Yamal V: … but they pull me back in …
Via Jeff at his blog The Air Vent, in the midst of a post that includes a nontrivial quantity of ALL CAPS and multiple exclamation points (no <blink> tag?), the following statement (since that time stated in a more reasonable tone here):
For some reason EVERY RCS CORRECTION Briffa can conceive of refuses to turn upward to fit the ACTUAL data. This lack of flexibility in the RCS curves is what creates the HS.
I’m not sure why he doesn’t take my post on growth curves at Yamal seriously, but I’ve already explained why this is probably incorrect. The problem is an intermingling of the signal and the noise. As I showed previously, when I constructed an RCS curve using only the recent, living trees from Yamal, because these trees were growing roughly at the same time, and at that during a period of increasing temperatures, estimating a growth curve from them potentially intermingles the climate signal with the geometric growth curve.
One way to look at this is to consider only those trees that ceased growing prior to 1900. I’ve extracted these from the full Yamal set and estimated the regional curve standardization using a 67% spline. Here it is:
Let me be as clear as I can be, there is no sign that I can detect that it is old trees that increase their growth at Yamal (even if identified, this phenomenon would require some hypothesis as to the cause), At Yamal, a portion of the old trees are those that were growing together during a period of climate warming. If you examine the raw ring width, there are a few fossil series that have rapid increases toward the end. If Jeff’s hypothesis were correct, we’d expect these to be the oldest, right? In fact, the seven subfossil samples I identified as having rapidly increasing growth in their later years, six had a wide range of ages from 90 to 180 years (this comes with the caveat that we don’t know the exact pith age).
Furthermore, and as my earlier posts have shown, and as I’ll show again below, using a 67% spline (so that the later part of the regional growth curve bends slightly upward) you still get an increase in the chronology values during the 20th century.
Finally, what of the claim that combining the mean-detrended series demonstrates that the RCS method is invalid? One way we can test this is by first aligning all the ring widths by age (as was done here), observing where the curve of the juvenile grow trend flattens in the majority of trees (eyeballing it at around 175 to 200 years), throwing away the ring widths for the time when every tree was between 1 and 199 years of age (you get essentially the same result if you use 175 years), then realigning by time (year A.D.), and removing the mean. Lets look at the truncated growth curves in time and the mean of these series:
What immediately jumps out at you is that the mean for the most recent century and half is noticeably higher than the earlier part of the millennium. This doesn’t a priori indicate warmer temperature, but as I explained to Jeff here, once again there is the potential for this approach to remove climate signal in the process of detrending. For the most part, the mean following truncation is lower for the non-living trees at Yamal, but for the living 17 trees, the mean is similar or in some cases higher after truncation. Why? At least partially because after truncation the mean of the recent, living trees is of a large portion of the growth occurring during 20th century warming.
So, what do the chronologies look like if we use three different types of detrending? Let’s try constructing an RCS chronology using a 67% spline, negative exponential, and a generalized negative exponential curve:
The top shows the regional curve fit to all the raw Yamal data, the bottom panel zooms in on the part of the chronology since A.D. 1700 (heavy lines are 20 year low pass Butterworth filtered values). The regional curve fit that I think Jeff would endorse (67% spline) to fit his ‘U shape’ hypothesis actually results in slightly higher chronology values following the Little Ice Age, and slightly higher values at the end of the chronology in the mid 1990s.
My point is not to indicate a ‘correct’ method here — that goes well beyond ‘Blog Science’. Rather my point is this: detrending and standardization is one of the most challenging tasks in accurately estimating past low frequency climate variability from tree rings. Divergence is a serious challenge worthy of further study. What I don’t understand, however, is why methods or analyses (mean detrending, identifying growth curves from a small number of simultaneously growing trees) that can be shown to have the potential biases in specific instances that I’ve demonstrated here and in a previous post can still touted by those with a visceral dislike of paleoclimatologists as proof that another method is incorrect. If this debate was a collegial one that might be one thing, but there is nothing collegial — or scientific — about the language and tone from the other side. Too bad. To endorse once again Rob Wilson’s comment from Climate Audit:
“In fact, the fatal flaw in this blog and what keeps it from being a useful tool for the palaeoclimatic and other communities is its persistent and totally unnecessary negative tone and attitude, and the assumption that our intention is faulty and biased, which keeps real discourse from taking place.”
UPDATE: Minor grammar corrections