Sunspots

Sunspot activity may be subtly linked to the earth's weather. Suggestive correlations between solar activity, global temperature, and rainfall have been observed, and analysis of tree-ring data spanning centuries seems to show the presence of an 11–13 year cycle. There is also geological evidence that the solar cycle may have been affecting terrestrial weather since Precambrian times. However, all these data have been disputed on statistical grounds, and there presently no consensus among scientists as to whether sunspots actually affect the earth's weather or not, or if so, how. The energy output of the Sun varies very little over the solar cycle (i.e., by about 0.1%), and some scientists doubt whether such slight changes can really affect the troposphere (lower atmosphere) of the earth, where precipitation occurs. A possible mechanism for amplifying the effects of the solar cycle on tropospheric weather is its influence on the stratosphere (the region of the atmosphere from an altitude of about 10 mi [16 km] to about 30 mi [50 km]). The stratosphere is home to the ozone layer, a diffuse shield of triatomic oxygen (O₃) that is an efficient absorber of ultraviolet radiation. Since the Sun's ultraviolet output varies 10 times more over the solar cycle than its overall radiation output, it is plausible—and has been confirmed by observation—that the temperature (and thus volume) of the stratosphere will vary significantly with the solar cycle. (Those involved in the launching and maintenance of Earth satellites are acutely aware that the upper layers of the earth's atmosphere respond to solar activity by expanding and thereby inflicting increased drag on satellites in low orbits.) However, the troposphere is many thousands of times more massive than the stratosphere, and scientists continue to investigate the question of whether temperatures in the frail film of the stratosphere can measurably affect surface weather.

One suggestion of a sunspot-weather link comes from historical records. There was a curious period of about 75 years shortly after Galileo's discovery of sunspots when few were observed. This era is called the Maunder minimum after the astronomer who first noted its existence. Other phenomena such as the aurora borealis (northern lights) that are associated with solar activity are also missing from European records during this period. The interval is also associated with what has long thought to have been time of unusually severe winters in both Europe and the North America that is sometimes termed the Little ice Age. However, there is now doubt as to whether the Little Ace Age ever happened at all, at least on a global level. Most of the evidence for its occurrence is anecdotal, and most comes from Western Europe. It is unlikely that historical evidence of this kind will resolve the scientific dispute over whether the solar cycle (or anomalies therein) significantly affect terrestrial weather.