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  3. The Sky’s Limit Air travel is a rapidly growing source of greenhouse gases. But it is also an indispensable way of travel. T

The Sky’s Limit Air travel is a rapidly growing source of greenhouse gases. But it is also an indispensable way of travel. T

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问题                             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 industry emerging from the fighting. The corollary was that aviation, unlike motor traffic and other forms of transport, would pay in a transparent manner for the infrastructure and services it required-air-traffic management, landing charges, flyover rights and so on. That was supposed to take care of the external costs. But no one in those days thought much about the environment.
Counting the cost
    It was not until 1999 that the United Nations Intergovernmental Panel on Climate Change (IPCC) attempted to reduce the effect of aviation on the environment. Transport as a whole was judged to be responsible for about a quarter of the world’s CO2 discharges. That makes it one of the biggest sources, alongside power generation and households, as a source of the gas. Within transport, aviation accounts for about 13%. Its contribution to total man-made emissions worldwide is said to be around 3%.
    So why all the fuss about so little? One reason is that high-altitude emissions are probably disproportionately damaging to the environment. The nitrogen oxides from jet-engine exhausts lead to the formation of ozone, another greenhouse gas. Contrails (飞行云) are also suspected of enhancing the formation of cirrus clouds, which some scientists think adds to the global warming effect. The IPCC estimated that the overall impact on global warming of aircraft could be between two and four times that of their CO2 emissions alone, though there is no scientific consensus about the size of this multiplier.
    Naturally, the airlines choose to measure the greenhouse gases they produce in the way that casts them in the best light — a trick they deploy on safety statistics, too. For instance, over half of aircraft accidents occur around take-off and landing. So accidents per passenger-mile compare very favourably with other means of transport. But at least one study has shown that, if accidents are measured per journey instead, aircraft are the second-most dangerous way of travelling, after motorcycles.
    Likewise on greenhouse gases. IATA says an aircraft’s fuel consumption is about the same as that of a family car, at 3.5 litres per 100 passenger-kilometres. So CO2 emissions are similar. But that is true only if the aircraft is full and the car’s passenger seats are empty. And even then, a jumbo jet flying from London to Sydney would be like nearly 400 Volkswagen Polos each travelling just over 16,000km—the average distance a European drives in a year. In other words, although cars and aircraft discharge roughly the same amount of CO2for each passenger-kilometre, the aircraft travel an awful lot farther.
Waiting to land
    Crowded airports compound the problem. Busy runways at places such as Heathrow mean aeroplanes have to circle wastefully. The possibility of being held up ensures that pilots carry extra fuel, thereby increasing the aircraft’s weight and, hence, its consumption of fuel. Other small changes could further save fuel and avoid carbon emissions: aircraft could be towed everywhere on the ground by electric vehicles. Consumers, too, can take a stand by voluntarily offsetting the carbon emissions associated with flying by paying, for instance, to have trees planted.
    This week IATA said the net loss of the world’s airlines in the past six years would amount to almost $44 billion. Carriers have been hit by terrorism, war, recession, the respiratory disease SAILS and soaring oil prices. There were hopes the industry could make a small profit in 2007, but having to pay for environmental costs could change that. Yet global warming is not something that airlines, or any other industry, can shake off for ever. Sooner or later, aviation will have to shoulder the burden it imposes on the planet.
Although aviation accounts for only 3% of the total man-made emission, its______ are believed to be disproportionately damaging to the environment.
选项
答案high-altitude emissions
解析 答案在第三部分第二段开头
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