Effect of trends of middle atmosphere gases on the mesosphere and thermosphere

We conducted model simulations to examine how changes in concentration of radiatively active trace gases in the middle atmosphere affect long-term changes in the upper atmosphere. We focused our model study on the impact of increases in carbon dioxide (CO₂), methane (CH₄), and water vapor (H₂O), and decreases in ozone (O₃) between 1983 and 2003. We used both the National Center for Atmospheric Research Whole Atmosphere Community Climate Model and the Thermosphere-Ionosphere-Mesosphere- Electrodynamics General Circulation Model, global mean version, in this study. The model simulations indicate that CO₂ is the main forcing mechanism of long-term changes in the thermsophere, with minor influences from O ₃, CH₄, and H₂O. At 400 km altitude, global mean thermospheric neutral density decreased by ∼4.5% due to CO₂ forcing alone, whereas it decreased by ∼4.8% due to the combined forcing from all four gases. O₃ depletion caused cooling in the stratosphere and mesosphere (maximum decrease of 0.5 K) due to reduced absorption of solar ultraviolet radiation, but had nearly no cooling effect in the thermosphere. However, due to thermal contraction in the stratosphere and mesosphere, O ₃ depletion caused a small decrease in thermospheric neutral density of ∼0.25%. Increases in both CH₄ and H₂O may slightly warm the upper mesosphere and thermosphere due to increased chemical heating and absorption of solar ultraviolet radiation.