Irony on high: global warming cools, thins upper atmosphere.

Increasing concentrations of carbon dioxide and other greenhouse gases in the air, which cause temperatures at Earth's surface to warm, will turn the upper layers of the atmosphere cooler and thinner in coming decades, new research suggests. This counterintuitive phenomenon, first predicted in the late 1980s and recently inferred from satellite data, will probably lead to longer orbital lifetimes for satellites and space junk.

Temperature trends in the highest levels of the atmosphere are difficult to measure, says Robert E. Dickinson, an atmospheric scientist at the Georgia Institute of Technology in Atlanta. He was one of the first scientists to propose a high-altitude cooling effect of greenhouse gases. Although direct temperature data aren't available, the thinning of the air at high altitudes has been detected. Dickinson notes that satellites that whiz around Earth in the upper atmosphere are experiencing less atmospheric drag than they, used to.

In the most recent report, Stanley C. Solomon, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colo., and his colleagues analyzed data gathered by satellites over more than 3 decades. The team presented its findings Dec. 12 at a meeting of the American Geophysical Union in San Francisco. Since 1970, the average density of the atmosphere at an altitude of 400 kilometers has decreased about 1.7 percent per decade.

Using their new model, which includes variations of the solar cycle, the researchers calculated that future increases in greenhouse gases could reduce the density of the air at that altitude--which is approximately one-billionth as dense as it is at sea level--by about another 3 percent by 2017.

"It's a paradox that there are different effects [of global warming] at high and low altitudes, but both are real," Solomon says. The dissimilar efffets result from the dispanty in atmospheric density at different altitudes, he notes.

Greenhouse gases are particularly efficient at absorbing infrared radiation, thereby trapping heat. Regardless of altitude, gas molecules increase their speed when they absorb radiation. The speeding molecules typically lose their extra kinetic energy in one of two ways: They transfer it as heat to another molecule via a collision, or they radiate a photon. If doesn't strike another molecule, the photon can carry the energy into space.

At altitudes below 15 km, gas molecules are closely packed and each molecule travels only a few dozen nanometers before it collides with another. Thus, the extra energy that a molecule gains via the greenhouse effect is usually passed on via a collision rather than radiated into space, says Solomon. So, warmth remains trapped in the lower atmosphere.

In contrast, at high altitudes, where the atmosphere is less dense, collisions between gas molecules are infrequent. There, energized molecules of gas are much more likely to radiate a photon, contributing to atmospheric cooling, Solomon explains. The cooling air contracts, leaving the atmosphere less dense at any given altitude.

The high-altitude atmospheric thinning that Solomon has documented indicates that global warming is really happening, Dickinson says.