Air travel, which makes up less than three percent of man-made CO2 emissions, has become the whipping boy for environmentalists, The Christian Science Monitor reports. 

Two factors have placed airline travel in the center of the global-warming debate: The number of airline tickets sold per year could double to more than 9 billion by 2025. In addition, experts see no viable jet-fuel alternative to kerosene.

In Britain, CO2 emissions from flights will surpass those from car trips in the next six to eight years, making it a very hot topic there. Britain has just increased a tax on airline flights from $19 to $76, depending on the length of the flight, in the name of reducing air travel and CO2 emissions.

Airlines are working on lighter composite materials. Other fuel-saving suggestions include pulling planes from the gate to the runway with their engines only idling. A new air traffic management system could yield a 12 to 15 percent improvement in environmental performance. Biofuels, possibly ethanol made from switch grass or biowaste, could provide an alternative. But no one knows when biofuels could replace Kerosene.

The European Union may pull aviation into its carbon-emissions trading plan, allowing airlines to “buy” the right to emit carbon from other industries.