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Calm Sun Is Bringing a Chill to England

14 April 2010 5:26 pm
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Pieter Bruegel the Elder

Brrrrrrrrrr. Pieter Bruegel the Elder’s Hunters in the Snow (1565) depicts frozen rivers and lakes during the Little Ice Age. Scientists now say that decreased solar activity may increase the frequency of cold winters and return Northern Europe to such conditions.

In 1801, astronomer William Herschel wrote that when sunspots were rare, the price of wheat in England jumped. Although his result is a historical footnote, a recent study has discovered a strong correlation between the sun’s cycle and temperatures in the United Kingdom. Decreased solar activity results in winter temperatures about half a degree colder than average. Because the sun is becoming less active, the study suggests that people in Northern Europe may have to brace for more frequent cold winters, despite global warming.

Scientists have long suspected a link between the sun’s operations and temperatures on Earth. During the Little Ice Age, which occurred between 1300 C.E. and 1850 C.E., Londoners celebrated winter frost fairs with horse races, football, and puppet plays—all performed on a frozen river Thames. This period partly overlapped with an era known as the Maunder Minimum, spanning from about 1645 to 1715, during which astronomers observed no sunspots—dark, relatively cool spots on the surface of the sun. Since then, the sun has picked up its activity, and scientists have had to wait until the solar cycle dipped again before they could show that this correlation was more than a coincidence.

In the current study, researchers used the world’s longest instrumental record of temperature: the Central England Temperature, recorded monthly in Lancaster, Bristol, and London since 1659. Although sunspots are typically used as a measure of solar activity, the team checked the temperatures against a model of variation in the sun’s magnetic field over time, which they felt was a better indicator of the sun’s total radiance. Long periods of cold winters in the United Kingdom coincided precisely with diminished solar activity, the team reports today in Environmental Research Letters.

The paper’s lead author, solar physicist Michael Lockwood of the University of Reading in the United Kingdom, says that Northern Europe will likely see more frequent cold winters in coming years. He explains that for the majority of the 20th century, solar activity has been unusually high. “But now the underlying trend is down,” he says. Mild winters will still occur, but they will be fewer and further between.

Lockwood and his team suggest that the sun is affecting Northern Europe by causing an increase in “blocking” of the northern jet stream, a fast-flowing river of air moving through the atmosphere. A decrease in solar radiation, they suspect, forces the jet stream to fold back on itself over the Atlantic, creating a large spiral of air that can stay stable for weeks and prevent westerly winds from reaching Europe. Northeasterly winds then come down from the Arctic, bringing colder weather.

Atmospheric scientist Sultan Hameed of Stony Brook University in New York state says the results provide good evidence that the sun’s patterns can affect local temperatures on Earth. “The analysis they have done is very straightforward and makes sense,” he says. Solar physicist Judith Lean of the United States Naval Research Laboratory in Washington, D.C., concurs. Despite Earth heating up, she says regional temperatures can still show great variability. “There’s not a uniform warming or cooling,” she says.