The Damage Done in Transportation -- Which Energy Source Will Lead to the Greenest Highways?

Pie chart showing world energy consumption by sectorAccording to automotive industry publisher WardsAuto, the world population of automobiles surpassed one billion for the first time in 2010. All those cars -- not even including all trucks, buses, trains, airplanes, ships and so on -- add up to a lot of energy consumption for transportation. According to the data shown in this pie chart (based on U.S. Energy Information Administration data, EIA), transportation sucks up more than a quarter of delivered energy consumption globally.

Pie chart showing world transportation energy by sourceNearly all of that transportation energy comes from petroleum, as you can see from this next pie chart from International Energy Agency (IEA) data.

And as you can see from the following trend chart based on EIA data, not much is expected to change over the next couple of decades -- projections show the world depending almost entirely on oil for its transportation energy through 2035.

Trend chart showing transportation energy through 2035


Of course, that's only a projection. What if oil prices skyrocket, while prices for natural gas, electricity, biofuels or other sources drop? What if greenhouse gas policies force a movement away from petroleum? What if market forces, government policy or a combination of the two push the transportation sector toward cleaner energy sources?

In such cases, alternative sources of energy could become more important, which means that their environmental effects will become more important and a focus.

Photo of trucks on highwayOver the past few weeks, my "Damage Done" series on the production and consumption of energy has focused on the effects of transportation energy. So far, I've written pieces on the environmental external effects, or "externalities," of petroleum fuels, natural gas, biofuels, electricity and hydrogen(Photo: Trucks in Toronto, Canada. Credit: Perry Quan, CC BY-SA 2.0.)

This article is kind of a wrap-up. How do the various potential energy sources for transportation stack up in terms of their environmental impact? Are gasoline and diesel really so bad? Are biofuels, electrical vehicles and hydrogen really "greener" than oil? And would natural gas be a cleaner way to get around?

Making comparisons is not as easy as one might hope. It's not hard to find information about the environmental effects of petroleum used in transportation. But because the other energy sources make up such small percentages of transportation energy, it's hard to find studies that analyze their environmental effects in the same terms that allow for true apples-to-apples comparisons.

Fortunately, the National Academy of Sciences (NAS) has come to the rescue with its voluminous 2010 study, "Hidden Costs of Energy: Unpriced Consequences of Energy Production and Use". It examines the life-cycle environmental effects of energy sources. In general, NAS presents its analysis in ways that allow you to make comparisons.

The following chart compares the various sources in terms of both greenhouse gas (GHG) emissions and non-GHG damages, which are primarily their effects on human health and mortality for the NAS. These estimates are based on 2005 data.

Energy Source Mean Life- Cycle Health and Other Non-GHG Environmental Damages in dollars per vehicle mile traveled ($/VMT) Mean Non-GHG Damages for Vehicle Operation Only in dollars per vehicle mile traveled ($/VMT) CO2 equivalents in grams per vehicle mile traveled (gCO2eq/VMT) CO2 equivalents in tonnes/barrel of gasoline equivalent
Gasoline (Light-Duty Vehicle)




Compressed Natural Gas




E85 Dry Corn Ethanol




Electricity (Grid-Dependent EV)





*Extrapolated from NAS data

As you can see, the metric that NAS uses most consistently is the non-GHG life-cycle environmental damages, expressed in dollars per vehicle mile traveled. Also, you can see that all of the five energy sources produce about the same damage estimates, ranging from 1.20 cents to 1.46 cents per mile. Natural gas comes in with the lowest non-GHG damages, which suggests that the environment would benefit somewhat from a move toward natural gas vehicles. In the GHG column, ethanol biofuel scores well.

Photo, charging an electric vehicleYou'll notice that in the second column, electric vehicles show very low damages when you consider just the operation of the vehicle -- only .22 cents per mile, as opposed to 1.46 cents per mile for full life-cycle damages. That's because an electric vehicle is powered by, well, electricity. So when you consider full life cycle, the vehicle ends up "inheriting" the life-cycle damages of the electricity used to power it. In the United States, that means, for the most part, coal and natural gas. (Photo: An electric vehicle being charged. Credit: Oregon DOT, CC BY 2.0.)

NAS doesn't provide a "vehicle operation only" figure for hydrogen, but I suspect it would be on the order of that for electricity. Hydrogen has to be produced from another energy source, so hydrogen inherits the life-cycle environmental damages of whatever source is used to produce it. If that's dirty fossil-fuel energy, whether from the grid or via another supply channel, then hydrogen's life-cycle damages will be high.

