Long-term changes in solar irradiance

Measurements of solar irradiance with the necessary precision to reveal the sun's intrinsic variability started in 1978. These measurements have revealed dips in solar brightness due to the passage of sunspots across the solar disc on a solar rotation time scale and a remarkable increase of the solar irradiance at solar activity maximum. In order to uncover a possible connection between solar irradiance variations and climate, however, it is necessary to extend the irradiance record to earlier times with the help of models. A brief introduction is given to the results of irradiance measurements and to our current understanding of the underlying physical causes, followed by an overview of the efforts to reconstruct irradiance or the underlying magnetic field at earlier times. The secular variation of solar irradiance, in particular the magnitude of the irradiance increase since the Maunder minimum, is critically discussed.