科学美国人60秒 SSS 改变飞行高度可消除影响气候的飞机云（在线收听）

This is Scientific American's 60-second Science, I'm Susanne Bard.

Airplanes ac???count for about 3 percent of the climate-altering carbon dioxide emissions we add to the atmosphere. But planes are warming the planet in another way.

"So if you look up in the sky, you probably see, at some point, an aircraft. And behind that aircraft are white, fluffy streaks. And that's what we call a contrail."

Imperial College London engineer Marc Stettler.

Contrails are made up of ice crystals that form when aircraft engines emit exhaust that hits the cold air. The ice crystals reflect incoming light from the sun back into space, which has a cooling effect on the atmosphere. But the contrails also stop heat coming up from the ground from escaping into space.

"It's reflected back down toward the ground. And so that's a warming effect."

Stettler says, on balance, contrails warm the atmosphere more than they cool it.

"And that's primarily because the cooling effect due to reflecting of sunlight can only happen during the day, when the sun's shining, whereas the warming effect due to trapping of outgoing heat happens all of the time."

Some contrails can form clouds that last for up to 18 hours. During that time, they spread out, trapping even more heat. This process allows contrails to warm the planet about as much as the carbon dioxide emissions from aircraft.

But when Stettler and his team analyzed flight data they obtained of Japan airspace, they found that most contrail warming was caused by just 2 percent of flights. And most of those flights originated in the late afternoon—because as the sun goes down, cooling can no longer offset the warming.

"And the warming effect persists throughout the evening, into the night."

But what if the contrails that contribute the most to warming could be eliminated? Such a change could be achieved if aircraft avoided flying in the thin layers of humidity where contrails form.

"By changing the altitude only by a couple of thousand feet, either up or down, it would no longer form a contrail. And so what we found in this study was that by changing the altitude of less than 2 percent of flights, we could actually get rid of just under 60 percent of the warming effect due to contrails."

The study is in the journal Environmental Science & Technology.

This improved understanding of how to manage contrails presents an opportunity for the aviation industry to reduce its global environmental impact. Think of it as a silver lining in those contrail clouds.

Thanks for listening for Scientific American's 60-second Science. I'm Susanne Bard.

这里是科学美国人——60秒科学系列，我是苏珊娜·巴德。

我们向大气中排放改变气候的二氧化碳，其中飞机排放约占3%。但飞机正在以另一种方式使地球升温。

“如果你仰望天空，有时可能会看到飞机。飞机后面跟着蓬松的白色条纹。我们称之为凝结尾（也称飞机云）。”

伦敦帝国理工学院的工程师马克·斯特特勒说到。

凝结尾由冰晶组成，而冰晶则由飞机引擎排放的废气遇冷空气凝结而成。冰晶将太阳的入射光反射回太空，这对大气产生冷却效应。但凝结尾也阻止地面热量逃逸到太空中。

“热量被反射回地面。因此这是种升温效应。”

斯特特勒表示，总的来说，凝结尾对大气的加温作用大于对大气的冷却作用。

“这主要是因为反射太阳光所产生的冷却效应只能在白天有太阳时产生，而困住流失热量所产生的升温效应全天都在发生。”

有些凝结尾可以形成持续18小时不散的云。在这段时间里，这些云扩散开来，困住更多的热量。这一过程使凝结尾对地球的加温程度与飞机排放的二氧化碳相等。

但是，当斯特特勒和团队分析他们从日本领空获得的飞行数据时，他们发现大多数凝结尾升温作用仅由2%的飞行造成。而这些航班大多在傍晚起飞，因为随着太阳下山，冷却不再与升温作用抵消。

“升温效应从晚上持续至深夜。”

但如果对升温效应贡献最大的凝结尾可以被消除，那会怎样？如果飞机避免在形成凝结尾的潮湿薄层中飞行，这一改变就可以实现。

“将飞行高度提升或降低几千英尺，就不会再形成凝结尾。因此，我们在这项研究中发现，改变航班中不到2%的飞行高度，就可以消除略低于60%的由凝结尾引起的升温效应。”

这项研究发表在《环境科学与技术》期刊上。

更好地理解凝结尾管理，为航空业提供了减少全球环境影响的机会。将其想象成飞机云中的银线吧。

谢谢大家收听科学美国人——60秒科学。我是苏珊娜·巴德。