Comment by woodchuck64

> What we don't understand (unless we decide GCMs are predictive and can be validated) is the impact of a given thermal forcing on planetary temperature. This is the "climate sensitivity." Unfortunately, this is the number we actually care about.

This is false, we do understand this well enough to be sure CO2 is a significant problem for warming, see http://www.realclimate.org/index.php/archives/2007/08/the-co...