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With the courts involved, at least for the time being with the issue of carbon emissions, it's a good time for us to take up a simple and somewhat naïve question: why can't we just remove carbon dioxide from the auto exhaust before it's released into the air? To find out, NPR's Elizabeth Shogren went for a ride with a climate scientist.

ELIZABETH SHOGREN: Daniel Kirk-Davidoff is a professor at the University of Maryland. I picked him up on his campus in the Washington, D.C. suburb of College Park.

Professor DANIEL KIRK-DAVIDOFF (Professor, University of Maryland): This is Baltimore Avenue, also known as US 1 and we're approaching a BP gas station.

SHOGREN: He says a gas station is the logical place to start talking about cars and carbon dioxide.

Professor KIRK-DAVIDOFF: The chain starts here. That's right. We pump the gas into the car. The car burns the stuff. It burns the gasoline and out comes CO2 and goes into the air.

SHOGREN: As we pump, he explains what the car does with the gasoline.

Professor KIRK-DAVIDOFF: When we burn it in the engine, we're mixing it with oxygen from the atmosphere and the oxygen gets stuck to the carbon and makes carbon dioxide.

SHOGREN: That's why it's CO2. The carbon atom from the gasoline and two oxygen atoms from the air. The car doesn't need the carbon dioxide to run, so it sends it out the tailpipe and into the atmosphere. There the gas absorbs infrared radiation and then sends some of that radiation to earth.

Professor KIRK-DAVIDOFF: So the first thing that happens when we add carbon dioxide to the atmosphere is it radiates some extra radiation back down to us at the surface and we warm up a little bit.

SHOGREN: That's the global warming that everyone's trying to avoid. So how do you get carbon dioxide out of auto exhausts?

Automakers already strip lots of other pollutants from cars, such as the nitrogen oxides that contribute to smog. To do that, cars use devices called catalytic converters. But Kirk-Davidoff says no one's coming up with similar gizmos for carbon dioxide. That's because to turn that greenhouse gas into something benign would use a lot of energy and that wouldn't be practical.

Professor KIRK-DAVIDOFF: So there's another thing you could do. You could say well, why don't we just grab the CO2, why don't we just hold onto it and not let it out into the atmosphere? And you could do that. You could tug about a cubic yard of water saturated with lye, with sodium hydroxide, and you could bubble the CO2 out through it and it would dissolve in the water. Maybe this would work. And then you'd be carrying around all the CO2 in your car, and maybe you could return it to the filling station and get it sucked out. But the problem is then you're carrying around almost a ton of water saturated with CO2.

SHOGREN: And that also seems out of the question. Auto engineers say the bottom line is that there's no way to avoid producing carbon dioxide if you want to use modern engines to run your car. But Kirk-Davidoff says there is a way to cut those emissions. Drive cars that use less fuel.

Professor KIRK-DAVIDOFF: Well my car, which is a big Honda Odyssey, it's actually a medium-sized one from the old days, gets about maybe 24 miles to the gallon. A Prius can get something like 50 miles to the gallon so that means it puts out only half the amount of carbon dioxide that my car does.

SHOGREN: In the future, there might be a more complete solution. Cars that run on new fuels that don't emit greenhouse gases at all, like hydrogen made with nuclear power.

Elizabeth Shogren, NPR News.

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