Wispy white jet contrails are a familiar sight, a sign of today's considerable air traffic and, to some people, a visible reminder of environmental threat.
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 the University of California-Riverside. "It's going to cool things in the daytime and warm things at night."
Scientists are studying contrails' impact on everything from climate change to crops.
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.
Contrails have increased since the 1970s, primarily because of a change in the atmospheric pressure pattern known as the Arctic Oscillation. It influences weather in northern, middle and high latitudes, Carleton said.
Climatologists believe the change may be related to global warming, he said.
Contrails keep daytime high temperatures lower than if the skies were clear, and they can raise the nighttime temperatures. Decreasing the temperature variation near the Earth's surface could change heating and cooling patterns and the local wind systems, which might affect evaporation rates and agriculture, Carleton said.
It is hard to talk about contrails without someone mentioning "chemtrails," which some say look similar to contrails but are really chemicals sprayed secretly by the government to control the weather and climate. Some go as far as saying it is a conspiracy to reduce the population.
Norman Meek, a geography and environmental studies professor at Cal State San Bernardino, dismisses the conspiracy theory.
"We know contrails have an effect. The chemtrails are something in the imagination," he said.
Rosalind Peterson, a former crop loss adjustor for the U.S. Department of Agriculture, has spent almost a decade documenting contrails. Persistent jet contrails, she said, "are keeping us in manmade cloud cover."
Peterson has amassed more than 19,000 documents on her websites -- CaliforniaSkywatch.com and AgriculturalDefenseCoalition.org -- that she says proves contrail coverage has been significant enough over the past 30 years to decrease U.S. agricultural production.
Peterson said she never uses the word "chemtrails" because it lacks scientific documentation. But she does believe the government is involved in widespread geo-engineering -- modifications of the Earth's energy balance to reduce temperatures and counteract human-caused climate change. Those efforts are causing increased cases of asthma and other adverse health effects, she said.
Peterson has lobbied for full disclosure and public debate about any climate-altering programs. She wants changes to jet flight elevations, routes and fuels to reduce the man-made cloud cover.
Global air traffic contributes 2 percent to 3 percent of carbon dioxide emissions, the main greenhouse gas linked to global warming. A May study of computer models by the American Chemical Society forecast that carbon dioxide emissions from air traffic likely will double or triple in the next 50 years.
Possible solutions include making planes fly lower, outside the atmospheric conditions that create contrails, or around those areas, Carleton said, adding that would increase fuel consumption and create more greenhouse gases.
Carleton worked with David Travis, a University of Wisconsin-Whitewater geography professor, on a study of the skies after the World Trade Center attacks on Sept. 11, 2001, when airline traffic was halted for three days.
They found the average daily temperature range between daytime highs and nighttime lows was almost 2 degrees greater than when jets fly. They concluded the contrails block some sunlight from reaching Earth during the day and prevent heat loss at night.
In 2004, NASA researcher Patrick Minnis isolated contrails on satellite data to calculate their effect on solar radiation, finding that overall the vapors contribute to warming.
"They're trying to transport people or goods from one place to another, and they happen to make these clouds," he said.
Now Minnis is working on the Aviation Climate Change Research Initiative, a venture of NASA and the Federal Aviation Administration to refine methods for detecting contrails and measuring their impact on climate.
"There's a lot of uncertainty," Minnis said. "If it turns out to be a higher number, things might need to be done to cut back on it."
(Distributed by Scripps Howard News Service, www.scrippsnews.com.)