[ UPDATE 22 Aug. 2010 3:20 PM EDT -- In the past hour, my comment passed moderation, and was slotted into position #42 (the comment count is currently at 60). Gavin Schmidt's inline commentary is reproduced at the tail of this post. -- AMac ]
"M&W10" is the recently-released preprint of "A Statistical Analysis of Multiple Temperature Proxies: Are Reconstructions of Surface Temperatures Over the Last 1000 Years Reliable?," by BB McShane and AJ Wyner. Because it questions the methodology of Mann08, it's already beloved by many critics of the AGW Consensus. However, this is a very lengthy and complex statistical treatment that is performed by two non-paleoclimatologists, done without the benefit of insights from insiders. It has, predictably, been slammed by the most-capable pro-AGW Consensus science-bloggers. It seems to me that many of these critiques have merit: the authors would benefit from a rewrite (and a shortening/focusing), if this is possible at this late stage in the submission process.
That said, what I take as M&W10's major point seems very timely: a focus on the tightness of the uncertainty estimates that accompany paleoclimate reconstructions like those published by Prof. Mann's group (M&W10 takes Mann08 as its jumping-off point). My sense is that the confidence expressed by the narrowness of these papers' uncertainty bars is very misplaced. Of course these are opinions. I don't claim the statistical chops to comment in detail on M&W10's methods.
The figure in "Doing it yourselves" that touches on the Lake Korttajarvi data series is reproduced (fair use) below. Here is the paragraph that accompanies it --
It’s also easy to test a few sensitivities. People seem inordinately fond of obsessing over the Tiljander proxies (a set of four lake sediment records from Finland that have indications of non-climatic disturbances in recent centuries – two of which are used in M&W). So what happens if you leave them out?
That figure seemed bereft of context. It is referring to which of M&W10's sections? Using M&W10's methods, the Realclimate authors are excluding the Tiljander proxies from... what, exactly?
So I submitted the following comment. Rereading it, I see that I edited away my "a question" during one of my rewrites. That is, "Could you show the output of your M&W10-style 'Lasso' reconstructions for Non-Dendro proxies and for Non-Dendro/Non-Tiljander proxies? Those would be comparable to the Non-Dendro and Non-Dendro/Non-Tiljander traces in the current version of Mann08's Fig S8a."
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21 August 2010 at 9:59 PM
A question, and two points of clarification.- - - - - - - - - -
First, thanks for the graphics that show various "Lasso" outputs with the Tiljander proxies omitted. This post's "No Tiljander Proxies" figure with the six reconstructions (1000 AD - 2000 AD) is an extension of M&W10's Figure 14. As such, it's a little hard to interpret without reference to that legend.
According to Fig. 14:
* The dashed Green line is the M&W10 backcast for Northern Hemisphere land temperature that employs the first 10 principal components on a global basis.
* The dashed Blue line is the backcast for NH land temperature that employs the first 5 PCs (global), as well as the first 5 local (5x5 grid) PCs.
* The dashed Red line is the backcast for NH land temperatures that employs only the first PC (global).
All three dashed lines appear to be based on the use of the entire data set used in Mann et al, (PNAS, 2008)--both Tree-Ring proxies and Non-Dendro proxies.
This is an important consideration, as there seems to be general agreement that Mann08's Dendro-Including reconstructions are not grossly affected by the exclusion of the Tiljander proxies. This point was made in Mann08's Fig. S8a (all versions).
However, there has been much contention over the extent to which NH land reconstructions restricted to non-dendro proxies are affected by the inclusion or exclusion of the Tiljander varve data. See, for example, the 6/16/10 Collide-a-scape thread The Main Hindrance to Dialogue (and Detente).
Mann08 achieved prominence because of its novel findings: the claim of consistency among reconstructions based on Dendro proxies, and those based on Non-Dendro proxies. Mann08 also claimed that the use of Non-Dendro proxies in could extend validated reconstructions far back in time. This is stated in Mann08's abstract (see also press release).
Thus, the matters of interest would be addressed if the dashed and solid traces in this post's "No Tiljander Proxies" figure were based only on the relevant data set: the Non-Dendro proxies.
(The failure of Non-Dendro reconstructions to validate in early years in the absence of Tiljander was raised by Gavin in Comment #414 of "The Montford Delusion" thread (also see #s 483, 525, 529, and 531). While progress in that area would be welcome, it is probably difficult to accomplish with M&W10's Lasso method.)
