"Efficiency is only set to improve at 1 or 2 percent per year at best, while the number of passenger kilometers is growing at 5 or 6 percent," says Peter Lockley, head of policy development at the Aviation Environment Federation, a British think tank. "So emissions are going up steadily in the gap between the two."
Then there are alternative fuels. Radical concepts like hydrogen-powered aircraft are still considered to be decades away. But serious work is being done on biofuels as an alternative to kerosene in aircraft. Last year, British entrepreneur Richard Branson promised to plough all profits from his air and rail companies into a new business, Virgin Fuels, that would fund development of biofuels.
Scientists are skeptical, though, of the potential for running jets on biofuels. Then there is the area of land required to produce fuel in sufficient volume. Already, environmentalists are concerned at the way rainforest is being destroyed to make way for palm oil, a biofuel crop.
Lockley says that one study concluded that supplying the US commercial fleet with a 15-percent mix of biofuel would require planting an area the size of Florida with soya beans.
Given the limited prospects for a technological solution, a growing body of opinion is arguing for efforts to manage demand for air travel. "What matters is the next 10 to 15 years, and technology can do very little in that time frame," says Kevin Anderson of Britain's Tyndall Centre for Climate Change Research. "The principal issue is to reduce the rate of growth of air travel."
Experts point to several options. Europe is planning to include aviation in its emissions-trading plan starting in 2011. The hope is to set an example to the rest of the world, chiefly China and India, where aviation growth is surging, that concerted efforts can make a difference.
Airlines will get a limited number of CO2 permits that can be traded; top polluters will have to buy additional permits, hurting their bottom line. The idea is to give airlines incentive to operate cleaner aircraft; higher ticket prices may result as well, reining in demand.
But experts note that caps will be set fairly high, weakening the imperative; ticket prices are expected to rise by only a couple of euros, if that. Consumer behavior may thus be little affected.