Fly the filthy skies

Globe and Mail, September 28, 2002, Science, Focus. F7

Airline travel is slowly returning to normal after last year’s terrorism shock. But there’s another reason to rethink our reliance on planes they may be the great neglected culprit in pollution and global warming.

By Roesmary Frei. Special to the Globe and Mail

When the World Trade Centre fallout hit the North American psyche and economy early last year, the airline industry suffered more than other sectors but not as much as many people expected. The International Air Transport Association (IATA) has calculated a 3-per-cent drop in passenger traffic this year, but expects a 6-per-cent rise next year and an average growth rate of about 4 per cent throughout 2006.

So, never mind turbulence, bad food and jet lag even the spectre of terrorist attack has not clouded long-term proposals for the major airlines. But the air-travel boom carry with it a significant impact that few fliers are aware of, and even fewer take into account when making vacation or business plans.

According to the Global Policy Forum, a non-governmental organization based in New York City, airplane travel is the fastest-growing source of carbon-dioxide emissions. Jet emissions currently stand at 4 per cent of the world total, according the GPF, but that contribution is expected to triple by 2015.

The good news, according to the IATA, is that the world’s air fleet now is about 65 per cent more fuel-efficient than it was in 1970, and efficiency increased by 17 per cent from 1990 to 2000. Furthermore, according to the Natural Resources Defense Council, based in Washington, D.C., Delta Airlines pilots practise reduced engine idling to the fullest extent possible before takeoff and after landing. The NRDC noted that this simple procedure can reduce ground-level air pollutants by as much as 40 per cent.

But a widely reported study this summer ironically made possible by the temporary cessation of commercial flight in the United States after the Sept. 11 2001, attacks put a damper on such optimism. Dr. David Travis and his colleagues at the University of Wisconsin-Whitewater found that during the three-day hiatus from Sept. 11 to 14, the average temperature range over the United States was more than 10 degrees (Celsius) wider than normal.

As the Globe and mail reported in August, they suggested the fluctuation was caused by the absence of jet-engine contrails (the ice-crystal streams that form around the particles expelled into the air by passing planes) during the three days of quiet skies. In regions with significant airline traffic, contrails may act like artificial cirrus clouds, preventing temperature from climbing during the day and from dropping at night.

Another team led by Dr. Patrick Minnis of the Langley Research Center, run by the U.S. National Aeronautics and Space Administration (NASA) in Hampton, Va., used satellite images to find that in just a few hours, six contrails formed by air-force jets spread out to cover more than 20,000 square kilometres.

The challenge for scientists now is to determine whether contrails and jet emissions affect more than just temperature, and also how they are interacting with other human influences on climate.

Dr. Katta Murty, a professor of industrial and operations engineering at the University of Michigan in Ann Arbor, said he believes that airplane travel is having a disproportionately large effect on global warming. He based his conclusions on a study he did in collaboration with NASA to compare the pollution airplanes generate with pollution from ground-level vehicles.

Using data supplied by Northwest Airlines Inc., he calculated that the average flight consumes 17.5 litres of jet fuel per kilometre. A public bus, by contrast, consumes an average of 0.53 litres/km -- 7.9 times more efficient than a plane. (A private automobile, carrying the U.S. average of 1.2 people, is one-third as fuel-efficient as a public bus.)

The impact of plane pollution, Dr. Murty added, is probably larger than that from cars and buses. Some of the ground-level gases can be absorbed by plants and trees. In contrast, airplanes release exhaust composed of carbon dioxide, carbon monoxide, water vapor, nitric oxide and ammonia into the “fragile and rarefied” region of the stratosphere. Air density at that height is only about one-sixth that at ground level, so cold gases there may remain in place for several years and form a stable ring of pollution that impedes the flow of energy from the lower atmosphere.

According to Dr. Murty, this may help explain why the Earth is experiencing much higher rates of increase in temperature than can be accounted for the moderate increase in greenhouse gases in the atmosphere as a whole.

“The increasing volume of airplane traffic worldwide has serious environmental consequences, perhaps more serious than the ozone-hole phenomenon on which the attention of scientific community and the public is riveted,” he said.

:”I believe that it is high time we realize the serious environmental impacts of increasing airplane traffic, and discuss widely whether anything can be done to make sure that it does not become a major crisis.”

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