That's what I was getting at as well.
As far as I know, the negative forcing of the aerosols was introduced into GCMs to replicate the cooling observed from the 1940's-1970's. I believe it was largely focused on sulphate aerosols, which was surmised to cause a cooling effect, by aiding in cloud formation, which increases the reflectivity of the globe. Now, it's been shown that aerosols (particulate) will also heat the atmosphere. Who woulda thunk it - dark coloured dust absorbing longwave radiation (and likely shortwave as well).
This is just another example of modellers "cranking the knob" in GCMs, without understanding the underlying processes.
Any other type of simulation model is calibrated to a specific time frame, then validated to another time frame - for the specific reason of testing whether the fundamental understanding is correct. It seems this isn't the case for GCMs.
