万物简史 第615期:冰河时代(12)（在线收听）

The Milankovitch cycles alone are not enough to explain cycles of ice ages. Many other factors are involved—not least the disposition of the continents, in particular the presence of landmasses over the poles—but the specifics of these are imperfectly understood. It has been suggested, however, that if you hauled North America, Eurasia, and Greenland just three hundred miles north we would have permanent and inescapable ice ages. We are very lucky, it appears, to get any good weather at all. Even less well understood are the cycles of comparative balminess within ice ages, known as interglacials. It is mildly unnerving to reflect that the whole of meaningful human history—the development of farming, the creation of towns, the rise of mathematics and writing and science and all the rest—has taken place within an atypical patch of fair weather. Previous interglacials have lasted as little as eight thousand years. Our own has already passed its ten thousandth anniversary.

单是米兰柯维契周期并不足以解释冰川期的周期。许多别的因素也必须纳入考虑的范畴——尤其是大陆的分布情况，特别是极地陆块的存在，但是对于所有这一切我们了解得并不完备。然而，一直有一种说法，如果你将北美大陆、欧亚大陆和格陵兰往北移500公里，我们就势必永远处于冰川期。看上去我们非常幸运，赶上了所有的好天气。对于一个冰川期内被称为间冰期的一段气候相对暖和的时期，我们对其周期尤其缺乏了解。说来也许有些令人沮丧，整个人类文明史——农业的发展，城镇的建立，数学、文学、科学和所有其他一切的兴趣——都发生在一段不大寻常的好天气时期。上几次间冰期只持续了8000年的时间，而我们这一次已过去了1万年。

The fact is, we are still very much in an ice age; it's just a somewhat shrunken one—though less shrunken than many people realize. At the height of the last period of glaciation, around twenty thousand years ago, about 30 percent of the Earth's land surface was under ice. Ten percent still is—and a further 14 percent is in a state of permafrost. Three-quarters of all the fresh water on Earth is locked up in ice even now, and we have ice caps at both poles—a situation that may be unique in Earth's history. That there are snowy winters through much of the world and permanent glaciers even in temperate places such as New Zealand may seem quite natural, but in fact it is a most unusual situation for the planet.

事实上我们依然处在一个冰川期，只不过这是一个范围已经缩小的冰川期——虽然缩小的程度不像许多人认为的那么大。在上一次冰川期的高峰期，也就是在大约2万年前，地球陆地表面约30%被冰雪覆盖。即便是现在，依然有10%的陆地覆盖着冰雪。更有14%的地区是永久冻土带。四分之三的地球淡水结成冰，南北两极有冰盖，一这种情况自地球诞生以来是极不寻常的。世界上许多地区冬天会下雪，像新西兰这样温润的地区也覆盖着永久性的冰盖，这一切对于我们来说起司空见惯的，可是在地球以往的历史中却极为罕见。