Before long, the airlines of the United States may be flying in environmentally friendlier skies -- whether they like it or not.
The European Union, which has taken several steps to address climate-change concerns, is preparing to crack down on greenhouse gas emissions by all airlines that do business in Europe, imposing cap-and-trade rules designed to encourage emissions reductions for commercial aircraft that serve the region. The European Parliament is scheduled to take its first vote on the plan this week.
U.S. carriers and the Bush administration don't like the proposal -- the White House has already called it a violation of postwar aviation agreements -- but they're unlikely to do anything that might jeopardize access to the lucrative European market.
The upshot is that airlines, which have been overlooked in much of the debate over climate change, will probably become one of the first test beds for a broader system of carbon restrictions in the United States. That's because Europe is likely to impose rules on them before Congress acts on any climate-change legislation that would affect the United States in a broad fashion.
"If airlines had to do it and it ended up not being a huge burden, that would be a very valuable lesson," said an economist with ties to the aviation industry who for that reason asked not to be identified. "We're already moving down that pathway."
The Federal Aviation Administration, in fact, is in partnership with airlines, aircraft manufacturers, airports, oil companies and universities on a Commercial Aviation Alternative Fuels Initiative. Representatives met in Washington last week to see how they might accelerate the development of fuels that would produce fewer emissions.
The aviation industry is responsible for about 3.5 percent of the greenhouse gas emissions and other agents, such as water vapor from hot exhausts, that can trap solar heat and contribute to global warming, according to the U.N.'s Intergovernmental Panel on Climate Change. That amount is small compared with automobile emissions but still substantial.
"That's about the same size of emissions as a medium-sized country, say Canada or the U.K.," said Annie Petsonk, international counsel for the advocacy group Environmental Defense. "If aviation were its own country, we would look to it and expect it to step up to the plate, to participate in capping and cutting emissions."
But airline travel is increasing, as is its effect on climate. World airports recorded 4.4 billion passengers in 2006 and expect the total to reach nine billion by 2025, according to the trade group Airports Council International.
"The concern is more, not what the impact is right now but what will the impact be 20 years from now, after many countries have put into place policies to try to reduce greenhouse gas emissions" for other industries, said Donald J. Wuebbles, director of the University of Illinois' School of Earth, Society and Environment. "At that point, aviation's percentage may have risen dramatically."
The U.N. panel estimates that by 2050, aviation could contribute up to 15 percent of the factors that lead to global climate change. The industry contends that its contribution would be about 5 percent.
Governments in several countries concerned about global warming have begun to assess aviation's impact. The European Union has been considering whether and how to add airlines to its existing cap-and-trade system, under which companies must meet emissions limits or buy carbon credits from those that do more than meet the standards.
The European Commission -- the EU's executive body -- proposed that European air carriers be brought under the plan by 2011, and international carriers by 2012.
The United States, Australia and China have balked at the EU plan and are girding for a trade fight, but the proposal expected to emerge from the European Parliament is even stricter and would extend the restrictions to foreign airlines at the same time as European carriers.