On Sep 9, 2008, I sent an FOI request to the University of East Anglia, requesting a copy of the MXD data set as provided to Mann et al. Today (Oct 2, 2008), I was notified that they would provide this data and, sure enough the data is now posted (Oct 2, 2008) at http://www.cru.uea.ac.uk/~timo/datapages/mxdtrw.htm under the heading Rutherford et al 2008.
There are some puzzles.
The website reports the use of 341 sites, in Rutherford et al 2005, while the text of Rutherford et al reports the use of 387 sites. So one or the other is incorrect. An earlier article (Briffa et al, Holocene, 2002a) used 387 sites, listed here. I presume that the website is correct and the article is wrong and that a small corrigendum on this matter should be issued. All 341 sites said to have been used in Rutherford et al 2005 are included in the list of 387. Why were 46 sites removed from the network? Surely some sort of explanation should be provided.
The website (as of Oct 2, 2008) states:
The values after 1960 are a combination of information from high-frequency MXD variations and low-frequency instrumental temperature variations. We recommend, therefore, that the post-1960 values be deleted or ignored in any analysis that might be biased by the inclusion of this observed temperature information, such as the calibration of these data to form a climate reconstruction, or comparision of these data with instrumental climate observations for the purpose of assessing the ability of these data to represent temperature variability.
This is also a very puzzling comment as Rutherford et al 2005 nowhere mentions the blending of instrumental temperature variations back into proxy data after 1960. And if this was done, it is rather troubling. The explanation in Rutherford et al 2005 (Briffa, Osborn being coauthors with the Mann crowd) said:
Because the age-banding method requires large numbers of samples throughout the time period being studied, it has been applied only at a regional scale for the MXD network used here, rather than at the level of the 387 original site chronologies.
OSB therefore worked first with the traditionally standardized data at the individual chronology scale and gridded them to provide values in 115 5° by 5° grid boxes (26 available back to A.D. 1400) in the extratropical NH (Fig. 1b). They then developed temperature reconstructions by the local calibration of the MXD grid-box data against the corresponding instrumental grid-box temperatures.
The “missing” low-frequency temperature variability was then identified as the difference between the 30-yr smoothed regional reconstructions of Briffa et al. (2001) and the corresponding 30-yr smoothed regional averages of the gridded reconstructions. OSB add this missing low-frequency variability to each grid box in a region. After roughly 1960, the trends in the MXD data deviate from those of the collocated instrumental grid-box SAT data for reasons that are not yet understood (Briffa et al. 1998b, 2003; Vaganov et al. 1999). To circumvent this complication, we use only the pre-1960 instrumental record for calibration/cross validation of this dataset in the CFR experiments.
As I read the above, it describes a sort of coercion of the individual gridded series at “low frequency” to the corresponding “low frequency” shape of the regional ABD data (which is available at WDCP by the way). However it’s hard to be sure right now.
Here is a plot of average of the Briffa-Osborn gridded data, with dotted red line showing the part deleted in Mann et al 2008 (where Osborn and Briffa are coauthors).
Note that Briffa and Osborn also archived today various data used in IPCC AR4 graphics – previously unavailable in these versions.