This is part one of my critical review of the palaeoenvironmental evidence for the influence of solar activity on climate.
Chen et al. 2011 Short term climate variability during “Roman Classical Period” in the eastern Mediterranean. Quaternary Science Reviews, 30, 3880–3891.
This paper was cited by Engels and van Geel (2012) as evidence of a solar-climate relationship. Chen et al. (2011) use the dinocyst record from a core from just outside the Adriatic Sea, off the Italian coast, to reconstruct climatic conditions during the “Roman Classical Period” (60 BC–AD 200). Spectral analysis of their results show 7-8 and 11 year cyclicities, which they suggest relate to variability of the North Atlantic Oscillation and make a link to the 11 year sunspot cycle. Chen et al. (2011) also report strong correlations between their reconstruction and Δ14C anomalies, a proxy for solar activity, and also with global volcanic activity.
Rather than use (dubious) transfer functions, Chen et al. (2011) make qualitative climate reconstructions from their dinocyst assemblages using several indices. For example, temperature patterns are estimated from the ratio of warm to warm plus cold indicating taxa. No information on the predictive skill of this index is given. I have no doubt that this index would have utility in determining that polar oceans are colder than the Mediterranean Sea, but I am doubtful about its utility for the relatively small changes expected during the Roman Classical period.
The spectral analysis in Chen et al. (2011) is done using redfit (which uses Lomb-Scargle Fourier transform – ideal for data that are not evenly spaced) and the multi-taper method (MTM). At least they write that they do, yet the figure is labelled MEM, which would suggest maximum entropy method. I have not used MTM or MEM, so cannot comment on them. Chen et al. (2011) also use wavelets.
The redfit spectral only have a significant (at the 95% level) peak at 11 years for one of the three indices. Significance levels are not shown the for MTM (or MEM) results (unless the redfit significance levels also apply to the MTM results), but two of the three indices show an 11 year peak. The wavelet analysis shows that this 11 year cycle does not occur throughout the record. This spectral analysis is weak evidence of an 11 year cycle that could be attributed to solar activity.
Chen et al. (2011) present a figure showing their dinocyst-temperature index and Δ14C anomalies. The correlation looks very impressive except for the period after AD 160, when the two curves diverge, however the correlation is not calculated (and would have to consider the autocorrelation in both records).
When viewing this figure, we need to remember that the calibrated radiocarbon dates (raw dates not given) which form the basis of the chronology have a 2-sigma uncertainty of up to 195 years. The chronological uncertainty is almost as large as the period being examined is long – the sample dated to 60 BC could actually date to AD 100 and the correlation would look very different! This means that the strong positive correlation between dinocyst-temperature index and Δ14C anomalies may be entirely fortuitous, a negative correlation is perfectly plausible given these data.
What is needed is an analysis that takes account of the chronological uncertainty when calculating the correlation between the dinocyst-temperature index and Δ14C anomalies. One procedure is to generate a family of age-depth models and calculate the correlation for each. If most of the age-depth models yield a positive correlation, the correlation is robust to chronological uncertainty. I did an analysis relating grebes and chironomids using this method.
Because the chronological uncertainty is so large, this correlation between dinocyst-temperature index and Δ14C anomalies is not useful evidence of a solar-climate relationship. Accepting this correlation as evidence risks a huge publication bias – if the dates had been slightly different and the correlation was lost, who would have published this as evidence against a solar-climate relationship?
This paper is not credible evidence of a solar-climate link.