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 researchers show that unusually cold winters are reported when sunspot numbers are minimal. For instance, the freezing of Germany's largest river, the Rhine, is the key, the journal Geophysical Research Letters reported.
"The advantage with studying the Rhine is because it's a very simple measurement," said Frank Sirocko, professor of Sedimentology and Paleoclimatology at the Institute of Geosciences of Johannes Gutenberg University, Mainz, Germany.
"Freezing is special in that it's like an on-off mode. Either there is ice or there is no ice," Sirocko, who led the study, said, according to a university statement.
From the early 19th through mid-20th centuries, riverboat men used the Rhine for cargo transport. And so docks along the river have annual records of when ice clogged the waterway and stymied shipping.
Sirocko and colleagues found that between 1780 and 1963, the Rhine froze in multiple places 14 different times.
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 -- they determined that 10 of the 14 freezes occurred during years when the Sun had minimal sunspots.
Using statistical methods, they calculated that there is a 99 percent chance that extremely cold Central European winters and low solar activity are inherently linked.
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