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Understanding the physical causes underlying solar spectral irradiance variations and the impact of these variations on terrestrial climate remain critical compelling challenges. On the solar side, the most fundamental unknown is the role of the “quiet-sun7 ,” a question brought into focus by spectral irradiance observations during the recent deep minimum, which show opposing trends in the infrared and ultraviolet portions of the spectrum (Harder et al. 2009). On the terrestrial side, while it is clear that the Sun played only a small role in late twentieth century warming, the signature of solar cycle variations in climate are now quite well established (Gray et al. 2010). How this coupling occurs is not fully understood, suggesting something incomplete in our understanding of the climate system.

We suggest that in the coming decade, through the focused efforts outlined below, we can successfully address both of these scientific challenges and emerge with a new more complete understanding of the Sun and its influence on Earth. The essential ingredients in this effort are poised at the edge of our abilities, challenging but not risky, and take full advantage of current technological capabilities.