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Link found between cold European winters and solar activity

Dr Amanda Doyle, Feature writer
Aug 25, 2012, 7:00 UTC

Sen—Scientists have determined a link between the Sun’s eleven year cycle and freezing European winters by analysing the freezing over of the river Rhine in Germany.

It has long been thought that the solar activity cycle could influence the climate in certain areas on Earth. However, trying to delve into the history of the coupling of solar activity and Earth’s climate is difficult given the paucity of records. While historical records of solar activity contained in ice cores and tree rings date back thousands of years, records of seasonal variations of temperature on Earth do not date back very far.

However, researchers have found a new way of dating the temperatures on Earth reliably by using the river Rhine in Germany. By determining when the Rhine froze over during harsh winters, scientists discovered a correlation between the regional climate and low solar activity.

“The advantage with studying the Rhine is because it’s a very simple measurement,” said Frank Sirocko at Johannes Gutenberg University in Mainz, Germany. “Freezing is special in that it’s like an on-off mode. Either there is ice or there is no ice.”

Scientists can date the freezing of the Rhine as far back as 1780, thanks to the meticulous records of the riverboat men who transported cargo up and down the river. Docks along the river noted whenever the waterway was blocked and shipping impeded. The logbooks show that the Rhine froze in multiple places fourteen different times between 1780 and 1963. It takes exceptionally chilly conditions for the Rhine to freeze, given the sheer volume of the body of water, which makes the icy river a very good indicator for harsh winters. 

Rhine river frozen

The frozen river Rhine. Credit: Warburg, via Wikimedia Commons

In addition to these logbooks, the scientists used records of diseases and migration among people to complete their data. “Disease can be transported by insects and rats, but during a strong freezing year that is not likely,” explained Sirocko. “Also, Romans used the Rhine to defend against the Germanics, but as soon as the river froze people could move across it. The freezing of the Rhine is very important on historical timescales.”

By comparing these freezing records to the eleven year cycle of solar activity, it was found that ten of the fourteen freezes occurred during solar minimum, i.e. when there were the least amount of sunspots for that cycle. Sirocko and colleagues found that there is a 99 per cent chance that the freezing is connected to the solar cycle.

“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,” said Sirocko.

The minimum level of solar activity will only cause regional, rather than global, variations in climate. During solar minimum, there is less ultraviolet radiation emitted, which causes less heating in the Earth’s atmosphere. These change circularisation patterns within the atmosphere which can influence the European climate.

“Due to this indirect effect, the solar cycle does not impact hemispherically averaged temperatures, but only leads to regional temperature anomalies,” said Stephan Pfahl, a co-author of the study.