The two perspective pieces are side-by-side, below.
|AMac's view |
|Certain aspects of the Tiljander story are straightforward, and easy for educated laypeople to grasp. |
(1) The paleotemperature reconstruction strategy employed by Prof. Mann’s research group in their 2008 PNAS paper (”Mann08″) absolutely required that each data series (”proxy”) be calibrated to the instrumental temperature record that spans 1850 to 1995.
(2) Mann08’s authors considered using the many-thousand-year record of lake-bed sediments that graduate student Mia Tiljander and her advisor collected and analyzed from the bottom of Lake Korttajarvi in Finland. In their paper (”Tiljander03″), Prof. Matti Saarnisto and his student warned that their lakebed data became progressively more unreliable from ~1720 on, as the sediments recorded nearby activites unrelated to climate: farming, peat cutting, roadbuilding, bridge reconstruction, and lake eutrophication.
(3) Mann08’s text acknowledges these issues as potential problems. However, the paper’s authors went ahead and incorporated four data sets from Tiljander’s work into their own reconstruction.
(4) The post-1720 contamination of the Lake Korttajarvi data series turns out to be much, much worse than Mann08’s authors suspected. Anyone who looks can quickly grasp the problem. This JPEG [copied below] is a graph of three of Tiljander’s series: “Darksum,” “Lightsum,” and “X-Ray Density” (in each case, Tiljander03’s “warmer” is plotted “up.”) It shows 2,000 years’ worth of data, using a running 20-year average to smooth out the curves. Look to the 1850-1995 interval at the right of the picture, where the instrumental temperature record is available for proxy calibration. Yikes!
(5) Mann08’s authors found a strong correlation between the rising temperature in southern Finland, 1850-1995 and the rising value of “Darksum” (r=0.3066). They got another one between rising temperature and rising “Lightsum” (r=0.2714). These are “spurious correlations,” unrelated to any pre-1850 climate signals that are contained in the sediments.
(6) In and of itself, this error is not a big deal. Scientists make mistakes all the time. The controversy has arisen over the refusal of Prof. Mann and his co-authors to acknowlege what happened, straighten out the mess, and incorporate any lessons into best scientific practices.
Judy Curry has written extensively about the woeful effects of Tribalism on Climate Science. The Tiljander/Mann affair is an object lesson of its dangers. Each reader will have their own take–and many partisans will bring their own spin to the analysis. In this Comment, I’ve sketched my own view of the matter. Perhaps, with Prof. Curry’s insights in mind, it will be possible to find more constructive ways of handling this (and future) controversies that arise at the interface between professional climatologists and citizen-scientists.
|For the sake of completeness, I will simply repeat what I have said before on various [RealClimate] threads, This is drawn entirely from the Mann et al (2008, 2009) papers (so you could just cut out the middle man). I’m not sure why people think that asking the same question a dozen times in different places will get a different answer, but here goes…|
1) Varved lake sediments often contain climate related signals, through changes in temperature, local runoff, stratification. Tiljander et al (2003) reported on records taken from a lake in Finland. They also reported possible anthropogenic contamination of their signal in more recent centuries. This makes them potentially useful, but also potentially dubious.
2) Mann et al (2008) used these proxies (4 out of 1209) as input data into two reconstruction methodologies. One (CPS) requires a local correlation to temperature before they can be used, the other (EIV) does not. In CPS, the local correlation requirement fixes the orientation of any proxy – if you have an a priori expectation that it should be a different way, that proxy cannot be used.
3) Since Mann et al (2008) were very aware of the potentially dubious nature of the modern portion of the Tiljander proxies, they performed their reconstructions without those proxies (and three others with potential problems) in sensitivity tests in the supplemental information (specifically Fig S8). Neither reconstruction (for NH mean (EIV) or NH land (CPS) temperature) is materially affected by the absence of the Tiljander proxies. This is the identical result to what you would have if you had a priori insisted on the opposite orientation of the proxies in CPS.
3) The reconstruction without the Tiljander proxies validates back to 700 AD (NH mean, EIV) or 400 AD (NH land, CPS).
4) If you think the Tiljander proxies are not usable or must be used in a different orientation, then Mann et al (2008) already showed what difference that makes to the overall reconstruction. There is nothing else left to do. All code and all data are available online for people to check this for themselves.
5) Please read the papers. Nothing stated in the RC posts or comments was incorrect. There is a sensitivity to how far back you can go without tree rings if you drop the Tiljander proxies as well. So if you don’t like them, and are convinced that tree rings are useless, these methodologies don’t allow you to say anything before 1500 (compared to 1760 in the original MBH) (though the structure is pretty similar back further (CPS)) (see SI in Mann et al 2009 for the EIV result). Other methodologies may still be useful (cf Osborn and Briffa, 2006; Moberg et al, 2004). If however, you think that tree rings do contain useful climate information (see Salzer et al, 2009 for instance), then you get validated reconstructions back to well before medieval times. All validated reconstructions show late 20th Century warmth as anomalous over the their range of validity.
(Consequences of all this)/(amount of time devoted to discussing it in the blogosphere) = a very small number.
Here's the picture of the three Tiljander data series that's linked within my piece. Darksum, Lightsum, and X-Ray Density are all oriented such that "up" represents "warmer," according to the interpretation specified in Tiljander03. To show them as they are employed in Mann08 (see that paper's Fig. S9), the images of Lightsum and X-Ray Density would need to be flipped so that high data values are near the top (Tiljander03 and Mann08 agree on the orientation of Darksum, as shown). The grey shaded area to the right covers the data on sediments that were deposited after 1720, about the time when Tiljander03 states that non-climate contamination began its growth. The very steep increase in non-climate signals through the 19th and 20th Centuries is strikingly obvious for Darksum and Lightsum.
In the thread following the Collide-a-scape post The Main Hindrance to Dialog (and Detente)”, Lucia memorably noted Gavin’s pronounced tendency to answer questions regarding the uses of the Tiljander proxies… but to not answer the questions that were actually being asked.
In that light, here are the six points I made, and my view of the response contained in Gavin's answer.
(1) Mann08′s methods require direct calibation of all proxies to the 1850-1995 temperature record.
(2) Tiljander warned about post-1720 contamination in the Tiljander proxies.
* Somewhat addressed -- but "potentially dubious nature”?
(3) Mann08 considered the warnings, then went ahead and used the proxies.
* Tacit agreement.
(4) The 19th and 20th Century contamination was really bad, as a glance at a figure will show.
(5) The correlations that Mann08 thought they found between 1850-1995 temperature and proxy signals were actually spurious correlations to contaminating non-climate signals.
(6) The mistake itself isn’t such a big deal, refusing to fix the problem is the issue.
Which perspective hews closer to the truth, and has greater explanatory power? For many people, I imagine the answer will rather depend on their pre-existing views. Time will tell...