Also, when a process has a maximum (in the case of CO2, total opacity in its spectral window), then going beyond the maximum has no further effect.
As for CO2, I also don't know why suddenly all attention is focussed on that. It seems to be just one part of the problem/puzzle.
We're told that if 80 % of scientists agree on something we cannot just dismiss it as nonsense. I would agree with that if even just a few of them had credible methods to simulate complex systems. They don't. Each of them models just a tiny piece of the system and their methods work only within that very narrow context. So what science did the IPCC use to join all those partial models?
What method would you propose for dealing with climate changes? What about changes in average temperature over the centures. Since you don't believe in any models, would you just ignore that information?
Is a very complex simulation even necessary? You don't need to simulate every atom in water to determine it's density in relation to it's temperature, for example.
Surely even a back-of-the-envelope calculation as to how difficult it would be to do an accurate mathematical model of climate, and the computing power needed to run such a simulation, would show that any prediction based on a computer model at this point is essentially bogus; much less being able to determine whether or not it is caused by man or by natural causes.
When you add in the fact that an accurate model of "how oceans work" is decades away (we don't even understand the Gulf Stream at this point) you have to wonder, exactly how can you mathematically model something that you don't understand?