Comments on "reconstruction of the extratropical NH mean temperature over the last millennium with a method that preserves low-frequency variability"

Abstract

Christiansen and Ljungqvist have presented an extratropical NH temperature reconstruction using a method (LOC) that they claim "preserves" low-frequency variability, at the expense of exaggerated highfrequency variability. Using theoretical arguments and a pseudoproxy experiment, it is demonstrated here that the LOC method is not guaranteed to preserve variability at any frequency. Rather, LOC reconstructions will have more variance than true large-scale temperature averages at all frequencies. This variance inflation, however, can be negligible at those frequencies where the noise variance in individual proxies is small enough to be effectively cancelled when computing an average over the available proxies. Because the proxy noise variance at low frequencies cannot be directly estimated, and thus has to be regarded as unknown, it is safer to regard a reconstruction with the LOC method as providing an estimate of the upper bound of the large-scale low-frequency temperature variability rather than one with a correct estimate of this variance. © 2012 American Meteorological Society.