In the ocean the residence time is thought to be more like a hundred years. 据认为，水分子在海洋里逗留的时间更可能是100年。

　　Altogether about 60 percent of water molecules in a rainfall are returned to the atmosphere within a day or two. 总的来说，下了一场雨以后，大约有60%的水分子在一两天内又回到了大气层。

　　Once evaporated, they spend no more than a week or so—Drury says twelve days—in the sky before falling again as rain.  一旦蒸发，它们在天空中待 不了一个星期左右——德鲁里说是12天，然后又以雨的形式落了下来。

　　Evaporation is a swift process, as you can easily gauge by the fate of a puddle on a summer's day.  蒸发是个很快的过程，你很容易根据夏日里一摊水的命运来测定。

　　Even something as large as the Mediterranean would dry out in a thousand years if it were not continually replenished. 要是不连续补充水的话连地中海这佯的大家伙也会在1000年里干涸。

　　Such an event occurred a little under six million years ago and provoked what is known to science as the Messinian Salinity Crisis. 这种情况在稍稍不到600万年以前发生过，产生了科学界所谓的“墨西拿盐度危机”，

　　What happened was that continental movement closed the Strait of Gibraltar. 原因是大陆移动阻塞了直布罗陀海峡。

　　As the Mediterranean dried, its evaporated contents fell as freshwater rain into other seas, 随着地中海干干涸，被蒸发的水汽以淡雨的形式落在别的海里。

　　mildly diluting their saltiness—indeed, making them just dilute enough to freeze over larger areas than normal. 稍稍降低了那些海的盐度——实际上，怡好稀释到能使更大区域结冰的程度。

　　The enlarged area of ice bounced back more of the Sun's heat and pushed Earth into an ice age. 冰区的扩大把更多的太阳热量反弹回去，从而把地球推进到了冰期。

　　So at least the theory goes. 至少从理论上说是这样的。

　　What is certainly true, as far as we can tell, is that a little change in the Earth's dynamics can have repercussions beyond our imagining. 就我们所知，有一点是肯定的，只要地球的动力稍稍发生变化，就可能产生难以想像的后果。

　　Such an event, as we shall see a little further on, may even have created us. 我们过一会儿将会看到，或许，连我们也就在这样的事件中诞生了。