Such statements are just about as poorly based as the IPCC’s pronouncements.
We just don’t know what the effects of 500 ppm and 1000 ppm CO2 will be. There are undoubtedly feedback systems, but we don’t know what all of them are or how they might interact.
We do understand the physics of sunlight interacting with CO2 though. Only 2 wavelenghts of light interact with CO2. When that light’s energy has been absorbed by the CO2 what will cause the additional heating?