Natural Gas for Cars

Does it make climate sense to drive cars with natural gas?

Our nation appears to be rapidly moving to a natural gas-powered economy. Advances in hydraulic fracturing (fracking) and horizontal drilling have made huge deposits of natural gas in shale and tight sands commercially viable. (See “Hydrofracturing: An Energy Revolution.”) Suddenly the United States is awash with a cheap form of domestic energy. Cheap enough to compete successfully against coal for generating electricity and climate friendly enough to make it the Environmental Protection Agency’s electricity-generating choice as well.

But there are some troubling questions, some related to the environmental damage that fracking might cause to local communities and others related to the climate benefits of natural gas itself.

Because the major ingredient of natural gas is methane, a powerful greenhouse gas many times more effective a global warmer than CO2, any leakage of natural gas during the extraction (production), transport or storage stage can offset or even overwhelm the climate benefits of burning natural gas instead of coal.

Where you end up coming down on the argument about climate benefits largely depends upon what you assume about the leakage rate for natural gas. Assuming a large leakage of as much as eight percent of the total production might lead you, as it did Cornell’s Robert Howarth et al, to conclude that sticking with “dirty” coal is more beneficial from a climate perspective. Assuming a more modest leakage of up to three percent, as EPA estimates, might result in the conclusion that switching to natural gas provides a climate benefit (although, because it is a fossil fuel, not a carbon-free one without carbon capture and storage).

Don’t count on that debate being resolved until the scientific community and gas industry team up to comprehensively determine just how much gas is actually leaking.

But while the debate roils over the climate benefit of substituting natural gas for coal in electricity generation, there has not been a similar assessment of whether it makes climate sense to substitute natural gas (actually compressed natural gas or CNG) for petroleum to power our cars and trucks. That is, until now, with the publication of a new paper by Ramon Alvarez of the Environmental Defense Fund and co-authors (including yours truly) in the Proceedings of the National Academy of Sciences.

Why Natural Gas-Powered Cars and Trucks?

At current prices, switching to compressed natural gas as a fuel for your car or truck would seem a no-brainer, as on an energy basis, natural gas costs about half as much as gasoline. And because natural gas is a domestic fuel, it makes great sense from a national security point of view. There are downsides:

Still it is an alternative, and one that some municipal transportation systems have opted for.So, does the switch make sense from a climate perspective?

An Estimate of Climate Impacts of Vehicles Powered by Compressed Natural Gas

When it comes to fossil fuels and carbon emissions, coal is the dirty fuel and natural gas the clean one with petroleum in the middle. Because coal is so “dirty,” it is relatively easy for natural gas to beat out coal in the climate-benefit contest for producing electricity. But the case is not that clear when it comes to natural gas and gasoline.

To assess the relative benefits, Alvarez et al used a metric we called the technology warming potential (TWP), which calculates the relative warming of a given fuel or technology choice over the lifetime of its use. The results were not encouraging for compressed natural gas-powered cars.

Even when adopting EPA’s modest leakage rates, we found that a full-scale U.S. move from gasoline to natural gas would initially lead to more warming than if we just stuck with gasoline.

Because methane is removed from the atmosphere more rapidly than CO2, the extra climate warming from natural gas leakage doesn’t last forever but eventually disappears. But that happens very slowly.

The break-even point between warming and cooling doesn’t happen in the Alvarez et al calculations until 80 years after the initial switch. And a 10 percent climate benefit compared to sticking with gasoline doesn’t accrue for 150 years. And that 10 percent benefit is approximately equivalent to improving a vehicle fleet that gets 30 miles per gallon by a mere three miles per gallon. Switching from diesel to compressed natural gas would take almost 300 years before seeing a climate benefit.

If we’re serious about cutting greenhouse gases in the coming decades, that, my friends, is simply not going to, as the phrase goes, cut it.*

Leakage the Key

Of course the wildcard in our study and all the related ones is the uncertainty surrounding the natural gas leakage rate. A colleague of mine commented on this subject saying, and I paraphrase, the more one learns about natural gas leakage, the more one realizes we don’t know much about it. It is conceivable that leakage rates are actually less than EPA’s estimate, in which case compressed natural gas might be a climate winner. And of course the opposite may also be the case.

And so the debate rages on, with claims and counterclaims (see here, here, and here). Of course one way to resolve the issue is for the scientific community and gas industry to team up and comprehensively determine just how much gas is actually leaking. (Here’s a study looking at methane leakage rates in Colorado.)

But there’s an easier way out that would make the whole debate over leakage moot: simply close up the leaks in the supply network and capture the natural gas before it escapes. In fact, in our paper we estimated that if leakage rates were cut to 1.6 percent (as opposed to the three percent estimate used in the paper) “CNG cars would result in climate benefits immediately and improve over time.”

You know what they say: a cubic foot of natural gas saved is a climate benefit earned.


End Note

* On the other hand, it should be noted that Alvarez et al did find an
immediate climate benefit in switching from coal to natural gas for
electricity generation when using EPA’s modest leakage rates.


  1. Bill Chameides
    May 14, 2012, 4:49 pm

    In response to the comment by John G. Dzwonczyk (April 18, 12:42 pm):

    You’re correct. There are a number of reasons to favor a switch to natural gas. However, one of the reasons given for switching to natural gas for vehicles is that there will be a climate benefit. It is just that aspect of the argument that is addressed in the post and paper.

    Sure, if your greatest concern is national security, you may not care about the climate impact and may want to make the switch regardless. But one could also argue that if you are really concerned about national security, you should be concerned about climate change.

  2. Bill Chameides
    May 14, 2012, 4:44 pm

    In response to the comments by Robert I Eachus above:

    The fact that methane is oxidized in the atmosphere on a time scale of roughly a decade and is converted into CO2 is fully accounted for in the calculations.

  3. Eideard
    Santa Fe, New Mexico
    April 19, 2012, 6:08 pm

    I love the dovetail between the Ivory Tower crowd, conservative politicians and know-nothings in the Kool Aid Party.

    Yup. Let’s research this for another decade. Cost reductions in the budgets of working class Americans aren’t really as important as a thorough discussion.

    Then, Congress can re-examine the concept for another decade, as well.

    Throw in populist pimps for Big Oil who are too lazy, too accustomed to subsidies for the inefficiencies of existing flywheel – but profitable – industries and we can have a jolly roundtable.

    The question of diminishing living standards for working people isn’t apparently worth including in the discussion.

  4. John G. Dzwonczyk
    Avon Lake, OH
    April 18, 2012, 12:42 pm

    As with anything else, there are going to be differences of opinion when forecasting is the game. Nevertheless, what natural gas vehicles can alleviate is so much pressure on oil resources, the kind that make higher-risk production such as deep-water wells and their inherent risks (cf. Macondo disaster), and the kind that make for higher fuel prices overall. The relatively precious nature of oil has caused a number of geopolitically undesirable developments in the past century, whereas gas is much more a plentiful, shared wealth worldwide.

  5. Robert I Eachus
    Nashua, NH
    April 17, 2012, 11:23 pm

    The issue isn’t how much methane leaks–although I tend to feel as an engineer that even three percent is wasteful. (If you keep the natural gas under pressure, it won’t leak, but you add weight. Allow the natural gas to boil off, and you get the high leakage scenario. But designing a car to keep the gas cool and under moderate pressure seems like a win.

    However, there is another factor which current climate models get very wrong. Methane oxidizes quickly in the atmosphere. For each methane molecule you get one of carbon dioxide and two of water. Still greenhouse gasses, but nowhere near the unoxidized methane.

    By the way, I think the best future transportation fuel is methanol.