Studies of Germany’s largest river reveal a correlation between periods of low activity of the sun and regional cooling
Scientists have long suspected that the Sun’s 11-year cycle influences climate of certain regions on Earth. Yet records of average, seasonal temperatures do not date back far enough to confirm any patterns. Now, armed with a unique proxy, an international team of researchers show that unusually cold winters in Central Europe are related to low solar activity – when sunspot numbers are minimal. The freezing of Germany’s largest river, the Rhine, is the key.
Although the Earth’s surface overall continues to warm, the new analysis has revealed a correlation between periods of low activity of the Sun and of some cooling – on a limited, regional scale in Central Europe, along the Rhine. “The advantage with studying the Rhine is because it’s a very simple measurement,” said Professor Dr. Frank Sirocko, lead author of a paper on the study and Professor of Sedimentology and Palaeoclimatology at the Institute of Geosciences of Johannes Gutenberg University Mainz. “Freezing is special in that it’s like an on-off mode. Either there is ice or there is no ice.”
But how to find this information? Easily done: From the early 19th through the mid-20th centuries, riverboat men used the Rhine river for cargo transport. And so docks along the river have annual records of when ice clogged the waterway and stymied shipping. The scientists used these easily accessible documents, as well as other additional historical accounts, to determine the number of freezing episodes since 1780. Sirocko and his colleagues found that between 1780 and 1963, the Rhine froze in multiple places fourteen different times. “The sheer size of the Rhine river means it takes extremely cold temperatures to freeze over making freezing episodes a good proxy for very cold winters in the region,” Sirocko said.
Mapping the freezing episodes against the solar activity’s 11-year cycle, a cycle of the Sun’s varying magnetic strength and thus total radiation output, Sirocko and his colleagues determined that ten of the fourteen freezes occurred during years when the Sun had minimal sunspots. Using statistical methods, the scientists calculated that there is a 99% chance that extremely cold Central European winters and low solar activity are inherently linked. “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.