Weather: Spotting a change in climate

The debate over the effect of sunspots on the earth's weather has been running for more than a century. Yet the issue is still clouded in mystery.

I mentioned yesterday Nigel Calder's recent book The Manic Sun, which makes a good case for sunspots having a strong effect on our weather. Its subtitle is "Weather Theories Confounded", and indeed a good deal of the book is devoted to arguing against the theories of greenhouse gases and global warming. But it is what Calder sees as the political manipulation of scientists that provokes his strongest reactions.

"Political correctness made life awkward for anyone trying to discover how nature really ran the planet," he writes. "If you so much as queried the greenhouse warming, you were an enemy of the environment. You probably panelled your study in fresh mahogany, ran your car on unleaded fuel and hunted endangered species for relaxation. You were undoubtedly in the pay of an oil company."

Politics apart, the argument is one of the great scientific debates of our time. We all know about the "greenhouse theory", of man-made gases in the atmosphere that prevent heat escaping from the Earth, which some say is just a theory and others claim as proven fact. But what about sunspots?

These black freckles on the sun's surface - big enough to be seen with primitive telescopes from Earth - were first identified by Galileo. They occur in groups, are generally short-lived, and have intense magnetic fields that seem able to create magnetic storms on earth. But what creates them in the first place is still unknown.

In 1848 Rudolph Wolf devised a method of counting the number of individual spots and groups of spots to give an estimate of solar activity. His method was simply to multiply the number of groups by 10, then add the number of lone spots. Calculations of the Wolf sunspot number have continued to the present day, and confirm Wolf's own finding that sunspot numbers occur in a cycle of just over 11 years. By combining his own findings with those of earlier astronomers, Wolf was able to extend his sunspot statistics back to 1700, and these figures have formed the basis for a long-running series of theories about sunspots and the weather.

In 1887, Robert H Scott wrote of attempts to link weather observations with the periodicity of the sunspot cycle: "It is maintained by many investigators of high authority - such as, among Englishmen, Meldrum and Balfour Stewart, not to mention foreigners - that such a periodicity does exist in the occurrence of cyclones, rainfall, the conditions of terrestrial temperature and of barometrical pressure." He then goes on to point out that some associated high sunspot activity with low temperatures, and others the exact opposite, and concluded: "We may venture to surmise that, whenever the connection may be really discovered, it will prove to be of a less simple nature than has hitherto been supposed."

Perhaps, as Calder maintains, the recent work of three Danish scientists is just that: a correlation between the lengths of sunspot cycles and the temperature on Earth. On the other hand, one of the strongest arguments put forward in favour of older sunspot theories is that of the Little Ice Age around 1700, which coincided with abnormally low sunspot numbers, the so-called Maunder Minimum of 1645-1715. But that depended on sunspot numbers, not cycle length.

Greenhouse or sunspot, the argument goes on. The sun-worshippers believe their correlations are conclusive, and cannot understand how any sensible person can reject them. Yet the greenhouse men demand hard physics, asking, how does it work? Read Calder's book (Pilkington Press, pounds 24.95) and form your own judgement.