How can you turn a massive, fuel-chewing Hummer into a zero-emissions vehicle? By buying “pollution credits” — via this wacky new company called TerraPass. The concept works the same way that the “cap and trade” emissions-trading system works between companies and countries: If you pollute more than you’re supposed to, you can buy a “credit” from someone who has voluntarily reduced their emissions to a nice green level. Theoretically, this keeps pollution to a regulated, accepted level, and encourages firms to voluntarily buy new superefficient technologies that reduce their emissions, because they can offset those costs by selling pollution credits.

Now TerraPass offers this deal — for cars. As a story in CNN reports:

The stickers TerraPass sends its customers do nothing to stop pollutants from coming out of a car’s tailpipe. Instead, the company offers its customers the chance to reduce pollutants from other sources, like power plants, in an amount equivalent to that produced by their car.

That way, you can drive your car while having no net effect on the amount of pollution in the air, the company says.

Apparently, it only costs $160 to effectively render your Hummer into a zero-emissions vehicle. Which to me is where you can see the fissures in this scheme — because I’m pretty sure that if you calculated the amount of carbon a Hummer spits out in its lifetime, it’d cost quite a lot more than $160 to remove it from the air. I’m sure the cost will go down as carbon-sequestration technology improves — and part of the goal of cap-and-trade schemes is to encourage the development and adoption of better sequestration tech, of course, so you could argue that we gotta start somewhere. Taken on its own terms, TerraPass is a pretty clever idea, I’d say.

But they’ve only sold 620 passes so far — and most were bought by eco-freaks who already buy super-low-emission cars. As founder Tom Arnold notes: “We fully expected to target SUV drivers with SUV guilt,” but “it just doesn’t exist.”