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Aircraft emissions: The dirty sky
All big ideas start life on the fringes of debate. Very often it takes a shocking event to move them into the mainstream. Until last year interest in climate change was espoused mainly by scientists and green lobbyists-and the few politicians they had badgered into paying attention. But since Hurricane Katrina, something seems to have changed, particularly in America.
There are plenty of anecdotal signs of change: Britain's pro business Tories have turned green; Al Gore is back in fashion in America. Companies are beginning to take action and encouraging governments to do the same. Europe already has an emissions trading system (ETS) for its five dirtiest industries. In America, although the Bush administration still resists federal legislation, more and more states do not.
So far the political about global warming have centred on two polluters, smoggy factories and dirty cars. Next month the European Parliament will vote on whether to extend its emissions trading system to airlines. If it decides in favor, the whole industry will feel the impact, for it will affect not just European airlines but all those that fly into and out of the EU. Talk about this prospect soured the International Air Transport Association's annual meeting this week in Paris. But whatever happens in the EU, the airlines look set to face vociferous demands that they should pay for their emissions.
In some ways, the airlines are an odd target for greens. They produce only around 3% of the world's man made carbon emissions. Surface transport, by contrast, produces 22%. Europe's merchant ships spew out around a third more carbon than aircraft do, and nobody is going after them. And unlike cars-potent symbols of individualism-airlines are public transport, jamming in as many people as they can into each plane.
What's more, many air travelers cannot easily switch. Car drivers can hop on the train or the bus, but transatlantic travelers can't row from London to New York. Nor can aircraft fuel be swapped for a green alternative. Car drivers can buy electro petrol hybrids but aircraft are, for now, stuck with kerosene, because its energy density makes it the only practical fuel to carry around in the air.
Yet in other ways, airlines are a fine target. They pay no tax on fuel for international flights, and therefore escape the "polluter pays" principle even more niftily than other forms of transport. Their emissions are especially damaging, too-partly because the nitrogen oxides from jet engine exhausts help create ozone, a potent greenhouse gas, and partly because the pretty trails that aircraft leave behind them help make the clouds that can intensify the greenhouse effect.
Slowly, businessmen and politicians are coming to agree with scientists. If this generation does not tackle climate change, its descendants will not think much of it. That means raising costs for all sources of pollution. Even those deceptively cheap weekend breaks cannot be exempt.
飞机排放物：肮脏的天空
所有的重要观念都在争论的边缘产生，而且通常发生了令人震惊的事件后这些观念才会成为主流。直到去年为止，关心气候变化的人群主要是科学家以及环保主义者--以及少数一些对此表示关心的政客。但是自从卡特里娜飓风出现以来，有些事情似乎开始改变，特别是在美国。
有足够的迹象表明变化的出现：英国的亲商保守党开始向绿党阵营转变；Al Gore在美国重新变得受欢迎。各个公司开始采取行动，同时也鼓励政府这样做。欧洲已经为它污染最为严重的5个工业制定了排放交易方案。在美国，尽管布什政府仍然抵制进行联邦立法，越来越多的州却开始地方立法。
到目前为止有关全球变暖的政治纷争主要集中在两种污染者身上：排放废气的工厂以及污染的汽车。下个月欧洲议会将投票决定是否在航空业适用排放交易方案。如果决定是肯定的，那么整个民航业将受到影响，因为这个方案不仅影响欧洲的航线，同样也对到达欧洲和飞离欧洲的航线产生作用。谈论这种可能性使在巴黎召开的国际航空运输协会年会的气氛变得紧张起来。但是不管欧盟如何决定，航空公司看起来必须面对他们应当为他们排放的废气付费的强大的呼声。
在某些方面，航空公司对绿色政治组织来说是一个奇怪的目标。它们排放的碳污染仅仅只占世界人为碳污染的3%。相较而言，地面运输占了22%。欧洲商船排放的碳比飞机排放的要多三分之一左右，但是却没有人追究它们。而且不像汽车--利己主义的重要标志--飞机是公共交通，在每架飞机内塞入尽可能多的人。
何况许多乘飞机出行的人无法那么容易地改乘其他交通工具。小汽车司机可以乘火车或者公车，但是一个横穿大西洋的旅行者不可能从伦敦划船到纽约。同样的，航空燃料也不能用无污染的替代品来替换。小汽车司机可以购买电力汽油混合动力的汽车，但是飞机只能用煤油，因为它的能量密度使其成为唯一可以在空中携带的燃料。
但是从其他方面来说，航空公司又是一个很好的攻击目标。他们不为国际飞行中的燃料付税，从而比起其他运输工具来更多地从"污染者付费"的原则中逃脱。而且他们的排放物特别具有破坏性，部分原因是喷气式发动机排放的氮氧化合物促使温室气体臭氧的合成，部分原因是飞机飞过后留下的漂亮尾烟帮助云层形成从而加剧温室效应。
慢慢地，生意人和政治家们都开始同意科学家的看法。如果这一代人不解决气候变化的问题，那下一代人也不会重视。这意味着将为所有形式的污染付出更高的成本。就算是现在似乎很便宜的周末旅行也不能例外地变得昂贵。
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