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Here Comes the Sun ... Again!
Reference
Shapiro, A.I., Schmutz, W., Rozanov, E., Schoell, M., Haberreiter, M., Shapiro, A.V. and Nyeki, S. 2011. A new approach to the long-term reconstruction of the solar irradiance leads to large historical solar forcing. Astronomy & Asrophysics 529: 10.1051/0004-6361/201016173.

Background
The authors write that "the variable Sun is the most likely candidate for the natural forcing of past climate changes on time scales of 50 to 1000 years," since "the terrestrial climate correlates positively with solar activity." But while they note that there is "general agreement on how solar forcing varied during the last several hundred years," they say "there is scientific controversy on the magnitude of solar forcing."

What was done
Assuming that "the minimum state of the quiet Sun in time corresponds to the observed quietest area on the present Sun," Shapiro et al. used "available long-term proxies of the solar activity, which are 10Be isotope concentrations in ice cores and 22-year smoothed neutron monitor data, to interpolate between the present quiet Sun and the minimum state of the quiet Sun," which procedure "determines the long-term trend in the solar variability, which is then superposed with the 11-year activity cycle calculated from the sunspot number," after which "the time-dependent solar spectral irradiance from about 7000 BC to the present is then derived using a state-of-the art radiation code."

What was learned
The seven Swiss scientists derived "a total and spectral solar irradiance that was substantially lower during the Maunder minimum than the one observed today," which difference between then and now they describe as being "remarkably larger than other estimations published in the recent literature." In addition, they say that "the magnitude of the solar UV variability, which indirectly affects the climate, is also found to exceed previous estimates."

What it means
The new findings provide an opportunity to resolve difficulties associated with attempts to link past changes in climate with solar variability. Or as Shapiro et al. state, their analysis "allows the climate community to evaluate the full range of the present uncertainty in solar forcing," adding that for such studies "the full dataset of the solar spectral irradiance back to 7000 BC is available upon request."

Reviewed 7 September 2011