Bloody Bastards

David Karoly, 7th June 2012 to Gergis and Neukom:

“The same argument applies for the Australasian proxy selection. If the selection is done on the proxies without detrending ie the full proxy records over the 20th century, then records with strong trends will be selected and that will effectively force a hockey stick result. Then Stephen McIntyre criticism is valid. I think that it is really important to use detrended proxy data for the selection, and then choose proxies that exceed a threshold for correlations over the calibration period for either interannual variability or decadal variability for detrended data. I would be happy for the proxy selection to be based on decadal correlations, rather than interannual correlations, but it needs to be with detrended data, in my opinion. The criticism that the selection process forces a hockey stick result will be valid if the trend is not excluded in the proxy selection step.”

Raphael Neukom, same day to Karoly:

“I agree, we don’t have enough strong proxy data with significant correlations after detrending to get a reasonable reconstruction.”

-Now, from Part 2a Journal Correspondence file, On June 14, Gergis back to editors:

Over recent days we have been in discussion with colleagues here in Australia and internationally about the use of detrended or non detrended data for proxy selection as both methods are published in the literature.
People have argued that detrending proxy records when reconstructing temperature is in fact undesirable (see two papers attached provided courtesy of Professor Michael Mann) .
While anthropogenic trends may inflate correlation coefficients, this can be dealt with by allowing for autocorrelation when assessing significance. If any linear trends ARE removed when validating individual proxies, then the validation exercise will essentially only confirm the ability of the proxies to reconstruct interannual variations. However, in an exercise of this nature we are also intrinsically interested in reconstructing longer-term trends. It therefore appears to be preferable to retain trends in the data, so that we are also assessing the ability of the proxies to reconstruct this information.