By Marc Kaufman
Washington Post Staff Writer
Tuesday, December 12, 2006
Far above Earth, where the atmosphere as we know it merges into empty space, lies the thermosphere -- home to space stations and satellites and very thin air. Hardly the kind of place, one might think, where human affairs would have much impact.
But just as all those cars and factories burning gasoline and coal are said to be creating a greenhouse effect, causing the lower atmosphere to warm, new research has concluded that the carbon dioxide released by humans is gradually changing the upper atmosphere, too.
The difference is that the effect is not to make it warmer, but rather to create a cooler -- and less dense -- environment. And there has already been enough of a change that those who control orbiting satellites have to take the effect into account as they do their work.
"This is of big interest to the satellite navigation community," said Stanley Solomon of the National Center for Atmospheric Research in Boulder, Colo.
"It will probably make it a little easier to keep large objects like the space station in orbit. But on the other side, space junk or debris gets cleaned out of the atmosphere when it becomes more dense. Because this makes it less dense, space junk may well have a longer lifetime and cause more problems."
Solomon added, however, that the cooling of the thermosphere -- which stretches from about 60 miles to nearly 400 miles above Earth's surface -- is a slow process and is "nothing to lose sleep over" on its own.
But on another level, the finding, he said, is quite important because climate researchers theorized in the late 1980s that carbon dioxide released by burning fossil fuels could have an effect on the outer atmosphere. Later scientists modeled what that effect might be, and now there is actual evidence that the thermosphere is cooling -- and that carbon dioxide is the cause. By 2017, Solomon and his colleagues predict, carbon dioxide emissions will produce a 3 percent reduction in the density of the thermosphere, with a resulting reduction in temperature.
"Carbon dioxide here will cause cooling rather than warming, but that's not what matters," he said. "What we have in common with research into the greenhouse effect is that predictions made by theoreticians were confirmed by observations. It lends credibility to the whole enterprise."
Solomon's work was presented yesterday in San Francisco at the annual meeting of the American Geophysical Union and is being published in the journal Geophysical Research Letters.
The thermosphere is the fourth layer of Earth's atmosphere -- only the exosphere is higher -- and it contains substantially less gas than the lower levels. The small amounts of residual oxygen and other gases present absorb the high energy radiation coming from the sun, which pushes temperatures up to 3,632 degrees Fahrenheit. (Astronauts do not feel the intense heat because the molecules are so far apart.) Because the thermosphere does not benefit from the protective shield of the lower atmosphere, it is prone to greater swings in density and temperature. It is also more susceptible to the effects of the sun's cyclical activity -- an 11-year ebb and flow of intensity that researchers know to expect. When solar activity is greatest, emissions of ultraviolet light and highly charged particles from the sun intensify, producing a warming and expansion of the outer atmosphere. When solar activity wanes, the thermosphere becomes more settled and cools.
This effect of solar radiation was first seen when sunspots modified the orbit of early Soviet Sputnik satellites. Now it is a well-known feature of satellite and space station life, requiring compensating maneuvers to keep the valuable equipment in proper orbit.
As Solomon explained it, this cycle of relative solar activity or inactivity had to be factored into his team's assessment of how carbon dioxide might be causing its own changes. Taking solar cycles into account, he said, the study still found a decrease of about 5 percent in the density of the thermosphere between 1970 and 2000.
How does this change result in a greater problem with space debris?
At high points in the solar activity cycle, Solomon said, the thermosphere becomes sufficiently dense that some of the space junk is slowed by atmospheric drag and falls to the lower atmosphere, where it burns up and is destroyed. But if carbon dioxide emissions gradually make the thermosphere cooler, that cyclical cleaning out of the orbiting junk will occur less and less. And with an estimated 10,000 unwanted objects larger than a grapefruit now orbiting Earth, anything that keeps them aloft is unwelcome.
Most important, the new research underscores the far-reaching effects of burning fossil fuels, said Stephen Maran, spokesman for the American Astronomical Society.
"This shows how interrelated things are in and around the Earth," he said of the new findings. "The same thing that is melting ice in Greenland or raising sea levels around atolls in the Pacific is actually causing effects in the outer atmosphere that you might have thought was way above the fray."