”Our goal is to remove the car from the environmental debate,” says Larry Burns, vice president for R&D and strategic planning at General Motors. His vision is that one day cars will emit no harmful pollutants from their tailpipes—or perhaps they’ll have no tailpipes at all. And if the beleaguered automaker survives that long, GM may be able to achieve that goal.

But no company can ever remove cars from the environmental equation. Public impressions are fleeting and malleable, but the laws of physics and chemistry are immutable. Cars require energy to move, and that energy—even if it’s stored in a battery pack rather than in fuel sloshing around in a tank—has to come from somewhere.

And therein lies the problem. Odds are those batteries won’t be recharged with solar or wind energy. In most places, grid power is for many decades going to come from the burning of fossil fuels, which generate their own emissions. So the question becomes: If you power a vehicle with electricity from the grid rather than with fuel from the tank, is that better or worse for the environment, particularly with respect to greenhouse gases like carbon dioxide?

It’s a question that dogs not just automakers but also policymakers all over the developed world. Companies and governments are already spending billions of dollars engineering the vehicles and infrastructures to kick off the transition from gasoline and diesel to electricity. Plug-in hybrids even became a mantra during the 2008 U.S. presidential election—with both candidates citing them as an environmental panacea.

A few analysts forecast that by 2020, plug-in vehicles, including plug-in hybrids and purely electric cars, will make up almost a third of new-car sales in the United States. And by 2050, plugâ¿¿ins could account for most of China’s burgeoning vehicular travel. But the environmental implications of such a massive shift are hardly straightforward.

The complexity stems from the multiplicity of vehicles, ­electricity-generating technologies, and assumptions behind future projections for both. Imagine that two years from now you’re comparing a newly available hybrid model that can recharge from wall current with a conventional gasoline car that consumes, say, 9.4 liters per 100 kilometers (25 miles per gallon). In this case, using grid power to drive electrically emits fewer greenhouse gases per kilometer—under any circumstances.

But if you compare the plug-in with an ultraeconomical European diesel or a conventional hybrid-electric like Toyota Motor Corp.’s Prius—either of which burns just 4 to 5 L/100 km—the picture is more complicated: The plug-in emits fewer greenhouse gases in some circumstances, but more in others.

The balance hangs on just what sort of power plants are being used to generate the electricity. So before you decide what to buy, you will need to answer a second question: How green is your grid?

Electric vehicles have been around for decades, although their limited ranges have made them impractical for most people. But now, with automakers preparing to introduce the first ­vehicles with automotive-quality lithium-ion batteries, the situation is about to change dramatically. Lithium-ion cells can store roughly four times as much energy as lead-acid cells and twice as much as nickel-metal hydride, the kind used in the Toyota Prius and most other currently available hybrid cars.

In the near term, lithium-ion offers the promise of plug-in hybrids that can achieve about 15 to 65 km (10 to 40 miles) of all-electric range, along with purely electric vehicles that can go about 160 km (100 miles) or more before recharging. And as battery costs diminish, the maximum ranges may improve considerably.

The advances in electric drive are unfolding in stages. The Prius, launched in Japan in 1997, was the first mass-­production car since the 1930s with an electric traction motor. It has a 1.5â¿¿L combustion engine, supplemented by two electric motors and a battery pack, which can provide only short bursts of pure electric travel—1 to 2 km at most. But the Prius can’t use grid power to charge its battery: It generates all its own electricity using both engine power and regenerative braking. Having a combination of electric motors and a combustion engine working in parallel is ­valuable, though, because it allows the Prius (and similar parallel hybrids) to use its fuel much more efficiently.

The next step in modern automotive electrification will be to add grid charging to hybrid-electric vehicles, turning them into plug-in hybrids. Toyota, for example, plans to offer a plug-in version of its Prius in 2010, but it will probably have a limited range in pure-­electric mode—up to 20 km (12 miles). Other production vehicles of this sort may go somewhat farther on battery power. (Their quoted electric ranges, however, are not necessarily continuous—at highway speeds or under heavy loads, the engine may switch itself on.)

A further move toward full electrification is the series hybrid. Assuming General Motors is still with us in late 2010, it will begin selling such a car: the Chevrolet Volt, which GM calls an ”extended-range electric vehicle.” The Volt supplements a 16-kilowatt-hour battery that provides 65 km (40 miles) of pure-electric range with a 1.4-L combustion engine. But as with all series hybrids, the engine isn’t mechanically connected to the wheels. Instead, the Volt’s engine spins a generator that provides enough current to the battery to sustain its charge and run the car for another 480 km (300 miles) or more on a tank. Even without the engine, the Volt would still function as a limited-range electric car.

Finally, there are models built as purely electric cars, also known as battery-electric vehicles, whose main drawback at present is the high cost of the battery pack. Analysts expect mass-­produced electric cars with reasonably affordable lithium-ion batteries—and consequently with ranges of 160 km or less—to enter the market by 2012. Nissan Motor Co. and its partner Renault have already announced one such compact sedan.

Calculating the green credentials of these different drivetrains is not at all a straightforward exercise. But let’s start with the simplest, and for many the most pertinent, yardstick: how much climate-warming carbon dioxide is generated for each kilometer driven—whether it’s delivered by gasoline or electricity.