The sun is the very engine of our climate, the powerhouse that drives our daily weather. It used to be thought that its motor hummed away at constant speed, that the energy surging forth in our direction never varied; indeed, that it was referred to as the solar constant. But then as the poet Richard Barnfield pointed out: Fortune is full of fresh variety,
Constant in nothing but
inconstancy.
Variations in the sun's radiative output are closely related to the phenomenon of sunspots. These dark, relatively cool areas on the solar disc increase and decrease in number over a cycle of about 11 years. Individual spots may last for anything from a day to several weeks, and are linked with local disturbances in the sun's magnetic field.
Now at any given time the spots cover, at most, about 2 per cent of the solar disc, and so, allowing for their slightly lower temperature, it used to be assumed that the solar output might decrease 0.2 per cent or thereabouts at sunspot maximum.
But 20 years ago the first measurements from space yielded the surprising result that the solar output is highest at the sunspot maximum, and falls as the number of spots decreases. We now know that this is because the dark and relatively cool sunspots are surrounded by very bright, mottled regions known as faculae, which more than compensate for the decrease in radiation from the spots themselves.
So to what extent do these ups and downs affect our climate? As yet, we really do not know, particularly as regards short-term variations in our weather, but on the scale of centuries solar advocates can point to a number of curious coincidences.
About 100 years ago, an astronomer called Edward Maunder, browsing through the records left by his predecessors over the centuries, noticed that between 1645 and 1715 hardly any spots at all were seen upon the surface of the sun.
Later research showed that this so-called Maunder Minimum was complemented by two further anomalies, the Sporer Minimum from 1400 to 1510 and the Medieval Maximum, a period of unusually high sunspot activity, from 1100 until 1250.
By chance or otherwise, the worst excesses of the Little Ice Age, a period of exceptionally cold and severe weather in Europe, coincided with the two minima, and the period around the Medieval Maximum was an era of very benign climatic conditions.
Others have noted that the Maunder Minimum coincided almost exactly with the reign of Louis XIV of France, the Sun King, from 1643 to 1715. But now that really is coincidence.