Research published in the July 2 issue of Nature reveals one reason our planet didn't succumb to an enveloping ice sheet during glacial ages.

发表在7月2日的《自然》（Nature）杂志上的一项研究揭示了地球在冰河时代没有完全封冻的一个原因。

Why Didn't Earth Freeze Completely?

During the last ice age our problem was too little carbon. Unlike today where too much carbon is causing global warming.

Past glacial ages occurred partly because the weathering of rocks, over millions of years, pulls CO2 from the atmosphere, locking it in ocean floor sediment. The rise of global mountain ranges during the last 25 million years should have sucked all the CO2, sending the Earth to an icy death.

But that never happened. CO2 levels stabilized at about 250 parts per million.

This week in the journal Nature, researchers announce one reason why this happened: plants.

Leafy greens need CO2 to live, and when CO2 levels drop significantly they starve. Researchers say that the plant numbers decreased to a level where volcanoes and other carbon-creating sources produced CO2 faster than the remaining plants could remove it. So the Earth remained somewhat warm.

It may seem that our leafy friends could help us now, this time from overheating. But ultimately we're producing too much CO2 too fast for natural weathering processes to remove it. Ultimately, we need a way to stop producing CO2 in the first place.

冰河时代为什么地球没有完全封冻？

在上一个冰河时代我们的问题是二氧化碳太少。而今天过多的二氧化碳正在导致全球变暖。

过去冰河时代发生的部分原因在于在几百万年的时间里，石头的风化吸收了大气中的二氧化碳，然后以沉积物的形式把这些二氧化碳固定在海底。在过去2千5百年的时间里，全球山脉的升高应该把大气中的二氧化碳全部吸完，从而让地球成为致命的冰窟。

但是这种情况根本没有发生。二氧化碳的浓度水平一直稳定在约百万分之250.

在本周的《自然》（Nature）杂志上，研究者们宣布了这种情况没有发生的一个原因：植物。

带叶子的绿色植物需要二氧化碳维持生命，当二氧化碳的浓度水平显着下降的时候，它们就会"挨饿".研究者们说，在冰河时代植物的数量下降到一定的水平，这样火山以及其它产碳源产生二氧化碳的速度就要比所剩植物消耗二氧化碳的速度快。因此地球仍然保持"温暖".

现在看来，似乎我们的植物朋友能帮助我们解决过热的问题。但是归根到底我们产生二氧化碳太多太快，以致于自然的风化过程不能消耗掉多余的二氧化碳。我们最终首先需要停止产生过多的二氧化碳。

Vocabulary:

Reveal:揭示；显示

Succumb:屈服

Enveloping:包裹着的；覆盖的

Glacial:冰川的

Freeze:结冰

Occur:发生

Weathering:风吹日晒

Sediment:沉淀

Stabilize:稳定

Leafy:带叶子的

Starve:饥饿

Volcano:火山

Ultimate:最终的