Shrinking Atmosphere

Scientists in Britain Say Sky May Be Shrinking, Outer Atmosphere Has Fallen 5 Miles
By Joby Warrick, Washington Post Staff Writer
September 17, 1998

Chicken Little may have been right: The sky appears to be falling, or at least shrinking. And global warming may be to blame. British scientists reported yesterday that the Earth's upper atmosphere contracted or dropped by nearly five miles in the past four decades - a decline they suggest is linked to "greenhouse gas" pollution on land. The long-term change in the Earth's outer "thermosphere" is apparently harmless, and, in fact, is barely noticeable against the daily ballooning and shrinking in the volatile outermost zone of the atmosphere. But researchers from the British Antarctic Survey said the shift appears to be another signal that human activity is profoundly influencing the planet's climate. "The closest you can get to explaining this phenomenon is greenhouse gases," said Martin Jarvis, an atmospheric research physicist in Cambridge, England, and the lead author of the report.

The findings, published in the September issue of the Journal of Geophysical Research, are based on 38 years of atmospheric measurements from research stations in the Falkland Islands and Antarctica. The data record changes in the thermosphere - the outer layer of the atmosphere that extends to roughly 300 miles above Earth -- by tracking the rise and fall of the ionosphere, a region of free electrons within the thermosphere that reflects certain kinds of high-frequency radio waves. The hottest and windiest part of the atmosphere, the thermosphere roasts by day as the sun's energy pushes temperatures as high as 800 degrees. The heating causes the thermosphere to expand by scores of miles and then contract at night. The amount of expansion varies according to seasonal cycles as well as long-term changes in the Earth's magnetism and in solar intensity.

But even after accounting for those fluctuations, Jarvis and his colleagues discovered that the thermosphere was in a long-term retreat. Compared with measurements from the late 1950s, the zone had "shrunk" by about 5 miles - an indication that the thermosphere is cooling. What does this cooling and shrinking have to do with global warming? Plenty, Jarvis said. Cooling in the upper atmosphere is one of the widely predicted consequences of global warming. In computer simulations of the so-called greenhouse effect, carbon dioxide and other gases trap heat near the Earth's surface while causing the upper atmosphere to cool, and therefore contract. Jarvis called the finding "not a shattering result, but another indicator that things are changing." Earlier, less ambitious studies in Europe also detected a contraction in the thermosphere, while a report earlier this month by two Indian scientists was unable to confirm the trend. The British scientist stressed, though, that the five-mile drop in altitude "is not in itself harmful to people." "It is however, another warning signal about what damage to the atmosphere can be caused by human impact," he said.

Jarvis conceded there could be alternative explanations for the trend, including a long-term change in the sun's intensity. So far, he said, scientists have not detected a shift in solar patterns large enough to account for the change in the thermosphere. Other scientists described the finding as interesting but said more research was needed to rule out other possible causes. "It does seem to be a noisy signal that we're trying to read," said Arthur Thomas, an atmospheric physicist and senior scientist at the National Center for Atmospheric Research in Boulder, Colo. "People at different locations have found different kinds of effects. We're still not 100 percent clear that we've pinned down the science."