Are trucks and buses that burn natural gas better for the climate than diesel ones?

Fossil natural gas brings minor climate benefits, if any. In theory, methane from organic waste can do much better—but it’s easy to mess up.

September 5, 2025

When you think of natural gas, maybe you imagine it heating a home or generating electricity in a power plant. You may also know that it burns cleaner than the other fossil fuels, coal and oil: It produces less climate-warming carbon dioxide (CO₂) for the same amount of energy.¹

Natural gas is less well-known as a vehicle fuel, at least in the United States. But it’s made inroads (pun intended) among certain kinds of vehicles—for instance, in some heavy-duty fleets like buses and garbage trucks, which have traditionally run on diesel.² So we might ask, is swapping diesel for natural gas better for the climate?

Unfortunately, there isn’t a cut-and-dried answer. “The devil is always in the details here,” says Daniel Cohn, a research scientist at the MIT Energy Initiative. Overall, his read of the evidence is that fossil natural gas can provide, at best, minor climate benefits over diesel. At worst, he says, it can actually be harder on the climate. 

Making things even more complex, not all the natural gas used in vehicles is the fossil kind extracted from the earth. Trucks and buses can also run on biomethane, an almost identical fuel made from organic matter.³ Biomethane can offer a clearer improvement over diesel, says Desirée Plata, professor of civil and environmental engineering at MIT. “But there are a lot of ways to screw it up” and undercut its climate benefits, she adds.

Why is this so complicated? It helps to start with the chemistry. Natural gas (once refined and injected into a pipeline) is more than 95% methane.⁴ Burning that methane produces energy—and CO₂. But methane itself is also a powerful climate-warming gas, around 80 times as potent as CO₂ over 20 years.⁵ Therefore, the climate impact of natural gas depends not only on how much you burn, but on how much escapes into the air before you burn it.

And there are many opportunities for methane to escape. It can leak, or be released for safety purposes, from the fields where natural gas is extracted or the pipelines it’s transported through. And before natural gas can fuel a vehicle, it needs to be put under high pressure to make “compressed natural gas” (CNG) or cooled to make “liquefied natural gas” (LNG); both of those processes can release methane, too. Unburned methane can also slip out of a truck’s tailpipe. (Diesel is not immune to these problems: Methane can escape during oil production and refining.)

How much methane is escaping? It’s hard to say. “There’s a wide range” of estimates, Cohn says. In the U.S. oil and gas system, recent research suggests official numbers are too low.⁶ But the answer matters: Researchers have found that a leaky fossil gas supply chain can more than cancel out any benefits of using cleaner-burning CNG on the road.⁷

Plus, those benefits at the tailpipe might be smaller than we’d expect. Let’s imagine a pair of city buses, one diesel and one CNG, chugging along the same route. The CNG fuel produces about 25-30% less CO₂ to provide the same amount of energy.¹

But there’s a road bump: Our two buses might not use the same amount of energy. Natural gas engines have historically been less efficient than diesel ones, says Cohn, though newer designs are beginning to close the gap.⁸ If the CNG bus is less efficient, its climate advantage narrows when you switch from measuring CO₂ per unit of energy to CO₂ per mile—which we care about more.⁹

Where does all this leave us? Depending on the details, Cohn says, trucks and buses using fossil natural gas can have a modest advantage over diesel. “But when you take into account the total cycle, which includes the fugitive methane emissions,” it’s easy to find cases where opting for fossil gas creates more climate pollution than diesel.¹⁰ That’s particularly true if you assume a leakier gas supply chain—or if you’re worried about climate change in the near future.¹¹ 

What about biomethane? Its methane comes not from underground, but from organic matter breaking down in low-oxygen conditions. In the U.S., most biomethane comes from organic waste, like rotting food in landfills or cow manure on dairy farms.¹²

In theory, biomethane can offer a much bigger improvement than fossil gas can. For one thing, the CO₂ from burning biomethane is often considered not to “count” toward climate change. Plants absorb CO₂ as they grow and release it again when they decay. Because biomethane taps into that relatively fast loop—instead of dredging up ancient carbon locked underground—it’s often assumed that everything comes out in the wash, with no “extra” CO₂ added to the atmosphere.

In some cases, using biomethane might ward off climate pollution. For instance, imagine a landfill where methane is leaking from rotting waste into the atmosphere. Left to escape into the air, that methane would exert a large warming effect before breaking down gradually into CO₂. Burning it as fuel skips over that more powerful warming phase.

The extent of these benefits, however, is controversial.¹³ What if the landfill’s methane would not have escaped to the atmosphere, even if it were never used as vehicle fuel? After all, many U.S. landfills are required to capture the gas from waste and use it, sell it, or burn it off.¹⁴ And what if making and burning biomethane does add CO₂ to the air? The assumption that you’ll “break even” on the CO₂ front can get dicey, especially if you’re not using actual biological waste. Say the biomethane comes from crops instead; that could mean clearing trees that, if left alone, would have kept absorbing more CO₂.¹⁵ Weighing the climate impacts of biomethane requires some judgment calls: Which sources of pollution should be considered part of biomethane’s “life cycle”? (For example, should the tally for manure biomethane include emissions from raising the cows in the first place?) And how do we assume the waste would be managed without a biomethane project?¹⁶

In Plata’s view, “If you have something that was going to be methane, that was going to be released to the atmosphere, and you can instead divert that to gas for energy utilization, that is probably a good end use.”

But she cautions that the amount of waste methane that meets all those standards is limited.¹⁷ She says policies should be careful not to incentivize the production of more waste and more methane, like increasing cattle herds just to make more manure. That’s especially important because biomethane supply chains are often leakier than those for fossil gas,¹⁸ which can dampen or even cancel out its climate benefits.¹⁹

In her opinion, the best use of the limited supply may not be strictly to fuel vehicles, especially if that means moving the gas long distances. She sees promise in using waste methane close to the source, in ways that require less processing than making vehicle fuel—for instance, a landfill burning it for onsite heat.

We are grateful to the following experts for their assistance with aspects of this piece: Andrew Burnham, Principal Environmental Scientist, Argonne National Laboratory, and Timothy Wallington, Research Specialist, University of Michigan.

Thank you to Ruth Reynolds of Oak View, California, for the question.

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