More importantly, there exist a number of feedback mechanisms, some of which are understood and some of which are not, which tend to regulate the Earth's climate pretty well even in the presence of severe perturbing factors. Some people claim that the feedback mechanisms will hit a runaway condition if CO2 gets much higher, but there is no particular basis for that claim. To the contrary, if such a runaway condition could exist under anything remotely close to current conditions, it would likely have already occurred (e.g. during the Medieval Warm Period). The fact that it has not is a pretty strong indication that it won't.
That's what I was referring to when I said it was not known quantitatively. The 'bare' effect, without feedback, can be calculated reasonably well (it's about 1.2C) but even the sign of the feedback is not known. There is some evidence that it is positive in the polar regions, where the air is dry, and negative in the tropical regions where cloud formation can offset the bare effect -- but no-one knows.