While I am not a regular reader of Pielke's blog, such efforts to establish dialog among 'Consensus scientists,' 'skeptics,' and 'lukewarmers' are commendable. Given my own interests, I was most taken by the following paragraph from Chick Keller's post.
What about global and NH temperatures in the past 2,000 years? It’s just not good science to dismiss all this very careful work with — well I’m uncomfortable with the way they match proxy and instrumental data. Here a careful read of the National Academy of Sciences PNAS paper in 2008 (see Mann et al. 2008) on the best work done to date on this is in order (as well as comparison with the several other different and mostly independent treatments of the subject by other authors–bore hole results, Moberg’s wavelet combination of low and high frequency proxies, Lonnie Thompson’s mountain glacier work, etc). Pretty hard to just dismiss out of hand all these people and their rather similar results."See Mann et al. 2008" is a link to the PDF of the Mann group's high-profile paleoclimate paper in the high-impact, peer-reviewed journal Proceedings of the National Academy of Sciences of the USA. This is of course the work that sparked a controversy because of its use of the four lakebed sediment data series from Lake Korttajarvi, Finland -- the "Tiljander proxies."
In the excerpted text, Chick Keller makes four claims about Mann et al (2008).
1. Very careful workI don't agree. The treatment by the Mann group of the Lake Korttajarvi sediment proxies provides a good explanation of why. In this recent comment at the Center for Inquiry Forum, I offered a precise formulation of this issue, and invited journalist Chris Mooney to ask Prof. Mann for his views (Mooney declined [12 Mar. 10 link corrected]). Here are those two questions, reprinted.
2. It's not good science to dismiss... the way they match proxy and instrumental data.
3. Here a careful read of... the best work done to date on [temperatures of the past 2,000 years] is in order.
4. Pretty hard to just dismiss out of hand all these people and their rather similar results.
BackgroundMy answers to these two questions are as follows.
In September 2008, Dr. Mann’s group published an important paper on Earth’s temperature history for the past 2,000 years in the prominent peer-reviewed journal Proc. Natl. Acad. Sci. USA. Temperature signals were extracted from many long-term data series, such as tree rings and ice cores. The paper has been strongly criticized for its inclusion of the lakebed sediments characterized by Finnish geologist Mia Tiljander. Critics claim that the four Tiljander proxies are uncalibratable due to contamination of the temperature signal by local activities, from the 1700s to the present. Critics also claim that the PNAS paper mistakenly uses two of the Tiljander proxies in an upside-down orientation, such that “warmer” information is added to the paleotemperature reconstructions as “colder”, and vice versa. In his Response published in PNAS in February 2009, Dr. Mann called these criticisms “bizarre,” but he did not explicitly rebut them.
1. Can the four Tiljander proxies be calibrated to the instrumental temperature record that spans 1850 to 1995?
2. Do the PNAS paper’s reconstructions use the temperature information in the “tiljander-2003-xraydenseave” and “tiljander-2003-lightsum” series in a manner that is consistent with the interpretation offered by Mia Tiljander in her 2003 paper?
1. No.Here's additional background. A compilation of literature and data links to this issue is here. Relevant blog posts are linked here. One useful entry point to understanding the contrasting stances of Consensus advocates and skeptics is this digest of the first Stoat blog post devoted to the issue. My opinion as to "why it matters" is covered in The Null Hypothesis. Jeff Id of the Air Vent has gone beyond Tiljander to discuss the underlying statistical approach taken by Prof. Mann's group.
With that in mind, I offer these comments on Chick Keller's four points concerning Mann et al (2008).
1. Mann et al (2008) is not careful work. It's slipshod work. The use of the Tiljander proxies is far from the only major error in the paper. It's just the glaringly obvious one. But experiences at Stoat, Lucia's Blackboard and elsewhere have shown that there's no point in discussing more complex topics if we can't first agree on simple concepts like, "Yes, the number sentence '2+2=4' is true, while '2+2=5' is false."
2. It is good science to dismiss the way Mann et al (2008) matched proxy and instrumental data. The Tiljander proxies are uncalibratable to the instrumental record. Their misuse proves that these Mann group methods are grossly in error. Did Mann et al make additional mistakes at the data-compilation step? Yes, almost certainly -- but they are subtler, thus not worth considering at this point.
3. To state that Mann et al is "the best work done to date on [temperatures of the past 2,000 years]" is not meant as a very harsh criticism of this field. Alas, I must take it that way. How did this paper pass peer review and editorial scrutiny? Why is it that in the 18 months since publication, not one "mainline" climate scientist has uttered a peep in public about it? The answers, whatever they are, cannot reflect well on practices in this branch of science.
4. As far as "dismissing out of hand" -- I've been seeking out logical, data-based defenses of Mann et al's use of the Tiljander proxies for months. I have encountered none. (Readers are urged to provide candidate links in the comments to this post.)
I invite Chick Keller to rebut these sharp criticisms of Mann et al. (2008). In particular (and as with Mann et al advocate Boris at Lucia's Blackboard), I'm glad to offer him a guest post here, or a prominent link to his discussion of these questions at another location.
A Stark Choice
If Chick Keller is right, high-quality paleoclimate reconstructions such as those in Mann et al (2008) can supply us with useful context for evaluating the likely effects on climate of the rising concentration of CO2. Many who share Dr. Keller's point of view also strongly believe that critics are muddying the waters with irrelevant, trivial, and incorrect objections -- perhaps out of an understandable but misguided effort to avoid facing what's needed to mitigate the climate-change threat.
If I'm right, Mann et al (2008) is not simply a fatally-flawed study that's overdue for correction or retraction. Because its worst flaws are obvious, its privileged treatment by journal editors, peer-reviewers, the paleoclimate community, and AGW Censensus science-bloggers should serve as a bright red flag. Some practices have gone very amiss in paleoclimatology. Until those problems are identified and addressed, no reconstruction of the past two millenia's climate should be accorded our trust.
Which is it? The best way to find out is through clear, respectful debates on the issues.
I've emailed Chick Keller a link to this post.