科学美国人60秒:If the Fuel Source Ain't Clean, Your Electric Car Ain't Green

As Tesla prepares to begin selling its Model S in smog-ridden China, the question is: do electric cars reduce air pollution? And the answer is: it depends.
 
That on-the-one-hand-yes-but-on-the-other hand-no conclusion comes from a study predicting the impacts of electric cars in the U.S. to 2050. It's in the journal Environmental Science and Technology. [Samaneh Babaee, Ajay S. Nagpure, and Joseph F. DeCarolis, How Much Do Electric Drive Vehicles Matter to Future U.S. Emissions?]
 
The reason it depends is that we don't know the future. Will batteries be cheap? Will the U.S. have a law limiting carbon dioxide? Which will cost less: oil or natural gas?
 
Depending on the answers, electric vehicles and hybrids might not reduce air pollution at all. They could even make it worse.
 
Researchers ran 108 different computer models of how driving might change in the next few decades. And it turns out the key to reducing air pollution isn't how many electric cars there are. What matters is whether there are regulations in place to mandate reductions in CO2 and other pollutants.
 
As it stands, battery cars that run on electricity from burning coal can be more polluting than cars that get good mileage on gas. And that's why a Tesla in China, where most electricity comes from coal, is no zero-emissions vehicle.

—David Biello