The sun is the primary source of energy for the climate of the earth. Variations in solar energy reaching the earth’s surface change the climate. Several factors control the influx of solar energy, including (1) variations in the earth’s albedo, (2) variations in earth’s orbit and rotation, and (3) variations in solar energy output.
In the short time since 1978, direct measurement of total solar irradiance (TSI) by satellites has shown cyclical variations in solar energy of 0.1% in conjunction with the 11-year sunspot cycle. Indirect evidence from the sun and other sunlike stars indicate that TSI has had significantly greater variation as the the sun goes through various cycles.
The correlations between climate and TSI variations are statistically solid. Small variations in TSI initiate indirect mechanisms on earth that yield climate changes greater than that predicted for the TSI change alone. At least three solar variables are known to affect earth’s climate: (1) TSI, which directly affects temperatures; (2) solar unltraviolet radiation, which affects ozone production and upper atmospheric winds; and (3) solar wind, which affects rainfall and cloud cover, at least partially, through control of the earth’s electrical field. Each affects the earth’s climate in different ways, producing indirect effects that amplify small changes in TSI. Individually, they do not cause the entire observed climate changes. Collectively, they appear to be sufficient, especially because solar forcing of earth’s climate is still an emerging science. Undoubtedly, other mechanisms of solar forcing are poorly understood, perhaps even unknown.