*Note added 22 April 2013 Here are the simple calculations, widely used, deployed by distinguished physicist Will Happer.
Extract, referring to delta-T-2, the expected temperature rise from a doubling of CO2 levels:
' In fact, the basic physics of the CO2 molecule makes it hard to justify a number much larger than ∆T2 = 1 C – with no feedbacks. The number 3 C comes from various positive feedback mechanisms from water vapor and clouds that were invented to make the eﬀects of more CO2 look more frightening. But observations suggest that the feedbacks are small and may even be negative.'
Further illustration is given here.