Dr David Evans writing in the Age: (LINK)
The warmists are correct that CO2 is a greenhouse gas and it causes warming, that CO2 levels have been rising, and that it has been warming.
Serious sceptics agree with all that, but point out that it does not prove that something else isn't causing most of the warming. By way of illustration, if the main cause of warming was actually Venusians with ray guns, then all those things would still be true.
We scientists can calculate how much warming results directly from an increase in CO2 levels.
We know how much CO2 levels and temperature have risen since pre-industrial times, but the warming directly due to CO2 is only a third of the observed warming. The theory assumes no other major influence on temperature changed, so the effect of the CO2 must have been amplified threefold, presumably by changes in the atmosphere due to humidity and clouds.
There is no observational evidence for this amplification, but it is nonetheless built into all the models. Sceptics point out that if the extra humidity simply forms extra clouds, then there would be no amplification.