The aviation industry may be more damaging to the environment than widely thought because aircraft not only release carbon dioxide but they also produce other harmful gases that warm the earth, experts said.
A tented camp of about 250 climate protests at London's Heathrow airport this week highlights pressures to include aviation in a global pact to fight global warming. But planes are among the least understood sources of emissions.
"Growth is going to continue, but it is complicated to estimate the effect of aviation on the climate," said Ivar Isaksen, a professor at Oslo University who is an expert in how aviation affects the atmosphere.
He said that aviation's impact went far beyond carbon dioxide, the main greenhouse gas, that many governments rely on for calculations.
Aviation accounts for about 2 percent of world emissions of carbon dioxide and projected passenger growth of 5 percent a year will far outstrip efficiency gains from better fuel or plane design, U.N. studies say.
Planes' climate impact may be magnified by factors including heat-trapping nitrogen oxides that are more damaging at high altitude. Jet condensation trails may contribute to the formation of a blanket of high-altitude cirrus clouds.
Cirrus clouds usually warm the earth's surface, increasing the impact of aviation on global warming.
A 1999 U.N. report, for instance, estimated that aviation's impact on the climate was two to four times greater than simply the carbon dioxide emitted by burning jet fuel.
