Don't we also know that an increase in CO2 causes a (slight?) increase in temperature?
The way I see it, the climate has to be hotter with the extra GHGs we've pumped in, than it would have been otherwise. Where else is all of the extra energy absorbed by the GHGs going? The question is over how much hotter, and the complex feedbacks due to water vapour etc. But to my mind the negative feedbacks can only limit the amount of extra heating rather than reduce it to zero.
So in effect, we know that the extra GHGs are heating the atmosphere by some amount, we're just not sure whether this is going to lead to runaway positive feedback, or just settle at some (slightly?) higher level?
