Climate change makes upper atmosphere thinner

Carbon dioxide leads to cooling and thickening of the thermosphere

Outer layer of earth's atmosphere becomes thinner. © UCAR
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The rising carbon dioxide concentrations not only heat the earth's climate, they also thin the atmosphere: a new study, now published in the "Geophysical Research Letters", revealed that by 2017, the density of the outer atmosphere layer will increase by around three percent will decrease. This also has consequences for the satellites in the low orbits.

"We see climate change manifesting in both the lower and upper atmosphere, " said Stan Solomon of the US National Center for Atmospheric Research (NCAR). That shows the far-reaching impact of greenhouse gas emissions. "Together with Liying Qian and Ray Roble, as well as colleagues from Pennsylvania State University, researchers explored how the upper atmosphere will change over the next few decades.

More greenhouse gas, cooler thermosphere

The thermosphere, the atmospheric layer that starts about 95 kilometers above the surface and reaches almost 650 kilometers, is gradually getting thinner. This has been shown by recent observations by scientists monitoring the satellite orbits. Such a development was predicted in 1989 by the researchers Roble and Robert Dickinson. They assumed that increasing CO2 levels would cool the thermosphere and reduce its density.

This apparent paradox - the greenhouse gas CO2 as a coolant - occurs because the atmosphere gets thinner and thinner as the altitude increases. In the dense air near the ground, the carbon dioxide molecules first absorb the heat radiated by the earth, but then, released by collisions with other gas particles, they quickly release it and the surrounding air. In the much thinner thermosphere, on the other hand, this process is reversed: in collisions with other particles, the CO2 does not release heat, but absorbs it, thereby depriving other molecules of the stored energy. Then it radiates the "stolen" heat into space. The result: it gets cooler. As this causes the thermosphere to sink somewhat downwards, so does the density at a given height.

Influence by solar activity

For the first time, researchers have now analyzed this development and investigated whether this trend will continue in the coming decades. The scientists paid particular attention to the eleven-year cycle of solar activity. For he too, that much was already known, also influences the thermosphere. During the time of high solar activity, the radiation of UV light and high-energy particles into the atmosphere increases. This warms the thermosphere and expands. Conversely, it cools down when the sun's activity decreases. display

Cooling trend stops

To find out how greenhouse gases and the solar cycle interact, scientists developed a computer model of the upper atmosphere that incorporates both factors. The forecasts showed that despite a relatively strong solar cycle, the density of the thermosphere will continue to fall within the next decade. During a solar minimum, this decrease accelerates even three to four times compared to the solar maximum.

Forecasts are crucial for the space industry. Because many satellites, but also the International Space Station ISS and the Hubble Space Telescope, follow an orbit in low Earth orbit, in heights of up to 380 kilometers. In order to keep them in their orbit course corrections have to be made every now and then, otherwise they lose their speed due to the frictional forces of the upper atmosphere and thus also to height. If the density of the thermosphere decreases, so does the friction and the course corrections have to be adjusted accordingly.

(National Center for Atmospheric Research (NCAR), 13.12.2006 - NPO)