This is part of my critical review of the palaeoenvironmental evidence for the influence of solar activity on climate.
Sirocko et al. (2012) Solar influence on winter severity in central Europe. Geophysical Research Letters, 39, L16704.
Sirocko et al. find that severe winters in Central Europe, identified by documentary evidence of the Rhine freezing, tend to occur during sunspot minima.
Via Stoat, I find news at Klimazwiebel and Simple Climate, that van Oldenborgh et al. (2013) [open access] have published a comment on Sirocko et al. The comment is published in Environmental Research Letters because, shamefully, Geophysical Research Letters does not publish comments.
Briefly, van Oldenborgh et al. (2013) express surprise that there is a strong relationship between sun-spot numbers and the Rhine freezing as the relationship is not detectable above weather noise in long instrumental series of temperature. They then show that there is evidence that the Rhine froze on more years than reported by Sirocko et al., whose data were not a homogeneous times series, but compiled from diverse sources including poems, postcards and paintings. If these extra years are included in the analysis, the relationship breaks down. There is more: read the paper or Klimazwiebel/Simple Climate for details.
I was interested by a quote from Pittock that literature reporting solar-climate links is full of common errors, such as ‘poor data quality, data selection, data smoothing and autocorrelations, and post hoc elaboration of hypotheses to explain discrepancies’. Pittock first expressed this view in 1978 in a review of 140 papers purporting to find a solar-weather relationship. It is a view he reiterates in 1983 and 2009. I suspect that these problems will be common in palaeoenvironomental solar-climate studies.