Normally, I would have classified the basic premise of Craig Loehle's recent paper, as summarized at Climate Audit, as a blinding glimpse of the obvious. Unfortunately, the climate science world is in desperate need of a few BGO's, so the paper is timely. I believe his premise can be summarized as follows:
- Many historical temperature reconstructions, like Mann's hockey stick, use linear regressions to translate tree ring widths into past temperatures
Linear regressions don't work when the underlying relationship, here between tree rings and temperature, is not linear.
The relationship between tree ring growth and temperature is almost certainly non-linear. For example, tree ring growth does not go up forever, linearly, with temperature. A tree that grows 3mm in a year at 80F and 4mm at 95F is almost certainly not going to grow 6mm at 125F.
However, most any curve, over a sufficiently narrow range, can be treated as linear for the purposes of most analyses. The question here is, given the relationship between tree ring growth and temperatures, do historical temperatures fall into such a linear region? I think it is increasingly obvious the answer is "no," for several reasons:
- There is simply not very good, consistent data on the behavior of tree ring growths with temperature from folks like botanists rather than climate scientists. There is absolutely no evidence whether we can treat ring widths as linear with temperatures over a normal range of summer temperatures.
- To some extent, folks like Mann (author of the hockey stick) are assuming their conclusion. They are using tree ring analysis to try to prove the hypothesis that historic temperatures stayed in a narrow band (vs. current temperatures that are, they claim, shooting out of that band). But to prove this, they must assume that temperatures historically remained in a narrow band that is the linear range of tree ring growth. Essentially, they have to assume their conclusion to reach their conclusion.
There is strong evidence that tree rings are not very good, linear measurements of temperature due to the divergence issue. In short — Mann's hockey stick is only hockey stick shaped if one grafts the surface temperature record onto the tree ring history. Using only tree ring data through the last few decades shows no hockey stick. Tree rings are not following current rises in temperatures, and so it is likely they underestimate past rises in temperature. Much more here.
Loehle's pursues several hypotheticals, and demonstrates that a non-linear relationship of tree rings to temperature would explain the divergence problem and would make the hockey stick a completely incorrect reconstruction.