Scientists have long suspected the sun's 11-year cycle influences the climate of certain regions on Earth. German researchers compared 14 freezing episodes of the river recorded from 1780 to 1963 against the solar activity cycle -- a variation of the sun's varying magnetic strength and radiation output -- and found 10 of the 14 freezes occurred during years around when the sun had minimal sunspots, the American Geophysical Union reported Thursday.
Using statistical models, they calculated a 99 percent chance that extremely cold Central European winters and low solar activity are inherently linked.
When sunspot numbers are down, the researchers said, the sun emits less ultraviolet radiation, which results in less heating of Earth's atmosphere, causing change in circulation patterns in the atmosphere that affect climate.
"Due to this indirect effect, the solar cycle does not impact hemispherically averaged temperatures, but only leads to regional temperature anomalies," study co-author Stephan Pfahl said.
Climate is a complex system, the researchers acknowledged, and solar activity should be considered in concert with other factors such as warming caused by greenhouse gas emissions.
"Climate is not ruled by one variable," study leader Frank Sirocko of Johannes Gutenberg University in Mainz, Germany, said. "In fact, it has [at least] five or six variables. Carbon dioxide is certainly one, but solar activity is also one."