In America, GHG emissions output lower despite driving activity increase

Generally speaking, American vehicle miles traveled (VMT) since the beginning of the industrial age, has followed an upward trajectory as has the growth in population.

NASA is advancing new tools like the supercomputer model that created this simulation of carbon dioxide in the atmosphere to better understand what will happen to Earth’s climate if the land and ocean can no longer absorb nearly half of all climate-warming CO2 emissions.

Global warming and/or climate change, meanwhile, like it or not and/or agree with it or not, are top-of-mind, front-and-center issues.

While true, the fact of the matter is, neither construct weighs heavier nor has left a deeper imprint in our collective consciousness than has the environmental fallout from transportation; namely, in the form of emissions from such entering the atmosphere – now or in the past.

So, the real question here is with population rise and growing traveled miles, have domestic connected emissions improved or worsened or remained the same which is tantamount to asking: Is American transport today cleaner or dirtier or has it gone unchanged?

Precisely what will be explored on the Air Quality Matters blog today. Two years will be considered: 2015 and 2017 (for these two years, all of the applicable data are in).

Miles driven

Consider for 2017, for a total of 261,229,000 cars, light trucks and SUVs plus heavy-duty trucks and buses (on-road vehicles), a sum total of 3.2 trillion miles were driven, the former and latter responsible for 90 percent and 10 percent of that total mileage, respectively (Transportation Energy Data Book Edition 38, “Quick Facts” page). This compares in 2015, to the 3.1 trillion miles logged by 253,203,000 total on-road vehicles, the breakdown here again being 90 percent and 10 percent done in the light- and heavy-vehicle classes, respectively (Transportation Energy Data Book Edition 36, “Quick Facts” page).

Average mileage ratings

Cars – 26.2 mpg (2015) (TEDB ed. 36, “Quick Facts” page); 27.3 mpg (2017) (TEDB ed. 38, “Quick Facts” page)

Light trucks – 18.8 mpg (2015) (TEDB ed. 36, “Quick Facts” page); 19.7 mpg (2017) (TEDB ed. 38, “Quick Facts” page)

The one bright spot here is that 2017 compared to 2015, with the increase in driving, cars, light-duty trucks and SUVs have gotten more fuel-efficient.

Greenhouse gas emissions

As long as combustion remains a viable process and part of transportation, produced among other emissions will be greenhouse gases. The six main GHGs of concern here are: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), perfluorocarbons (PFC), hydrofluorocarbons (HFC) and sulfur hexafluoride (SF6) – the last three taken together alternatively known as high global warming potential or High GWP gases.

In 2015, U.S. CO2 emissions from transportation-based fossil fuel combustion amounted to 34.7 percent of contributed or sourced GHGs. (TEDB ed. 36, p. 11-1). In 2017, the percentage emissions of CO2 in the U.S. from the same modes in the very same sector totaled 37.1 percent, 2.4 percent higher in just two years’ time and a per-year increase of 1.2 percent. (TEDB ed. 38, p. 12-1) This is among all of the six GHGs. Modes here include aircraft, buses, cars, heavy trucks, light trucks, marine vessels, pipelines, SUVs and trains.

In raw numbers, U.S. GHG output total in 2015 was 6,539.8 million metric tons of carbon dioxide equivalent (MMTCO2e) GHGs (TEDB ed. 36, p. 11-5), while in 2017, that number slid to 6,410.2 MMTCO2e (TEDB ed. 38, p. 12-5), a reduction of 129.6 MMTCO2e.

Interestingly, the biggest GHG producer domestically wasn’t even transportation; the industrial sector claimed the heavy weight title in both years weighing in at 1,915.7 MMTCO2e in 2017 and 1,931.2 MMTCO2e in 2015. Transportation’s total share was 29.2 percent or 1,870.6 MMTCO2e (TEDB ed. 38, p. 12-6) and 27.7 percent or 1,810.4 MMTCO2e (TEDB ed. 36, p. 11-6), respectively.

All of which means with respect to the emissions-reduction effort, the sector activity being what it is, transportation still has its work cut out.

As can be seen below by the representative numbers (in MMTCO2e), not surprisingly, the trains category was the only one, 2015 versus 2017, to see its GHG output drop. (TEDB ed. 38, p. 12-10)

  • Passenger vehicles: 1,038.1 (2015); 1,059.0 (2017)
  • Heavy trucks: 428.3 (2015); 449.7 (2017)
  • Marine vessels: 30.8 (2015); 40.2 (2017)
  • Aircraft: 159.1 (2015); 173.1 (2017)
  • Trains: 43.6 (2015); 41.3 (2017)*
  • Pipelines: 38.5 (2015); 41.4 (2017)

*There could have been less reliance on train transportation in 2017 compared to 2015 which, if there was, would explain the reason for the decline. It should be noted CO2 emissions produced from trains operating in the United States in 2016 (TEDB ed. 38, p. 12-10) was 40.2 MMTCO2e.

The question is: With the presumption here being that America’s transport emissions will continue to rise, how long before emissions overall, again, presumably, do likewise?


Sources for the data above: Davis, Stacy C. and Robert G. Boundy, (TEDB) Transportation Energy Data Book, Edition 36, published Dec. 2017 and Edition 38, published Jan. 2020, both from the Oak Ridge National Laboratory.

The statement: “That transportation’s total contribution to domestically produced GHG was less in 2017 versus that in 2015 suggests that, despite a presumed uptick in transportation activity, all modes (with the exception of pipelines) are becoming cleaner and hence the overall improvement,” that was included in the text originally, is, in fact, incorrect. The post has since been revised and the revised title reflects this change.

Image above: NASA/GSFC

This post was last revised on Mar. 9, 2020 @ 9:52 a.m. Pacific Daylight Time.