Solar dimming
October 10, 2011
Cyclical changes in the amount of radiation emitted from the sun affect weather patterns in the northern hemisphere, according to a study published on Monday in the journal Nature Geoscience.
Researchers analyzed recent satellite data to conclude that when ultraviolet emissions from the sun drop, parts of Europe and North America experience harsher winters.
Sunspot cycles
Scientists have known for some time that radiation from the sun peaks and falls in a cycle that runs around 10 or 11 years.
Sunspots are an indicator of solar activity, and ultraviolet radiation varies in concert with these cycles of activity.
The new study relies on fresh data that suggests this activity is more variable than previously thought.
"We're actually measuring the direct amount of ultraviolet radiation that's reaching the Earth," said Jeff Knight of the Met Office Hadley Centre in Exeter, one of the study's authors.
The scientists found that a reduction in ultraviolet light from the sun makes cold air form in the atmosphere about 40 kilometers above the tropics.
"It kicks off a chain of influences that lead to a reduction of the westerly winds that blow across the Atlantic into Europe in the winter," Knight told Deutsche Welle.
In a normal winter, Europe receives westerly winds that cause a warming influence.
"When we lose those winds, you lose that warming influence, bringing that cold, Siberian-type of wind into northern Europe," Knight said.
These frigid winds are part of a larger circulation that reaches all the way across the Atlantic Ocean, to the Eastern seaboard of the United States. "So you get similar types of weather in that region too – as demonstrated last winter," Knight said.
During this time of absent westerly winds, Canada and the Mediterranean experience milder weather.
Periods of stronger solar radiation, the scientists believe, cause the opposite effect.
Predicting the weather
The study could assist long-term predictions for winter weather around the world by helping illuminate the sun's impact on climate.
"We can't draw a direct one-to-one parallel between the sun and our weather – it's more complicated," Knight said.
He added that the study has more relevance for the northern hemisphere and mid-latitudes of the Earth.
Some skeptics of man-made global warming have pointed to variations in solar activity as an alternative explanation for climate change.
Knight said the new study did not challenge the role of greenhouse gases in affecting the climate.
"This study doesn't say anything at all about global climate," Knight emphasized. "Air is being moved around, not changed – it's just a rearrangement."
Knight said he and other researchers planned to look into speculation that the sun may be entering a period of prolonged relative inactivity, like what happened during the "little ice age," between the 16th and 19th centuries.
Satellite data
The study was made possible thanks to new satellite data from 2004 that has been able to measure the ultra violate radiation levels more accurately than before.
The researchers ran the new data through computer models that simulate interactions between the atmosphere and the ocean. Their findings were able to account for the extreme winters experienced in parts of Europe and North America in recent years.
The relatively small size of the data sample spells a potentially high rate of statistical uncertainty. The scientists hope to complete further studies as more data become available.
The paper involved five researchers with the Met Office (the UK public weather service), one with the University of Oxford and the other with the University of London.
Author: Sonya Angelica Diehn
Editor: Nathan Witkop