Inside the highest layer of the atmosphere is a region called the ionosphere, where charged particles help reflect radio waves back to Earth.
(Related photos: auroras, lights in the ionosphere.)
Changes in the ionosphere caused by solar storms or other cosmic radiation have been known to affect the way radio signals travel through the atmosphere (related news: "Stronger Solar Storms Predicted; Blackouts May Result" [March 7, 2006]).
But there are ways to take such fluctuations into account when calculating GPS relays, said study co-author Rashid Akmaev of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado.
"You would presumably be able to do the same thing in the future if the ionosphere changes" due to cooling temperatures, he said.
Emmert agreed, noting that "GPS is most sensitive to rapid small-scale fluctuations in the ionosphere.
"So I suspect that long-term, it probably would be something that would easily be adapted to. But who knows; there might be unforeseen consequences."
Hot and Cold
Atmospheric cooling seems contrary to prevailing news about global warming.
But what many people might not know is that the upper and lower atmospheres react differently to carbon dioxide emissions, Akmaev said.
In the Earth's lower atmosphere, carbon dioxide traps solar energy, causing the air to heat.
But in the upper atmosphere the greenhouse gas causes the thin upper air to radiate energy more rapidly back into space, becoming cooler.
Overall, Akmaev said, the upper atmosphere is cooling at a rate of 9 to 18°F (5 to 10°C) a decade—and perhaps even up to 30°F (17°C) a decade—according to one observational study.
As the cooler gases hug more closely to Earth, the density at any given altitude is dropping by about 2 to 3 percent a decade, he added.
For Akmaev, the study's take-home message is simply that human activities are affecting the atmosphere at all altitudes from surface to space.
"If we continue monitoring it," he said, "we will learn more about how the whole atmosphere changes, not just at the surface."
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