Wednesday's Supreme Court argument about whether to regulate carbon dioxide from auto emissions might have made you wonder why we don't just remove the greenhouse gas from auto exhaust the way we do a lot of other pollutants.
Auto engineers and climate scientists say that to understand that, you have to realize that producing carbon dioxide is unavoidable if you want to use a modern engine to power your car.
These engines use gasoline or diesel, and both fuels are full of chains of carbon atoms known as octane.
"When we burn [octane] in the engine, we're mixing it with oxygen from the atmosphere. And the oxygen gets stuck to the carbon and makes carbon dioxide," says Daniel Kirk-Davidoff, an assistant professor of climate dynamics at the University of Maryland in College Park, Md.
That's why it's called carbon dioxide, or C02 — one 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 tail pipe and into the atmosphere. There, the so-called greenhouse gas absorbs infrared radiation, and then sends some of that radiation to earth.
"The first thing that happens when we add carbon dioxide into the atmosphere is it radiates some extra radiation back down to the surface, and we warm up a little bit," Kirk-Davidoff says.
The result is the global warming that scientists and political leaders around the world are trying to avoid.
Automakers already strip lots of other pollutants from cars, such as the nitrogen oxides that contribute to smog, and carbon monoxide, which is poisonous. To do that, cars use devices called catalytic converters.
"Those do chemistry on the exhaust coming out of the cars, and turn the nasty stuff into things that we don't mind so much. With carbon dioxide, that's a little bit harder," Kirk-Davidoff says.
In fact, he says, no one has come up with similar gismos for carbon dioxide. That's because turning carbon dioxide into something benign would use up a lot of energy. That wouldn't be practical.
So, if you can't convert the carbon dioxide into something harmless, can you capture it?
"You could do that. You could tug about a cubic yard of water saturated with lye, sodium hydroxide, and you could bubble the CO2 out through it and it would dissolve in the water," Kirk-Davidoff says. But this wouldn't be practical, either.
"You'd be carrying around all the CO2 in your car, and maybe you could take it back to the filling station and get it sucked out. But the problem is, then you're carrying around a ton of water saturated with CO2," he adds.
But Kirk-Davidoff says there is a way to reduce emissions on carbon dioxide: Drive cars that use less fuel.
For instance, he says he drives a Honda Odyssey minivan that gets about 24 miles per gallon. If he drove a hybrid car that runs on gasoline and electricity, his car would emit much less carbon dioxide.
"A [Toyota] Prius can get something like 50 miles per gallon, so it puts out something like half the carbon dioxide that my car does," Kirk-Davidoff says.
Another option, of course is driving fewer miles. Kirk-Davidoff does that by commuting by bus.
"If I drive my car by myself, I put out about eight-tenths of a pound of carbon dioxide per mile I drive. But if I take a bus, the bus only emits about a quarter of a pound per mile that the bus drives per person in the bus. So, I save maybe three-quarters of the CO2 that I would have put into the air by taking the bus," Kirk-Davidoff says.
In the future, there might be a more complete solution: Cars that run on new fuels that don't emit greenhouse gases at all, such as hydrogen made with nuclear power, or cars that run on electricity that was generated with wind or solar power.