Since the Department of Transportation announced last Thursday that new-car buyers had bankrupted the “cash for clunkers” rebate initiative in its first week, the media have devoted countless articles to analyzing the federal program’s value.

The deal—which offers consumers up to $4,500 for exchanging older vehicles for new, more fuel-efficient ones—was designed to stimulate the economy and protect the environment by reducing heat-trapping greenhouse-gas emissions. Last Friday, the House of Representatives voted to inject an additional $2 billion (on top on the $1 billion already claimed) into the program and the Senate is “expected” to second that, according to news reports on Tuesday. In assessing the wisdom of that decision, however, the press has focused almost entirely on the economic impacts of cash for clunkers, paying relatively little attention to the program’s environmental upshot.

It’s unsurprising, of course, that news pages would tilt toward the pocketbook analysis. Industry-wide auto sales—a bellwether of economic health—in July were still down twelve percent from a year ago, according to CNN, but the best they’ve been since last August. Admittedly, that isn’t all that legislators care about. Senators Dianne Feinstein (D-California) and Susan M. Collins (R-Maine), who had authored an early version of a cash-for-clunkers bill that called for higher efficiency requirements, released a statement last week saying that their support for extending the program would be contingent upon raising the minimum fuel economy improvement for newly purchased vehicles by at least two miles per gallon.

Feinstein and Collins abandoned their condition on Monday, however, after learning, as the former told The New York Times, that “The statistics are much better than anybody dreamed they would be.” According to the Times’s article:

[T]he Transportation Department reported that the average gas mileage of the vehicles being bought was significantly higher than required to qualify for a rebate… Of 120,000 rebate applications processed so far, the department said the average gas mileage of cars being bought was 28.3 miles per gallon, for S.U.V.’s, 21.9 miles per gallon, and for trucks, 16.3 miles per gallon.

That is wonderful, of course, but most news outlets have taken the environmental benefit of those statistics for granted. Fortunately, a handful of reporters have looked closer at that assumption. Reuters’s Richard Valdmanis was perhaps the quickest to weigh in, with a report last Friday concluding that the clunkers program would provide only a “tiny boost for U.S. fuel efficiency.” The article went on to report that:

Even if the program is extended, analysts said the scheme will shave little more than 0.05 percent, or between 4,000 and 5,000 barrels [of oil] per day, off the nation’s daily consumption of 9 million barrels…

The U.S. Energy Information Administration, the statistical wing of the Department of Energy, said it is not planning to alter its short-term outlook as a result of the program.

On Tuesday, the Associated Press’s Seth Borenstein crunched the numbers, also coming to the conclusion that “replacing those fuel hogs will reduce carbon dioxide emissions by just under 700,000 tons a year. While that may sound impressive, it’s nothing compared to what the U.S. spewed last year: nearly 6.4 billion tons (and that was down from previous years).” In other words, as Borenstein cleverly put it in his lede:

“Cash for clunkers” could have the same effect on global warming pollution as shutting down the entire country — every automobile, every factory, every power plant — for an hour per year. That could rise to three hours if the program is extended by Congress and remains as popular as it is now.

Climate experts aren’t impressed.

And even Borenstein might be overestimating the program’s environmental benefit. “While some people have worried that there might be an added environmental and energy cost to recycling the metal in the junked cars, experts said that is not the case,” he wrote. But what about the energy cost on the front end, during manufacturing?

An op-ed in The Washington Post on Tuesday argued that “[E]ven when new cars and appliances are more efficient than the ones they replace, the act of replacing them entails environmental costs not accounted for in the stimulus programs.” The piece did not offer any hard numbers on the total lifecycle costs of automobiles, however. Its author, a program researcher in environmental history and policy at the Chemical Heritage Foundation’s Center for Contemporary History and Policy in Philadelphia, later told FOXNews.com “that figures on energy costs in production and disposal were not readily available.”

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 is the editor of The Observatory, CJR's online critique of science and environment reporting. Follow him on Twitter @cbrainard.