Solar forcing of climate change in recent millennia

Solar radiation varies continuously; so too does Earth's global environment. Numerous empirical relations exist among assorted solar and climate parameters. Reliable specification of the true extent of Sun-climate connections requires continuous solar monitoring with sufficiently high stability to specify long-term solar irradiance variability and to characterize spectral irradiance variability at all wavelengths. A suite of cavity-type electrical substitution radiometers on various spacecraft have tracked day-to-day and solar cycle fluctuations in the Sun's total irradiance during the past two decades. Space-based spectroradiometers have also measured concomitant changes in the shorter wavelength ultraviolet spectrum. Primary irradiance variability mechanisms - dark sunspots and bright faculae - have been identified, and models developed that replicate much of the observed variance in the extant irradiance databases. When extended to centennial time scales, the models predict irradiance variability in the range 0.2 to 0.5%. General circulation model simulations are beginning to explore the physical pathways through which the climate system responds to changes in total solar irradiance. These models suggest that surface warming of order 0.5degreesC may be associated with increasing solar activity since the 17(th) Century Maunder minimum. Improved knowledge of Sun-climate connections requires future simulations that include an adequate representation of the spectral nature of the Sun's radiative output changes, and of the indirect pathways that solar UV irradiance variability may influence climate via its control of ozone concentrations and the stratosphere. Since the increase in solar activity from anomalously low Maunder Minimum levels to presently high Modem Maximum levels is the most recent of similar episodes apparent throughout the Holocene, improved understanding of both solar forcing and climate response in the past 400 years, will likely contribute new understanding of climate change in past millennia.