EXCERPT: 'SuperFreakonomics,' by Steven Levitt and Stephen Dubner

You could also switch from eating beef to eating kangaroo -- because kangaroo farts, as fate would have it, don't contain methane. But just imagine the marketing campaign that would be needed to get Americans to take up 'roo-burgers. And think how hard the cattle ranchers would lobby Washington to ban kangaroo meat. Fortunately, a team of Australian scientists is attacking this problem from the opposite direction, trying to replicate the digestive bacteria in kangaroos' stomachs so it can be transplanted to cows.

An incredibly simple dilemma lies at the heart of global warming. Economists fondly call it an externality.

What's an externality? It's what happens when someone takes an action but someone else, without agreeing, pays some or all the costs of that action. An externality is an economic version of taxation without representation.

If you happen to live downwind from a fertilizer factory, the ammonium stench is an externality. When your neighbors throw a big party (and don't have the courtesy to invite you), their ruckus is an externality. Secondhand cigarette smoke is an externality, as is the stray gunshot one drug dealer meant for another that instead hit a child on the playground.

The green house gases thought to be responsible for global warming are primarily externalities. When you have a bonfire in your backyard, you're not just toasting marshmallows. You're also emitting gases that, in a tiny way, help to heat the whole planet. Every time you get behind the wheel of a car, or eat a hamburger, or fly in an airplane, you are generating some by- products you're not paying for.

Imagine a fellow named Jack who lives in a lovely house -- he built it himself -- and comes home from work on the first warm day of summer. All he wants is to relax and cool off. So he cranks the air conditioner all the way up. Maybe he thinks for a moment about the extra dollar or two he'll pay on his next electricity bill, but the cost isn't enough to deter him.

What he doesn't think about is the black smoke from the power plant that burns the coal that heats the water that turns to steam that fills the turbine that spins the generator that makes the power that cools the house that Jack built.

Nor will he think about the environmental costs associated with mining and trucking away that coal, or the associated dangers. In the United States alone, more than 100,000 coal miners died on the job over the past century, with another estimated 200,000 dying later from black lung disease. Now those are externalities. Thankfully, coalmining deaths have plummeted in the United States, to a current average of about 36 per year. But if Jack happened to live in China, the local death externality would be much steeper: at least 3,000 Chinese coal miners die on the job each year.

It's hard to blame Jack for not thinking about externalities. Modern technology is so proficient that it often masks the costs associated with our consumption. There's nothing visibly dirty about the electricity that feeds Jack's air conditioner. It just magically appears, as if out of a fairy tale.

If there were only a few Jacks in the world, or even a few million, no one would care. But as the global population hurtles toward 7 billion, all those externalities add up. So who should be paying for them?

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