Solar variation refers to fluctuation in the amount of energy emitted by the Sun. The Sun is a star undergoing hydrogen to helium fusion reactions due to compression deep inside. The rate of reactions is sufficiently constant that the amount of solar radiation emitted at the surface does not change much. Small variations have been measured from satellites during recent decades.
Sunspots are relatively dark areas on the surface of the Sun. They have been determined to be cooler than its average surface. Variations in their numbers alter the brightness of parts of the Sun.
Climate models are computer simulations which are used to examine understanding of climate behavior. Some models use constant values for solar irradiance, while some include the heating effects of a variable Sun.
