We’ve noted that Briffa’s gridded MXD has high correlations to temperature, much higher than run-of-mill proxies. We’ve also noted that Mann (like Briffa) truncated this data at 1960 because of divergence. At the time that Mann et al 2008, the gridded MXD data was not available anywhere – Mann cited a webpage as follows:
The gridded maximum density dataset was developed differently by Osborn et al. (http://www.cru.uea.ac.uk/~timo/datapages/mxdtrw.htm) [as used by Rutherford et al. (2)] with the specific goal of capturing century to multicentury variability.
While Briffa has published many articles on his MXD data, the only article providing even a cursory description of the gridded version was Rutherford (Mann) et al 2005, which stated:
The version of the MXD dataset used here was compiled using a combination of grid-box estimates based on traditionally standardized MXD records (with limited low frequency information) and regional estimates developed to retain low-frequency information (OSB; the data in the MXD network are available online at http://fox.rwu.edu/~rutherfo/supplements/jclim2003a). The latter were developed using the age-band decomposition (ABD) method of standardization, wherein density data from trees of similar ages are averaged to create long chronologies with minimal effect of tree age and size (Briffa et al. 2001).
Unfortunately, the information that the data was available at http://fox.rwu.edu/~rutherfo/supplements/jclim2003a was untrue. Previous attempts to get this data had failed. In September, as noted previously, I sent an FOI request to CRU in England for any gridded data that they had sent to Mann. (Note that requesting in terms of documents is more effective). This led to a new tranche of data and file information at Osborn’s webpage, and, of course, the usual puzzles.
The creation of the gridded series (by coauthors Osborn, Briffa and Jones) is described in Rutherford et al 2005 as follows:
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.
You have to watch a bit carefully here – you expect “OSB” to be an acronym for a peer reviewed publication describing the gridding process, but OSB (which confusingly is Briffa, Osborn and Schweingruber GPC 2004, as noted by PAt Frank below) contains no description of the gridding method. I was wrongfooted a bit by this and tried to find additional descriptions elsewhere in the Briffa opus, before deciding that this was the description of the gridding operation.
To understand what happened in the gridding process, I picked a cell with only one site (72N, 132E), but there are many others and plotted the underlying chronology of the one site, together with three versions of the gridded data: the Osborn version, the “Mann original” version and the Mann infilled version. The 4 plots are shown below and give a variety of puzzles.
The earliest value of the underlying site chronology (omoloya) is in 1496 (end-1991). The gridded version is almost identical in the period 1496-1991 (correlation of about 0.99), but it begins in 1400. Where did the first 96 years come from? Is there another tranche of unarchived data? Or was there some “infilling” of the early data? Rutherford et al 2005 is silent on this.
In any event, we know that Mann truncated post-1960 values and used “infilled” data for 1960-1995 instead of real data. However the effect in this particular series is a little unexpected: the infilled data doesn’t go up as much as one would have expected given the trouble that they’ve gone to replacing real data with imaginary data. It “diverges” from temperature as well. In fact, for this gridcell, the “original” original data has a higher correlation to temperature than the replacement data.
Update – Osborn’s webpage (and I think that this info was provided in Sept 2008 in response to my FOI request) mentions that the 341 sites used in Rutherford et al 2005 were selected as follows:
the MXD data from the 341 sites determined by Briffa et al. (2002a) to exhibit correlations with nearby gridded April-September temperature of at least +0.22 were used in this study. See Briffa et al. (2002a) and the data description above for further details
This is helpful, as Rutherford et al do not mention 341 sites, instead saying:
it [ABD] 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.