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As global warming and greenhouse gases raise temperatures near the Earth's surface, they cool the outer atmosphere and may change the orbits of global-positioning satellites, a group of scientists said.

Falling temperatures in the upper atmosphere create lower air density, according to the commentary in the journal Science, and that may affect the paths of satellites circling the Earth.

"Temperature directly affects atmospheric density," wrote lead author Jan Lastovicka, of the Institute of Atmospheric Physics in Prague.

"At altitudes between 200 and 800 kilometers, atmospheric drag causes measurable decay of the orbits of satellites and space debris."

Twenty-four satellites orbit the Earth to form the Global Positioning System, which is used for navigation, according to the Federal Aviation Administration's Web site.

Changes in the ionosphere, where satellites orbit, can affect how radio waves travel, making GPS systems less effective, the commentary said.

The scientists estimated that the air cools about 63 degrees every decade at distances of 217 miles from the surface, the commentary said.

In the atmosphere closer to the Earth, the air density has decreased by 2 percent to 3 percent a decade, a figure that accelerates both after 1980 and as measurements are taken farther from the Earth, the scientists wrote.

"The upper atmosphere is generally cooling and contracting," the scientists wrote.

The "dominant driver" of the trend is buildup of greenhouse gases, they said; thus, human-generated gases "influence the atmosphere at nearly all altitudes between ground and space, affecting not only life on the surface but also the space-based technological systems on which we increasingly rely."

Satellites' lifetimes are also affected by their energy sources, instrument failures and the competence of command centers, the commentary said.