万物简史 第616期:冰河时代(13)（在线收听）

For most of its history until fairly recent times the general pattern for Earth was to be hot with no permanent ice anywhere. The current ice age—ice epoch really—started about forty million years ago, and has ranged from murderously bad to not bad at all. Ice ages tend to wipe out evidence of earlier ice ages, so the further back you go the more sketchy the picture grows, but it appears that we have had at least seventeen severe glacial episodes in the last 2.5 million years or so—the period that coincides with the rise of Homo erectus in Africa followed by modern humans. Two commonly cited culprits for the present epoch are the rise of the Himalayas and the formation of the Isthmus of Panama, the first disrupting air flows, the second ocean currents. India, once an island, has pushed two thousand kilometers into the Asian landmass over the last forty-five million years, raising not only the Himalayas, but also the vast Tibetan plateau behind them. The hypothesis is that the higher landscape was not only cooler, but diverted winds in a way that made them flow north and toward North America, making it more susceptible to long-term chills. Then, beginning about five million years ago, Panama rose from the sea, closing the gap between North and South America, disrupting the flows of warming currents between the Pacific and Atlantic, and changing patterns of precipitation across at least half the world. One consequence was a drying out of Africa, which caused apes to climb down out of trees and go looking for a new way of living on the emerging savannas.

直到相当近的一段时期前，地球表面的温度在绝大多数时候都比较高，哪里也没有永久性的冰川。当前的冰川期——实际上是冰河时代一开始于大约4000万年前，从极其严酷变得不大严酷。我们就生活在一个为数不多的不大严酷的时期。新的冰川期总是会抹掉以前的冰川期所留下的痕迹，因此时间越是往前推移，展现在你面前的图景就越是不完整。但是，在过去250万年左右的时间里，我们似乎已经历了至少17个严酷的冰河时代——在这段时期，正是非洲直立人及随后的现代人生一活的时期。人们常说，造成当前冰川期的两个嫌疑犯是喜马拉雅山的隆起和巴拿马地峡的形成。前者干扰了气流的畅通，后者打乱了洋流的走向。在过去4500万年的时间中，曾经是一个岛屿的印度漂移了2000公里与亚洲大陆相撞，其结果不仅使喜马拉雅山上升，并且在它的后面形成了广袤的青藏高原。据认为，高原的海拔增高不仅使气候变得更加寒冷，而且改变了风向，使风吹向北方，吹向北美，使得那里更容易长时期处于严寒的控制下。接着，从大约500万年前开始，巴拿马地区从海里隆起，将南北美洲连在一起，这样就影响了太平洋的大西洋之间的暖流的流向，改变了至少半个世界的降雨量的分布情况。其结果之一是造成了非洲的干旱，使得那里的类人猿纷纷从树上来到地上，到正在形成的大草原寻找新的生活方式。

At all events, with the oceans and continents arranged as they are now, it appears that ice will be a long-term part of our future. According to John McPhee, about fifty more glacial episodes can be expected, each lasting a hundred thousand years or so, before we can hope for a really long thaw.

无论如何，随着海洋和大陆变成目前的分布情况，我们似乎还将经历一个漫长的冰河时代。根据约翰·麦克菲的观点，我们还将经历大约50个冰河时代，每一个冰河时代持续1O万年左右，之后我们才能有希望迎来一个极其漫长的解冻期。