From CAWSES

What are the uncertainties in establishing the long-term direct solar effect upon climate?

A major uncertainty in modeling of solar influence on climate is related to the long-term reconstructions of solar irradiance (total and spectral) which differ markedly. We do not propose to become involved with the reconstructions but we aim to evaluate the uncertainties in the corresponding climate impacts. Other effects also need to be considered. In particular, long-term studies may be useful in disentangling direct solar irradiance and EP effects.