Correct
They have not proven that CO2 causes the surface to be warmer than it otherwise would be. They use the -18C argument to say that it has to. Then they assume it does. Then they base everything else on this assumption. I traced it back from the latest IPCC here - How We Know the Effect of CO2 on Global Temperature
They just refer to past IPCC reports and papers that are just calculations based on earlier calculation. There is no evidence provided anywhere in the last 4 decades of reports.