Slide 2 in this series.
The main assumption is that CO2 can control the increase in temperature. Not the sun or the ocean or other natural causes that might affect cloud cover for instance.
Also the various cases being considered like the RCP amount of CO2 increase for instance and how much coal and population is contributing to these cases, makes little sense given the amount of ACO2 that is in the atmosphere.
Even the assumption that relative humidity is constant is not born out in reality. This has a great impact on any computer result, as we even saw in the CO2 Science section.
NEXT: If CO2 dominates then how does the energy balance be achieved?
The topics cover a wide range of material on the GCM climate models.
Alternative Models Last Century