The archived information for Wilson et al 2007 contains interesting new information on an unpublished West Siberian series (Putorama, 70 31 N, 92 57E). In this case, I was actually able to obtain a better correlation to gridcell temperature than the one reported by Rob by using a gridcell closer to the actual location. This series has no 20th century trend. Wilson noted a divergence from instrumental records at the end of the record. However, there is a remarkable divergence between the new West Siberian series which is touching lows at its end and the West Siberian series beloved by multiproxy reconstructions (Briffa’s Yamal series), which touches new highs at the end of its record.
Rob reported the following:
This series correlates with mean May–September gridded temperatures at 0.43 (Table 1). The Western Siberia series tracks the gridded temperature series quite well (Figure 3) except for the last two years, where the proxy values are substantially lower than the actual instrumental data. These two years of misfitting are too short to identify whether this is a significant divergence.
The temperature comparison was said to be to two gridcells (62-67N; 82E) over the period 1938-2000. However, there are available gridded values from 1881 on for the gridcell 67N, 87E (derived mainly from Turuhansk 66N 89E), obviously much closer to Putorama than the gricells used in the study. The correlation to the Turuhansk gridcell (over 1881-2000 exccept for a few years) for the May-Sept period specified in Table 1 was 0.54 – better than the reported values.
Here is a plot comparing the tree ring and gridded temperature series. While there is a good correlation to temperature, there is also no 1920-2000 trend in the tree ring data (actually slightly negative.)
Rob noted that the data tracked well “except for the last two years” where there is a divergence. Visually, it looked to me like the divergence was more than just the last two years. Below is a plot of the residuals from a fit of RW against gridded temperature. While the negative residuals are more pronounced in the past few years, there does appear to be somewhat of a negative divergence trend in the residuals for more than the past two years. The Durbin-Watson statistic using the lmtest package in R was 1.50 – right at the red zone value – and differs substantially from the reported DW value of 1.98. I wonder how Rob calculated his DW statistic.
Now for the most interesting divergence. I’ve discussed the Yamal series (Briffa variation) on many occasions. This is a staple of multiproxy studies and is individually shown in the IPCC AR4 spaghetti graph. Although the Yamal chronology is usually attributed to Hantemirov (2002) e.g. by Juckes et al 2007, the staple version was calculated by Briffa, who has only reported the chronology and refused to provide the supporting measurement data. It has a pronounced HS shape with its medieval-modern differential very different from the Polar Urals Update. While the Putorama series is some distance to the east of the Yamal-Urals series, any new light on West Siberian chronologies is welcome.
The figure below compares the Briffa Yamal version with the new Wilson et al chronology. Whereas the Briffa chronology is exploring new highs at its close, the Wilson West Siberian series is exploring new lows. One looks forward to the commentary if and when a study is published on the new West Siberian chronology.