In a little noticed paper, Charles Perry and renown geologist, Kenneth Hsu study the proxies for solar output and created a solar output model based upon that information. They use solar cycle harmonics to create their model, which matches the proxy data quite well.
Although the processes of climate change are not completely understood, an important causal candidate is variation in total solar output. Reported cycles in various climate-proxy data show a tendency to emulate a fundamental harmonic sequence of a basic solar-cycle length (11 years) multiplied by 2N (where N equals a positive or negative integer). A simple additive model for total solar-output variations was developed by superimposing a progression of fundamental harmonic cycles with slightly increasing amplitudes. The timeline of the model was calibrated to the Pleistocene/Holocene boundary at 9,000 years before present. The calibrated model was compared with geophysical, archaeological, and historical evidence of warm or cold climates during the Holocene. The evidence of periods of several centuries of cooler climates worldwide called “little ice ages,” similar to the period anno Domini (A.D.) 1280–1860 and reoccurring approximately every 1,300 years, corresponds well with fluctuations in modeled solar output. A more detailed examination of the climate sensitive history of the last 1,000 years further supports the model. Extrapolation of the model into the future suggests a gradual cooling during the next few centuries with intermittent minor warmups and a return to near little-ice-age conditions within the next 500 years. This cool period then may be followed approximately 1,500 years from now by a return to altithermal conditions similar to the previous Holocene Maximum. source
They note that the warming seen this century doesn't match that which occurred in the Medieval Optimum. In other words, it was warmer a thousand years ago than it is now.
"However, geophysical, archaeological, and historical evidence is consistent with
warming and cooling periods during the Holocene as indicated by the solar-output model. The current warm period is thought to have not reached the level of warmth of the previous warm period (A.D. 800-1200), when the Vikings raised wheat and livestock in Greenland. Therefore, the magnitude of the modern temperature increase being caused solely by an increase in CO2 concentrations appears questionable. The contribution of solar output variations to climate change may be significant.” Charles A. Perry and Kenneth J. Hsu, “Geophysical, archaeological, and historical evidence support a solar-output model for climate change, PNAS, 97(2001):23, p. 12436
An interesting comparison of their model with the C14 production, which is related to the strength of the sun's output, shows that their model fits the known variations.
Their comment that the observed warming can't be due to CO2 alone is spot on. CO2 hysteriacs rarely seem to look at the source of all our heat--the sun.