As the climate warms near Earth's surface, the upper atmosphere is getting less dense, a change that will mean less drag on satellites, scientists announced today.
Carbon dioxide emissions from the burning of fossil fuels will cause a 3 percent reduction in air density in the outermost layer of the atmosphere by 2017, the researchers predict.
"We're seeing climate change manifest itself in the upper as well as lower atmosphere," said Stan Solomon of the National Center for Atmospheric Research (NCAR). "This shows the far-ranging impacts of greenhouse gas emissions."
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The finding was presented today at a meeting of the American Geophysical Union in San Francisco.
The thermosphere extends from about 60 miles above Earth to 400 miles. The air is incredibly thin, but still causes drag on satellites in low Earth orbit. NASA routinely boosts the orbit of the space station as it is constantly degrading. Other satellites have limited life spans in part because the thin air up there eventually drags them down.
A thinning thermosphere means satellites can stay aloft longer.
Carbon dioxide molecules absorb radiation. Near Earth's surface, the molecules collide frequently with other molecules and the energy is released as heat, warming the air, the scientists explained. In the much thinner thermosphere, a carbon dioxide molecule has ample time to radiate energy to space because collisions are infrequent. The result is a cooling effect. And as it cools, the thermosphere settles, so that the density at a given height is reduced.
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The effect varies with changes in the 11-year cycle of the Sun's activity, too. Being able to now predict the changes will help satellite operators plan better, the researchers said.
"Satellite operators noticed the solar cycle changes in density at the very beginning of the Space Age," Solomon said. "We are now able to reproduce the changes using the NCAR models and extend them into the next solar cycle."
The findings are also detailed in the journal Geophysical Research Letters and explained in a video presentation.