The United States isn’t running out of natural gas anytime soon, according to an article in today’s Wall Street Journal. As recently as three years ago, domestic gas production seemed to be in permanent decline. But now, thanks to a series of discoveries in Texas, Louisiana, and elsewhere, it’s looking likely that natural gas will remain plentiful long into the future. One (industry-backed) study estimates that the United States has enough natural gas to meet its current level of demand for the next 100 years. This is fairly good news from a climate perspective, since natural gas is less carbon-intensive than either coal as an electricity source or oil as a transportation fuel. So does that mean it's time to dust off this summer’s Pickens Plan and start converting the nation’s vehicle fleet to natural gas?

Not if the goal is to reduce greenhouse-gas emissions as cost-effectively as possible. It’s certainly true that natural-gas vehicles emit less carbon-dioxide than the average gasoline-powered car. For instance, a natural-gas version of the Honda Civic, with an EPA fuel-economy rating of 28 miles per “gasoline gallon equivalent," will emit about 0.48 pounds of carbon-dioxide per mileabout two-thirds the per-mile emissions of a comparable gasoline-powered vehicle getting 27.5 miles per gallon. So that's not bad. Trouble is, there’s basically no scenario in which a car running on natural gas emits less than an electric vehicle, or even a plug-in hybrid like the Chevy Volt.

If you assume that an electric vehicle or a plug-in hybrid running in all-electric mode gets 4 miles to the kilowatt-hourand according to this NRDC study, that's a fairly conservative assumptionand assume that each kilowatt-hour results in the current national average of about 1.3 pounds of carbon-dioxide, then driving an electric car produces about 0.327 pounds of CO2 per mile. That’s significantly less than a natural-gas vehicle, and  without any need for building an expensive network of natural-gas fueling stations.

And what happens when the battery on that plug-in hybrid runs out and the car starts burning gasoline? Given that there are 19.4 pounds of carbon-dioxide per gallon of gas, any gasoline-fueled car that gets more than 40.8 miles per gallon will emit less carbon-dioxide per mile than our natural-gas Honda Civic. So even if it ran its internal combustion engine full-time, the Volt would still do better than that. And even some non-plug-in hybridslike the Prius, at 46 miles per gallon, or the hybrid Civic, at 42 miles per gallonend up having lower per-mile emissions than a similar car running on natural gas.

This isn’t to say that natural gas isn’t useful. Until large-scale energy storage gets a lot cheaper, instant-on natural-gas turbines will be an important source of backup power for intermittent renewables like wind and solar. But unless electricity production gets inexplicably more carbon intensive, it just doesn’t make sense to use natural gas in cars.

(Flickr photo credit: John Christopher Jones)

--Rob Inglis