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.