Global Warming Could Disrupt GPS Satellites, Study Says

Richard A. Lovett
for National Geographic News
November 29, 2006
Part of the Digital Places Special News Series
More Digital Places Stories>>

A buildup of carbon dioxide in Earth's atmosphere could require changes in the way satellites are launched and might impact the function of global positioning systems (GPS), an international team of atmospheric scientists suggests.

Networks of orbiting GPS satellites send signals back to Earth that allow everything from jetfighters to cell phones to pinpoint their exact locations.

The same carbon dioxide that is a prime culprit for global warming in Earth's lower atmosphere is also causing the upper atmosphere to cool and contract, the team reported in last week's issue of the journal Science.

This change will be both good and bad for the orbiters, Jan Lastovicka, lead study author and researcher at the Institute of Atmospheric Physics in Prague, Czech Republic, said in an email.

As the upper atmosphere pulls in closer to Earth, the air at altitudes where low-orbit satellites reside will be less dense, meaning the craft can more easily maintain orbit and therefore last longer, Lastovicka said.

But spacecraft—including those that deliver new satellites into orbit—currently jettison booster rockets and other debris at about the same altitude.

The craft drop debris at just the right height to ensure that it will fall back to Earth relatively quickly and burn up in the atmosphere.

"If the atmosphere contracts, there will be less atmosphere up there to get rid of all the junk," said study co-author John Emmert of the U.S. Naval Research Laboratory's E. O. Hulburt Center for Space Research in Washington, D.C.

As conditions continue to change, space agencies will need to reevaluate their launch procedures to avoid increased risk, he said.

Rapid Changes

Changes in the upper atmosphere could also affect radio signals being sent from GPS satellites.

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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