Pollution Is Blamed for Thinner Air at Edge of Atmosphere

February 10, 2004

Scientists say they have found strong new evidence that carbon dioxide, the main smokestack and tailpipe emission linked to global warming, is cooling and shrinking the atmosphere's outermost layers in ways that could aid as well as endanger space activities.

The average density of the air in the region more than 60 miles up - just a trillionth of that near the surface - has dropped 10 percent over the last 36 years, and it could decline by a total of 50 percent by the end of the century, scientists at the Naval Research Laboratory in Washington report.

The effect had long been predicted, but observations showing that it is occurring have been scant.

Satellites, less affected by friction with the outer atmosphere, should be able to stay aloft longer but may be in more danger from collisions with orbiting debris, which would also last longer, the scientists said.

The only logical cause for the contraction and cooling is the rising level of carbon dioxide, the researchers said. The other possible influences, including variations in the sun, have been accounted for, said Dr. John T. Emmert, a George Mason University researcher working at the Navy lab, who is the lead author of a paper in the current Journal of Geophysical Research - Space Physics.

The link to carbon dioxide is very likely, Dr. Emmert said, adding, "There are not that many other options left."

The findings provide some of the clearest evidence that emissions of carbon dioxide and other so-called greenhouse gases can significantly change the dynamics and even the dimensions of the atmosphere, Dr. Emmert said. "We're impacting the environment all the way out to space," he added.

Earlier studies by American and European researchers had found spotty evidence of the cooling and contraction, but the new analysis appears to cement the case, several experts not involved in the latest study said.

"This is pretty compelling evidence for the effects of carbon dioxide," said Dr. Gerald M. Keating, a research scientist at George Washington University and the Langley Research Center of NASA, who made some of the original measurements of the phenomenon. "The whole structure of the upper atmosphere will change as this effect becomes stronger and stronger."

The changes in density were measured in the thermosphere, a layer of hot, rarefied air 60 to 400 miles above the surface, including the region where many satellites and spacecraft orbit. The anticipated effects on spacecraft and debris come through the relationship between the density of air and the drag it exerts.

The lower the density of the thermosphere, the less drag on everything that orbits there, from the International Space Station to old rocket casings. Dr. Emmert said a 50 percent decline in density, by reducing friction, could halve the rate of descent of satellites and debris, extending the life of orbiting equipment but also hazards.

The thermosphere changes were detected by sifting precise Air Force records on the orbits of thousands of objects like satellites, meteors and spent rocket sections. The researchers focused on 27 objects that had records dating from the 1960's and crisscrossed the thermosphere in a variety of orbits, assessing orbital changes compared with changes in the sun and other factors.

Dr. Keating's study in 2000 looked at five objects. British researchers in the late 1990's used measurements of the ionosphere, the blanket of charged particles in the thermosphere, to estimate that that part of the atmosphere was contracting, probably from building greenhouse gases.

But those measurements sample only a small portion of the atmosphere, experts said.

The link between carbon dioxide and high-altitude cooling is much clearer than the gas's link to warming near the surface, scientists said, because the highest atmospheric layer, barely more than a scattering of ions and molecules, sees no confounding influence from clouds, weather systems and oceans.

"When the composition of gases changes, their radiation effects show up quickly," said Dr. John R. Christy, director of the Earth System Science Center at the University of Alabama.

Near the surface, the influences driving temperature shifts are far harder to discern.

"Down here," Dr. Christy said, "energy transfers are affected by a lot of other things."