Solar Activity Linked To Colder European Winters
August 24, 2012

Rhine River Freezing Provides Clues To Solar Activity And Colder Winters

April Flowers for - Your Universe Online

Scientists have long suspected that the Sun's 11-year cycle influences climate of certain regions on Earth. Records of average, seasonal temperatures do not date back far enough to confirm any patterns, though.

An international team of researchers, armed with a unique proxy, show that unusually cold winters in Central Europe are related to low solar activity — when sunspot numbers are minimal. Germany's Rhine River freezing is the key.

The new analysis reveals 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 Frank Sirocko professor of Sedimentology and Paleoclimatology at the Institute of Geosciences of 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.”

Riverboat men used the Rhine for cargo transport from the early 19th through mid-20th centuries. Docks along the river have annual records of when ice clogged the waterway and stymied shipping. The research team used these easily-accessible documents along with other historical accounts to determine the number of freezing episodes since 1780.

Sirocko's team found that between 1780 and 1963, the Rhine froze in multiple places fourteen 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.

The researchers mapped the freezing episodes against the solar activity cycle — a cycle of the Sun's varying magnetic strength and thus total radiation output. They determined that ten of the fourteen freezes occurred during years when the Sun had minimal sunspots and 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.

“There is some suspension of belief in this link,” said Thomas Crowley, Director of the Scottish Alliance for Geoscience, Environment, and Society (SAGES), who was not involved with the study, “and this study tilts the argument more towards thinking there really is something to this link. If you have more statistical evidence to support this explanation, one is more likely to say it´s true.”

The study is set to be published August 25 in Geophysical Research Letters, a journal of the American Geophysical Union.

The sun emits less ultraviolet radiation when sunspot numbers are down. 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.

“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 who is now at the Institute for Atmospheric and Climate Science in Zurich.

The study shows that this variation in atmospheric circulation leads to cooling in parts of Central Europe but warming in other European countries, such as Iceland. This suggests that sunspot influence is a localized phenomenon. The data links the low solar activity to the extremely cold winters of 2010 and 2011 which were so cold they resulted in record lows for the month of November in certain countries. Some who dispute the occurrence of anthropogenic climate change argue that this two-year period shows that Earth's climate is not getting any warmer.  Climate is a complex system, however, and a short-term localized dip in temperatures only temporarily masks the effects of a warming world.

“Climate is not ruled by one variable,” said Sirocko. “In fact, it has [at least] five or six variables. Carbon dioxide is certainly one, but solar activity is also one.”

Moreover, the researchers also point out that, despite Central Europe´s prospect to suffer colder winters every 11 years or so, the average temperature of those winters is increasing and has been for the past three decades. As one piece of evidence of that warming, the Rhine River has not frozen over since 1963. Sirocko said such warming results, in part, from climate change.

Hoping to establish a more complete record of past temperature dips, the team is looking to other proxies, such as the spread of disease and migratory habits.

“Disease can be transported by insects and rats, but during a strong freezing year that is not likely,” said 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.”

The Rhine wasn't Sirocko's inspiration for this study, however. It was 125 mile ice skating race he attended over 20 years ago in the Netherlands.

“Skaters can only do this race every 10 or 11 years because that´s when the rivers freeze up,” Sirocko said. “I thought to myself, ℠There must be a reason for this,´ and it turns out there is.”