Happy Thanksgiving!

A step function would have a different impact on the tree ring PC calculations and on the regression step. I’m offline now.

And more generally, isn’t it right that the MBH procedure will mine for any series where the data points during the 20th century “training period” (or did they call it “validation period” ?) are markedly offset from the remainder of the data seres ? It seemed to me that you could replace the bristlecones with a step function with the diccontinuity in the year 1901, and that is what the end result would look like.

Or did I get hold of the wrong end of the (ahem) stick ?

We have tried to maintain attention to both data and methodology issues. I don’t like the idea of a “beauty contest” (or an “ugliness contest”) to say which is the REAL problem. We don’t say that their results are “simply” an artifact of their PC method, since you can get low 15th century results with a centered PC method. A phrase that I like is: the flawed method interacts with the flawed proxies to create a “perfect storm”. We discovered the role of the bristlecones by actually following the flawed method to see what it did: it highlighted the bristlecones. I don’t think that a sensible answer can or should depend on how you apply Preisendorfer’s Rule N (and I’m not persuaded that it was ever used in the tree ring networks in the first place – I think that it’s been proposed ad hoc to try to salvage inclusion of the PC4).

In MM05 (EE), we characterized these results in terms of robustness rather than correctness – although this buance seems to have been totally lost in debate, largely because of realclimate disinformation. One of the warranties of MBH (and I like to use contractual terms like warranty) was that it was robust to the presence/absence of dendroclimatic indicators. (I’ve cited reliance of Bunn et al [2005] on exactly this claim.) But it’s results are obviously not robust to presence/absence of bristlecones.

So – simple question that has probably been answered a thousand times – but I can’t find it. What happens if you use centred PCA without the Bristlecones?

Cheers

As far as I remember it’s the using a centered PCA that results in the hockey-stick moving from PC1 to PC4 (or is it PC5) and that’s what resulted in Preisendorfer’s rule entering into the picture. IOW most of the variance with a centered PCA get explained by other proxies, but it still takes the bristlecones to explain the hockeystick. This either means that the other proxies aren’t very good temperature measurers or that the instrumental record, with its pronounced hockeystick-blade shape, is flawed and takes a flawed proxy like bristlecones to reproduce it.

Can Steve or someone confirm whether you still get a hockey stick shape using centred PCA? The dialogue above suggests you do. And if that is true, isn’t TCO right, that the real issue is the Bristlecone pine series? The use of non-centred PCA might exacerbate the problem but the hockey stick still exists without it.

Is there a diagram somewhere showing what the hockeystick looks like using centered PCA? Presumably it is different.

Thanks