The 40m scheduled(预定的，定期的) passenger flights which took place in 2019 added up to(总计达到) 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.(超过40%的航班在部分航程中留下尾迹，因为发动机废气中的水蒸气凝结成冰晶。)
2019年，全球共执行了4000万次定期客运航班，总航程约610亿公里——相当于全年每天飞行距离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尾迹，因为发动机排出的水蒸气在低温下凝结成微小的冰晶。
Many of these trails were ephemeral(短暂的，转瞬即逝的). They streamed out behind their progenitor(祖先，先驱；此处指母机) 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.(但在约5%的航程上，尾迹持续并扩展，因为大气中的水蒸气加入冻结。) Thin white lines became broad translucent(半透明的) stripes of cirrus(卷云) 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.(将这些条纹绕地球一圈，你会发现持久性尾迹覆盖天空的千分之一。) In doing so(这样做，如此一来) they warm the planet.
许多尾迹转瞬即逝。它们在母机身后延伸，随后迅速消散。但在约5%的总航程上——足以每天往返月球十次——尾迹持续存在并扩展，因为大气中已有的水蒸气也加入了冻结过程。细长的白线变成了宽阔的半透明卷云条纹。将这些条纹像毛线绕球一样缠绕地球，你会发现，在任何时刻，持久性尾迹可能覆盖天空的千分之一。它们因此使地球变暖。
Contrails are white because their constituent(组成的，构成的) ice crystals reflect sunlight. That can have a cooling effect during daytime. But the crystals also absorb infrared(红外线的) radiation and re-emit it, rather as(正如，有点像) greenhouse gases do. Some is thus radiated(辐射，发射) into space, but the rest travels back towards Earth. This produces 24-hour warming that, overall, handily(轻易地，方便地) 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.(净增温量难以计算，但现有证据表明尾迹和卷云的增温作用可能超过发动机排放的二氧化碳。)
尾迹呈白色，因为其组成的冰晶反射阳光。这在白天可能产生冷却效应。但冰晶也吸收红外辐射并重新发射，就像温室气体一样。部分辐射被射向太空，但其余部分返回地球。这产生了24小时的增温效应，总体而言，其增温效应轻松超过冷却效应（非尾迹卷云也是如此）。净增温的确切数值难以计算，但现有证据表明，飞机尾迹及其引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。
The nucleus of the problem
问题的核心
The Winter’s tale
冬天的故事
The ideal is the enemy of the good
理想是好的敌人