Spotting a climate change

The role sunspots may play in determining our weather has long been a source of debate. A discovery this week may shed light on the matter

MENTION SUNSPOTS to any respectable meteorologist and you will be liable to fall under intense suspicion of heresy.

For at least a hundred years, an assortment of non-establishment weather forecasters have advocated a variety of theories linking sunspot activity with weather on Earth. They have generally, in some degree at least, been treated as crackpots because of an absence of hard science to back their theories.

This week, however, there has been a sudden change in the scientific climate, for two researchers at Leicester University have outlined a theory that appears to support much of what the alleged crackpots have been saying.

The case of the sunspot worshippers has until now been based mainly on statistical correlations between sunspot activity and the Earth's temperature. We'll come to just what sunspots are in a moment. For the time being, all you need to know is that if you look at the sun through a telescope you will see a varying number of freckles on its surface, caused by some sort of local disturbance. The number of these freckles is called the sunspot number, and it has long been known that the number increases and decreases roughly according to an 111/2-year-cycle.

Since the late 19th century, attempts have been made to match variations in the Earth's temperature to fluctuations in the sunspot cycle. It seemed to many scientists that it could not be a coincidence that a period of abnormally low sunspot numbers, from about 1,645 to 1,715 (the so-called Maunder Minimum), coincided with the Little Ice Age on Earth. But arguments have continued as to the strength of any general correlation between sunspot numbers and temperatures on Earth.

A modicum of success was obtained in attempts to make the changing Earth climate fit the 111/2-year sunspot cycle - the worst droughts in the American West, for example, appear to have happened at the same time in alternate cycles, every 33 years - but the theory left too many questions unanswered. In the case of those droughts, for example, why only every alternate cycle?

The most important objection, however, was an absence of any convincing theory of causality. A sunspot seems to be an area of cooler gas at the sun's surface, caused by strong magnetic fields that block the flow of heat. Magnetic storms on Earth happen at times of high sunspot activity. But although sunspots may involve some blockage of the flow of heat from the sun, calculations show that the effect on the amount of solar energy reaching the Earth would be no more than a change of 0.1 per cent. And that's hardly likely to cause an ice age.

Yet in recent years, new theories of sunspot effects have been gaining ground. The meteorologist Piers Corbyn has been a thorn in the side of the Met Office for years now with his well-publicised bets on the weather with William Hill. His method of long-term weather prediction based on sunspot theories has been consistently outperforming conventional forecasting methods.

Still more recently, three Danish scientists have produced some convincing correlations not between the Earth's temperature and sunspot activity, but between its temperature and the varying length of sunspot cycles.

All the time, however, there were strong grounds for reasonable doubt. The scientific establishment could never convict sunspots of interfering with our weather until an explanation was given of how they did it. And that's what we may now have, thanks to the work of Professor Terry Robinson and Dr Neil Arnold at Leicester University.

They have constructed a computer model of the Earth's climate that stretches far higher above the surface of the Earth than existing models, and takes into account the high levels of electromagnetic radiation that are associated with sunspots. This radiation is known to heat up the outer atmospheric layers, but by the time the effect had drifted down to Earth, the effect on our weather would, according to earlier theories, have all but vanished.

What the Leicester model does is to show how pressure waves the size of the whole planet can build up and vary according to solar activity. Global pressure waves then interact with jet streams to produce large changes in climate.

So far this is all just a computer model, but its predictions fit well with observations. As Dr Arnold says: "There have been many observations linking climate to solar activity, but without a mechanism a lot of people have dismissed it as a fluke. Our model has come up with something which might offer an explanation."

Corbyn is characteristically less cautious: "This is great news. It confirms everything we've been saying for years," he says. "I think the forecasting establishment are going to have to wise up."

Caution, however, is still advisable. The recent history of weather forecasting is littered with too many mathematical models that have fitted beautifully with the data of the past but have gone on to fail the test of predicting the future. In the Seventies, when the Earth's climate took a sudden dip into cooler regions, it became fashionable to predict an imminent ice age. Almost any model that fitted the observed data of a cooling Earth would have been bound to do so. Just as the Nineties have been full of predictions of runaway global warming. The test of a computer model comes not in how well it fits the past, but in its accuracy in predicting the future.

And that is why the consequences of this discovery, if it stands up to rigorous tests, will have far deeper implications than settling an old squabble between meteorologists. If proponents of the various sunspot theories are correct, then recent changes in the Earth's climate may be fully explained by solar activity.

For anyone who thought that Kyoto went too far in trying to restrict the production of greenhouse gases, this new theory provides another potential weapon to their arguments.

The oil lobby will be looking with great interest in the direction of Leicester over the coming years.