The historical evidence we have simply does not support a high sensitivity to CO2. The IPCC quotes climate sensitivity numbers (degrees C per doubling of CO2 concentrations) in the 3-5 range. These kind of numbers imply that, given the amount of CO2 we have produced in the past, we should already have seen 1.5-2.5C of warming. In reality, we have seen about 0.7C, and likely not all of that is due to CO2.
The debate has become so wrapped in politics that it is impossible to have an intelligent discussion with some of the proponents.