Abstract
Abstract. In the upper atmosphere, greenhouse gases produce a cooling effect, instead of a warming effect. Increases in greenhouse gas concentrations are expected to induce substantial changes in the mesosphere, thermosphere, and ionosphere, including a thermal contraction of these layers. In this article we construct for the first time a pattern of the observed long-term global change in the upper atmosphere, based on trend studies of various parameters. The picture we obtain 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. Together with the large body of lower atmospheric trend research, our synthesis indicates that anthropogenic emissions of greenhouse gases are affecting the atmosphere at nearly all altitudes between ground and space.
Highlights
Life on Earth is more directly affected by climate change near the surface than in the middle and upper atmosphere, but as the story of the Earth’s ozone layer illustrates, changes at higher levels of the atmosphere may be important for life on Earth, as well
The trends described in previous Sections allow us to construct a scenario of global change in the upper atmosphere at heights above 50 km, i.e. in the mesosphere, thermosphere and ionosphere
The known potential drivers of upper atmospheric long-term global change include increasing concentrations of greenhouse gases (CO2, CH4 and others), anthropogenic changes of the ozone layer and to some extent of water vapour, and natural long-term variations of geomagnetic activity, which increased throughout the 20th century; the role of long-term change of general solar activity is unimportant in the trends observed in the second half of the 20th century
Summary
Life on Earth is more directly affected by climate change near the surface than in the middle and upper atmosphere, but as the story of the Earth’s ozone layer illustrates, changes. We identify gaps in our knowledge and understanding of long-term trends in various parameters, as well as possible inconsistencies among trends or their substantial changes with time, and we investigate whether the observed trends are consistent with the theoretically predicted effects of increasing greenhouse gas concentrations. The height cannot be directly scaled from ionograms; it is computed from the critical frequencies of the E and F2 layer as well as the propagation parameter M(3000)F2 using empirical formulae These formulae were derived in the 1970s based on various regional data sets and to be simple and straightforward enough to be used by the computers of the time, but these formulae are by no means globally valid, and their use results in different trends. At present the geomagnetic activity may play an important or even dominant role only in foF2 and hmF2 trends
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