The first is that some property of the data causes the statistical methods (which look good) to fail unexpectedly. I doubt any UV-based mechanism is plausible, and the relationship between the sunspot cycle and weather seems to be so weak that any weather mediated mechanism is also unlikely. Given the above, I am convinced that the mechanism proposed by Skjærvø et al is not plausible. Year-on-year variability in UV irradiation due to cloud cover is about an order of magnitude larger than that due to solar variability. If we assume that an overcast sky blocks 75% of the UV ( Kuchinke and Nunez 1998), then there should be about 40% more UV irradiance from a sky with mean of four octas of cloud (50% cover) than one with six octas. Mean May-August cloud cover at Sunndal, western Norway, in octas (0-8 scale).Ĭlouds do not stop all UV. Nineteenth century Norway was not exactly a bikini culture: the doses received by the Australian sunbathers is likely to be much higher than the Norwegian mothers received. The link between UV radiation and folic acid depletion is established, but the doses received by some of the Australian participants in the study cited by Skjærvø et al (over 600 Joules UV radiation per day), suggest they were sunbathing. The link between folic acid deficiency and problems during pregnancy are well known.
Skjærvø et al hypothesise that changes in survival and fertility were caused by enhanced ultraviolet radiation at the solar maxima which depleted the Norwegians mother’s folic acid reserves. Further, low-status women born in years with high solar activity have a reduced fertility rate and life-time reproductive success (defined as the number of children surviving 20 years). They show that men and women born during years with high solar activity have a reduced probability of reaching adulthood and a reduced life expectancy. Skjærvø et al use church records of births and deaths from two districts in western Norway. That’s not quite how Skjærvø et al, authors of “Solar activity at birth predicted infant survival and women’s fertility in historical Norway” describe how Apollo’s arrows join war, famine, pestilence and death in ending the lives of 17th-19th century Norwegians.
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