What was done
Noting that "the most direct mechanism for climate change would be a decrease or increase in the total amount of radiant energy reaching the earth," the authors developed a simple solar-luminosity model and used it to estimate total solar-output variations over the past 40,000 years, as well as 10,000 years into the future.  The model was derived by summing the amplitude of solar radiation variance for fundamental harmonics of the eleven-year sunspot cycle throughout an entire 90,000-year glacial cycle.  The results of this exercise were compared with geophysical, archaeological and historical evidence of climate variation during the Holocene.

What was learned
The model output was well correlated with the amount of carbon 14 (which is produced in the atmosphere by cosmic rays that are less abundant when the sun is active and more abundant when it is less active) in well-dated tree rings gong back to the time of the Medieval Warm Period (about A.D. 1100), which finding, in the words of the authors, "supports the hypothesis that the sun is varying its energy production in a manner that is consistent with the superposition of harmonic cycles of solar activity."  The model output was also well correlated with the sea-level curve developed by Ters (1987).  Present in both of these records over the entire expanse of the Holocene was a "little ice age"/"little warm period" cycle with a period of approximately 1,300 years.  In addition, the climate changes implied by these records correlated well with major historical events.  Specifically, the authors note that "great civilizations appear to have prospered when the solar-output model shows an increase in the sun's output," while they state that such civilizations "appear to have declined when the modeled solar output declined."

What it means
"Current global warming commonly is attributed to increased CO2 concentrations in the atmosphere," the authors note.  "However," they continue, "geophysical, archaeological, and historical evidence is consistent with warming and cooling periods during the Holocene as indicated by the solar-output model."  They therefore conclude that the idea of "the modern temperature increase being caused solely by an increase in CO2 concentrations appears questionable."  Their findings also clearly suggest that as far as humankind is concerned, warmer is better.

Reference
Ters, M.  1987.  Variations in Holocene sea level on the French Atlantic coast and their climatic significance.  In: Rampino, M.R., Sanders, J.E., Newman, W.S. and Konigsson, L.K. (Eds.) Climate: History, Periodicity, and Predictability.  Van Nostrand Reinhold, New York, NY, pp. 204-236.