The principle is like the emissions credits that car companies themselves can trade and use to relieve themselves of CAFE debts. TerraPass buys credits from clean power sources like windmills and, yes, dairy farms that use anaerobic digesters to produce electricity. There's even a carbon-emission calculator on their site to properly gauge how much you need to contribute to the good of the earth.
The company, started by MBA grads from Penn's Wharton business school, is a for-profit that keeps itself in greed check by limiting profits to 10 percent. Only 620 of the passes have been sold thus far - and according to TerraPass, it's almost impossible to convince drivers of the biggest SUVs that they're a menace to the environment.
So if you have extra cash lying around, as well as a massive backlog of auto-induced doubt, go ahead and venture to the TerraPass Web site. Me, I'd rather put it towards some regular unleaded.