Car off cliff.Should the clunkers plan be junked?Support “cash for clunkers” as an auto-industry bailout if you must, but don’t call it green.

So say the Wall Street Journal, the American Council for an Energy-Efficient Economy, and U.K. environmentalist George Monbiot, all critical of the environmental benefits of the proposal President Barack Obama endorsed yesterday.

Obama and Democratic House lawmakers reached compromise on a plan that would pay drivers $3,500 to $4,500 to trade in gas-guzzling older vehicles for more fuel-efficient new ones. The idea has been tossed about as a way to give automakers a boost and retire the nation’s dirtiest vehicles.

But the House plan, which could end up in the larger Waxman-Markey climate and energy bill, sets almost laughably low standards.

A House Energy and Commerce Committee fact sheet [PDF] reveals just how low. The plan would require a new passenger car to get only 22 miles per gallon (according to EPA “window sticker” estimates) and be at least 4 mpg more efficient than the car it’s replacing. For light trucks, the threshold drops to 18 mpg and a 2 mpg improvement. For large light-duty trucks, it’s 15 mpg and a 1 mpg improvement.

“Toothless,” the WSJ’s Keith Johnson calls the plan:

The problem with all this, as Duke’s Bill Chameides pointed out last month, is that making a new car produces, on average, about 6.7 tons of carbon dioxide. By his calculations, it would take at least five years to “pay off” the environmental impact of building the new car with a 22-mile-per-gallon purchase. That SUV might be even worse — the estimated payback time is almost 20 years.

ACEEE, an efficiency advocacy group, preferred the ‘clunkers’ plan put forth earlier this year: “Unlike the scrappage bill introduced in January, which aims to accelerate the modernization of the U.S. fleet to a more fuel-efficient one, the program just announced aims primarily to clear Detroit’s unsold inventory from the storage lots,” it said in a press release.

“I can’t see using taxpayer dollars to sell a Hummer H3T,” ACEEE Transportation Program Director Therese Langer said in the release. “We would welcome incentives to retire gas guzzlers and encourage the purchase of efficient vehicles, but the proposal just isn’t there yet.”

The Guardian’s George Monbiot tore apart a similar U.K. proposal and a German cash-for-clunkers program already in effect:

A paper published in 2000 by the journal Transportation Research comes to even grimmer conclusions: that replacing old cars with new ones increases carbon pollution. Because between 15% and 20% of a car’s emissions are produced during its manufacture, the optimal age for a car, the paper says, is 19 years. (The average age of the UK’s fleet is 4.9 years). If the paper’s assumptions hold (they may be out of date now), it would make more sense for the government to pay us to keep our old bangers on the road.

Wonkette’s analysis is less heavy on the numbers: “Congress Will Buy Your Crappy ‘Ride’ For $4,500!

And more debunkery, for good measure, from Common Tragedies.