The Sky's LimitAir travel is a rapidly growing source of greenhouse gases. But it is also

The Sky's Limit

Air travel is a rapidly growing source of greenhouse gases. But it is also an indispensable way of travel.

The new A380

The double-decker A380, the biggest airliner the world has seen, landed at Heathrow last month to test whether London's main airport could handle the new 550-seater, due to enter commercial service at the end of this year. It was a proud moment for Britain's Rolls-Royce, the makers of the aircraft's Trent 900 engines. Rolls-Royce says the four Trents on the A380 are as clean and efficient as any jet engine, and produce "as much power as 3,500 family cars". A simple calculation shows that the equivalent of more than six cars is needed to fly each passenger.

Take the calculation further: flying a fully laden A380 is, in terms of energy, like a 14km (nine-mile) queue of traffic on the road below. And that is just one aircraft. In 20 years, Airbus reckons, 1,500 such planes will be in the air. By then, the total number of airliners is expected to have doubled, to 22,000. The huge airplane alone would be pumping out carbon dioxide (CO2) at the same rate as 5 million cars.

That may not seem much compared with the 60 million vehicles that pour off assembly lines every year—or the 1 billion vehicles already on the world's roads. But whereas cars are used roughly for about an hour or so a day, jet airliners are on the move for at least 10 hours a day. And they burn tax-free, highoctane (高能量的) fuel, which dumps hundreds of millions of tonnes of CO2 into the most sensitive part of the atmosphere.

Aviation is a relatively small source of the emissions blamed for global warming, but its share is growing the fastest. The evidence is strong. As a result, aviation is increasingly attracting the attention of environmentalists and politicians. Amid much controversy, CO2caps (最高限制) and carbon-trading could soon be used to help curb aircraft emissions.

Frequent flyers, free riders

Airlines are accused of having a free ride in terms of air pollution because they pay no tax on the fuel they use for international flights. Even though today's aircraft are about 70% more efficient than those of 40 years ago, concerns over emissions have grown. Despite booming demand for air travel, many airlines are losing money. Now green campaigners want people to think twice before they fly. The opposing voice is particularly loud in Europe, where low-cost carriers are expanding fast on busy shorthaul (短距离) routes. The European Parliament will vote in July on a proposal to limit aircraft emissions.

America is deeply unhappy at the prospect of its airlines being affected. Sharon Pinkerton, a senior representative of the Federal Aviation Administration insisted, on a visit to Brussels last year, that American carriers should be exempted from the scheme. This sets the scene for another transatlantic aviation dispute, to add to the two bitter and long-running disputes over subsidies to Europe's Airbus and the liberalisation of air traffic between the two continents.

The airlines are growing nervous. The big international carriers represented by the International Air Transport Association (IATA) would rather Europe waited for the deliberations of a United Nations body, the International Civil Aviation Organisation (ICAO), which has set technical, legal and safety rules for more than 50 years. International aviation was excluded from the Kyoto protocol on global warming, but only on condition that, by the end of 2007, countries and airlines worked under the umbrella of ICAO to come up with a way of reducing emissions through a trading scheme.

Soon after the end of the second world war the member governments of ICAO agreed that airlines should be free of fuel taxes. Some say this was to outlaw unilateral taxes that could distort markets, but others reckon it was done to boost the fledgling airline in

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