THE IMPORTANCE OF DYNAMICAL FEEDBACKS ON DOUBLED CO2-INDUCED CHANGES IN THE THERMAL STRUCTURE OF THE MESOSPHERE

The Garcia-Solomon two-dimensional model was used to study the effect of doubled carbon-dioxide on the middle atmosphere. The model. has been improved to include non-LTE CO2 cooling in the 15 micron band above 70 km and new chemical heating and heating efficiencies. The effect of doubling CO2 on the temperature is found to be large at the stratopause (about 10-12K cooling) and at the mesopause (about 6-12K cooling). In the stratosphere, dynamical feedbacks on the heating rate caused by the temperature changes are small compared to the radiative changes while in the mesosphere they can be large. In fact, calculations with the present dynamical heating rates used in the doubled CO2 energy equation indicate that the radiative forcing alone could cause a temperature increase of about 10K in the polar summer mesopause region. The dynamical feedbacks which oppose this positive radiative forcing are discussed.