Environmentalists and many state and local air pollution officials argue that the standards are too weak. The EPA says limits now in place will slow the growth of aircraft emissions, but more stringent standards "will likely be necessary and appropriate in the future," says Margo Oge, director of the agency's Office of Transportation and Air Quality.
Last month, FAA Administrator Marion Blakey proposed changes in air traffic control procedures and expansion of U.S. airports to accommodate the projected increase in commercial flights, a strategy widely decried by critics.
"The FAA protects its customers: the airports and the industry," says Jack Saporito, executive director of the Alliance of Residents Concerning O'Hare, a Chicago group that opposes plans to expand O'Hare airport. "It does not protect the public, their families' health or our environment, though it pretends to."
In written answers to questions from USA TODAY, the FAA says aircraft emissions "are not expected to be the fastest-growing contributor to global warming."
Don Wuebbles, a University of Illinois professor of atmospheric science who chaired the panel of scientists brought together by NASA and the FAA, says the projected growth in aviation could make aircraft emissions one of the fastest-growing contributors. But he acknowledges many uncertainties, including aviation's role in global warming and the growth of other pollution sources abroad.
What is known, he says, is that it's "much harder" to reduce carbon dioxide emissions from aviation. Jet engines are already energy efficient, and technology to significantly reduce carbon dioxide from them isn't as far along as it is for land-based pollution sources.
Besides carbon dioxide, jet engines emit many pollutants into the atmosphere, including nitrogen oxides, sulfur oxides, soot and even water vapor. Carbon dioxide and water vapor are called greenhouse gases, because they trap heat and contribute to global warming.
Though planes contribute to air pollution while on the ground, scientists studying global warming are most concerned about pollutants emitted when a plane is airborne. Jets are the major source of emissions deposited into the upper atmosphere, where some pollutants have a greater warming effect than when they are released in the same amount from the ground, according to a 1999 scientific report sponsored by the United Nations.
Some pollutants emitted from engines during flight warm the Earth by adding to the heat-trapping gases, both natural and man-made, already in the atmosphere. Also, jet contrails -- the vapor trails they leave in the sky -- add to cloud cover and may contribute to the warming of the planet. A contrail forms when water vapor from the engine cools and mixes with air and the humidity becomes high enough for condensation.
NASA scientist Patrick Minnis has studied contrails and believes they may have a prominent role in global warming. A 2002 report by the British scientific commission agrees, concluding that "aviation-induced cirrus clouds will be a significant contributor to warming." But Minnis says another NASA study concludes that the contrails have little effect on global warming. Further research is being done.
Carbon dioxide is a heat-trapping gas that can remain in the atmosphere about 100 years. Scientists say planes' engines emit up to 3% of all carbon dioxide that contributes to global warming, but the figure appears to be on the rise.
University of Washington scientist Richard Gammon says carbon dioxide emissions from aircraft are rising more rapidly than those from any other source.
Nitrogen oxides emitted from aircraft engines react with other gases in the air to form another heat-trapping gas, ozone. Their effect on global warming is unclear because nitrogen oxides have another effect that may be beneficial: They remove methane, which can cool the air.
Except for carbon dioxide's contribution to global warming, "There remain significant uncertainties on almost all aspects of aircraft environmental effects on climate," according to the report this year by Wuebbles and other scientists.
Though uncertainties about global warming abound, there's no doubt that jet engines must have stricter emission standards, says the National Association of Clean Air Agencies, which represents pollution control officials in 49 states and 165 metropolitan areas. The group is suing the EPA.
Clouds of ice formed in the trails of jet exhaust trap heat and prevent the earth from cooling.
Aviation is going green from ticketing to takeoff by modernizing equipment, reducing fuel consumption and exploring new technologies.
Aviation expert Mike Boyd said he thought high-profile announcements by airlines are akin to building a firebreak to show the industry is taking some action.