Since AR4, there have been a series of new multiproxy studies, several of which were cited in AR5 (Mann et al 2008; Ljungqvist et al 2010; Christiansen and Ljungqvist 2012; Shi et al 2013). A distinctive feature of these and other recent multiproxy studies is the incorporation of varve thickness and near-equivalent mass accumulation rate (MAR) series, in which varve thickness (positively oriented) is interpreted as a direct proxy for temperature. The following table shows the usage of varve thickness and near-equivalent mass accumulation rate (MAR) series in post-AR4 multiproxy studies (“long” series shown below). It is evident that the varve thickness data in multiproxy studies is anything but “independent”.
Table 1. Varve thickness and MAR (mass accumulation rate) series used in multiproxy studies which are both “long” (including the medieval period) and which have not been truncated in the modern period. Both logged and unlogged versions are used. In a couple of cases, the mass accumulation rate is limited to organics (“dark”). I’ve also included the Igaliku pollen accumulation rate series, because it appears to me to be closely related to MAR series. XRD (Xray density not included).
One of the most obvious features of the above table is the repeated use of a small number of varve thickness series used in Kaufman et al 2009: Big Round, Blue, C2, Donard, Iceberg and Lower Murray Lakes. Five of the six series were used in Shi et al 2013. In my recent discussion of Shi et al 2013, I observed that a composite of the five series (and the same is true for all six) had something of an HS-shape, though the series otherwise had negligible common “signal” (as demonstrated clearly by their eigenvalues). Further, several of the series (especially Iceberg which had been discussed in prior CA posts) had serious problems, compromising or potentially compromising any potential utility as a temperature proxy. This certainly suggested to me that the somewhat HS-ness of the varve thickness composite was more likely to be an artifact of selection from a noisy network rather than actual scientific knowledge. Skeptic blogs have long discussed this phenomenon, but it is one to which academic literature in the field has been wilfully obtuse.
Blog discussion has been mostly based on red noise examples. So I think that readers may be interested in seeing the phenomenon at work with actual data.
In the course of examining literature on varves, it quickly became evident that specialist literature prior to the relatively recent multiproxy articles had regarded thick varves as evidence of glacier advance (rather than “warmth”). Readers (and myself) wondered how the prior consensus (so to speak) that thick varves were related to glacier advance (and vice versa) had been replaced by a model in which thick varves were now interpreted as evidence of warmer temperatures. This proved to be an interesting backstory. I’ll also contrast the varve thickness series from Iceberg Lake, a canonical series in Kaufman et al 2009 and subsequent multiproxy studies, with “non-canonical” varve thickness series from Silvaplana, Switzerland and Hvitarvatn, Iceland, where thin varves are interpreted as evidence of warmth. Continue reading