Abstract
Information can be inferred on the timing and amplitude of solar total irradiance changes over 1880- 1993 by simulating the global terrestrial surface temperature changes produced by these irradiance changes and comparing them with observed temperatures. The profiles of solar irradiance variations used in the climate simulations are adopted from several different proxies: (1) the length of the sunspot cycle, (2) the mean sunspot number, and (3) a composite proxy that includes the two previous indicators plus the equatorial solar rotation rate, the fraction of penumbral spot coverage, and the rate of decay of the sunspot cycle. We use a seasonal energy-conservation climate/upwelling-diffusion ocean model, forced by the assumed profiles of solar total irradiance variations, combined with variations in anthropogenic greenhouse gases. Optimized cases imply total irradiance changes during 1880-1993 in the range 0.18%-0.77%.
If the solar irradiance profiles found from the climate simulations are required to be consistent with recent satellite observations, then the composite solar profile reconstructed by Hoyt & Schatten, combined with the anthropogenic greenhouse forcing, explains the highest fraction of the variance of observed global mean temperatures. In this case, the solar and greenhouse combination accounts for 92% of the observed long-term temperature variance during 1880-1993. The simulation implies that the solar part of the forcing alone would account for 71% of the global mean temperature variance, compared to 51% for the greenhouse gases' part alone. It also suggests a solar total irradiance variation of 0.5% during the interval 1880-1993. Such an amplitude of solar total irradiance change is consistent with astrophysical limits of brightness changes on timescales of decades to centuries independently derived from observations of solar-type stars (including the Sun).