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Airplane contrails and their effect on temperatures

Contrails from airplanes impact temperatures at Earth's surface. But do they raise or lower them?

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Immediately after 9-11, other scientists looked at contrails left by military planes, the only aircraft allowed to fly. They noticed that when planes flew in a cloudless area west of Washington D.C., within just a few hours, contrails that had begun as vapor trails just a few meters in width covered 20,000 square kilometers (7,722 square miles).

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A 2006 study published in Nature, meanwhile, provided more details. The authors concluded that when a contrail forms — day or night, for example — determines whether the net effect will be warming or cooling. (Here's Scientific American's write-up [PDF] of the article.

Planes flying between 6 p.m. and 6 a.m. comprised only one-quarter of total flights examined. But they were responsible for 60 to 80 percent of contrails' warming effect ontemperatures. That's because contrails at night trap outgoing heat radiation.

Contrails during the day, meanwhile, offset their heat-trapping effect by reflecting incoming sunlight. Likewise, winter flights, which account for only 22 percent of annual air traffic, were nonetheless responsible for half the warming by contrails.

"[F]light rescheduling could help to minimize the climate impact of aviation," conclude the authors.

Or, as the Scientific American article puts it, "Kiss the red-eye good-bye."

Of course, aviation's real impact on climate probably has nothing to do with contrails. In 2005, NASA's James Hansen published a study to that effect. He found that, even if the number of contrails were quintupled, global mean temperature would increase by just 0.03 degrees C (0.05 degrees F.). Aviation emissions, which are rising dramatically, are the true culprit.

According to this Wired story, aviation is on track to have a 1.5 billion-ton carbon footprint by 2025. The entire 27-nation, 457-million-person European Union emits some 3.1 billion tons of CO2 yearly at this point.

And yet, although the net impact of contrails may pale in comparison to the net impact of carbon emissions, engineers are already thinking up solutions to contrail formation. One has proposed a powerful microwave beam mounted on the jet engine. The idea: Evaporate the condensing water and soot particles before they become contrails.

Scientist Frank Noppel, who's researching this idea, says the job would cost just 0.1 percent of a jet engine's power. Sound crazy to have a microwave pointing back the way you came? Rolls-Royce has reportedly filed a patent on the technology.

Editor's Note: The Monitor's Environment section has a new URL. And there's also a new URL for our Bright Green blog. We hope you'll bookmark these and visit often.

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