As globe warms, atmosphere may shrink.

With tongues nowhere near their cheeks, British scientists report this month that the sky is actually falling. Radar measurements show that the upper atmosphere has contracted since 1958, matching the predictions of greenhouse warming theory.

The atmospheric physicists used radar installations in the Falkland Islands and along the Antarctic coast to probe the thermosphere--the region of the atmosphere above 85 kilometers, where atoms are scarce and the sky blends seamlessly into space. The radar waves shot upward and reflected off a layer of charged atoms, called the ionosphere, at an altitude of about 300 kin. By measuring the time it took the radar signal to return to Earth, the researchers could track changes in the height of the ionospheric layer within the thermosphere.

From 1958 through 1995, the average ionosphere height dropped by 8 km, according to the group's report in the Sept. 1 JOURNAL OF GEOPHYSICAL RESEARCH. "There does appear to be a global reduction in the altitude of the ionosphere. The best explanation is linked to the increase in greenhouse gases," says Martin J. Jarvis of the British Antarctic Survey in Cambridge, England.

As carbon dioxide and other pollutants trap heat in Earth's lower atmosphere, they conversely cool the middle and upper atmosphere, according to computer climate models. These simple models forecast that the thermosphere will chill by 50[degrees]C with a doubling in the atmospheric concentration of carbon dioxide, expected sometime in the next century. That would shrink the thermosphere and cause the height of the ionosphere to drop by 10 to 20 km, according to calculations.

Researchers have previously detected decreases in ionospheric height over Finland and Germany. The Southern Hemisphere data "show that it's a global phenomenon, not just over Europe," says Jarvis.

The British researchers say that although their measurements are consistent with greenhouse warming predictions, there could be other factors causing the thermosphere to contract.

Another recent study, in fact, failed to find a consistent trend in ionospheric height. When Indian scientists analyzed records from 31 stations around the world, they found the ionosphere rising in some regions and falling in others. Their report appears in the Sept. 1 GEOPHYSICAL RESEARCH LETTERS.

Taken at face value, the Indian study suggests there has been no global shrinking of the thermosphere, says Thomas Ulich of the Geophysical Observatory in Sodankyla, Finland. Ulich cautions, however, that researchers need to check the ionospheric records for changes in instruments and personnel doing the measuring. At Sodankyla, the same technician has conducted measurements since 1957, during which time the ionospheric heights have dropped, he says.

Measurements of the middle atmosphere have been more consistent, with different techniques showing a strong cooling, says Guy P. Brasseur of the National Center for Atmospheric Research in Boulder, Colo.

A more definitive picture of the middle and upper atmosphere could come in 2000, when NASA plans to launch a small satellite to study these regions.