As a matter of certain fact, yes, they do consider that as a possibility. Cloud feedback effects are considered the top uncertainty in all GCMs. It is possible that cloud feedback effects could cancel out the effects of increasing CO2 in the atmosphere. It will be VERY interesting to see what the next IPCC report says about clouds.
As if there is any doubt. The IPCC will conclude that clouds will accelerate global warming. It is not a scinece, it is an agenda.