While ice never completely engulfed the planet, 尽管冰雪从未覆盖整个地球
compacted to form permanent ice sheets several kilometers thick. 压缩成了几千米厚的永久性冰层
it still endures some of the same freezing conditions that an ice age brings. 这里仍经受着 冰河时代带来的冰冻状态
So when the Ice Age hit two and a half million years ago, 所以 2500万年前 当冰河时代突然袭来之时
it has been too warm for ice to form. 温度都太高 无法形成冰冻
almost half the planet's ice had melted. 地球上几乎一半的冰已经融化