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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?

In principle, this shouldn’t be such a hard problem. If we knew how much it cost humankind every time someone used a tank of gas, we could simply levy a tax of that magnitude on the driver. The tax wouldn’t necessarily convince him to cancel his trip, nor should it. The point of the tax is to make sure the driver faces the full costs of his actions (or, in economist-speak, to internalize the externality).

The revenues raised from these taxes could then be spread out across the folks who suffer the effects of a changing climate—people living in Bangladeshi lowlands, for instance, who will be flooded if the oceans rise precipitously. If we chose exactly the right tax, the revenues could properly compensate the victims of climate change.

But when it comes to actually solving climate-change externalities through taxes, all we can say is good luck. Besides the obvious obstacles—like determining the right size of the tax and getting someone to collect it—there’s the fact that greenhouse gases do not adhere to national boundaries. The earth’s atmosphere is in constant, complex motion, which means that your emissions become mine and mine yours. Thus, global warming.

If, say, Australia decided overnight to eliminate its carbon emissions, that fine nation wouldn’t enjoy the benefits of its costly and painful behavior unless everyone else joined in. Nor does one nation have the right to tell another what to do. The United States has in recent years sporadically attempted to lower its emissions. But when it leans on China or India to do the same, those countries can hardly be blamed for saying, Hey, you got to free-ride your way to industrial superpowerdom, so why shouldn’t we?

When people aren’t compelled to pay the full cost of their actions, they have little incentive to change their behavior. Back when the world’s big cities were choked with horse manure, people didn’t switch to the car because it was good for society; they switched because it was in their economic interest to do so. Today, people are being asked to change their behavior not out of self-interest but rather out of selflessness. This might make global warming seem like a hopeless problem unless—and this is what Al Gore is banking on—people are willing to put aside their self-interest and do the right thing even if it’s personally costly. Gore is appealing to our altruistic selves, our externality-hating better angels.

Keep in mind that externalities aren’t always as obvious as they seem.

To keep their cars from being stolen off the street, a lot of people lock the steering wheel with an anti-theft device like the Club. The Club is big and highly visible (it even comes in neon pink). By using a Club, you are explicitly telling a potential thief that your car will be hard to steal. The implicit signal, meanwhile, is that your neighbor’s car—the one without a Club—is a much better target. So your Club produces a negative externality for your non-Club-using neighbor in the form of a higher risk that his car will be stolen. The Club is a perfect exercise in self-interest.

A device called LoJack, meanwhile, is in many ways the opposite of the Club. It is a small radio transmitter, not much larger than a deck of cards, hidden somewhere in or beneath the car where a thief can’t see it. But if the car is stolen, the police can remotely activate the transmitter and follow its signal straight to the car.

Unlike the Club, LoJack doesn’t stop a thief from stealing your car. So why bother installing it?

For one, it helps you recover the car, and fast. When it comes to auto theft, fast is important. Once your car has been missing more than a few days, you generally don’t want it back, because it likely will have been stripped. Even if you don’t want your car to be found, your insurance company does. So a second reason to install