Certified Clean Car’s comment above is similar to the answers given by Terrapass. They’re buying not just carbon credits but more expensive solar and wind in the mix.

That’s an interesting question. While many people who would buy these credits would be thrilled to be supporting solar, wind or other renewables, this ignores the main reasoning behind pollution credits. That is to create a market, and encourage the most efficient means of reducing output.

If my goal is to reduce total CO2, I’m not going to want to spend $500 on solar to remove a ton of CO2 if I could remove 200 tons with the same money by paying a polluting plant to cut its output.

Indeed, this creates an interesting problem for the Prius buyer. You spend about $3,000 more on a Prius over a comparable car, including saved gasoline, or so I have read. With that $3,000, you could offset the output of over 100 ordinary cars. By spending it on the Prius you’re making a statement but increasing pollution over what you could do.