Jet traffic may be having a negative impact on the environment by causing the atmosphere to warm up, according to a recent study conducted by Penn State geography Professor Andrew Carleton and David Travis, geography and geology professor at University of Wisconsin-Whitewater.

"As [contrails] persist and spread out, they become thinner, which means they probably trap more long-wave radiation from Earth's surface, having a warming effect," Carleton said.

He added that as the amount of vapor trails increases, it covers more surface area, which traps more sunlight and allows for even more warming.

Over the course of three years, the research has looked at the effect of jet traffic on the environment by looking at air temperature. Carleton and Travis used satellite data to look at how jet contrails, or jets' vapor trails, have changed with increased air traffic.

Since similar research had been completed in the mid-1970s, the eastern part of the country had a dramatic increase in amount of jet contrails while the western part of the country has only changed minimally -- fluctuating between a slight decrease and slight increase.

Carleton said this change probably occurs because the eastern atmosphere is colder now than it was then and the western atmosphere has actually warmed some.

"But no one really knows why this has occurred," he said. "Now there is even some research to show that it is going the other direction and a reversal is occurring."

Initially, their research was just going to be compared against contrail data from the mid-1970s, Carleton said. However, after Sept. 11, 2001 all flight traffic was grounded for several days so they included that in their research as well.

"We studied the change in temperature range that occurred during the three-day period immediately following the 9/11 terror attacks when airplanes were grounded and compared those to the three-day period immediately before and afterwards," said Travis in an e-mail message.

Before conducting the research, the pair hypothesized that when the planes were grounded, a lack of clouds would be produced, causing the temperature range to be greater, Travis said. He added that their research confirmed the prediction.

After studying about 5,000 weather stations around the United States, their research showed that the change in temperature range between when the planes were grounded and the before and after when planes were flying produced the largest change in temperature range in the past 30 years for the fall season, Travis said.

"On average across the country, there was a one-degree Celsius increase in the diurnal -- or daily -- temperature range," Carleton said. That can be translated to almost a two-degree Fahrenheit change.

He also said that where more jet contrails occurred, over areas that receive more air traffic, there was more than a two-degree Celsius increase.

In an effort to protect the environment from the buildup of jet contrail formations, Travis mentioned several possibilities for commercial jet plane companies.

"It is possible that airplanes could alter their flight paths or fly at significantly higher or lower altitudes to reduce the likelihood of persisting contrail formation. However, this would pose new environmental risks such as higher altitudes threatening the ozone layer and lower altitudes causing increased air and noise pollution at the surface," he said.

Also, Travis said that if flight patterns were altered, it would result in longer flights and more fuel usage.

"Unfortunately, none of these alternatives are ideal, but since airplanes typically fly at altitudes that make persisting contrail formation most likely, it is necessary to at least consider these alternatives," he said.

Carleton agreed with his colleague.

"Commercial companies are running a business, not knowing the science behind it," he said.

He also mentioned that in Germany, the scientific community is trying to use other contrail research to have government policy implemented in an effort to decrease contrail formation.

"That is a long way off for the United States, though," Carleton said.

Carleton and Travis said they will be collaborating on future research.

"We are currently finishing up research that is looking at the causes of increases in jet contrail coverage beyond simply increased number of flights," said Travis. "These results are currently in review and will hopefully be published later this year."