Cool times ahead for the upper atmosphere.
Not all parts of the atmosphere will warm as carbon dioxide and other greenhouse gases accumulate above Earth. A new computer modeling of the upper atmosphere shows that the thin air high above the planet's surface should cool substantially -- an effect that may help some satellites stay in orbit longer.
Most climate projections for the next century have focused on how accumulating greenhouse gases will affect the troposphere and the stratosphere, the two layers closest to Earth. This is the first time researchers have used complex computer models to predict the effect on the two highest regions, the mesosphere and thermosphere, says Raymond G. Roble of the National Center for Atmospheric Research in Boulder, Colo. Roble and colleague Robert E. Dickinson discuss their findings in the December GEOPHYSICAL RESEARCH LETTERS.
Their computer simulations suggest that when carbon dioxide and methane reach double their 1970s levels throughout the entire atmosphere, the mesosphere (about 50 to 90 kilometers in altitude) will cool by about 10 degrees C and the thermosphere (90 to 500 km) by 50 degrees C. Simulations by others have indicated that temperatures in the stratosphere (about 12 to 50 km) will also drop as greenhouse gases accumulate, but temperatures in the troposphere (reaching from Earth's surface to 12 km) will rise.
The predicted cooling in the stratosphere, mesosphere and thermosphere stems from carbon dioxide's ability to absorb atmospheric heat-energy and emit it as infrared radiation, Roble explains. As the number of carbon dioxide molecules in the upper atmosphere increases, this region will send more of its heat-energy toward space and therefore will cool.
Closer to home, a doubling of carbon dioxide should warm the troposphere because this layer holds far more carbon dioxide and other greenhouse gases than does the upper atmosphere. In the troposphere, radiation emitted by these gases cannot travel far before it hits another molecule and is absorbed, becoming trapped at that level. In the upper atmosphere, where greenhouse gases are more diffuse, emitted radiation stands a much greater chance of escaping into space.
A cooling in the stratosphere will likely worsen the ozone depletion around Earth's poles. But researchers say they cannot tell what will happen to the planet as a result of a cooling in the mesosphere and thermosphere. "We really are fairly ignorant about a lot of the major processes occurring in that region," says Darrell Strobel, an atmospheric physicist at Johns Hopkins University in Baltimore.
Strobel says some researchers have jokingly used the term "ignorosphere" to describe the mesosphere and the lower portion of the thermosphere, which are too high for research balloons and too low for satellites to study.
Several scientists say an upperatmosphere cooling could influence certain space missions. As the thermosphere cools, it will contract, pulling the edges of the atmosphere closer to the planet. This should extend the lifetime of low-orbiting satellites, says Roble. As their orbits weaken with time, such satellites slowly descend toward Earth until atmospheric friction eventually knocks them out of orbit.
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|Date:||Jan 13, 1990|
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