Contrail-free flying could help the climate

学习目标

• 读懂冰晶、辐射与持续性尾迹的形成机制
• 拆解长主语、嵌入从句与条件表达
• 区分航班数、航程、覆盖率和增暖份额
• 核对数量级，并说明短期收益不能替代长期减排

THE 40M scheduled passenger flights which took place in 2019 added up to about 61bn kilometres of travel—exceeding the distance from Earth to the Sun every day of the year. More than 40% of those flights left contrails behind them for at least some of the journey as water vapour in the exhaust from their engines froze into tiny particles of ice.

2019年执行的4000万班定期客运航班，飞行总里程约为610亿公里——平均到一年中的每一天，都超过地球到太阳的距离。 其中超过40%的航班，在至少部分航程中留下了凝结尾迹，这是因为发动机尾气中的水汽冻结成了微小冰粒。

Many of these trails were ephemeral. They streamed out behind their progenitor planes before fading quickly away. But over about 5% of the total distance flown—comfortably enough for ten round-trips a day to the Moon—they persisted and widened as water vapour already in the atmosphere joined in the freezing. Thin white lines became broad translucent stripes of cirrus cloud. Wind those stripes around Earth like yarn around a ball and you will find that, at any one time, persistent contrails cover maybe a thousandth of the sky. In doing so they warm the planet.

许多尾迹转瞬即逝。 它们在产生自己的飞机身后延伸，随后迅速消散。 但在约占总飞行距离5%的航程上——换算成每天往返月球十次还绰绰有余——大气中原有的水汽也加入冻结过程，使尾迹持续存在并不断扩宽。 纤细白线变成了宽阔、半透明的卷云带。 把这些云带像纱线绕球那样缠在地球上，你会发现，在任一时刻，持续性尾迹或许覆盖了天空的千分之一。 它们在这个过程中使地球变暖。

Contrails are white because their constituent ice crystals reflect sunlight. That can have a cooling effect during daytime. But the crystals also absorb infrared radiation and re-emit it, rather as greenhouse gases do. Some is thus radiated into space, but the rest travels back towards Earth. This produces 24-hour warming that, overall, handily outweighs the cooling (something which is true for non-contrail cirrus, too). The exact amount of net warming is hard to calculate, but current evidence suggests aircrafts’ contrails and the cirrus clouds they induce may do more to raise the planet’s temperature than carbon dioxide emitted by the aircrafts’ engines.

尾迹呈白色，是因为组成它们的冰晶会反射阳光。 这在白天可能产生降温效果。 但冰晶也会吸收红外辐射并重新发射，颇像温室气体的作用。 其中一部分因而辐射到太空，其余则返回地球方向。 这带来全天候的增暖作用，总体上明显超过降温作用（非尾迹形成的卷云也如此）。 净增暖的确切大小很难计算，但现有证据表明，飞机的尾迹及其诱发的卷云，对地球升温的作用可能超过飞机发动机排放的二氧化碳。

This warming is, though, to some extent, optional. For contrails to persist the air they are in must be well below freezing point and yet contain a significant amount of not-yet-frozen water vapour. The thing which stops this water vapour freezing is a lack of particles that might “seed” the process by acting as nuclei around which crystals can form. A contrail provides such seeds by the trillion, and so creates cloud where there was none.

不过，这种增暖在一定程度上是可以选择避免的。 尾迹若要持续存在，它所在的空气必须远低于冰点，同时仍含有相当数量尚未冻结的水汽。 阻止这些水汽冻结的，是缺少能够充当冰晶形成核心、为冻结过程“播种”的颗粒。 一条尾迹能提供数以万亿计的这类“种子”，从而在原本无云的地方制造出云。

Such ice-supersaturated regions (ISSRs) tend to be found at altitudes of 8-13km. They are a few hundred kilometres across, but only a few hundred metres deep. Keep aircraft out of them and you should get rid of a lot of persistent contrails—which means getting rid of some warming, too.

这样的冰面过饱和区（ISSR），通常位于8至13公里的高空。 它们横向延伸几百公里，垂直厚度却只有几百米。 让飞机避开这些区域，就应该能去掉大量持续性尾迹，也就意味着避免一部分增暖。

On August 18th a consortium based in Britain announced Operation Blue Skies, the most important trial of this approach to cooling the climate so far undertaken. It will take place in Shanwick, a piece of airspace that covers 1.8m square kilometres of the North Atlantic west of the British Isles (see map).

8月18日，一个设在英国的联合团队宣布开展“蓝天行动”，这是迄今针对这一气候降温思路最重要的试验。 试验将位于Shanwick空域：不列颠群岛以西、覆盖北大西洋180万平方公里的一片空域（见地图）。

There have been smaller trials of the idea. One was run by Germany’s aerospace agency in 2021 and another, in 2025, by American Airlines and Contrails.org, a not-for-profit research organisation largely funded by Breakthrough Energy, a network of investors founded by Bill Gates. The results were promising. Blue Skies, which is considerably bigger, should show whether ways to predict and route around ISSRs are both effective and suitable for operational use.

