Spacecraft measurements have established that the total radiative output of the Sun varies at the 0.1-0.3% level. The observed fluctuations are well modelled as radiative deficits proportional to the area of the solar disk covered by sunspots. Historical records of projected sunspot areas allow an accurate reconstruction of these short-term fluctuations over the past century. Such changes can be expected to perturb the terrestrial surf ace temperature by a fraction of a degree centigrade and probable evidence of this solar-induced signal has been found. The effect, though important in terms of understanding the climate system, is too small to be significant in practical weather or climate predictions.
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