The debate continues over what the effect of known changes in the Sun’s activity have on the Earth. As the primary source of heat and light and energy, it sustains life on the planet, and variations in its output can be expected to influence climate in significant ways. A common example of this is the 11- and 22-year cycles of sunspot activity, during which the number of dark visible spots on the Sun’s surface varies between maximum and minimum values (which are different for each cycle). Higher numbers of sunspots are evidence of a more active Sun.
Economists have recognised for many years (from at least Adam Smith’s time) that there is a link between sunspot activity and crop yields, having seen that wheat prices rose at times of low activity (the quiet Sun). And when there are a number of years of low activity, the effect on weather patterns can be very significant significant. The best known example of this is the so-called Maunder minimum. From 1645 to about 1715, very few sunspots appeared (only about 0.1% of the normally expected number). This lack of solar activity coincided with the Little Ice Age, when there are well-documented accounts of cool summers and bitterly cold winters afflicting the northern hemisphere.
For some time, it has been clear that the Sun has moved from being highly active in the later part of last century to a current quiet period. The current sunspot cycle (24, marked by a reversal of the magnetic field) has started much later than usual and, by some reckoning may not even have started at all. Certainly, there have been only occasional small spots for an extended period. This coincides with a report from scientists responsible for the Ulysses satellite that the solar wind - the stream of charged particles which extends for billions of kilometres to form the heliosphere - is weaker than it has been for 50 years. Confirmation of this comes from the Voyager probes, launched in the 1970s, and now moving beyond the heliosphere into interstellar space earlier than was originally expected.
Whether this cyclical change in the Sun’s behaviour is behind the current climate pattern - an apparent halt to any global warming trend after the peak average temperatures of 1998 - is a moot question, but certainly cannot be ruled out. The jetstream has stayed further south than normal, leading to two miserable summers in northern Europe, and such alterations in wind patterns seem also to be at least a contributory factor (and perhaps even the primary driver) of the changing patterns of summer ice loss in the Arctic, including irregular opening of the North-West passage.
But perhaps the most intriguing prospect is to be able to observe the effect a quiet Sun has on cloud formation. Svensmark and others have hypothesised that a weakening of the solar wind allows more high-energy cosmic rays to enter the Earth’s atmosphere and that these create a greater number of nuclei around which clouds can form. Put simply, if the Sun is quiet, cloud cover will on average be greater and average temperatures will be lower. The credibility of this is due to be tested at CERN in the CLOUD experiment, but real-life observations may already have provided much of the evidence by the time the results are in. Many mainstream scientists dismiss the Sun as the major driver of climate, whether directly via radiance changes or indirectly by a mechanism such as that proposed by Svensmark. But all professional scientists, whatever their current views, should look forward eagerly to the collection of climate data as the Sun continues through its quiet period, however long that may be. The scientific method relies on careful and systematic gathering of evidence to try to falsify hypotheses. No true scientist should ignore the observations of solar activity and weather patterns over the next few years.
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