Reconstructions of Past Solar Variability

Abstract

Spacecraft observations of present-day solar irradiance document variations at all wavelengths, on all measured time scales. Historical solar activity surrogates, and observations of Sunlike stars and of the Sun itself, provide circumstantial evidence for larger irradiance changes than have been observed thus far during a single, contemporary 11-year solar activity cycle. Reconstructions of past solar irradiance variations are needed to assess solar forcing of paleoclimate and to investigate the range of natural solar forcing that may potentially compete with anthropogenic effects in the future. Effort has focused primarily on reconstructing total (spectrally integrated) solar irradiance and, recently, the more variable ultraviolet (120 – 300 nm) spectrum during the past 400 years. Estimates of past solar variability amplitudes are obtained from knowledge of the wavelength-dependent effects on the present-day solar irradiance of dark sunspots and bright faculae, combined with mechanisms speculated to generate larger facular emission variability over longer time scales. Relative temporal variations in past irradiance are derived from solar activity proxy data such as the sunspot number. Reconstructions of past solar irradiance variability are compared with the ¹⁰Be and ¹⁴C cosmogenic isotope records.