这一想法此前已有较小规模的试验。 其中一项由德国航空航天机构于2021年开展；另一项于2025年由美国航空与非营利研究机构Contrails.org开展，后者主要由比尔·盖茨创立的投资者网络Breakthrough Energy资助。 结果令人鼓舞。 规模大得多的“蓝天行动”应能检验，预测并绕开冰面过饱和区的方法，是否既有效又适合实际运行。

Shanwick is a bit of airspace well suited to such a project. It is prone to the right sort of damp chilliness at airliners’ cruising altitudes, and it is used by a lot of planes. In combination, those factors mean that, although it covers just 0.4% of Earth’s surface, models suggest Shanwick is responsible for about 5% of contrail-associated warming. What is more, European and American weather satellites monitor the area day and night in both visible-light frequencies and infrared, making it easy to see what is going on.

Shanwick空域很适合这样的项目。 在客机巡航高度，它容易出现所需的那种潮湿寒冷天气，而且有很多飞机经过。 这些因素结合起来，意味着虽然它只覆盖地球表面的0.4%，模型却认为它产生了约5%的尾迹相关增暖。 此外，欧洲和美国的气象卫星使用可见光波段与红外线，昼夜监测这一区域，使人们容易看清发生了什么。

The plan is that on between 20 and 40 days over the coming winter and again the one after that air-traffic controllers at NATS, the company which looks after British airspace, will alter the flight paths of aircraft passing through Shanwick to keep them out of suspected ISSRs. These changes will route the planes a few hundred metres below their filed flight plans when conditions suggest it and when there is no reason not to (such as turbulence).

计划是在即将到来的冬季选取20至40天，并在下一个冬季再次这样做，由负责英国空域的NATS公司的空管人员，调整经过Shanwick的飞机的飞行路径，让它们避开疑似冰面过饱和区。 当气象条件提示应当调整、且没有湍流等不宜调整的理由时，这些改变会使飞机比已申报飞行计划中的高度低几百米飞行。

Comparing contrails made during periods when these changes are happening and periods when they are not should provide reliable data on how much the contrails are being reduced. “The perfect outcome”, says Sebastian Eastham, part of the team working on Blue Skies at Imperial College, London, “is that a satellite image robustly and reliably has fewer contrails in it, time after time after time.”

比较实施这些调整的时段与未实施调整的时段中形成的尾迹，应能可靠地反映尾迹究竟减少了多少。 伦敦帝国理工学院参与“蓝天行动”的Sebastian Eastham说，“最理想的结果”，就是“一次又一次，卫星图像都以稳健、可靠的方式显示更少的尾迹”。

As well as NATS and Imperial, the consortium behind Blue Skies consists of Contrails.org, Google (which, as well as its software expertise, is chipping £1.4m, or $1.9m, of hardware and computing time into the £5m budget), the University of Cambridge and Britain’s Met Office.

除了NATS和帝国理工学院，“蓝天行动”的联合团队还包括Contrails.org、谷歌、剑桥大学和英国气象局；谷歌除提供软件专长外，还向500万英镑的预算投入了价值140万英镑（190万美元）的硬件和计算时间。

If contrail avoidance is to go mainstream, the Blue Skies trial needs to show not just that days with avoidance measures in place have fewer contrails, but also that integrating the process into normal operations can be done safely and straightforwardly. That would open the way to more widespread, and eventually everyday use.

尾迹规避若要进入主流，“蓝天行动”就不仅需要证明实施规避措施的日子尾迹更少，还必须证明这一流程能够安全、简便地融入日常运行。 这将为更广泛、最终成为日常做法的应用铺平道路。

Adoption does not have to be universal to be useful. The warming attributed to contrails is concentrated. By one estimate 80% of the temperature-raising effect on the climate comes from just 2% of flights. Awkwardly, you cannot know in advance exactly which 2% they are. But Marc Shapiro, who runs Contrails.org, says that a system eliminating 70% of the world’s contrail warming might require changing the flight plans of fewer than 5% of flights.

不必所有航班都采用，这种方法也能有用。 尾迹造成的增暖高度集中。 一项估计认为，气候升温效果的80%，仅来自2%的航班。 棘手的是，你无法事先确切知道是哪2%。 不过，Contrails.org负责人Marc Shapiro说，一个消除全球70%尾迹增暖的系统，或许只需要改变不到5%航班的飞行计划。

According to a recent paper by Jessie Smith of Cambridge and colleagues elsewhere, if aviation does not change its ways it will be responsible for about 0.01°C of total warming above preindustrial levels in 2050. If the world is seeing about 2°C of warming by then, that contribution comes in at 5%, with more than half of it induced by contrails. A 75%-effective contrail-avoidance programme that began in 2035 would reduce the total by about a third, though there is a lot of uncertainty.

