The radiant energy output by the sun varies, especially over the 11-year cycle of growth and decline of solar activity as manifested by large variations in sun spots, solar flares and other phenomena. Such changes can contribute significantly to the increase (or reduction) in global surface temperatures (the greenhouse effect) and to interference in radio communications. There is evidence indicating that the increase in solar radiation associated with sunspots may drive viruses (especially influenza viruses) from the upper atmosphere down to the earth's surface resulting in periodic epidemics.
The amplitude of the observed variation is about 0.1% during the current solar cycle. There is no direct evidence for greater variations over longer periods, although it is known that there have been substantial changes in solar activity over the centuries and that they correlate positively with temperature changes on Earth. During the period 1600 to 1700, with the almost total absence of sun spots in the latter half of the century, the coldest period in the last 1000 years was experienced. A much lesser degree of cooling was experienced from the 1940s to the late 1970s, accompanied by a substantial reduction of solar activity. The statistical relationship between sunspot peaks and pandemics has been explored from 1761 in relation to the 11-year cycle.