The argument over CO2 is what its effect is on temperature (Equilibrium Climate Sensitivity). In 1979, a off-hand guess of 3C +- 1.5C per doubling of CO2 concentration in the atmosphere was made. Despite significant research on this topic (which might very well be the single most important equation in all of climate science), the most recent IPCC report in 2014 is still basically in that range. There's a significant difference in the impact to civilization of 1.5C increase per doubling versus 4.5C.
https://en.wikipedia.org/wiki/Climate_sensitivity