 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. "The science around this isn't very clear," said Sarah Brown, spokeswoman for CarbonNeutral Co, an offset company that allows travellers to invest in renewable energy projects to soak up emissions from flights. The company uses British Environment Ministry data that excluding climate side-effects of aviation. "The science of radiative forcing is currently uncertain," it said, referring to the effects that go beyond carbon dioxide. Germany's Atmosfair (www.atmosfair.de), whose patrons include former U.N. Environment Programme chief Klaus Toepfer, covers factors such as the release of nitrogen oxide. "We're trying to estimate the overall effect," said Robert Muller at Atmosfair. He said airlines such as British Airways or Scandinavian SAS worked with companies with low estimates when offering customers offsets. Take a one-way flight from Sydney to London, for instance -- CarbonNeutral estimates each passenger is responsible for 1.9 tonnes of greenhouse gases, costing 20.95 euros ($28.46) to offset. The same route with Atmosfair works out at 6.4 tonnes, and a charge of 130 euros to offset. Outside Heathrow, about 250 campaigners are camping in tents on the path of a proposed third runway for the world's busiest international hub. More and more people fly, partly because companies have axed ticket prices despite high fuel costs. International flights are now excluded from the Kyoto Protocol, the main UN plan for curbing climate change to 2012. The European Union is among those aiming to include aviation after 2012 while the United States is opposed. A report by the UN climate panel in May said extra charges for fuel or the inclusion of the aviation sector into a greenhouse gas trading scheme "would have the potential to reduce emissions considerably". "A first possible approach is where initially only carbon dioxide from aviation is included in for example an emission trading system," it said. Parallel measures could be differing airport charges according to nitrogen oxide emissions.