If ice sheets advanced again, we have nothing in our armory that could deflect them.

如果冰盖卷土重来，我们没有任何武器可以改变它们的方向。

Mountains rise and melt away, ocean basins come and go, ice sheets advance and withdraw.

大山隆起又复平，海洋出现又消失，冰原前进又后退。

About the past million years, massive ice sheets advanced and retreated across the Northern Hemisphere during natural cycles of climate change.

大约在过去的一百万年里，在气候变化的自然循环中，北半球的巨大冰盖进退。