A global average model of the coupled mesosphere, thermosphere and ionosphere is used to examine the effect of tract gas variations on the overall structure of these regions. In particular, the variations caused by CO2 and CH4 doublinds and halvings from present day mixing ratios are presented. The results indicate that the mesosphere and thermosphere temperatures will cool by about 10 K and 50 K respectively as the CO2 and CH4 mixing ratios are doubled. These regions are heated by similar amounts when the trace gas mixing ratios are halved. Compositional redistributions also occur in association with changes in the temperature profile. The results show that global change will occur in the upper atmosphere and ionosphere as well as in the lower atmosphere during the 21st century. ¿ American Geophysical Union 1989