One of the aspects of the Yamal discussion that is perhaps clearer to regular CA readers than to new readers is that Briffa’s Yamal chronology was very different from ring width chronologies previously reported in the area (including by Briffa itself.)
Shortly after the publication of Osborn and Briffa 2006 and D’Arrigo et al 2006 in February 2006, I reviewed the findings of Briffa et al (1998) on the wide-spread decline of ring-widths and MXD since 1960 (the “divergence problem”), an issue that was discussed at the NAS panel presentations the following month (and very unsatisfactorily in the NAS report).
Briffa et al 1998 reported on the very large Schweingruber survey – a survey of 314 NH sites selected ex ante to be temperature sensitive. See here for list.
At the time, I excerpted the following graphic from Briffa et al 1998 showing the decline:
Briffa et al. 1998 Original Caption. Figure 6. Twenty-year smoothed plots of averaged ring-width (dashed) and tree-ring density (thin solid line), averaged across all sites in Figure 1, and shown as standardized anomalies from a common base (1881-1940), and compared with equivalent-area averages of mean April-September temperature anomalies (thick line). [SM - it looks to me like the labels in the caption are reversed between density and temperature]
Figure 2 of Briffa et al 1998 breaks this down into regions. The figure below is an excerpt from their Figure 2 showing Siberia – Yamal would be in West Siberia. The left half shows density (MXD), the right half ring width (RW). The figure of particular interest to us is WSIB ring width (third row, right half). The thick line in the left panel shows temperature, the thin line ring width (both smoothed), showing that ring widths in this region, as elsewhere in the world, had not kept pace with temperature. The right panel shows the difference (the “divergence problem”). The “divergence problem” affects both ring width and density.
From Briffa et al 1998 Figure 2. Figure 2 Regional tree growth and temperatures over the past 120 years. Decadally smoothed tree growth (thin lines), maximum-latewood density or ring width, plotted against mean summer temperatures (thick lines), April–September for density and June–August for ring width, for each of the regions described in Fig. 1. The difference series (growth minus temperature), shaded to emphasize negative values, are shown to the right of each pair of curves. All data series have been scaled to have zero mean and unit variance over the period 1881–1940 (except the short ESIB temperature series which uses 1932–75
One of the sites included in this survey is Khadyta River, Yamal. I’ll do a count of how many series are included in the WSIB region, but it is obviously a considerable number.
The “divergence problem” has been discussed on many occasions at this site. If ring widths have gone down in the last half of the 20th century despite increasing temperatures, how can we use information from prior periods to reconstruct past temperatures? Kurt Cuffey was much puzzled by this conundrum at the NAS panel hearings.
In the present case, we’re talking a different sort of divergence entirely. Here we’re not talking about temperature. We’re talking about the discrepancy between Schweingruber’s large-scale network of both ring width and density ( a network involving hundreds of cores and thousands of measurements), with a WSIB network with dozens of sites where late century ring widths and MXD go down, as compared to Yamal – one site where late century ring widths go strikingly up.
I got an email this morning in which Hantemirov told a correspondent that they used 120 cores in a forthcoming study and only used long cores for corridor standardization because that’s what you need for this method. This confirms my prior point that the requirements of the corridor method were different than the RCS method and that a much larger population of cores was available, though, for some reason, not used in Briffa et al 2008.
However, Hantemirov also says that the results with a larger population are very similar to the Briffa results – raising the question of why the Yamal results are so different from Polar URals and the Schweingruber network – a question that I’ll ask him. Hantemirov:
Low number of used for reconstruction subfossil series is explained by standardisation method (“corridor method”). We had to select the longest series. The same concerns to living trees. There are not much old living trees in this area (in contrast to Polar Urals), therefore we used only 17 (not 12) samples from living trees. At that time we had close collaboration with CRU and I sent to Keith Briffa these raw data.
So, selection of samples has been made by me taking into account length of individual series as well as common requirements to increment cores (exclusion samples with compression wood, rotten wood etc.).
As to reliability of recent increase in tree growth – we have updated our data using many additional subfossil and living trees and using RCS-method. I.e. we used not only long series. Therefore many (120) living trees have been used. Finally, we have got almost the Briffa’s result. These results not published yet. I’m going to prepare paper at the end of this / beginning next year. Some preliminary data you can find in some kind of report in Russian
fig 2 – sample replication, fig 5 – temperature reconstruction (smoothed by three filters – 50-, 100- and 200-year)