For a long time it was thought that we moved into and out of ice ages gradually, over hundreds of thousands of years, but we now know that that has not been the case. Thanks to ice cores from Greenland we have a detailed record of climate for something over a hundred thousand years, and what is found there is not comforting. It shows that for most of its recent history Earth has been nothing like the stable and tranquil place that civilization has known, but rather has lurched violently between periods of warmth and brutal chill.

在很长一段时期内，我们认为地球是渐渐地进入和脱离冰川期的，其周期在数十万年以上。但是，现在我们知道实际情形并不是这样。通过对格陵兰岛冰核的测量，我们有了一份10多万年以来地球气候变化的详细记录。结果并不令人乐观。记录表明地球在最近一段历史时期根本不是人们以前所认为的那样，是一个风调雨顺的安身之处，相反，它的气候总是在温暖和严寒之间剧烈地摇摆不停。

Toward the end of the last big glaciation, some twelve thousand years ago, Earth began to warm, and quite rapidly, but then abruptly plunged back into bitter cold for a thousand years or so in an event known to science as the Younger Dryas. (The name comes from the arctic plant the dryas, which is one of the first to recolonize land after an ice sheet withdraws. There was also an Older Dryas period, but it wasn't so sharp.) At the end of this thousand-year onslaught average temperatures leapt again, by as much as seven degrees in twenty years, which doesn't sound terribly dramatic but is equivalent to exchanging the climate of Scandinavia for that of the Mediterranean in just two decades. Locally, changes have been even more dramatic. Greenland ice cores show the temperatures there changing by as much as fifteen degrees in ten years, drastically altering rainfall patterns and growing conditions. This must have been unsettling enough on a thinly populated planet. Today the consequences would be pretty well unimaginable.
大约12000年以前，地球快要结束最近的一次大规摸的冰川期，气候开始变暖，而且暖得很快。可是，接着它又突然一下子回复到长达1000年左右的酷寒期。那个时期在科学史上被称做新仙女术期。（这一名称来自一种名为仙女术的北极植物，是冰川消退后第一批重新生长起来的植物之一，科学史上还有一段被称为老仙女木期，不过其特征并不那么显著。）在这个千年酷寒期快结束的时候，平均气温再次突然攀升，20年内上升了4摄氏度之多。这听起来或许并不那么可怕，但足以在短短的20年内使斯堪的纳维亚半岛的气候变成地中海地区的气候。在局部地区，这种变化更加惊人。格陵兰的冰核显示，那里的气温在10年内改变了8摄氏度之多。气温的变化改变了那里的降水形式，也改变了那里的生长环境。这种情形在人烟稀少的过去就已经足以令人不安，而在今天，其后果简直无法想象。