Fewer contrails would be good for the climate

学习目标

• 区分降温作用、增暖作用和净效果
• 拆解三组how从句与让步结构
• 理解反事实估值为什么难
• 保留试验前提、数字分母与作者的论证转折

GOOD NEWS on global warming is thin on the ground. But there may be some up in the air. Operation Blue Skies, announced on August 18th, is a scientific study being undertaken by Britain’s air-traffic-control service, NATS, and various organisations where people study the effects on climate of the streaks of cloud created by aircraft—known as contrails.

关于全球变暖的好消息难得一见。 不过，空中或许就有一些。 8月18日公布的“蓝天行动”是一项科学研究，由英国空中交通管制机构NATS与其他多个机构共同开展；这些机构研究飞机制造的条带状云迹——即凝结尾迹——对气候的影响。

Over the next two winters the consortium aims to see if it is possible to reduce the number of persistent contrails—the class with the largest such effects—over a swathe of the North Atlantic, thereby measurably reducing the climate impact of civil aviation. If this experiment works, a worldwide application of the same techniques might cut the climate effects of air travel by as much as half.

未来两个冬季，联合研究团队希望看看，能否减少北大西洋一大片区域上空的持续性凝结尾迹——这是气候影响最大的一类——从而可测量地降低民用航空的气候影响。 如果试验成功，在全球应用同样技术，或许能使航空旅行的气候影响减少多达一半。

On a sunny day the wispy cirrus clouds into which some contrails are transformed can look as if they might be cooling the planet in the manner of a delicate parasol. And some contrails do indeed cool things down beneath them a bit. But they also warm things up by trapping some of the upwelling infrared radiation that would otherwise escape into space. On average, that warming effect, found in all contrails (and the cirrus clouds they turn into), handsomely outweighs the cooling provided by some. In the climate scheme of things, contrails are a liability.

晴天时，一些凝结尾迹转化成的纤细卷云，看起来可能像一把轻巧的阳伞，正在给地球降温。 有些尾迹确实会让下方稍微凉快一些。 但它们也会截留一部分原本将散逸到太空的向上红外辐射，从而产生增暖作用。 平均而言，所有尾迹及其转化成的卷云都有的这种增暖作用，明显超过部分尾迹产生的降温作用。 从气候角度看，凝结尾迹是一项不利因素。

Happily, it appears that much harm can be avoided by small changes to flight paths. Persistent contrails appear only in what are known as “ice-supersaturated regions” (ISSRs). Often planes can avoid such regions just by flying a little bit lower. The purpose of Operation Blue Skies is to find out how well the presence of ISSRs can be predicted, how effective such little shifts in altitude can be in avoiding persistent contrails·, and how easily and safely the changes in flight path needed to bring them about can be integrated into routines for airlines and air-traffic control.

令人欣慰的是，小幅改变飞行路径，似乎就能避免很多危害。 持续性尾迹只出现在所谓的“冰面过饱和区”（ISSR）。 飞机往往只需稍微降低高度，就能避开这类区域。 “蓝天行动”的目的，是弄清楚能多准确地预测这些区域的存在，小幅高度调整能多有效地避免持续性尾迹，以及为实现前述调整所需的飞行路径变化，能多方便、多安全地纳入航空公司和空管的常规操作。

The cut of up to half in the climate-effects of air travel would come at a very small cost. The number of flights at lowered altitude might be small—as little as 5% of the total. Because altitude buys efficiency, those flights would consume more jet fuel, but less than 2% more.

航空旅行气候影响最多减少一半，所需代价可能很小。 需要降低高度的航班比例或许很低——最低可能只占总数的5%。 由于较高的飞行高度能带来效率，这些航班会消耗更多航空燃料，不过增加幅度不到2%。

Burning even that little extra fuel would be anathema to airlines. So some suggest the costs of action might be minimised, and the airlines incentivised, by setting up a market which somehow links contrail cooling to carbon prices. The European Union, which has had some success and plenty of experience with carbon markets, recently took a small step towards something along these lines. It is also the sort of idea to which economists—and The Economist—tend to be partial. But it would be a mistake nonetheless.

即使只多烧这一点燃料，航空公司也会深恶痛绝。 因此，有人建议建立一个以某种方式把尾迹相关降温效益与碳价联系起来的市场，从而尽量降低行动成本，并给航空公司提供激励。 欧盟在碳市场方面有过一些成功，也积累了大量经验，最近朝着这一思路迈出了一小步。 这也是经济学家——以及《经济学人》——通常偏爱的一类想法。 但即便如此，它仍会是一个错误。

Markets for pollutants have worked best when both the aim and the remedy are fairly simple—most famously, reducing sulphur-dioxide emissions from power stations in America. Contrails, by contrast, are complicated. Though there is no doubt that their net effect is warming, saying how much warming a particular contrail might produce is often difficult and sometimes next to impossible.

当目标和解决办法都相当简单时，污染物市场运行得最好；最著名的例子，是减少美国发电厂的二氧化硫排放。 相比之下，凝结尾迹要复杂得多。 虽然毫无疑问它们的净效果是增暖，但要说清某一条尾迹可能造成多少增暖，往往很难，有时几乎不可能。

The difficulties are made yet worse when the contrail being assessed is a purely notional one—as would be the case for the contrails avoided by changing flight paths. Valuing counterfactuals is always hard.

当被评估的尾迹纯属假设时，困难还会更大；通过改变飞行路径而避免的那些尾迹，正是这种情况。 为反事实情形估值，总是很难。

If contrail-avoidance turns out to be as useful as it looks, it would be much simpler to simply have air-traffic controllers assign routes that limit persistent contrails when it is possible and safe to do so. If some airlines choose to avoid the affected routes, others will surely be happy to take their place. And if such a scheme were also to show the benefits of innovations in air-traffic control (quite a few have been suggested) the industry would end up better off, as well as less polluting.

如果尾迹规避被证明像看起来那样有用，那么在可行且安全的条件下，直接让空管人员安排能限制持续性尾迹的航路，会简单得多。 如果一些航空公司选择避开受影响的航线，其他公司想必很乐意取代它们。 而且，如果这样的方案还展示出空管创新的好处——人们已经提出了不少创新建议——那么这个行业最终不仅污染更少，处境也会更好。

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