It has been assumed that the Sun's 11-year cycle influences climate of certain regions on Earth. Until now, scientists couldn't find strong evidence to prove it.
Now, armed with a unique proxy, an international team of researchers has shown 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. Freezing is special in that it's like an on-off mode. Either there is ice or there is no ice," said Frank Sirocko lead author of a paper on the study and professor of Sedimentology and Paleoclimatology at the Institute of Geosciences of Johannes Gutenberg University in Mainz, Germany.
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. The scientists used these easily-accessible documents, as well as 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 14 different times. The sheer size of the 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 10 of the 14 freezes occurred during years around when the Sun had minimal sunspots.
Using statistical methods, the scientists calculated that there is a 99 percent 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.
When sunspot numbers are down, the Sun emits less ultraviolet radiation. Less radiation means less heating of Earth's atmosphere, which sparks a change in the circulation patterns of the two lowest atmospheric levels, the troposphere and stratosphere. Such changes lead to climatic phenomena such as the North Atlantic Oscillation, a pattern of atmospheric pressure variations that influences wind patterns in the North Atlantic and weather behavior in regions in and around Europe.
In fact, studies have suggested that the extremely cold European winters of 2010 and 2011 were the result of the North Atlantic Oscillation, which Sirocko and his team now link to the low solar activity during that time.
The study will be published in Geophysical Research Letters, a journal of the American Geophysical Union.