The changes arising from variations in solar activity and human activities on atmospheric composition and thermal structure of the middle atmosphere, especially in the ion composition, are assessed using interactive one- and two-dimensional chemical models of the stratosphere, mesosphere, and lower thermosphere. The changes from anthropogenic activities are less in magnitude but permanent in nature, and changes due to variations in the solar activity are comparatively higher but periodic. Our results may have bearing and could explain some of the results reported in recent literature on observed lower ionospheric data and strengthen the global change concept in this region. Model response to available observations is quite reasonable, and a good agreement on temperature trend and ionization parameters is found. The impact of increasing concentrations of several greenhouse gases (anthropogenic origin) and variations in solar activity on the ion composition, temperature, and neutral species (of interest to ionization) is examined in two different modeling studies. In each case, the effect of the other forcing is filtered out. A cooling of the order of -10 ¿K in the mesosphere and a maximum of -14 ¿K in the stratosphere is predicted for the double-CO2 scenario. A decreasing trend in nitric oxide number density from -40% to -95% with altitude is noticed for the double-CO2 case contrary to an increasing trend (10% to 140%) for high solar activity (HSA) as compared to low solar activity (LSA) conditions in the mesosphere and the lower thermosphere. As a result of changes in temperature and in the concentrations of NO, O2, H2O O, CH3CN, a decreasing trend of -50% for the double-CO2 case and an increasing trend of 60% for HSA as compared to LSA are predicted in NO+ ion number density near the mesopause for the topics. The variations in water cluster ions are negligible in the stratosphere but reach to -50% in the lower thermosphere for the double-CO2 scenario. The total ion density varies from -30% at 40 km to +30% at 90 km for HSA as compared to LSA conditions and from -5% to 12% from the stratosphere to the mesosphere for the double-CO2 scenario. ¿ 2000 American Geophysical Union