The first point of clarification is that the Tiljander proxies cannot be meaningfully calibrated to the instrumental temperature record, due to increasing influence of non-climate factors post-1720 (Discussion). This makes them unsuitable for use by the methods of Mann08 -- and thus by the methods of M&W10 -- which require the calibration of each proxy to the 1850-1995 temperature record.
The second point of clarification is concerns this phrasing in the post:
People seem inordinately fond of obsessing over the Tiljander proxies (a set of four lake sediment records from Finland that have indications of non-climatic disturbances in recent centuries – two of which are used in M&W).The "four lake sediment records" used in Mann08 are "Darksum," "Lightsum," "XRD," and "Thickness." The authors of Tiljander et al (Boreas, 2003) did not ascribe meaning to "Thickness," because they derived "Darksum" by subtracting "Lightsum" from "Thickness." Thus, "Thickness" contains no information that is not already included in "Lightsum" and "Darksum."
In other words, there are effectively only three Tiljander proxies (Figure).
M&W10's Figure 14 is reproduced (fair use) below:
- - - - - - - - - -FIG 14. Backcasts to 1000 AD from the various models considered in this section are plotted in grey. CRU Northern Hemisphere annual mean land temperature is given by the thin black line with a smoothed version given by the thick black line. Three forecasts are featured: regression on one proxy principal component (red), regression on ten proxy principal components (green), and the two stage model featuring five local temperature principal components and five proxy principal components (blue).
[ UPDATE 22 Aug. 2010 3:20 PM -- My comment passed moderation and was slotted into position #42, with its original timestamp of 21 August 2010 at 9:59 PM. Gavin Schmidt's inline commentary follows, with [Notes] for my follow-ups inserted. -- AMac ]
[Response: We aren't going to go over your issues with Mann et al (2008) yet again [Note 1] - though it's worth pointing out that validation for the no-dendro/no-Tilj is quite sensitive to the required significance, for EIV NH Land+Ocean it goes back to 1500 for 95%, but 1300 for 94% and 1100 AD for 90% (see here). But you missed the point of the post above entirely. The point is not that M&W have the best method and it's sensitivities need to be examined, but rather that it is very easy to edit the code and do what ever you like to understand their results better i.e. "doing it yourself".[Note 2] If you want a no-dendro/no-Tiljander reconstruction using their methodology, then go ahead and make it (it will take just a few minutes - I know, I timed it - but to help you along, you need to change the selection criteria in R_fig14 to be
sel < - (allproxy1209info[,"StartYear"] <= 1000) & (allproxy1209info[,2] != 7500) & (allproxy1209info[,2] != 9000)(no_dendro) and change the line
proxy < - proxy[,-c(87:88)]to
proxy < - proxy[,-c(32:35)](no_tilj)). [Note 3] Note that R_fig14 does not give any info about validation, so you are on your own there. The bottom line is that it still doesn't make much difference (except the 1PC OLS case, which doesn't seem very sensible either in concept or results anyway). [Note 4] - gavin]
[Note 1] There is no "yet again" to discuss. The problems with Tiljander have never been addressed. Not by Dr. Schmidt, not by Prof. Mann, not by any of Mann08's other authors. As stated in my comment, the main issue is that the Tiljander proxies cannot be calibrated to the instrumental record, and thus are wholly unsuited to Mann08's methods.
[Note 2] It is silly to propose that I have "missed the point of the post entirely." It is silly to suggest that I've claimed that M&W10 "have the best method and it's sensitivities need to be examined."
[Note 3] Like most pro-AGW-Consensus advocates, most lukewarmers, and most skeptics: I am not conversant in either "R" or MatLab. Thus, I cannot immediately profit from Dr. Schmidt's sincere and well-meaning advice.
[Note 4] Dr. Schmidt links to the No-Dendro/No-Tiljander variant of M&W10's Figure 14 that he generated:
That is an informative figure! It suggests that exclusion of the Tiljander proxies does not greatly alter the Non-Dendro reconstructions obtained by M&W10's "Lasso" method, if the first 10 global PCs are used (Green), or if the first 5 global PCs and the first 5 gridded PCs are used (Blue). On the other hand, the use of only the first global PC shows something interesting. Adding in the uncalibratable Tiljander proxies completely changes the character of the first principal component. Without it, PC1 of the Non-Dendro proxies
Perhaps those better-versed than me in principal component analysis will be able to make more sense of this figure.
[ UPDATE 23 Aug. 2010 -- Wording two paragraphs up altered. In reviewing Gavin's new No-Dendro/No-Tilj reconstruction figure, I am discussing the shapes of the anomaly traces governed by the principal component(s). These are not the PCs themselves -- AMac ]