Does commercial jet traffic affect climate?

Is this atmospheric graffiti a problem? Research by Penn State geography professor Andrew Carleton suggested it could be. Contrails "can extend the natural cirrus cover," Carleton explained, and unlike most clouds, cirrus tend to warm the surface overall because they trap heat more than they reflect the sun's radiation. "This is a concern to climate scientists because it could mean that a lot more contrails would make global warming worse."

Although scientists had suspected that contrails affect regional temperatures, there was no way to truly test the idea until the terrorist attacks on Sept. 11, 2001. In the enforced no-fly period following the collapse of the World Trade Center, air traffic was completely stopped for three days and scientists were able to directly compare temperatures logged in the presence of contrails against temperature data collected with contrail-free skies.

"I remember walking to and from my office (during that time) and thinking how incredibly clear the skies were," recalled Carleton. He mentioned this to a colleague and former doctoral-degree student of his, David Travis of the University of Wisconsin, who had noticed the same thing. "Then we started thinking that we should look at the temperature conditions" during those days in September and compare them to years past, Carleton said.

Looking at daytime highs and nighttime lows, Carleton and Travis found the average daily temperature range across the no-fly period to be almost 2 degrees Fahrenheit larger than when jets do fly. This implies, Carleton explained, that contrails lower daytime maximum temperatures and increase nighttime low temperatures -- probably in the same way that cirrus clouds do, by blocking some solar radiation from reaching earth's surface during the day and insulating against heat loss at night.

Cirrus coverage also rose in the North Pacific and North Atlantic flight corridors. The trends in cirrus cover and warming over the United States were greatest during winter and spring, the same seasons when contrails are most frequent. These results, along with findings from earlier studies, led to the conclusion that contrails caused the increase in cirrus clouds.

"This study indicates that contrails already have substantial regional effects where air traffic is heavy, such as over the United States. As air travel continues growing in other areas, the impact could become globally significant," Minnis said.

Humidity is the amount of water vapor in the air and determines how long contrails remain in the atmosphere. Contrails that persist for an extended period of time are most likely to impact the climate.

Contrails form high in the atmosphere when the mixture of water vapor in the aircraft exhaust and the air condenses and freezes. Persisting contrails can spread into extensive cirrus clouds that tend to warm the Earth, because they reflect less sunlight than the amount of heat they trap. The balance between Earth's incoming sunlight and outgoing heat drives climate change.
http://www.nasa.gov/home/hqnews/2004/apr/HQ_04140_clouds_climate.html

In 1993, a study of toxic emissions at Chicago's Midway Airport revealed that arriving and departing planes released more pollutants than the industrial pollution sources in the surrounding 16-square-mile area. A more recent study at London's Heathrow airport showed that aircraft contributed between 16 and 35 percent of ground level NOx concentrations.
The Advanced Subsonic Combustion Rig Image right: The Advanced Subsonic Combustion Rig is a state-of-the-art facility for analyzing and fine-tuning the performance of aircraft engine combustors. Credit: NASA

Because of local concerns about the gases exhausted by airplanes, the expansion plans of several U.S. airports—Atlanta, Boston, Chicago, Houston, Los Angeles, New York, Philadelphia, Phoenix, and Washington—have been stopped. Internationally, NOx emission limits set by the ICAO could result in strategies to limit air traffic, such as per flight emissions fees and curfews. Environmental concerns together with the prediction that world air traffic will grow 5 percent annually for the next decade have made reducing aircraft NOx emissions a priority.
http://www.nasa.gov/centers/glenn/about/fs10grc.html