was not a complete population of living trees, but been subsetted by the Russians for a purpose different than RCS standardization. To test potential bias in this procedure, I tested the results without the 12 cores ending in 1988 and after and with the Schweingruber data. This indicated a dramatic difference between the versions.
Whether the McIntyre version is any more robust a representation of regional tree growth in Yamal than my original, remains to be established.
I note that McIntyre qualifies the presentation of his version(s) of the chronology by reference to a number of valid points that require further investigation. Subsequent postings appear to pay no heed to these caveats.
I did not propose the results of these sensitivity studies as an “alternative” and “more robust” chronology. I am not arguing that the Yamal versions using the Schweingruber data provide the “correct” climate history for the region. I am arguing that the version constructed by Briffa, and relied on so extensively in the literature since then, is not robust in its late-20th century portion to a small and reasonable inclusion of additional data. To accuse me of using “inappropriately low weights” for the cores selected into the CRU archive is beside the point. I could equally argue that Briffa used “inappropriately low weights” (i.e. zero) on the Schweingruber samples.
We have not yet had a chance to explore the details of McIntyre’s analysis or its implication for temperature reconstruction at Yamal but we have done considerably more analyses exploring chronology production and temperature calibration that have relevance to this issue but they are not yet published.
Like nomads, the Team has moved on. With respect to his new analyses, let’s hope that Briffa archives the measurement data and results concurrent with publication and that it doesn’t take 9 years from publication to see the data.
On a closing note, as I said from the outset, I did not say or imply that Briffa had “purposely selected” individual cores into the chronology and clearly said otherwise. Unfortunately for himself, Briffa’s tactic of withholding data and obstructing requests for data has backfired on him, as some people (not myself) have interpreted this as evidence of malfeasance, as opposed to my own interpretation that this only shows stubbornness on Briffa’s part and ineffective compliance administration by funding agencies and journals.