Official 40 Set 5

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Greenhouse Effect

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What is the main purpose of the lecture?
  • A. To argue that carbon emissions today are not the only factor responsible for global warming

  • B. To provide evidence that rates of global carbon emissions fluctuate over time

  • C. To show the importance of distinguishing between different types of carbon emissions

  • D. To describe different ways carbon is removed from the atmosphere

显示答案 正确答案: C

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    NARRATOR:Listen to part of a lecture in an environmental science class.

    MALE PROFESSOR:Now, over the next few weeks we're gonna focus on carbon, and its role in what's known as the greenhouse effect.Everyone knows what that is, right? But let's make sure, anyway.[pause, waiting for someone to raise their hand]Yes, Carly?

    FEMALE STUDENT:The greenhouse effect is when gases in Earth's atmosphere act like the glass in a greenhouse, or hothouse—they trap in heat, which warms up the Earth.

    MALE PROFESSOR:[leading] Gases like ?

    FEMALE STUDENT:[listing] Um, water vapor, carbon dioxide.

    MALE PROFESSOR:Right. [sort of a musing tone] Carbon dioxide,We hear a lot about carbon these days, no?Carbon emissions,carbon burning,leaving a carbon footprint. so it'd be easy to assume that any form of carbon burning is necessarily a bad thing.But the fact is, it's not quite that simple.

    So, we're gonna focus today on the difference between good, bad, and,uh, well, the not-so-bad, the potentially OK carbon burning.

    OK, good carbon burning well, we all have a personal stake in this, because burning carbon is the basis of life.We wouldn't be here if we ourselves weren't burning carbon.Uh, basically all living things burn carbon to survive.Usually this happens at the cellular level, and what's burned is carbon in the form of sugars—glucose.Oxygen gets chemically combined with sugars in our cells, and the energy produced from that reaction is then used to power the cells.So just by breathing you could say we're all guilty of carbon emission, and contribute to the amount of carbon in the atmosphere.

    Um, another thing about atmospheric carbon: It keeps us from freezing.Because without carbon and other greenhouse gases, our planet would be the same temperature as outer space around 4 degrees above absolute zero.And there’s nothing going on at that temperature, no possibility of life of any sort.So without atmospheric carbon, life couldn't exist on Earth.

    Now another thing to remember is that carbon is always being removed from the atmosphere: It gets used up.Largely it's consumed by plant life,through photosynthesis.Also, it dissolves in the oceans.Or gets stored deep in the ocean—you have shellfish that use the CO2 dissolved in the water to make carbonate shells, and when they die, they fall to the ocean floor and the carbon gets sequestered down there.So, with all this carbon constantly being depleted from the atmosphere, we really need to keep carbon output up, to a certain point.

    So why all the bad press for burning carbon?Well, turns out it's the source of the carbon being burned that's the key.It's in fact the burning of what we call fossil carbon that creates the imbalance —fossil fuels—coal, oil, natural gas— these substances are all mined; we have to dig them up, or drill a well to release them.And this is carbon that was in the atmosphere millions of years ago.So what happens is, when we burn this carbon—OK, it doesn't really release a whole lot, compared to the amount that's already there in the atmosphere, but it adds to the pool. And over years it accumulates.Think of the atmosphere as a big bathtub: It's basically already filled to the brim with carbon; when we start adding fossil carbon into the mix, it starts to spill over.

    Uh, That being said, there's actually a category that's in between: what we call biomass fuels.Probably the most common one is wood.Another example, uh, on the-the North American plains, the Native Americans used to collect buffalo droppings to burn.Uh, in Ireland they cut up peat from bogs, and they burn that.So what's the big distinction between this and fossil carbon?Well, biomass carbon is what we might call current carbon—it's always going in and out of the atmosphere.So if we burn one of these fuels, we're putting its carbon into the atmosphere, right?But in a balanced system, somewhere else in the world this same amount of carbon is going back—it gets taken in by growing vegetation.So burning biomass fuel produces sort of "not-so-bad" carbon.In fact it can become good carbon, if we endeavor as a society, as humanity—to allow forests to recover this carbon—if we don't, say, pave over all the surfaces to, uh, prevent things from growing.So whenever we cut down a tree and burn its wood, we have to allow another tree to grow, to keep things in balance.That way you're-you're carbon neutral.

