The Coastal Post - July 2000

More Than Noise:
Environmental Effects of Aircraft

By Jim Scanlon

Except for the heavy smell of jet fuel that sometimes permeates the air close to major airports, the most noticeable environmental effect of civil aircraft is the loud thunderous noise of the heavy jets as they climb laboriously to cruising altitude after take off. They make much less noise while landing, although the wining noise can be disturbing to those who live beneath their landing routes, especially when traffic is heavy and the wining repeats every few minutes. When we fly ourselves, we don't notice these effects. We are happy to be on our way, or we are glad to be on our way home.

Besides noise, a striking visual effect of commercial aircraft is the formation of contrails. Who has not stood fascinated, looking up, transfixed, as a tiny silvery jet draws a straight white chalk line across the blue sky. Sometimes the condensed vapor from the engine exhausts, the contrail , disappears after a few seconds. At other times, the white line hangs there and lazily curves, meanders, feathers and broadens into a cirrus cloud. This type cloud may be indistinguishable from a normal cirrus cloud. Persistent contrails have been imaged from satellites from their birth, and tracked as they evolve and drift across California into Nevada, Utah and beyond.

Sometimes, when conditions are right the entire sky is criss-crossed with tic-tac-toe lines marking the paths of airplanes long passed.

Clouds reflect sunlight from their tops, shading and cooling the air below, as when we open a beach umbrella. At night clouds keep the surface warm like a blanket, preventing warmth from escaping the surface.

While scientists still haven't been able to clearly distinguish a human induced warming trend from the steady increase in "greenhouse" gases such as carbon dioxide, they have measured a clear change in the range of high and low temperatures over the United States. That is the daytime highs are not as high as they used to be, and the nighttime lows are not as low. Overall there has been a small increase. This regional change in our atmosphere is widely accepted as resulting from contrails formed by the hundreds of daily flights along the North American Flight Corridor

Air traffic is especially heavy over the North Atlantic where hundreds of aircraft burn thousands of tons of fuel just below the tropopause, the boundary between the dynamic, ever changing lower atmosphere where our weather occurs and the stratosphere, a dry, much less changeable part of the atmosphere. Pollutants in the lower atmosphere tend to stay for days, or weeks until they react with other substances and fall out or are washed out as rain or acid rain. In the stratosphere pollutants may drift around for months or even years.

In the 1970s there was great concern over what effect a fleet of supersonic aircraft might have on the stratospheric ozone layer. The fear was that oxides of nitrogen would destroy ozone allowing greater amounts of ultraviolet radiation to reach down towards the surface of earth and harm living things. The fleet of supersonic aircraft were never built, mostly for financial reasons, but during the 1980s and 1990s, as commercial jets got bigger and flew higher, they began to fly in the stratosphere. In 1993, German scientists, after conducting an exhaustive study, estimated that aircraft flying the North Atlantic Flight Corridor fly in the stratosphere 44 percent or the time. It has been estimated that all sub sonic jet aircraft fly in the stratosphere 17-25 percent of the time.

Oxides of nitrogen from jet exhausts produce ozone, a greenhouse gas, while flying in the upper reaches of the troposphere. If they fly a little higher in the stratosphere, which is a little warmer due to the presence of naturally formed ozone, these same oxides of nitrogen destroy ozone which results in a cooling effect. There are many other substances emitted in jet exhausts that produce a variety of effects including, soot, unburned fuel, acid droplets and probably the worst of all-water vapor.

Aircraft emissions account for only a small fraction of global emissions, however, aside from chloroflurocarbons, aircraft are the most significant source of man made pollutants in the upper troposphere and stratosphere. It was recently estimated that the air travel industry burns 170 million metric tons of fuel a year-in the most sensitive, least polluted part of our atmosphere. This industry is expected to increase by 200 percent over the next 15-20 years.

While our engineers continue to develop aircraft that are safer, more powerful, less noisy, more economical, more fuel efficient and less polluting, there is a limit to what can be accomplished. As fuel efficiency is gained, and soot and unburned fuel is reduced, more oxides of nitrogen and water vapor are produced.

What to do about the environmental impact of aviation is an extremely difficult question. Air travel is vital to the way we live, important to developed and developing countries. Sooner or later, we will be forced to address this problem.

Aircraft noise, coming and going, may be the most noticeable effect, but it is far from being the most important.

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