The good news for enthusiasts of both electric and hydrogen vehicles is that if, in the coming decades, the energy supply can shift from fossil fuels to clean sources like nuclear and renewables, then these two transportation technologies will naturally become less environmentally damaging, in both the GHG and non-GHG columns.

To get an idea of how the green chops of EVs and hydrogen would change based on the feedstock used for their energy, let's look at the environmental impacts of the major electrical generation energy sources. The following chart is based on data cited by Benjamin J. Sovacool in an article in Energy Policy. The chart shows GHG emissions for the principal sources of electric power:

Source/Technology Life-cycle CO2 equivalents in grams per kilowatt hours (gCO2eq/kWh)

960 to 1,050

Natural gas






Solar Photovoltaic (PV)



10 to 13



As you can see in that chart, the climate-change effects of electric power are cut in half if generation is migrated from coal to natural gas. Effects are cut even more drastically if it migrates to nuclear or renewable sources.

Considering non-GHG damage results in a similar picture. The following data are from the NEEDS study (New Energy Externalities Developments for Sustainability, Project no. 502687, "External costs from emerging electricity generation technologies," March 24, 2009):

Quantifiable External Costs From Electricity Generation Technologies in Year-2000 Euro-cents (€) per kWh
Technology Health Impacts Bio-diversity Crop Yield Losses Material Damage Land Use Total Non-Climate Cost
Coal - Lignite














Natural Gas Combined Cycle














Offshore Wind







Solar PV







Concentrating Solar







As with climate-change impacts, these non-GHG damages (looking primarily at the far-right column) show nuclear- and most renewables-generated electricity as much less damaging than coal. Natural gas and solar photovoltaic (PV) generation represent a middle ground.

All of this suggests that electric and hydrogen vehicles could become much less environmentally damaging if electrical generation were to move from fossil fuel technologies to nuclear power and renewables.

The following chart from the U.S. Department of Energy (DOE) bears out that conclusion when it comes to hydrogen and greenhouse gases:

Bar chart showing how hydrogen GHG is affected by feedstock

The chart shows how the life-cycle greenhouse gas emissions of a hydrogen fuel cell vehicle (HFCV) are affected by various feedstock (or "pathway") choices for creating the hydrogen. The yellow bars show whether hydrogen's GHG emissions are better or worse than a standard gasoline vehicle under the various options.

For example, the bar on the far left shows that hydrogen generates about 55 percent less GHG than a gasoline vehicle if the hydrogen is gaseous and is produced from natural gas in a centralized plant ("Central GH2, NG"). The third bar from the right shows that hydrogen creates about 62 percent more GHG than a gasoline vehicle if the hydrogen is in liquid form and is produced using average U.S. grid electricity ("Electro LH2, U.S. Mix").

Electric and hydrogen vehicles offer an additional environmental benefits in that they can move the location of damaging emissions away from concentrated populations. Locally, where the vehicle is being operated, emissions are nearly zero. So in a city, for example, EVs and HFCVs present lower risk for the human population. The life-cycle emissions get displaced to some central production facility, such as a power plant, which might be in a location with a lower population density. That displacement results in lower damages.

Another thing to think about: The NAS figures I've cited here represent, more-or-less, current technologies, but transportation technologies -- both conventional and alternative -- are continually changing and improving to yield greater efficiencies and lower emissions. So we can expect the environmental damages from vehicles powered by all energy sources to decrease in the future.

To review the "Damage Done" series of articles on the effects of production and consumption of energy for transportation, follow these links:

The Damage Done Down the Road - Can Green Energy Reduce Environmental Damage in Transportation?

The Damage Done, Gas Addiction Edition - How Detrimental Is Petrol?

The Damage Done - Natural Gas Vehicles, Cleaner and Greener?

The Damage Done, Biofuels Edition - a Green Bumper Crop or a Harvest of Environmental Problems?

The Damage Done - Electric Vehicles, the Green Way to Get Around?

The Damage Done - Hydrogen Vehicles, Good for the Planet?

And you can access the series of "Damage Done" articles on electricity through the summary piece here:

The Damage Done, Part 10 - Are Renewables Really Better for the Environment Than Fossil Fuels?


Photo of traffic in Singapore

(Photo: Traffic in Singapore. Credit:, CC BY 2.0.)


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