The Sun is the only source of energy for Earth’s ocean-atmosphere system. Any variation in this quantity of energy causes a variation in mean temperature, which is more or less direct due to the complexity of the climate system, in which relief, the oceans, atmosphere, aerosols, ground albedo and the biosphere are all likely to create positive and negative feedback.
The ice ages are explained by periodic variations in Earth’s orbital parameters. During shorter periods, variations in solar activity are reflected in observed climate variations, but we also need to understand the amplitude of these variations and for that we have to model climate. In climate models, we need to know the amount of energy entering the ocean-atmosphere system, so we have to reconstruct it for past periods, since the first measurements only go back to 1978. We do this using information like observations of sunspots and faculae, the length of the undecennal cycle and records of variations in the concentration of cosmogenic isotopes (Hoyt and Schatten (1993), Lean et al., (1995), Lean (2000), Solanki and Fligge (2000), Usoskin et al. (2004), Solanki et al. (2004)). These reconstructions disagree for certain periods and the differences are such that they can significantly alter the temperature calculated by the model, the validity of which may thus be in doubt. We therefore need to be able to precisely reconstruct the energy received by Earth at least for the historical period, i.e., for which the climate is best understood.