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5,000 miles away, in the forests of Mexico, monarch butterflies, in millions, also huddle for survival.
8000公里外的墨西哥树林中，数百万只黑脉金斑蝶也为了生存挤作一团。
All of them have travelled across North America to escape the winter that is coming there.
它们集体飞越北美大陆，躲避冬季的严寒。
This forest, at an altitude of 3,000 metres, might seem to offer little comfort, for temperatures here can drop below freezing, and that could kill the butterflies.
这片海拔3000米的森林，似乎并不能提供舒适的环境，因为这里的气温可降至零度以下，如此低温能置蝴蝶于死地。
But in fact, a thin layer of warm air trapped by the tree canopy protects them, and will do so throughout the winter, provided that the forest remains undisturbed.
然而事实上，树冠保有薄薄的一层暖空气保护着蝴蝶，只要森林不受干扰，就能一整个冬天为蝴蝶提供保护。
Unpredictable extremes of weather, however, can disrupt this delicate balance.
然而，不可预测的极端天气，会破坏这一微妙平衡。
Strong winds can push deep into the forest, chilling the air.
强风吹进森林深处，降低气温。
Hundreds of thousands of butterflies may be blown from the branches.
数十万只蝴蝶受强风席卷，掉下树枝。
In some years, storms kill 75% of hibernating monarchs.
某些年，75%冬眠中的黑脉金斑蝶死于风暴。
This one has survived, but on the ground.
这只蝴蝶得以幸存，但也掉落在地。
And night-time frosts are fatal. It can't stay here and live.
夜间的霜冻足以致命。因此它不能留在这里。
It is too cold to fly, but it must get back into the tree branches somehow. So it starts to climb.
尽管天气过冷，无法飞翔，但它必须找个办法回到树枝上。因此它开始爬树。
It's the equivalent of a human climbing up a skyscraper. Safety, at last.
困难程度相当于人类徒手爬上摩天大楼。终于安全了。
As spring returns, the monarchs wake from their long slumber.
春天来临时，黑脉金斑蝶从漫长的沉睡中苏醒。
And now starts one of the Earth's greatest natural spectacles.
地球上最伟大的自然奇观之一就此拉开帷幕。
But this mass awakening is under threat.
然而，这种集体复苏的生活习性正面临威胁。
The changing climate is creating more extreme weather, and storms are becoming more frequent.
气候变化引发越来越多极端天气，风暴也越来越频繁。
But, for now, this miracle of nature continues.
但就目前而言，这一自然奇迹仍在继续。
