万物简史 第617期:冰河时代(14)（在线收听）

Before fifty million years ago, Earth had no regular ice ages, but when we did have them they tended to be colossal. A massive freezing occurred about 2.2 billion years ago, followed by a billion years or so of warmth. Then there was another ice age even larger than the first— so large that some scientists are now referring to the age in which it occurred as the Cryogenian, or super ice age. The condition is more popularly known as Snowball Earth.

在5000万年前，地球上并没有很有规律的冰川期，可是一当它们在地球上出现，其规模和持续时间都是十分惊人的。第一次大范围的冰川期出现在大约22亿年前，之后就是长达10亿年左右的温暖期。在这之后出现的冰川期比第一次来得更大——事实上，它是如此之大，以至于如今有些科学家提到那个时代时，都用上了覆冰纪或超级冰川期这样的字眼。这种状况更经常被称为“雪球世界”。

"Snowball," however, barely captures the murderousness of conditions. The theory is that because of a fall in solar radiation of about 6 percent and a dropoff in the production (or retention) of greenhouse gases, Earth essentially lost its ability to hold on to its heat. It became a kind of all-over Antarctica. Temperatures plunged by as much as 80 degrees Fahrenheit. The entire surface of the planet may have frozen solid, with ocean ice up to half a mile thick at higher latitudes and tens of yards thick even in the tropics.

然而，“ 雪球”很难说明那一时期环境极其恶劣的程度。那种理论认为。由于阳光的照射量减少了约6%，产生（或保留）温室气体的能力降低，地球实际上堆以保持其热量。地球变成了如同南极那样的冰天雪地，气温降低了45摄氏度。整个地球表面都被冻得严严实实，高纬度地区的海洋结冰厚达800米，热带地区的也有几十米厚。

There is a serious problem in all this in that the geological evidence indicates ice everywhere, including around the equator, while the biological evidence suggests just as firmly that there must have been open water somewhere. For one thing, cyanobacteria survived the experience, and they photosynthesize. For that they needed sunlight, but as you will know if you have ever tried to peer through it, ice quickly becomes opaque and after only a few yards would pass on no light at all. Two possibilities have been suggested. One is that a little ocean water did remain exposed (perhaps because of some kind of localized warming at a hot spot); the other is that maybe the ice formed in such a way that it remained translucent— a condition that does sometimes happen in nature.

这里存在一个严重问题：从地质学角度来看，整个地球，包括赤道地区在内，都被冰雪所覆盖；可是从生物学角度来看，却又确定无误地表明在某些地方一定存在着未曾冰冻的水域。首先，藻青菌存活了下来，还进行了光合作用。进行光合作用需要阳光，可是要是你透过冰块看，你会发现光线很快变得越来越暗，几米之外就根本看不见光线了。对于这个问题，可能有两种解释：一是有一小部分水域确实未曾冰冻（也许是因为当地某个地方很热）；一是某些结构的冰块是半透明的这种现辅时在大自然中确实存在。

If Earth did freeze over, then there is the very difficult question of how it ever got warm again. An icy planet should reflect so much heat that it would stay frozen forever. It appears that rescue may have come from our molten interior. Once again, we may be indebted to tectonics for allowing us to be here. The idea is that we were saved by volcanoes, which pushed through the buried surface, pumping out lots of heat and gases that melted the snows and re-formed the atmosphere. Interestingly, the end of this hyper-frigid episode is marked by the Cambrian outburst—the springtime event of life's history. In fact, it may not have been as tranquil as all that. As Earth warmed, it probably had the wildest weather it has ever experienced, with hurricanes powerful enough to raise waves to the heights of skyscrapers and rainfalls of indescribable intensity.

如果地球确实被冰冻过，那它又是怎样重新变得温暖起来的呢？这是一个很难于回答的问题。由于反射的热量太多，一个处于冰冻状态的星球会永远保持这种状态。挽救这种局面的力量似乎来自地球内部的岩浆。我们能在这里也许要再次感谢地壳的构造。我们认为，是火山救了我们。火山的喷发突破了冰川的封锁，喷涌而出的热量和气体融化了地表的冰雪，使得大气层重新得以改变。非常有意思的是，这一次高度寒冷的时期是以寒武纪大爆发——生命发展史上的春天——为结束的标志。当然，这样一个春天并非总是风和日丽，因为随着地球变暖。它经历了有史以来最狂暴的天气，强烈的飓风掀起摩天大楼高的巨浪，到处下着不可思议的瓢泼大雨。