Assessing the Jet Threat

Gazing into a clear blue Wisconsin sky, David Travis was amazed by what he did not see: not one fluffy airliner contrail. Not that day or in the two days that followed the 9/11 terror attacks, when commercial airliners in the United States were grounded.

For Dr. Travis, a climatologist at the University of Wisconsin at Whitewater, that tragedy had a tiny silver lining. A sky without jet contrails became a once-in-a-lifetime opportunity to see if the skinny, man-made clouds really did affect climate, as he had long suspected.

Little is known about the global climate effects of airliner exhaust. Although jets create far less greenhouse gas than power plants or automobiles, they have an outsize impact because of where they spew it – the delicate upper troposphere and lower stratosphere, five to seven miles up from Earth's surface. And an expected boom in airline travel in coming years is likely to swamp any efficiency gains from the next generation of airliners, such as the just unveiled Airbus A380.

The result: growing scientific concern that jets may be turning the skies into a hazier, heat-trapping place.

"Airliners are special because even though their total emissions are relatively small, compared to other sources, they're putting their emissions directly into the upper troposphere," says Joyce Penner, a University of Michigan professor of atmospheric science and lead author of a landmark report on aviation and the atmosphere. "It's a special location."

When injected together into the icy atmosphere, the mix of exhaust gases – including water vapor, unburned hydrocarbons, particulates, sulfates, nitrogen oxides (NOX), and carbon dioxide – produces clouds and has two to three times the warming effect of carbon dioxide alone, Massachusetts Institute of Technology researchers reported last year.

That finding meshes with what Travis found. Comparing ground temperature readings during the 9/11 flight ban with those after and before it, Travis found that those seemingly inconsequential wisps fanning out miles above the earth were like a blanket, reducing temperature fluctuations nationwide.

Travis's findings heightened scientists earlier suspicions that the cirrus clouds formed from contrails did much more than just suppress temperatures – perhaps playing a bigger role in global climate change than many had suspected. A key 1999 international report had cited airliner exhaust as responsible for 3.5 percent of the climate warming shift.

By 2050, carbon-dioxide emissions from airliners are expected to grow two to 10 times the 1992 level, thanks to increasing air traffic, according to the Intergovernmental Panel on Climate Change report co-authored by Dr. Penner. By then, aircraft emissions will have risen to 5 percent of the cause of global warming, IPCC says.

New research suggests the problem could be even bigger. "Contrails can be called a cause of warming and definitely need to be considered in climate-change models," says Patrick Minnis, an atmospheric scientist at Langley Research Center in Hampton, Va., part of the National Aeronautics and Space Administration.

Hot Under the Contrail

Contrails not only can reduce temperature variations, but also increase surface temperatures – enough to account for the entire warming trend in the U.S. between 1975 and 1994, according to a study Dr. Minnis published last year. Still, he notes, additional research is needed. Just because contrails "could account for all the warming, it's not absolutely certain they did," he says.

Other scientists say neither contrails nor airliner exhaust poses much of a warming threat.

"If you're worried about the planet warming up, airplanes are not the first place to look to reduce the impact," says Andrew Gettleman, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colo. "It's a fairly small piece of the puzzle. ... More than 95 percent of global warming is caused by other things, like power plants."