 A satellite in the Navstar global positioning system (GPS) orbits above Earth.
The
same carbon dioxide gas that is a main contributor to global warming is
also causing the planet's upper atmosphere to cool and contract, new
research says. The shift could impact how satellites and other
spacecraft are launched and how well GPS satellites send their signals
back to Earth.
Photograph courtesy Air Force Research Laboratory
November 29, 2006
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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.
nside the highest layer of the atmosphere is a region called the
ionosphere, where charged particles help reflect radio waves back to
Earth.
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."
*
http://news.nationalgeographic.com/news/2006/11/061129-warming-gps.html
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