Hm.

Over the last two days, a question has circulated around the NEW office, asked by green architect and NEW friend Rob Harrison. His quandary: Which car should he buy to replace an automobile that was totalled?

He’s narrowed his choices to four — a super-efficient Toyota Prius, a VW or Subaru station wagon, or a 1992 Honda Accord — and is weighing factors including price, reliability, safety, utility, and environmental performance.

I can’t claim any special expertise on the subject, but I can say this much (and I’m preparing to duck when people start throwing blunt objects at me): For most city dwellers, buying a new Prius is a fairly expensive way of reducing your environmental impacts.

Here’s what I mean. A new Prius starts at about $20,295. To keep comparisons simple, I Iooked at similar Toyotas — they’re likely to have similar safety, performance and reliability to the Prius. The closest match given Rob’s needs was a new Toyota Corolla, which is just a bit smaller than the Prius. But the Corolla starts at $6,600 less.

Of course, a Prius’ operating costs are lower than the Corolla’s: both have solid reputations for reliability, but the Prius sips gasoline. Over the course of 10 years, the Prius might save $2,000 or more in gasoline costs alone, depending on how much you drive it and how expensive you think gas is going to be. (I’ve assumed that gas will cost $1.80/gallon in real terms, a future discount rate of 5 percent, and that the car will be driven 12,000 miles a year.) Buying a Prius may also make you eligible for a federal tax credit, though that benefit is being phased out. But excluding any tax credit, the Prius is about $4,500 more expensive than a comparable Corolla, after factoring in its lower fuel costs.

But saving gasoline also has the side benefit of preventing global warming emissions — roughly 15 tons of CO2 over a decade, plus lots of particulates and smog-forming compounds. Those benefits aren’t included in the market price of the vehicles — but if they were, they’d make the Prius an even better buy.

Or, maybe not.

In reality, the cost of offsetting a ton of CO2 emissions isn’t all that high. Yesterday’s L.A. Times reports on a company that’s selling what it calls a TerraPass: essentially, a pricey bumper sticker that identifies the driver as a volunteer in the fight against global warming." When you buy a TerraPass, the parent company buys up CO2 credits in the newly established Chicago Climate Exchange, whose member corporations have committed to reducing greenhouse emissions. The rub: A TerraPass that offsets 10 metric tons of CO2 emissions costs just $79.95. If the program really works as advertised (a big if, obviously) $120 would be more than enough to offset the increase in emissions from buying a Corolla vs. a Prius. If you were willing to commit just one-tenth of the cost difference between the Prius and the Corolla, you could make your driving climate neutral for 10 full years. For one-fifth the savings vs. a Prius, you could offset both your emissions, plus a neighbor’s. And so on.

If you have qualms about the relatively untested Climate Exchange, there are other options. Bonneville Environmental Foundation’s Green Tags program lets you buy credits to support new wind and solar power projects. More renewable power means less coal and natural gas burned in power plants — and consequently less particulates, smog-forming chemicals, mercury, and the like. Buying enough green tags to cover the 10-year difference in CO2 emissions between the Prius and the Corolla costs about $420.

Or if you’re into tree planting, Reuters reported yesterday that, according to a new Pew Charitable trust study, large forest-based carbon sequestration projects could remove CO2 from the atmosphere for somewhere between $25 and $75 per ton. And the list of cost-effective carbon-sequestering projects (including contributing to organizations working for legislative and policy changes) goes on and on.

Now, before you start getting all mad that I’m not being fair to the Prius, I do want to point out two other things to consider. First, driving a Prius — which qualifies as a "super ultra low emissions vehicle" under California rules — prevents pollution where you live. For CO2, that’s irrelevant. But for other pollutants, it might matter: driving a Prius is a sure-fire way of reducing your contribution to local air pollution. The other options — Green Tags and the like — still prevent pollution, but possibly not in your neighborhood.

Second, in the abstract I still love the Prius, and wish it (and its hybrid successors) well. I want gas-sipping hybrids to be so successful that they completely transform the automotive marketplace. They’re already starting to — and by buying a hybrid, you’re helping speed that transformation along. But that doesn’t mean that there aren’t other, more cost-effective ways of achieving the benefits that the Prius brings.

Of course, your mileage may vary. If you compare the Prius with the roomier Camry, the the Prius stacks up pretty well — the Camry is more expensive than the Corolla, and gets worse mileage. Farther afield, the Subaru Outback (another of Rob’s possible choices) is a relative gas-guzzler which makes the potential cost savings of choosing a Prius a little higher.

Nevertheless it seems like a sound environmental choice would be to buy a safe, inexpensive car that gets decent — but not outstanding — mileage. Then, commit some or all of your long-term savings to a program that’s working to reduce CO2 emissions (either directly, or through changes in policy).

Bonus: Just because it seemed like it might be helpful, I’ll include a handy guide to gasoline costs and CO2 emissions from your car. Notice the diminishing returns: trading an SUV that gets 15 mpg to a midsized car that gets 30 mpg is twice as beneficial as switching from 30 mpg car to a hybrid that gets 60 mpg.

Estimated tons of CO2 emissions, and present cost of gasoline, for a car driven 12,000 miles per year for 10 years (guesstimating that gas will cost $1.80 per gallon in constant dollars, and discounting future purchases at 5%):

UPDATE: Some follow-up math here.