  • 旁白:请听环境科学课上的部分内容。

    教授:在接下来的几个周,我们会关注碳,以及它在"温室效应"中所起的作用。每个人都知道那是什么,对吧?不过不管怎样,我们还是确定一下吧。Karlie,请说。

    学生:温室效应是指地球大气层中的气体起着像温室的玻璃一样的作用,它们会把热量困住,而这就让地球变暖了。

    教授:哪些气体呢?

    学生:水蒸气?二氧化碳?

    教授:没错,二氧化碳我们最近经常听到碳,不是吗?碳排放、碳燃烧、留下碳足迹。所以一个人很容易会以为任何形式的碳燃烧都一定是一件坏事。但事实是:这远没有那么简单。

    所以,今天我们会集中讨论一下好的、坏的、不那么坏的、可能没问题的碳燃烧之间的区别。

    好的,首先是好的碳燃烧,我们所有人在这里面都有份,因为碳燃烧是生命的基础。如果我们自己没有在烧炭,我们就不会站在这里了。基本上,所有活着的生物都需要烧碳来生存。这通常发生在细胞的层次,燃烧的是糖、葡萄糖形式的碳。氧气会以化学的方式在我们的细胞中和糖结合,这种行为产生的能量接着会被用来给细胞提供力量。可以说仅仅通过呼吸我们所有人都有排放碳的罪过,并且增加了大气中的碳含量。

    另一个关于大气中的碳的事情是:它能保护我们不被冻死。因为没有碳和其他温室气体,我们星球的温度会和外太空一样,大约在绝对零度以上四度左右。在那种温度之下,没有任何事情能够得以进行,任何形式的生命都没有存在的可能性。所以没有了大气中的碳,地球上就不会存在生命。

    要记住的另一件事是,碳经常都被从大气中清除了:它会被耗尽。很大一部分都是被植物通过光合作用消耗掉了。它还会溶解在海洋中。或者被存储在深海中,贝类会用溶解在水中的二氧化碳做碳化的壳,当它们死后会掉落在海床上,而碳就被隔离在那下面了。所以,随着大气中碳的不断减少,我们真的需要把碳的排放量保持在一定的水平上。

    那么,为什么所有关于燃烧碳的报道都是负面的呢?原来关键在于碳燃烧的来源。事实上,燃烧"化石碳"才会产生这种不平衡。化石燃料、煤、石油、天然气,这些物质全都埋在地底下,我们必须把它们挖起来或钻个井把它们释放出来。而这是几百万年前存在于大气中的碳。所以会发生的事是:当我们烧这种碳时,与已经存在于大气中的相比,它真正释放出的并没有那么多,但它增加了大气中的含量。经过数年它会累积。可以把大气层想成一个大浴缸:它基本上已经加满了碳,当我们开始往里面添加化石碳时,它就开始外溢了。

    话虽这么说,实际上还有一类中间的:我们称它为生物燃料。最常见的一种可能就是木头了。再举个例子,在北美平原上,美洲土著以前会捡拾牛粪用来烧。在爱尔兰,他们会切碎从泥塘中捞来的泥煤,然后烧这个。那么这个和化石碳之间的大区别是什么呢?我们也许会把生物碳叫做活性炭,它一直进出于大气层。所以如果我们烧了这种燃料中的一种,我们是在把它的碳放进大气中,对吧?但在平衡机制下,同样数量的碳会回到世界上其他地方,它会被生长中的植被吸收。所以烧生物燃料会产生不那么坏的碳。事实上,它会成为好碳,如果我们作为一个社会、作为人类努力起来,让森林恢复这种碳,如果我们不铲平所有的地面,阻止植物生长的话。所以无论何时我们砍掉一棵树,燃烧木头时,我们必须让另一棵树生长以保持平衡。这样你就能做到碳平衡了。

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  • 本题对应音频:
    4 感谢 不懂
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    题型分类:主旨题

    音频定位:So, we’re gonna focus today on the difference between good, bad, and, uh, well, the not-so-bad, the potentially OK carbon burning.

    选项分析:教授说今天将会主要研究有利碳燃烧,无利碳燃烧,潜在有利碳燃烧之间的区别,所以C项正确。

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