Jet trails may change global climate
By Dr Claire Gilbert
Jet planes leave behind vapour condensation trails called "contrails" which often stay in the sky for many hours and spread out to become icy cirrus clouds. They are not as innocent as they appear.
Current research indicates the problem of clouds is an important one for global climate change.
There are 62 million commercial and military flights across US skies each year.
A National Science Foundation study by Steven Ackerman of the University of Milwaukee has found that jet contrails influence regional climate. "It's been estimated that in certain heavy air traffic corridors, cloud cover has increased by as much as 20%", he explained.
Patrick Minnis of NASA's Langley Research Centre, in the June 13 issue of Science News, says cloud cover is up about 5% in the US since jet travel mushroomed several decades ago. Most of the new clouds are in the form of cirrus that contrails evolve into, says Minnis.
Imaging large areas every 15 minutes with the advanced Geostationary Operational Environmental Satellite has detected extremely long-lived contrails.
Ackerman's research is considering whether the contrails are changing the chemistry in the upper atmosphere.
Jet motors expel invisible oxides of nitrogen, carbon (dioxide and monoxide), soot and unburned hydrocarbon fuel in addition to the water vapour which becomes the white trails we see. Jet planes pollute just as autos do, but the location of jet pollution is more serious in its impact on atmosphere and climate.
At cruising altitude, either slightly below the stratosphere or in it, the temperature is very low. The air is also very dry and relatively much cleaner than on the ground. The waste gases in aircraft exhaust have very complicated effects that are different from the effects of vehicle exhaust.
It appears that high clouds originating from contrails spread to cover tens of thousands of square kilometres. They trap heat. Warming a part of the atmosphere that is normally much colder may break down its structure. This effect is more pronounced where a large number of planes fly fixed routes.
Also, water from jet exhausts is being introduced in massive and ever increasing amounts at the boundary of the stratosphere.
The gases emitted from the jet aircraft flying in the stratosphere reduce the natural ozone.
The thousands of aircraft traversing the North Atlantic flight corridor fly an estimated 44% of the time in the stratosphere. Civil aircraft fly in the stratosphere from 17% to 25% of the time. This flight time will increase as newer generations of jets fly higher and longer. Some estimates predict the total flight time of civil aircraft in the stratosphere could double by 2020.
[Abridged from the Environment News Service.]