Progress in observations and simulations of global change in the upper atmosphere

The steady increase of CO₂ is the dominant cause of upper atmosphere trends; other drivers are long-term changes of radiatively active trace gases such as CH₄, O₃, and H₂O, secular change of solar and geomagnetic activity, and evolution of the Earth's magnetic field. Observational and model studies have confirmed that in the past several decades, global cooling has occurred in the mesosphere and thermosphere; the cooling and contraction of the upper atmosphere has lowered the ionosphere and increased electron density in the E and F₁ regions.

Trends of other parameters, including the F₂ region, mesospheric clouds, and mesopause wave activity, have been more controversial. Modeling investigations have demonstrated that both greenhouse gas forcing and secular change of the Earth's magnetic field can cause regional, diurnal, and seasonal variability of trends in F₂ region density and height, which may contribute to discrepancies regarding ionospheric trends.