Will people really donate $200 after buying a $300 air conditioner?

Robert Frank defends carbon offsets at the sister blog. I'm sympathetic to much of Frank's argument; in particular, the fact that Al Gore has a big house isn't much of an argument against carbon offsets. (If the crops are failing and the flood waters are rising, it won't be much help to stand on a street corner shouting: But Al Gore had a big house!)

But I'm not happy with the example that Frank chooses to illustrate his point. He writes:

Suppose you live in a northern city with normally mild summers and are considering buying a bedroom air conditioner to ease you through the occasional brutal heat wave. Your choices are between a highly efficient model that sells for $500 and a less efficient one that sells for only $300. Because you're concerned about global warming, you feel obligated to buy the more efficient model. But because you use your air-conditioner so infrequently, buying that model won't actually help much. You'd do much more to curb global warming if you bought the cheaper model and used the money you saved to buy carbon offsets.

My first thought is that you'd be better off buying a couple of high-quality fans for $79 each. Beyond this, I'm troubled by the example because what makes it work is the willingness of somebody to spend $200 extra for the environment. This can't be how large change can come. Usually the argument I hear made to encourage people to buy energy-efficient appliances is that they will save you money in electricity costs after a year or two. It's hard for me to see carbon offsets making a difference if they're being driven by voluntary contributions. Although I guess the idea is that if voluntary contributions become some sort of norm, they can gradually become mandatory contributions. I don't claim to have any handle on the policy issues here; I just thought it was an odd example.