万物简史 第614期:冰河时代(11)（在线收听）

The cause of ice ages, Koppen decided, is to be found in cool summers, not brutal winters. If summers are too cool to melt all the snow that falls on a given area, more incoming sunlight is bounced back by the reflective surface, exacerbating the cooling effect and encouraging yet more snow to fall. The consequence would tend to be self-perpetuating. As snow accumulated into an ice sheet, the region would grow cooler, prompting more ice to accumulate. As the glaciologist Gwen Schultz has noted: "It is not necessarily the amount of snow that causes ice sheets but the fact that snow, however little, lasts." It is thought that an ice age could start from a single unseasonal summer. The leftover snow reflects heat and exacerbates the chilling effect. "The process is self-enlarging, unstoppable, and once the ice is really growing it moves," says McPhee. You have advancing glaciers and an ice age.

柯本认为，产生冰川期的原因是因为凉快的夏季，而不是严寒的冬季。如果在某一地区夏季非常凉快，射来的阳光就会被地表反射，这样就加剧了寒冷的程度，使得更多的雪降下来，结果往往是地表的冰雪永久化。随着积雪成冰盖，整个地区就会更加寒冷，以致冰雪越积越多了。正如冰川学家格文·舒尔茨所说的那样：“冰盖的产生并不取决于下了多少的雪，而是取决于有多少未融化的雪——不管多么少。”冰川期被认为开始于某个季节反常的夏天，未融化的雪反射了热量，加剧了寒冷的效果，麦克菲说：“这是一个不断自我扩大的过程，而冰盖一旦形成，它就开始移动。”这样，就有了移动的冰川，也就进入到了一个冰川时代。

In the 1950s, because of imperfect dating technology, scientists were unable to correlate Milankovitch's carefully worked-out cycles with the supposed dates of ice ages as then perceived, and so Milankovitch and his calculations increasingly fell out of favor. He died in 1958, unable to prove that his cycles were correct. By this time, write John and Mary Gribbin, "you would have been hard pressed to find a geologist or meteorologist who regarded the model as being anything more than an historical curiosity." Not until the 1970s and the refinement of a potassium-argon method for dating ancient seafloor sediments were his theories finally vindicated.

20世纪50年代，由于检测年代技术的不完善，科学家们并没有能把当时所掌握的有关冰川期的数据与米兰柯维契精确计算出来的周期进行对比，因此，米兰柯维契及其计算结果越来越不吃香。直到1958年去世时，米兰柯维契知未能证明其周期的正确性。到了这个时候，用一部记述该时期的史书里的话来说：“你会迫切需要找到一位认为那个计算结果不仅仅是一个古董地质学家或气象学家。”直到20世纪70年代，随着用于确定古代海底沉淀物的年代的钾氩测年方法的改善，米兰柯维契的理论才最终得以确证。