Long-Term Trends
A theoretical modeling study by Roble and Dickinson [1989] suggested a major greenhouse cooling in the upper atmosphere in response to increases in CO2 and CH4. Such a cooling effect is associated with a global reduction in neutral densities including O and N2 and total neutral mass density as the neutral temperature Tn decreases. If greenhouse gas concentrations double, as is predicted to happen by the end of the 21st century, the authors predict a decrease in the thermospheric Temperature of up to 50K, and a decrease in thermospheric densities at a fixed heightofs 40-50%.
Preliminary analysis of long-term ionospheric ion temperature changes from a database of incoherent scatter radar observations spanning 2 solar cycles during 1979-2001 at Millstone Hill indicates a decreasing trend in altitudes from 300-500 km. Such a decrease is associated with a decrease in the exospheric temperature, and appears to be partly due to the solar cycle variability. However, after the solar cycle effect is removed, the decrease tends to present as a long-term trend suggestive of possible anthropogenic greenhouse gas effect.
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