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Kai Ryssdal: The uber-popular Cash for Clunkers program is set to get another $2 billion from the Senate before lawmakers go home for a month. The aim is to get gas guzzlers off the road and at the same time give the economy a boost with new car sales. Dealers love it. Car makers love it. Car buyers certainly love it. How environmental is it, though? Sam Eaton has that story from the Marketplace Sustainability Desk.

SAM EATON: Being green often comes down to this: reduce, reuse, recycle. In that particular order. But Cash for Clunkers skips all the way down to the last step. It requires gas guzzlers, engines and all, to be shredded into scrap metal for recycling, even if the engines are good enough to be reused.

And that has scrap yard owners like Rick Morrow questioning just how green the program is. He's the owner of M and M Auto Parts in Stafford, Va. He says reusing car parts is far better for the environment than recycling them. And he's built his business on that model.

RICK MORROW: Long before green was popular, I mean this kind of operation, even though a lot of people said, oh, junk yard. But they were actually were making use of what the component was built for in the first place.

And he's got a point. It takes a lot of energy to melt metal from old cars and make new ones. But David Friedman with the Union of Concerned Scientists says in the case of Cash for Clunkers, scrapping the cars comes out ahead.

DAVID FRIEDMAN: For a gas guzzler you're talking 90 to 95 percent of the energy that's used over its life comes from actually burning fuel and only 5 to 10 percent comes from making the vehicle.

So getting it off the road, he says, makes more sense environmentally, as long as the new car that's replacing it is more efficient. That's where Friedman says there's room for improvement. To qualify for the Cash for Clunkers program, the new car you buy only has to get 22 miles per gallon. Still a guzzler by most standards.

Friedman says the program is still worthwhile. But don't expect any meaningful reductions in greenhouse gas emissions. He says for that you have to wait until 2016, when new federal standards for fuel economy kick in.

I'm Sam Eaton for Marketplace.