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 byDr. 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.
Contrails not only can reduce temperature variations, but also increase surface temperatures - enough to account for the entire warming trend in the US 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."
Much of the concern over contrails' potential impact on climate is coming from the European Community, which, ironically, unveiled the world's largest airliner last month. The four-engine Airbus A380 - far larger than a Boeing 747 - carries up to 840 passengers. Not to be outdone, Boeing is ramping up production of its twin-engine 7E7 Dreamliner, the first carbon-composite airliner that will be lighter and burn far less fuel per passenger mile than older airliners.
Improving fuel efficiency is one of the big environmental success stories in aviation. Better engines, aerodynamics, and other factors have improved airliner fuel efficiency 60 percent in the past 35 years, says Ian Waitz, a professor of aeronautics at the Massachusetts Institute of Technology and an authority on airliner emissions impact on the atmosphere.
Although the A380 and 7E7 will sport even more fuel-efficient engines, they will add to an already burgeoning global fleet of some 12,000 airliners. Airbus hopes to sell more than 700 of its megaliners; and Boeing, more than 2,500 of its Dreamliners.
"Even with a 40 to 50 percent improvement in fuel efficiency, you're faced with a 3 to 20 factor increase in the amount of travel, so we are going to have to have big increases in emissions with that kind of growth," says David Greene, a coauthor of the IPCC report and scientist at the Center for Transportation Analysis at Oak Ridge National Laboratory in Tennessee. Because it will take decades to turn over the global fleet, pollution will moderate only very slowly, he says.
Future contrail scenarios depend much on how much fuel is burned - and at what altitude. One possible solution, noted by researchers in England, would be to fly at lower, warmer altitudes. Contrails require moist yet very cold air to form - prevalent over some regions, such as the American Midwest, or Northern and Western Europe. By flying 6,000 feet lower, aircraft would produce fewer contrails, a team of scientists from Manchester Metropolitan University reported in 2003.
But flying lower in denser air would cut fuel efficiency. Burning more fuel would increase carbon dioxide output, possibly neutralizing benefits, other point out.
Even today's more efficient engines have a downside. The ever higher pressures and internal fuel-combustion temperatures that such engines require tend to increase NOX emissions - a major ingredient in smog. Special combustion-chamber designs and technology can reduce NOX formation. And such "low-NOX" exhaust options have been available for one of today's most popular engines - yet few are purchased because of the extra cost, experts say.
Unless low-NOX engines become more popular, NOX could grow more than fourfold over 1992 levels by 2050, IPCC forecasts.
Even so, the NOX issue is small potatoes, says Professor Waitz in an e-mail. "We should be concerned about all potentially important environmental impacts, but we must also recognize that aviation is a relatively small contributor."
For others, however, contrails pose a threat of growing gradually into a murky blanket that reduces earth's temperature swings and dims the sun - a scenario that would hurt crops and even maple-sap harvests by helping harmful insects survive.
"The jet-contrail problem is not really a pollution problem - it's a cloud problem," notes Dr. Travis back in chilly Wisconsin. "We're disrupting the natural radiation and energy balance of the planet ... trapping outgoing radiation and blocking incoming sunshine. And that makes the world a cloudier, warmer, less enjoyable place."
1952: British Overseas Aircraft Corporation (BOAC) inaugurates the first commercial jet service with a flight from London to Johannesburg, South Africa. The De Havilland Comet 1 could cruise at 480 miles per hour, 2-1/2 times as fast as the propeller-driven DC-3. Repeated crashes force BOAC to suspend flights within two years.
1956: The Soviet Union's Aeroflot begins the first regularly scheduled and sustained passenger jet service with flights of its Tupolev Tu-104 between Moscow to Irkutsk.
1958: Pan American flies the first Boeing 707 from New York to London with 111 passengers, the largest number ever to board a single regularly scheduled flight. The fare: $272 (or $1,778 today).
1970: The jumbo jet era begins as Boeing's 747, capable of carrying more than 400 passengers, enters commercial service with a Pan Am flight from New York to London.
2005: Airbus unveils a super-jumbo - a double-decker A380, able to carry 555 people - while Boeing pushes ahead with its smaller and more efficient 7E7, due in 2007.
2008: Passenger air traffic is forecast to grow some 20 percent from today's levels. China and Poland should see a rise of more than 50 percent.
Sources: US Centennial of Flight Commission; Aeroflot; Finfacts.com; International Air Transport Association