Scientists have discovered a link between changes in solar activity and recent cold temperatures in parts of Europe and North America.
A study has found that low ultraviolet radiations from the sun can contribute to colder winters over parts of the Northern Hemisphere, with the opposite occurring when UV levels are higher.
The researchers have known for a while that the sun has an 11-year cycle of solar activity, during which radiation measured by sunspots on the surface reaches a peak before falling. But until now pinning down a link has proved difficult.
"Our research confirms the observed link between solar variability and regional winter climate," said Sarah Ineson, lead author of the UK Met Office. "It's more than just coincidence, there's a real correlation between ultraviolet levels and meteorological variables."
The study, which is published in the journal Nature Geoscience on Monday, found that UV radiation from the sun can affect high-altitude wind patterns in the Northern Hemisphere, which can trigger cold winters.
The data also showed that the changes in ultraviolet radiation levels were considerably larger than previously thought.
"While UV levels won't tell us what the day-to-day weather will do, they provide the exciting prospect of improved forecasts for winter conditions for months and even years ahead. These forecasts play an important role in long-term contingency planning," said Ineson.
The researchers cautioned that there could be several factors that have influenced cold winters too, such as declining sea ice, but that satellite measurements of UV radiation in the upper atmosphere support the idea that low solar activity played a major role.
"There are a lot of different factors that affect our winter climate. However, the solar cycle would probably have been acting in a way that gave us those cold winters," said Ineson.