Take a look at this transcript of a BBC documentary - http://www.bbc.co.uk/sn/tvradio/programmes/horizon/dimming_trans.shtml I thought it was a joke, but it sure doesn't read like one... Here's an excerpt...
This is rather long, but important for understanding high altitude weather effects from: http://www.sciencenews.org/pages/sn_arch/7_6_96/bob1.htm Although it sounds like military flight training, Anderson and his colleagues were actually conducting a high-tech emissions check-measuring the gases and particles spewing out of jet engines. Their mission resembles the pollution tests that states routinely perform on cars, except that the NASA-run experiment happened at 400 miles per hour, 40,000 feet above the ground. And whereas car emissions are well understood, scientists have little information on the pollution from jet engines. Toward that end, NASA gathered four planes and 120 scientists in Kansas during April and May to make the most detailed measurements yet of jet engine exhaust at cruising altitude. This project and future ones are addressing the question of whether aircraft emissions are increasing the number of clouds and are perturbing atmospheric chemistry-both of which could affect the weather down on the ground, says project scientist Randall R. Friedl of NASA headquarters in Washington, D.C. "There are 10,000 large-size commercial aircraft in operation today. It's expected that this number will double by the year 2020. It's a natural question to ask whether these are having an environmental impact," says Friedl. Fueling this investigation are several sketchy studies hinting that ground temperatures have shifted in the last few decades in regions beneath well-traveled jet routes. "There is some concern that aircraft may play a role in some of the changes that have been seen," notes Friedl. Although commercial jets have been sailing through the skies since the 1950s, scientists have only just started wondering about their widespread effects on the atmosphere. NASA launched its environmental investigation of subsonic aircraft 2 years ago and plans to continue the $140 million program through 2001. European researchers began similar studies in 1992 and are running a project parallel to the NASA work. In flights over the central United States, the Rocky Mountains, and the Pacific Ocean, the NASA team measured emissions of sulfur and soot, with the aim of understanding how these affect high-altitude clouds. The scientists also analyzed the makeup of condensation trails, or contrails, those long, straight clouds often created by jets. NASA, ever eager for a catchy acronym, labeled the mission SUCCESS, for Subsonic Aircraft: Contrail and Cloud Effects Special Study. Contrails develop when hot, humid fumes from a jet engine meet the cool air of the upper troposphere. Water vapor in the exhaust and atmosphere freezes to create tiny cloud particles, much like the mist that forms when a person exhales on a cold winter day. As turbulence in the upper atmosphere tears contrails apart, they can spread into wispy sheets essentially identical to natural cirrus clouds. Engines can also stimulate cloud growth indirectly, by way of tiny aerosol particles within the exhaust. These aerosols-droplets of sulfuric acid and specks of soot-serve as seeds. They provide surfaces upon which water molecules can condense or freeze to create cloud particles, explains Eric J. Jensen, a participant in SUCCESS and a researcher at NASA's Ames Research Center in Mountain View, Calif. Scientists do not know the fate of the aerosols once they leave the back end of a jet engine and start mixing with the ambient air. The specks and droplets may be among the ingredients necessary for creating contrails. They may also thicken natural cirrus clouds, rendering them more opaque to sunlight and making them last longer. In fact, so little is known about the clouds produced by aircraft exhaust that researchers cannot say whether, on balance, they cool or warm the climate. The uncertainty exists because high-altitude clouds have numerous and contrary effects. Contrails and cirrus help cool the globe by reflecting sunlight that would otherwise hit Earth's surface. At the same time, they exert a warming influence because they absorb infrared radiation emitted by the ground, thus trapping energy and heating the atmosphere.
