It is well established that sunspots – cool dark spots on the Sun's surface caused by intense magnetic activity – wax and wane over an 11-year cycle. The global mean temperature is influenced by the sunspot cycle, varying by around 0.1 degrees between peak and trough. And it's thought that the weather in the stratosphere and upper troposphere is also influenced to a small degree by the 11-year cycle. However, at ground level any solar cycle effects seem to disappear, drowned out by the "noise" of other climate variables.

So when Frank Sirocko from Johannes Gutenberg University, Germany, and colleagues published their paper in Geophysical Research Letters last August, claiming that sunspots were influencing European winters, there was widespread surprise. According to the accompanying press release (as detailed on environmentalresearchweb at the time), Sirocko's team used historical documents to find that the Rhine froze in multiple places fourteen different times between 1780 and 1963. Ten of the fourteen freeze years occurred close to the point in an 11 year cycle when there are fewest sunspots. "We provide, for the first time, statistically robust evidence that the succession of cold winters during the last 230 years in Central Europe has a common cause," Sirocko said in the press release.

The claim set Geert Jan van Oldenborgh of the Royal Netherlands Meteorological Institute (KNMI) thinking. "Fifty years ago, Edward Norton Lorenz [the American mathematician and meteorologist, father of chaos theory] showed that a large part of the variability of European winter weather is due to deterministic chaos (unpredictable fluctuations in atmospheric circulation) particularly in the mid-latitude westerly flow," said van Oldenborgh. "The role of solar activity in determining the warmth of individual winters is expected to be minor, or even negligible, so these new results were highly unexpected."

Along with colleagues, van Oldenborgh decided to try and reproduce the findings presented by Sirocko and his team. Almost immediately the scientists found flaws in the data. "The supplementary information in the paper shows that the data record of Rhine freezing is constructed merely on searching the internet for miscellaneous documents containing information about Rhine river freezing, including an oil painting," explained van Oldenborgh.

These data appeared not to have been checked against other well-established long-term temperature records, such as the Pfister reconstruction of Swiss winter temperatures and the Dutch winter temperatures by van Engelen and colleagues, which are both based on freezing of waterways among other sources. "When we compared Sirocko's data to these other records we found that Sirocko's record missed around half of the documented severe winters," said van Oldenborgh.

Delving deeper into the paper, van Oldenborgh and colleagues found inconsistencies in how the sunspot minimum was defined. Sometimes it was defined as the four years surrounding the minimum and other times it was the single year following the minimum. "With their original definition their 'frozen Rhine' time series would show no connection with the 11-year cycle," explains van Oldenborgh, whose analysis of the study is published in Environmental Research Letters (ERL).

  • environmentalresearchweb contacted Frank Sirocko before publication of this news article but he declined to comment at this time.

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