By JANET ZIMMERMAN
Wispy white jet contrails are a familiar sight, a sign of today's considerable air traffic and, to some people, a visible reminder of the environmental threat that comes with it.
The trails -- formed when moisture condenses around aircraft engine exhaust -- create cirrus clouds that block solar energy from above and trap heat below. They may be contributing to warming of the Earth's surface temperature, NASA studies show.
"There is absolutely an effect," said David Mrofka, a climate change lecturer at UC Riverside. "It's going to cool things in the daytime and warm things at night."
Numerous theories surround the trails. Scientists are studying contrails' impact on everything from climate change to crops, and conspiracy theorists contend they are the result of dangerous government experiments and cause health problems.
Contrails occur in clusters because of favorable atmospheric conditions -- temperatures below minus 40 degrees and high humidity at 30,000 feet altitude, said Andrew Carleton, a climate science professor at Penn State University in University Park, Pa. Those clusters occur over the United States, Europe and, increasingly, over Asia and Southeast Asia as air traffic grows. More than 8.3 million domestic and international flights crossed U.S. skies in 2010, according to the federal Department of Transportation. In 2003, the last year analyzed in a NASA study, there were about 27,000 flights per day over the United States that could cause contrails.
There has been an increase in contrails since the 1970s, primarily because of a change in the atmospheric pressure pattern known as the Arctic Oscillation, which influences weather in the northern, middle and high latitudes, Carleton said.
Climatologists don't know why it has changed but believe it may be related to global warming, he said. Read More
Report: Peer into the 'deep past' to divine future warming: Study Reports by 2100 the Earth will have more greenhouse gases in its atmosphere than at any time in the past 35 million years.