根据剑桥大学的Jessie Smith与其他机构同事最近的一篇论文，如果航空业不改变现有做法，到2050年，它造成的相对于工业化前水平的总增暖将约为0.01°C。 如果届时全球升温约为2°C，那么这项贡献就占5%，其中超过一半由尾迹造成。 如果一项有效率为75%的尾迹规避计划在2035年开始实施，就会将航空业造成的总增暖减少约三分之一，不过其中有很大不确定性。

Fractions of fractions sound small, but Dr Smith and her colleagues argue that the effect is similar in size to that of any other single measure proposed to reduce warming as a result of air transport—and could be realised faster.

小比例中的小比例听起来不多，但Smith博士及同事认为，这种效果与任何其他单项降低航空运输增暖的提议相比，规模相近，而且能更快实现。

The benefits are not free. Flying at lower altitudes increases the amount of fuel used, which is a cost to airlines, and the amount of carbon dioxide emitted, which is a cost to the planet. But neither cost is huge. Estimates from models suggest that avoiding ISSRs makes flights only slightly longer and increases the amount of fuel used by less than 2%.

收益并非没有代价。 降低飞行高度会增加耗油量，给航空公司带来成本，也会增加二氧化碳排放，给地球带来代价。 但两种代价都不算大。 模型估计认为，避开冰面过饱和区只会让飞行稍微变长，耗油量增加不到2%。

Using 2% more fuel on 5% of flights would increase the industry’s total fuel bill by a thousandth. In the American Airlines trial the difference in fuel use between the flights that were diverted and those which were not was undetectable.

如果5%的航班多用2%的燃料，整个行业的燃油账单就会增加千分之一。 在美国航空的试验中，改道航班与未改道航班之间的耗油差异未能被检测出来。

Similar calculations hold for warming. According to Olivier Boucher, a climate scientist who has worked on the matter for 30 years and now runs Klima, a small French company dedicated to it, “If you really focus on the most impactful flights, maybe you can save like 100 tonnes of carbon-dioxide equivalent for an extra tonne of carbon dioxide.”

类似的计算也适用于增暖。 Olivier Boucher是一名研究这一问题已有30年的气候科学家，如今经营专注于此的法国小公司Klima；他说：“如果你真正聚焦影响最大的航班，或许每多排放一吨二氧化碳，就能避免大约100吨二氧化碳当量的气候影响。”

There are also opportunity costs. Some worry that focusing on contrails will distract attention from airlines’ carbon-dioxide emissions and efforts either to reduce them (by using sustainable aviation fuels, or SAFs) or offset them (by removing CO2 from the atmosphere and putting it into permanent storage). Eliminating contrails without attacking these points—or reducing the amount of flying—provides a one-off gain but does not change the fundamental fact that, in the long term, more flying means more warming.

此外还有机会成本。 有人担心，把注意力集中在尾迹上，会分散人们对航空公司二氧化碳排放的关注，以及对减排或抵消排放工作的关注：前者可使用可持续航空燃料（SAF），后者可从大气中移除二氧化碳并将其永久封存。 如果不处理这些问题，也不减少飞行量，单独消除尾迹只会带来一次性收益，而不会改变一个根本事实：长期而言，更多飞行意味着更多增暖。

Doubters argue as well that SAFs can be designed to produce fewer particles around which ice can form, thus reducing contrails as well as carbon dioxide. This is true. But steering clear of ISSRs looks like a far more cost-effective approach to contrail reduction. The paper by Dr Smith and her colleagues found that adding SAFs to a system which already practised avoidance resulted in at most 0.001°C of cooling.

质疑者还指出，可持续航空燃料可以通过设计减少供冰晶形成的颗粒，因而在减少二氧化碳的同时减少尾迹。 这确实如此。 但就减少尾迹而言，避开冰面过饱和区看起来是一种成本效益高得多的方法。 Smith博士及同事的论文发现，在已经实行规避的系统中加入可持续航空燃料，最多只多带来0.001°C的降温。

There is, then, no free lunch. But the idea that giving passengers a minute or two longer to savour the in-flight cuisine that they have paid for on a few of the world’s many many flights might result in some good is about as appealing as climate interventions get.

所以，天下没有免费的午餐。 不过，在全世界众多航班中的少数几班上，让乘客多花一两分钟品味他们已经付钱的机上餐食，或许就能带来一点好处——作为气候干预措施，这几乎已是最诱人的想法了。

学习资料

精读音频 · 精读笔记