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The science take: Solar activity and ice ages

  • 10 Oct 2011, 14:50
  • Verity Payne

A new paper, published online today, has been interpreted by some journalists as heralding a "mini 'ice age'". This echoes similar claims made earlier this year after the press became aware that the sun's activity has been unusually low over the last few years.

But as numerous commentators have pointed out, the claim that we face a new ice age simply does not fit with what the science shows. So does this latest research proclaim a mini ice age? We take a look at what the paper actually says.

It has long been known that the sun's activity fluctuates on a roughly 11-year cycle, and periods of low solar activity have been associated with changes in winter weather patterns. Originally solar activity level was determined by counting sunspots, with less sunspots meaning less solar activity. However, recent advances in satellite technology have enabled researchers to monitor solar activity more accurately, and have found that the change in UV radiation from the sun over the solar activity cycle is between 4 and 6 times larger than previously calculated.

British scientists have used the new satellite data to estimate the difference between UV radiation when solar activity is at its peak, and when it is at a minimum. The researchers then ran a climate model which simulates the ocean and atmosphere under both scenarios, and compared the two sets of results.

The researchers found that the solar minimum scenario prompts a specific climate pattern during the winter months. UV is absorbed in the stratosphere, around 50 km up in the atmosphere, so during solar minimum when there is less incoming UV radiation, a layer of relatively cold air forms in the stratosphere. This changes how large masses of air flow, as Sarah Ineson, UK Met Office, explains:

"What we're seeing is UV levels affecting the distribution of air masses around the Atlantic basin. This causes a redistribution of heat - so while Europe and the US may be cooler, Canada and the Mediterranean will be warmer, and there is little direct impact on global temperatures."

Under a solar maximum scenario, the researchers found the pattern reversed.

The findings might go some way to explaining the severe winters experienced in Northern Europe and the US since 2008. It's possible it could also help scientists to more accurately predict the severity of coming winters, as Adam Scaife from the UK Met Office puts it:

"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. 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."

In summary, if the satellite observations are correct, then solar minima can cause more severe winters in the US and northern Europe. 

So just how do some journalists jump from that to predicting a "mini 'ice age'"?

During the late 17th and early 18th centuries there was a period of low sunspot activity which lasted for around 70 years and coincided with part of the what was (unhelpfully) labelled the 'Little Ice Age' - a period of cooling affecting parts of the globe that lasted around 300 years.

Now there seems to be a common mistaken belief that low solar activity leads inevitably to the world entering a new ice age, which confuses an actual ice age with the 'Little Ice Age'. This conclusion disregards the wealth of scientific research in this area. 

Whilst the sun's effect on climate is important, providing as it does nearly all of the energy to power Earth's climate system, its temperature effect is actually superimposed on the more dominant climate forcing provided by increasing greenhouse gas emissions. 

Recent research disputes any notion that solar activity was the dominant driver of the Little Ice Age. Furthermore, climate model simulations suggest that a Maunder Minimum-type solar minimum would impose a cooling of around 0.3 ˚C, which, although noteworthy, is marginal compared to the projected warming of 2 - 4.5 ˚C likely to result from man-made greenhouse gas emissions over the coming century.

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