Carbon dioxide warms the lower atmosphere by trapping heat in a well-known process called the greenhouse effect. However, scientists believe that carbon dioxide cools the upper atmosphere — the region about 50-800 km above Earth’s surface. Here the density of carbon dioxide is too low to maintain greenhouse warming and instead the gas absorbs heat from its surroundings and radiates much of it away from Earth with a net cooling effect.

Most atmospheric physicists believe that increasing carbon dioxide levels in the upper atmosphere will boost this radiative effect and cause the temperature and density of that region to decrease. However, obtaining global historical data on the temperature, density and size of the upper atmosphere can be a tricky business, which has made these effects difficult to confirm.

Now, Jan Laštovička of the Institute of Atmospheric Physicists in the Czech Republic and colleagues in the US, India and Germany believe that these effects have been observed in a sufficient number of independent studies to paint a consistent picture of the manmade changes occurring in the upper atmosphere.

According to the researchers, comprehensive studies of the mesosphere – the portion of the upper atmosphere at 50-90 km altitude – reveal that most of this region is cooling at a rate of about 3 Celsius degrees per decade. This cooling is in agreement with models based on increasing carbon dioxide levels. Further up in the thermosphere (90-800 km) there have been no direct measurements of temperature. However, a dramatic drop in the “ion temperature” of 17 degrees per decade has been observed at heights of about 350 km. The ion temperature is a measure of the thermal motion of ions and is related directly to temperature. This drop is also consistent with rising carbon dioxide levels.

Careful studies of the orbital trajectories of satellites in the thermosphere reveal that the satellites are experiencing less atmospheric drag than before. This implies that the density of this part of the upper atmosphere has been dropping at 2-3% per decade, which is also in line with theoretical predictions based on rising carbon dioxide levels. Laštovička and colleagues also point out that the contraction of the upper atmosphere has been indirectly observed in terms of the downward movement of the ionized layers of the upper atmosphere – referred to as the ionosphere.

Laštovička told Physics Web that the thinning of the thermosphere could be good news for operators of some low-Earth-orbit (LEO) satellites such as the International Space Station, which orbits about 350 km above Earth. LEO satellites are slowly falling back to Earth because of atmospheric drag, which is proportional to atmospheric density. The Space Station, for example, must fire a booster rocket every so often to maintain its orbit – something that it may have to do less often as the thermosphere continues to thin.