During Ben Santer's Question and Answer session Anthony Watts asked the following question.
As it happens over at SkepticForum.com there was a detailed comment that seems to me speaks to Anthony's question but more importantly addresses many of the crazy-making claims/charges surrounding this important topic.
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1:02:05 - Anthony Watts: Would you describe the Radiative Return Effect of carbon dioxide in the Earth’s atmosphere as being linear or logarithmic?