~sighs~
CO2 absorbs more IFR at different bands than does water alone. Water vapor is not neglected in General Circulation Models, and neither are negative feedbacks from various derivatives from S04's (sulfates) which create temporary cooling effects due to dimming. CO2 has a far longer atmospheric life than various aerosols... CH4 is 25 times more efficient than C02 in absorbing radiation and trapping in heat. CH4 is not neglected in GCM's either.
In addition to GCM's there are numerous satellite data and observations of CH4 emitting in greater quantities in the Artic empirical in nature.
There is also the issue of sea level rise due to thermal expansion and other stearic considerations.