September's clear skies showed contrails' climate impact.© GettyImagesThe grounding of commercial flights for three days after last September's terrorist attacks in the United States gave David Travis at the University of Wisconsin-Whitewater and colleagues a chance they never thought they'd have: to study the true impact that contrails from jet engines have on our climate1.
"It was a tarnished golden opportunity," recalls Patrick Minnis, an atmospheric scientist at NASA'a Langley Research Center in Hampton, Virginia. Despite a wealth of experiments, it had been virtually impossible to gauge the effect of contrails because air traffic, particularly over regions such as Europe and North America, never stopped. Until 11 September 2001, that is.
Contrails left high in the atmosphere spread out into cirrus-like clouds under the right atmospheric conditions. Natural cirrus clouds - thin layers of wispy water vapour that often resemble fish scales - trap heat being reflected from the ground and, to a lesser extent, reflect some of the Sun's rays.
Travis's team compared the average daily high and low temperatures over North America from 11 to 14 September 2001, with climatic records from 1977 to 2000, matching the weather over those three days with similar weather in September from historical records.
They found that the difference between daily high and nightly low temperatures in the absence of contrails was more than 1 oC greater than in the presence of contrails. Comparing the three-day grounding period with the three days immediately before and after, the impact was even larger - about 1.8 oC.
"It's obviously a significant effect," says Andrew Carleton, an atmospheric scientist at Pennsylvania State University in University Park and a member of Travis's team.
The researchers suggest that in regions with crowded skies, contrails work just like artificial cirrus clouds, preventing days from getting too hot by reflecting the Sun's rays, and keeping nights warmer by trapping the Earth's heat.
Averaged over the globe, which is largely free of air traffic, the effect is negligible. "But locally, contrails are equally as significant as greenhouse gases," says Carleton.
The discovery is important, says Minnis, "especially when you consider that air traffic is expected to increase at about five per cent a year". But making use of the information by incorporating it into climate models, for example, will be difficult. Little is known about what conditions lead to contrail formation, how long they last, and whether they affect more than just temperature.
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