Recent general circulation model (GCM) study suggests that CO2 maintains the present regional climate differently than trace gases CH4, N2O, CFC-11 and CFC-12, thus raising the question of the adequacy of using ''effective'' CO2 to assess future climate changes due to the greenhouse effect of these other radiatively active gases. Our GCM simulations indicate that, although the effective CO2 can provide global mean surface temperature responses in good agreement with that when these other trace gases are explicitly considered, it calculates smaller surface warming in several continental regions, notably in northwestern North America by 20--30% during winter and 10--15% during summer. ¿ American Geophysical Union 1992