Thankfully, at least one reporter has tried to find them. In April, when Congress was still in the process of debating two competing clunkers bills, The Christian Science Monitor’s Gregory Lamb had the good sense to inquire about the amount of carbon dioxide released in manufacturing. He found that:

Estimates of the “carbon cost” of a new vehicle range from 3.5 tons to 12.4 tons of CO2 expended per vehicle, says William Chameides, professor of the environment and dean of the Nicholas School of Earth & Ocean Sciences at Duke University. He’s averaged them as 6.7 tons per vehicle.

Depending on the fuel efficiency of the car or truck, and how many miles it will be driven, it could take from a few years to beyond the lifetime of the vehicle to make up the “carbon cost” and begin saving on emissions.

Judging by an April post at Chameides’s blog, The Green Grok, however, most of the cars sold last week under the clunkers program would take under five years to amortize the energy cost of their manufacturing. But there are other considerations. Conceivably, buyers could avoid the manufacturing cost altogether by trading in their clunkers for more efficient albeit used vehicles such as the Toyota Prius (or even the ‘94 Geo Metro), but the program does not allow that to happen. And Time’s Bryan Walsh worries that with the money they’re saving on gasoline, new-car buyers could start driving more, nullifying the effect of owning a more efficient vehicle.

The upshot, as the director of the Center for Climate Change Law at Columbia University explained it to the AP’s Borenstein, is that:

“It’s not that [cash for clunkers] is a bad idea; just don’t sell it as a cost-effective energy savings method. From an economic standpoint it seems to be a roaring success. From an environment and energy perspective, it’s not where you would put your first dollar.”

Indeed, the program may be worth the $2 billion extension (Lamb’s article in the Monitor pointed out that even if it doesn’t do much for carbon pollution, it could reduce conventional air pollutants like nitrogen oxide and sulfur oxide). But reporters must do a better job analyzing the other side of the cash-for-clunkers coin—paying as much attention to the environmental impacts as the economic ones—so that taxpayers can make more informed decisions about where they stand.

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Curtis Brainard writes on science and environment reporting. Follow him on Twitter @cbrainard.