Just before Climategate, we were reflecting on the apparent non-“robustness” of the Yamal chronology to inclusion of Khadyta data. Briffa’s response was that he could still “get” a HS by adding in Yadoyayakha and Porzayakha ring width data that hadn’t been used in Briffa (2000). As noted in other posts, Khadyta isn’t the only site with a divergence problem. Divergence, if anything, is the rule, rather than the exception. Briffa didn’t discuss why there were such differences between the sites – something that I’ll consider briefly today.
First here is a plot showing the ten YAD core chronologies (the chronologies, as usual, divide ring width by a negative exponential to adjust for aging.) YAD061, the “most influential tree in the world”, surges off the page, closing at an astonishing 8 units.
Next is a plot showing 15 Khadyta River core chronologies that go to 1990. None of these core chronologies are rocketing off the top of the page – an entirely different impression to the YAD chronologies.
Statistically, the inconsistency between the two sites creates all sorts of problems. The idea behind using chronologies as a proxy for temperature is that ring widths, after an allowance for aging, are a linear function of temperature. However, something else needs to be invoked in order to explain the huge difference between nearby sites. How can one “thermometer” read 8-sigma, while the other is below normal?
This is a difficult and interesting statistical question – one that isn’t even posed in Team literature.
A relatively recent study, Devi et al 2008 (needless to say, not cited by Oxburgh), appears to me to shed considerable light on the conundrum. here’s their Figure 8, showing ring widths for Siberian larch that switched from a creeping (krummholz) form to an upright form in the 20th century. The generic shape of these diagrams is highly reminiscent of the YAD core plots at issue.
Devi et al described their observations as follows:
Vertical stems started to appear after 1900, with most of them appearing in the 1920s and 1930s (Figs 7 and 8). The change from creeping to vertical growth led to significant growth enhancements of the multi-stemmed trees, as indicated by the increases in the ring widths of the horizontal stems of multi-stemmed trees (Fig. 8). Approximately one to two decades after the stems had started to grow vertically, ring widths increased 2- to 10-fold, which were much greater than the increases in ring width of single-stemmed trees during the same period.
2- to 10-fold increases in ring width! The same sort of thing that we’re seeing at the YAD site. Look back at core YAD061. It grows at relatively low rates for its first 100 years or so and then surges to 8 times the growth rate around 1950 – in this case, a little later than the growth surge in the Polar Urals cores shown here.
Now look at core YAD041 – its fluctuations are highly correlated to YAD061, but it experienced a growth surge early in the 20th century and has dramatically tailed off in the second half of the 20th century.
The shapes of these curves is very similar to the Polar Urals krummholz-to-upright curves. It sure seems plausible to me that the huge “late-life” growth surge in important YAD cores might be due to the same effect.
Reference: NADEZHDA DEVI, FRANK HAGEDORNw, PAVEL MOISEEV, HARALD BUGMANN, STEPAN SHIYATOV, VALERIE MAZEPA and AND REA SRIGLING, 2008. Expanding forests and changing growth forms of Siberian larch at the Polar Urals treeline during the 20th century, Global Change Biology, 14, 1581–1591