In the upper atmosphere, greenhouse gases produce a cooling effect, instead of a warming effect. Increases in greenhouse gas concentrations induce changes in the mesosphere, thermosphere, and ionosphere. We constructed for the first time a pattern of the observed long-term global change in the upper atmosphere, based on trend studies of various parameters, which shows general cooling and thermal contraction of the upper atmosphere and change sin the ionosphere due to chemical changes in minor atmospheric constituents as a consequence of cooling. The picture we obtained is qualitative, and contains several gaps and a few discrepancies, but the overall pattern of observed long-term changes throughout the upper atmosphere is consistent with model predictions of the effect of greenhouse gas increases.
The observed effects are predominantly caused by the increasing concentration of greenhouse gases, but in the mesosphere, lower thermosphere and lower ionosphere some role is played also by ozone depletion and its development, and in the F2 region ionosphere still the geomagnetic activity long-term changes may play dominant role. Our results indicate that anthropogenic emissions of greenhouse gases are affecting the atmosphere at nearly all altitudes between ground and space.
Results of various authors on trends in foF2 and hmF2 are controversial. We tested various methods at the same high-quality data set and found that the results reasonably agree each other except for one erroneous wavelet-based method and a special method by Mikhailov, which differs principally in the way of removing the solar and geomagnetic activity impact on trends. This means that part of controversies are in the use of different periods (trends are not stable on long-term scale and different periods make also problems in removing solar cycle effects), part is in data quality and homogeneity, and part of the differences might be real regional differences.
We also showed that the impact of long-term changes of solar and geomagnetic activity on trends in the atmosphere-ionosphere system weakens from above (F-region ionosphere) down to the troposphere, and weakens from the beginning to the end of the 20th century.