Modeling the cooling due to CO2 increases in the mesosphere and lower thermosphere

Sensitivity of the upper-atmospheric structure to the increase of carbon dioxide amounts over the last four decades and to the standard 2 x CO2 scenario has been investigated with the Spectral Mesosphere/Lower Thermosphere Model. The global thermal response is negative and its magnitude is much stronger than in the lower atmosphere. There is a pronounced seasonal- latitudinal structure, especially at solstice, that perhaps may explain some differences in long-term temperature trends derived from observations at different locations. The strong overall cooling results in hydrostatic contraction of the atmosphere and in considerable downward displacement of constant-pressure levels. Attention is again drawn to the fact that upper- atmospheric temperature changes differ substantially depending on whether the temperatures are compared at a given altitude as in many observations or at a constant pressure commonly used as the vertical coordinate in numerical models. Lower temperatures correspond to greater densities at fixed pressure. Combined with the hydrostatic contraction, however, the cooling results in significant mass density reductions as a function of height in the thermosphere similar to those recently derived from historic satellite drag data.