Brian, most of the text of his posts are cut-and-paste jobs from online journals. This might be why he's doing a fairly poor job of answering your questions.

Climate scientists do not "know" CO2 is the bad guy. They DO know that CO2 can force warming slightly in a controled setting. They DO know that measured temperatures have increased on average over the past 100 years.

They do NOT know, however, how this slight increase in atmospheric CO2 increase resonates in our extremely chaotic climate system. In the models that project runaway warming, they ASSUME a strong net positive feedback mechanism that accelerates/amplifies the initial CO2 input. It is these assumptions about feedback that lead to the catastrophic scenarios.

You can prove that CO2 can cause an initial preturbance, but the response of the climate system to that input CANNOT be proven empirically.

The reality is that when scientists encounter an unknown, but longterm stable system, they assume the system is regulated by NEGATIVE FEEDBACK! Which makes sense of course, since systems dominated by positive feedbacks are not stable over time. Prior irregular inputs to the climate system have not turned the Earth into a Venus-like inferno, or a Mars-like ice rock. It seems like a strong positive feedback mechanism is a pretty dim assumption - but don't tell that to AGW alarmists!