It is precisely because of the complexity of the climate that one cannot just plot CO_2 concentraions against temperature. One has to make detailed models and validate those. Now, I've read that climate models do an excellent job in describing past climate. Of course, one can object that one can always adjust parameters in such models to fit observations. But these models are based on physics and are thus well motivated.
I'm glad to hear you say you just can't look at CO2/temp correlations. But this is exactly what is held up to the public as confirmation of GW.
But onto models. Models are only as good as the basic understanding of the system they are replicating, and the response functions that the modeller inputs into the model. If we don't have the complete picture, or have the wrong response functions, well the model output will be wrong.
GCMs are not physically based models. For a physically based model, the modeller must understand all components of the system. I hope everybody here realizes that is not the case whatsoever. GCMs are simplifications, extreme simplifications, of our climate. Heck 9 forcing components are used within GCMs. Do we really think our climate is determined by the interplay of only 9 components?
You will need a lot more than a model, whose algorithms specify that increased CO2 causes temperature increases (wonder why they predict increased temperature??), to convince me that CO2 is the
primary driver for the current warming.
To sum up, models tell us that CO2 is causing the temperature increase, because we told the model that in the first place!!!!
