A DISCUSSION OF PLAUSIBLE SOLAR IRRADIANCE VARIATIONS, 1700-1992

From satellite observations the solar total irradiance is known to var y. Sunspot blocking, facular emission, and network emission are three identified causes for the variations. In this paper we examine several different solar indices measured over the past century that are poten tial proxy measures for the Sun's irradiance. These indices are (1) th e equatorial solar rotation rate, (2) the sunspot structure, the decay rate of individual sunspots, and the number of sunspots without umbra e, and (3) the length and decay rate of the sunspot cycle. Each index can be used to develop a model for the Sun's total irradiance as seen at the Earth. Three solar indices allow the irradiance to be modeled b ack to the mid-1700s. The indices are (1) the length of the solar cycl e, (2) the normalized decay rate of the solar cycle, and (3) the mean level of solar activity. All the indices are well correlated, and one possible explanation for their nearly simultaneous variations is chang es in the Sun's convective energy transport. Although changes in the S un's convective energy transport are outside the realm of normal stell ar structure theory (e.g., mixing length theory), one can imagine vari ations arising from even the simplest view of sunspots as vertical tub es of magnetic flux, which would serve as rigid pillars affecting the energy flow pattern by ensuring larger-scale eddies. A composite solar irradiance model, based upon these proxies, is compared to the northe rn hemisphere temperature departures for 1700-1992. Approximately 71% of the decadal variance in the last century can be modeled with these solar indices, although this analysis does not include anthropogenic o r other variations which would affect the results. Over the entire thr ee centuries, approximately 50% of the variance is modeled. Both this analysis and previous similar analyses have correlations of model sola r irradiances and measured Earth surface temperatures that are signifi cant at better than the 95% confidence level. To understand our presen t climate variations, we must place the anthropogenic variations in th e context of natural variability from solar, volcanic, oceanic, and ot her sources.