SAN FRANCISCO - Carbon dioxide emissions from global warming are cooling and shrinking the outermost atmosphere, keeping orbiting spacecraft airborne longer but also increasing the threat that space junk poses to satellites, scientists reported Monday.

In a signal of the wide-ranging impacts of climate change, the thinning of the thermosphere, which begins about 60 miles above Earth and extends up to 400 miles, reduces the drag on orbiting spacecraft but also extends the lifespan of space junk — leftovers from space missions, old satellites, items astronauts lose during spacewalks and the like.

"It's a bit of a two-edged sword," said Stanley Solomon, a senior scientist at the National Center for Atmospheric Research, who presented the new results at an American Geophysical Union meeting in San Francisco.

Using a computer model, Solomon and his colleagues estimated that the air density of the outer atmosphere declined about 5 percent over the past three decades and could decrease 40 percent by the end of the century.

Knowledge of how the outer atmosphere responds to carbon dioxide levels could help NASA and international space agencies time their spacecraft launches and calculate their fuel needs.

Solomon said that a less dense outer atmosphere should not affect launches in the near term, but that it could be problematic in the future with the increase of space litter.

"In the long haul, it means we have to be even more assiduous about not letting miscellaneous pieces of metal float around," Solomon said.

Researchers have long predicted that carbon dioxide, produced when fossil fuels such as oil and natural gas are burned, would cool the outer atmosphere. Solomon's conclusions mirror previous research that predicted similar effects, including recent observations that measured the drag of satellites over time.

Robert Dickinson, a pioneer in the field and a professor of earth and atmospheric sciences at the Georgia Institute of Technology, said the latest work is unique because it looks at the effects of solar activity on the atmosphere. An active solar cycle could spawn magnetic storms that will be more severe and disruptive to communication systems.

"We're getting a more detailed description," said Dickinson, who had no role in the new study.

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On the Net:

American Geophysical Union: http://www.agu.org

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