That is not an accurate assessment of AGW.  The current thinking basically balances an increase in particulate emissions which over the short term reduce global temperature average with greenhouse gasses which increase global temperature (plus a host of other factors).  The fact that CO2 is extremely stable means than it accumulates in the atmosphere so its effects accumulate over time.  Several other greenhouse gasses are modeled including water and methane, but their long term impact is reduced with time.<p>Now the long term stability of CO2 vs. methane is based on simple chemistry, and water rains. As to why CO2 would increase the greenhouse effect see: <a href="http://en.wikipedia.org/wiki/Absorption_spectrum" rel="nofollow">http://en.wikipedia.org/wiki/Absorption_spectrum</a> and its overall concentration in the atmosphere.<p>PS: Looking back in history the accumulation effect of CO2 works in reverse so other man made impacts could have easily dominated it.