Yamal Emulation II: Divergence
As I mentioned in my previous post, the Khadyta River chronology appears to suffer from the classic signs of what dendrochronologists have come to call ‘the divergence problem‘. Before I continue, however, I should emphasize a few points.
 I didn’t collect the Khadyta River chronology. Neither did Steve McIntyre. The problem with treating tree ring chronologies as nothing more than received time series downloaded from the internet to be manipulated in various ways is that the context of the original investigators can be lost. Moreover, there are several subdisciplines within dendrochronology that collect tree ring data for different reasons, and in their fieldwork emphasize different site or individual tree characteristics during sampling.
 In any case, the concept of what comprises a single ‘site’ is ill defined (Wikipedia covers this in the context of archaeology). There is no hard-and-fast rule for how to geographically delimit which group of individual trees belong to a given chronology site.
Let’s return to Yamal and Khadyta. Remember that adding the Khadyta raw data to that from Yamal gives us the following situation (zooming in on the last century and a half now):
What becomes clear immediately is that the two chronologies diverge in approximately the early 1970s. Why does this occur?
We can look at the raw data from each site separately. First, I’ll graph the standardized raw data (to account for differences in tree size and mean growth rate) and their mean from Khadyta:
Again, what jumps out here is that the trees at this size show a decrease in growth in the late 1960s and early 1970s. The step change that the mean takes jumps out at me immediately.
Lets look at the same type of plot for the Yamal data:
Here, the persistent low growth years seen at Khadyta after 1970 are not present in the majority of trees (although, interestingly, perhaps in a few of them). As a consequence, the mean raw value continues to climb through the 1980s and 1990s.
We can now look at the two mean raw value time series together, as well as compare them to the gridded temperature data from the region. For this purpose, I’ve extracted summer (JJA) temperature for the grid box [65-70N, 65-70E] from CRUTEM3– a bit crude, but for the comparison to the raw mean data, it will serve our purpose:
Considered individually, and remember here with no detrending, both mean time series track each other — and the gridded summer temperature — well at both low and high frequencies. The two mean tree ring series diverge starting in the 1960s. Both track years with lower temperatures in the late 1950s and 1960s, but by the 1970s, Khadyta fails to mirror regional warming in the gridded summer temperature. Yamal continues to track increasing temperatures from the 1970s through the 1990s.
This quick-and-dirty analysis emphasizes a few points. Khadyta River does display the divergence problem, in that it ceases to track temperatures as it did from 1883 to the 1960s. At first glance, in this case, the divergence doesn’t seem to be related to detrending [PDF], and really does seem to reflect a decline in growth in the most recent decades. Second, adding Khadyta River data to Yamal is unlikely to reflect temperatures in the region more accurately.
I want to emphasize that neither Yamal nor Khadyta River are ‘the problem’ — divergence is the problem, and for this reason is a major [PDF] area of focus in dendrochronology. Relatively little more can be said about the specific case of Yamal at this point — I’ll leave that to the scientists working in this part of the world — but even my quick review of these data here shows that including Khadyta River raw data in the Yamal chronology does not result in a more accurate nor precise understanding of past temperatures in the region. This isn’t to say that some time in the past that Yamal didn’t experience divergence (this after all is a large part of the concern about divergence), but we can clearly see that Khadyta River does exhibit modern divergence.
A very interesting paper with relevance to this issue appears to be in press in Global Change Biology:
Esper, J., Frank, D., Buntgen, U., Verstege, A., Hantemirov, R., and Kirdyanov, A. (2009). Trends and uncertainties in Siberian indicators of 20th century warming. Global Change Biology, in press [subscription wall]
Check it out if you can.