It will have escaped no-one's attention that the past couple of winters have been quite harsh. Snow piled up around the country in glistening white drifts, culminating in the 'snow covered Britain' image that flashed around the world last December. It turns out that, as always, it is the sun that is behind these overly chilly winters. But not in the way that you might expect. It seems that Northern Europe's recent harsh winters may have their roots in the sun's varying ultraviolet emissions. Recent data shows the sun's UV output is far more changeable than scientists had previously thought, and now it appears that these variations might be very useful.
A team of UK scientists has shown how changes in UV levels can result in warmer winters in some places and colder winters in others. Dr Adam Scaife, who heads the UK Met Office's Seasonal to Decadal Prediction team and was one of the researchers on the paper, emphasises that ultraviolet emissions are not the sole reason why winter temperatures vary. But understanding this UV link may improve meteorologists' capacity to predict winter weather accurately. "Assuming these new satellite data are correct... then as the 11-year solar cycle is predictable, it's going to contribute some predictability for European and indeed UK weather," he said. "You'll never be able to predict the precise temperature of the third week in January or whatever, but you might be able to say 'this winter is more likely to be warm' or 'more likely to be cold' with more accuracy."
The new findings come from a Nasa satellite called the Solar Radiation and Climate Experiment. Among the satellite's instruments is the Spectral Irradiance Monitor (SIM), which analyses the sun's output at frequencies in the infrared, visible and ultraviolet parts of the spectrum. SIM is giving scientists a detailed picture of how the sun's ultraviolet emissions vary over an 11 year cycle and have found that it is about five times larger than previously thought.
Incoming UV radiation is absorbed by ozone in the stratosphere, warming that part of the atmosphere. In the quiet parts of the solar cycle, when there is less UV to absorb, the stratosphere is relatively cooler. A Met office Hadley Centre model shows that the effects of these cooler temperatures percolate down through the atmosphere, changing wind speeds, including the jet stream that circles the globe above Europe, North America and Russia. The result is a reduced air flow from west to east, which brings colder air to the UK and northern Europe and re-distributes temperatures across the region.
It seems that the sun has recently been in a quiet phase of an 11-year cycle. This quiet period has coincided with three years in which Northern Europe and parts of the US experienced unusually cold conditions, but in which warm weather was felt both further south, around the Mediterranean, and further north in Canada and Greenland. But researchers were keen to stress that these new findings had no direct impact on global warming. "The key point is that this effect is a change in the circulation, moving air from one place to another, which is why some places get cold and others get warm," said Dr Scaife. "It's a jigsaw puzzle, and when you average it up over the globe, there is no effect on global temperatures."