英语听力文摘 English Digest 247、呼吸甲烷

　　A Fresh Breath of Methane¹
很早很早以前地球有很微量的氧气，而且地表状况十分恶劣。大气主要是N2和Co2，后来通过复杂的变化，地球开始出现了厌氧型生物。在生命的作用下，地球的氧气开始不断积累。后来又出现了藻类，在各种（必须注意是各种）生物的作用下，氧气开始积累。据科学家们说，甲烷生产可能是一系列导致形成可呼吸大气的事件中一个被忽略了的环节。地球的早期环境中，氧气是相当稀薄的，甚至在光合作用的细菌出现后也是这样。为了解释这个滞后，David C. Catling和同事们提出这段时间的大部分可能被甲烷的生产所占据。根据这个假说，当其它微生物分解有机物质与可能的火山气体时，光合生物产生的氢被转化成甲烷。然后紫外线的轰击将甲烷分解成二氧化碳、氧气和氢气。氢气会逃逸到太空中，而重一些的氧气将在大气中积聚，使大气日益适宜陆地的生命。

　　Y:Open the window and let some fresh air down.
D:Ok!
Y:A!Take a deep breath of oxygen.
D:It might not always been there,you know.
Y:What do you mean?
D:According to climate researcher James Kasting, when the earth first formed there might not have been any oxygen around.
Y:Before the begining of the life？
D:No,there would have been life, just no oxygen.For example, take methanogens, the microbes that make methane. They only thrive when there’s no oxygen around.
Y: Is that why they live in airless places like the stomachs of cows?
D: Correct!And according to the theory, back when the earth was young and there was no oxygen around, methanogens ruled and the atmosphere was filled with methane.
Y:So when did Oxygen come on the scene?
D:Not untill about a billion years later. But without methane, oxygen-producing microbes might not have evolved as quickly as they did. See, methane is a powerful greenhouse gas, meaning that it could have helped keep the sun’s rays trapped near the earth’s surface. This was important, since billions of years ago the sun was weaker than it is today. So without the methane-induced greenhouse effect, the earth might have frozen² solid until the sun got hotter.
Y:But when the earth would have become way too hot?
D:Yes. If not for the fact that the methane rich atmosphere also could have helped to reflect enough incoming sunlight to maintain a delicate temperature balance.
Y:So maybe the earth stayed warm, but not too warm, until oxygen-producing microbes evolved and filled the atmosphere with oxygen, which drove away most of the methane.
D:Right.
Y:Well!You've just made my breath of fresh air more interesting.
译文：
呼吸甲烷
Y：打开窗户让新鲜空气进来。
D：好的。
Y：啊！深吸一口氧气。
D：你知道的，它不是一直都有的。
Y：什么意思？
D：气候研究员的 James Kasting指出当地球初始形成时，周围并没有氧气。
Y：在生命开始之前吗？
D：不，有生命，只是没有氧气。比如产烷生物，一种制造甲烷的微生物。它们只在周围没有氧气的时候才能生存。
Y：这就是它们为什么生活在像牛胃那样不通风的地方的原因吗？
D：正确。根据理论，当地球年轻并且没有氧气的时候，产烷生物统治着地球，空气里面充满了甲烷。
Y：那么氧气是什么时候出现的呢？
D: 直到大约十亿年后。但是如果没有甲烷，制氧微生物就不会进化的如此之快。甲烷是一种强有力的温室气体，它可以维持地球表面的太阳光线。这是非常重要的。因为几十亿年前太阳的光线比现在弱。因此如果没有甲烷制造的温室效应，地球可能会被冻成一个坚硬的固体，直到阳光变强为止。
Y：但是地球变得太热了怎么办呢？
D：是的。甲烷丰富的大气同样有助于折射足够多的射入阳光，使温度保持一种微妙的平衡。
Y：所以地球保持着温暖，但不至于太温暖。直到制氧生物进化完成，赶走了绝大多数的甲烷，让大气充满氧气。
D：是的。
Y: 嗯，你让我呼吸新鲜空气变得更有趣了。

1 methane   

n.甲烷，沼气

参考例句：

• The blast was caused by pockets of methane gas that ignited.爆炸是由数袋甲烷气体着火引起的。
• Methane may have extraterrestrial significance.甲烷具有星际意义。

2 frozen   

adj.冻结的，冰冻的

参考例句：

• He was frozen to death on a snowing night.在一个风雪的晚上，他被冻死了。
• The weather is cold and the ground is frozen.天寒地冻。