We have a new article from the Mannian school, this time involving supposedly “independent” NAS panelist, Doug Nychka, and geologist Caspar Ammann, who is very enthusiastic about calculating covariances using Mannian proxies. The lead author is a statistician, Bo Li. The article purports to use MBH99 proxies and says
we do not critically evaluate these proxies but simply apply them as the best available set at the time of 1999
They then proceed to use what appears to be Mann’s PC1, plus no less than 4 different time series from Quelccaya (Core 1 dO18 and accumulation, Core 2 dO18 and accumulation) with each different core remarkably and mysteriously teleconnecting in different ways with “climate reconstruction fields” such that mere covariances are interpreted as having physical meaning.) But more on the statistics on another occasion. The article concludes by saying:
The authors thank Dr. Michael Mann for providing the proxy data.
Jean S observed that the proxy series do not match the MBH versions that he had. I checked this as well. The figure below compares MBH99 Quelccaya Core 2 accumulation (with its characteristic pattern that Hans Erren figured out a few years ago) and the Quelccaya Core 2 version used by Li et al. The two proxy series are clearly not the same; the correlation is 0.70. Correlations of about this value are typical. It’s hard to say what Mann has done this time. I checked the a.c.f’s of a couple of versions to see if Mann had given them smoothed versions, but, in the examples that I looked at, the new series did not have additional autocorrelation. The correlation of the original version to the new version was consistently about 0.70 for all of the series – which is close to the square root of 1/2 – but why? All these proxies could have been obtained from WDCP so I don’t know why they didn’t use original data. Pretty mysterious.
Here’s another example:
UPDATE: The above surmise that noise was added to the data has proved correct. Bo Li has confirmed that noise was added to the proxies and that they archived a sample of synthetic data and did use original proxies. She has accordingly corrected the description of the data sets on her webpage to “synthetic” instrumental and “synthetic” proxy and responded cordially, apologizing for any inconvenience. This resolves the proxy issue, which was an odd one. When I get a chance, I’ll look at the statistics.
The data sets that I posted on my website are not the real Northern Hemisphere temperature and the MBH99 proxies. They are generated by adding white noise with unit variance to the standardized real data. The pseudo data sets on my website only serve as a toy example to try the R code that I used in my paper. However, the results in Li et al. (Tellus, in press) are based on the real data instead of the pseudo data. I am sorry that I did not explain very clearly what the data set on my webpage is and also sorry for the confusion that I brought to you as a consequence. I have modified my webpage to make the point more explicitly.
In our paper, we looked at the residuals in calibration (14 proxies against not PC1 of instrumental, but full N-Hem average) and found that there is enough serial correlation that an AR2 process is warranted to represent the “noise”. Then, the approach was to simulate a set of ensembles that fulfill the criterion during calibration so that the explained variance is the same but that the noise is different (using that AR2). Then, because we wanted to establish a method to study the maximum decadal values, we then simulated all series. Simulating time-evolving series was necessary because we have serial correlation, and thus decadal maxima can be computed. The goal was not to be producing the best reconstruction that is currently possible, but to demonstrate a way of how one could go and address the decadal max question. This question was not well addressed in the NRC report, and thus this work is a followup as collaboration between stats and geoscience to show how one could better answer the significance question for the old MBH99 framework.