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第一段
1 .Listen to part of the lecture in an environmental science class.
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第二段
1 .Now, over the next few weeks, we're gonna focus on carbon and its role in what's known as the "greenhouse effect."
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2 .Everyone knows what that is, right?
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3 .But let's make sure anyway.
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4 .Yes, Karlie?
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第三段
1 .The greenhouse effect is when gases in Earth's atmosphere act like the glass in a greenhouse or a hothouse.
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2 .They trap in heat which warms up the earth.
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第四段
1 .Gases like?
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第五段
1 .Um, water vapor? Carbon dioxide?
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第六段
1 .Right. Carbon dioxide.
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2 .We hear a lot about carbon these days, no?
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3 .Carbon emissions, carbon burning, leaving a carbon footprint.
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4 .So it'd be easy to assume that any form of carbon burning is necessarily a bad thing.
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5 .But the fact is: it's not quite that simple.
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6 .So we're gonna focus today on the difference between good, bad, and well, the, not so bad, the potentially okay carbon burning.
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第七段
1 .OK. Good carbon burning.
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2 .Well, we all have a personal stake in this because burning carbon is the basis of life.
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3 .We wouldn't be here if we ourselves weren't burning carbon.
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4 .Basically, all living things burn carbon to survive.
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5 .Usually this happens at the cellular level and what's burned is carbon in the form of sugars, glucose.
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第八段
1 .Oxygen gets chemically combined with sugars in our cells and the energy produced from that reaction is then used to power the cells.
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2 .So just by breathing you could say we are all guilty of carbon emission and contribute to the amount of carbon in the atmosphere.
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第九段
1 .Um, another thing about atmospheric carbon: it keeps us from freezing.
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2 .Because without carbon and other greenhouse gases, our planet would be the same temperature as outer space,
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3 .around four degrees above absolute zero.
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4 .And there's nothing going on at that temperature.
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5 .No possibility of life of any sort.
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6 .So without atmospheric carbon, life couldn't exist on Earth.
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第十段
1 .Now, another thing to remember is that carbon is always being removed from the atmosphere.
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2 .It gets used up.
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3 .Largely it is consumed by plant life through photosynthesis.
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4 .Also, it dissolves in the oceans or gets stored deep in the ocean.
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5 .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.
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第十一段
1 .So with all this carbon constantly being depleted from the atmosphere, we really need to keep carbon output up to a certain point.
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第十二段
1 .So why all the bad press for burning carbon?
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第十三段
1 .Well, turns out it's the source of the carbon being burned that's the key.
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2 .It's in fact the burning of what we call "fossil carbon" that creates the imbalance: fossil fuels, coal, oil, natural gas.
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第十四段
1 .These substances are all mined.
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2 .We have to dig them up or drill a well to release them.
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3 .And this is carbon that was in the atmosphere millions of years ago.
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第十五段
1 .So what happens is: when we burn this carbon,
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2 .ok, it doesn't really release a whole lot compared to the amount that's already there in the atmosphere,
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3 .but it adds to the pool.
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4 .And over years it accumulates.
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第十六段
1 .Think of the atmosphere as a big bathtub.
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2 .It's basically already filled to the brim with carbon.
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3 .When we start adding fossil carbon into the mix, it starts to spill over.
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4 .That being said, there's actually a category that's in-between: what we call biomass fuels, probably the most common one is wood.
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第十七段
1 .Another example, um, on the North American plains, the Native Americans used to collect buffalo droppings to burn.
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2 .Uh, in Ireland, they cut up peat from bogs and they burn that.
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第十八段
1 .So what's the big distinction between this and fossil carbon?
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第一十九段
1 .Biomass carbon is what we might call current carbon.
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2 .It's always going in and out of the atmosphere.
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3 .So if we burn one of these fuels, we're putting its carbon into atmosphere. Right?
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4 .But in a balanced system, somewhere else in the world the same amount of carbon is going back.
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5 .It gets taken in by growing vegetation.
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6 .So burning biomass fuel produces sort of not-so-bad carbon.
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7 .In fact, it can become good carbon if we endeavor as a society, as humanity,
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8 .to allow forests to recover this carbon,
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9 .if we don't, say, pave over all the surfaces to prevent things from growing.
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10 .So whenever we cut down a tree and burn its wood, we have to allow another tree to grow to keep things in balance.
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11 .That way, you're...you're carbon neutral.
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