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.

    發(fā)表在7月2日的《自然》（Nature）雜志上的一項(xiàng)研究揭示了地球在冰河時(shí)代沒有完全封凍的一個(gè)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.

    冰河時(shí)代為什么地球沒有完全封凍？

    在上一個(gè)冰河時(shí)代我們的問(wèn)題是二氧化碳太少。而今天過(guò)多的二氧化碳正在導(dǎo)致全球變暖。

    過(guò)去冰河時(shí)代發(fā)生的部分原因在于在幾百萬(wàn)年的時(shí)間里，石頭的風(fēng)化吸收了大氣中的二氧化碳，然后以沉積物的形式把這些二氧化碳固定在海底。在過(guò)去2千5百年的時(shí)間里，全球山脈的升高應(yīng)該把大氣中的二氧化碳全部吸完，從而讓地球成為致命的冰窟。

    但是這種情況根本沒有發(fā)生。二氧化碳的濃度水平一直穩(wěn)定在約百萬(wàn)分之250.

    在本周的《自然》（Nature）雜志上，研究者們宣布了這種情況沒有發(fā)生的一個(gè)原因：植物。

    帶葉子的綠色植物需要二氧化碳維持生命，當(dāng)二氧化碳的濃度水平顯著下降的時(shí)候，它們就會(huì)"挨餓".研究者們說(shuō)，在冰河時(shí)代植物的數(shù)量下降到一定的水平，這樣火山以及其它產(chǎn)碳源產(chǎn)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"溫暖".

    現(xiàn)在看來(lái)，似乎我們的植物朋友能幫助我們解決過(guò)熱的問(wèn)題。但是歸根到底我們產(chǎn)生二氧化碳太多太快，以致于自然的風(fēng)化過(guò)程不能消耗掉多余的二氧化碳。我們最終首先需要停止產(chǎn)生過(guò)多的二氧化碳。

    Vocabulary:

    Reveal:揭示；顯示

    Succumb:屈服

    Enveloping:包裹著的；覆蓋的

    Glacial:冰川的

    Freeze:結(jié)冰

    Occur:發(fā)生

    Weathering:風(fēng)吹日曬

    Sediment:沉淀

    Stabilize:穩(wěn)定

    Leafy:帶葉子的

    Starve:饑餓

    Volcano:火山

    Ultimate:最終的