改變飛行高度可減少影響氣候的飛機云

Airplanes account 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.” said Imperial College London engineer Marc Stettler.

人類排放到大氣中的二氧化碳正改變著氣候，其中3%的碳排放來源于飛機。然而，飛機還正以另一種方式加劇全球變暖。倫敦帝國理工學院的工程師馬克·斯泰特勒介紹：“當你仰望天空時，某個時刻可能會看到飛機。飛機后面拖著幾條毛絨絨的白色拉線，我們稱之為飛機云。”

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.”

飛機云由冰晶組成，而這些冰晶則是由飛機引擎排放的廢氣遇冷空氣凝結而成。冰晶將太陽的入射光反射回太空，對大氣產生冷卻效應。但飛機云也阻止了地面熱量逃逸到太空中。“熱量被反射回地面。因此這是種升溫效應。”

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.” 

斯泰特勒表示，總的來說，飛機云對大氣的升溫作用大于冷卻作用。“主要是因為反射太陽光所產生的冷卻效應只能在白天有太陽時產生，而由于阻擋散發(fā)熱量所產生的升溫效應則全天都在發(fā)生。”

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.

有些飛機云可以形成持續(xù)18小時不散的云團。在這段時間里，這些云團擴散開來，阻擋了更多熱量的散發(fā)。這一過程使飛機云給地球帶來的升溫效應與飛機排放的二氧化碳基本持平。

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.

然而，當斯泰特勒及其團隊分析從日本領空獲得的飛行數據時，他們發(fā)現飛機云產生的大部分升溫效應僅由2%的航班造成。這些航班大多在傍晚起飛，隨著太陽下山，飛機云的冷卻作用無法抵消其升溫作用。“升溫效應從傍晚一直持續(xù)到深夜。”那如果對升溫作用最明顯的飛機云可以被消除呢？只要飛機避免在形成飛機云的潮濕薄層大氣中飛行，就可以實現這一改變。

“By changing the altitude only by couple of thousand feet, either up or down, it would no longer form 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.

“將飛行高度提升或降低幾千英尺，就不會再形成飛機云。因此，我們在這項研究中發(fā)現，改變不到2%的航班的飛行高度就可以消除接近60%由飛機云引起的升溫效應。”這項研究發(fā)表在《環(huán)境科學與技術》期刊上。

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.

這一研究使我們加深了認識，了解到如何更好地控制飛機云的形成，也為航空業(yè)減少其對全球氣候的影響提供了機遇。正如烏云周邊都有一線光亮，我們也可以把這項成果視作是解決飛機云問題的一絲希望吧！