How Much Carbon Does Cycling Really Save?

Introduction

Fig. 1: Cycling can be a convenient, economical, and fun way to get around, particularly in urban areas. (Source: Wikimedia Commons)

With mounting concerns about the effects of climate change, many individuals and governments are attempting to find ways to decrease greenhouse gas (GHG) emissions in every sector of society. In the U.S., the leading sector producing GHGs is the transportation sector, contributing 27% of the nation's carbon dioxide (CO₂) emissions. [1] Within the transportation sector, light-duty vehicles, i.e. regular passenger vehicles, produce 57% of GHG emissions, by far the single largest contributor among transportation modes.

One commonly promoted option for an individual to decrease their carbon footprint is to commute by bike instead of by car. [2] (See Fig. 1.) It seems obvious that riding a bike produces less carbon than driving a car. But how much less does it actually produce? After all, the human body is a machine like any other; the energy to pedal a bicycle must come from somewhere. By employing a few simple estimations, we shall attempt to address the questions(1) how much carbon does riding a bike produce per kilometer? and(2) how does this compare to driving the same distance? With these numbers in hand, we can better evaluate how helpful switching one's commute to cycling actually is purely from the perspective of decreasing carbon emissions.

CO₂ Emissions From Cycling

To determine the GHG emissions associated with operating a bicycle, we must look at the two main sources of emissions: (1) the manufacture of the bicycle, and (2) the extra food consumed to fuel the cyclist. Using the average emissions produced by manufacturing bicycle materials, such as steel and aluminum, the Dutch research firm TNO found that production and lifetime maintenance of an average commuter bike, weighing about 20 kg, emits about 96 kg CO₂ equivalent (CO₂e). [3,4] Using the study's assumption of a bicycle lasting for 19,200 km over eight years, a single bicycle's production results in about 5 g CO₂e km^-1.

To estimate the carbon emissions caused by increased food intake, we must look at two factors: (1) the amount of extra calories a person burns while cycling a kilometer, and (2) the GHG emissions associated with the average diet per calorie. Average caloric use during exercise varies widely between individuals of different age, weight, fitness level, metabolic rate, etc. However, we can estimate a 70 kg person riding at a moderate 15 km/h will use about 20-40 kilocalories (kcal, equivalent to 1 calorie on American nutrition labels) per kilometer. [5] The GHG contribution of the average diet in countries of high economic development such as the U.S. is 3.17 g CO₂e kcal^-1. [6] Using these values, we find the average GHG emissions per kilometer of cycling is about 95 g CO₂e km^-1. However, this assumes that a cyclist will consume extra calories equal in amount to those burned during cycling. This is often not the case; on average, active commuters compensate only about 57% of the extra calories burned. [6] This leads to a lower average emission rate of 54 g CO₂e km^-1. It is also important to note that the emissions per calorie consumed are highly dependent on an individual's diet. Meat- and dairy-rich diets have very high associated CO₂ emissions (5.3 g CO₂e kcal^-1) whereas plant-based diets contribute significantly lower emissions (1.5 g CO₂e kcal^-1). [7] Thus, a vegan cyclist would produce about 25 g CO₂e km^-1 while a meat enthusiast would produce 90 g CO₂e km^-1. Adding on the contribution from the bicycle manufacturing emissions, this gives us a range of about 30-100 g CO₂e km^-1.

CO₂ Emissions From Driving

How does this rate of CO₂ emission compare to that of the average car? We can look at the analogous sources of carbon emissions for cars: those produced during manufacture and those released during vehicle operation. Woody et al. calculate the production and operation GHG emissions for passenger vehicles of different classes (sedan, SUV, and pickup truck) and powertrains (internal combustion engine (ICE), hybrid-electric, and electric). [8] This study determines a range of 80-573 g CO₂e km^-1, with battery electric sedans being the lowest emitters and ICE pickups being the highest. However, this uses a (weighted) average of city and highway fuel economy. As cycling essentially only replaces urban driving, we can compare to city fuel economy, which is less efficient for ICE vehicles, but more efficient for electric vehicles (EVs). This adjustment leads to a range of average CO₂ emission rates for urban driving of 70-640 g CO₂e km^-1.

Carbon Comparisons

When considering the GHG savings of cycling, it is key to make accurate estimates of the carbon emissions associated with the activity. From the estimates above, we can see that cycling actually has non-negligible GHG contributions, largely due to the emissions caused by the agricultural sector in producing food and raising livestock. At the extremes, a vegan cyclist will produce only 5% of the emissions a conventional pickup truck will produce, while a meat-loving cyclist will actually produce 42% more GHGs than the most efficient EV. For the more ordinary case, the average cyclist's GHG contribution (~60 g CO₂e km^-1) is nearly one quarter that of the average car (~250 g CO₂e km^-1). Thus, for the average case, switching from driving to cycling decreases the carbon footprint of the trip by 75%.

How much CO₂ does one cycling commuter save in a year? For a round trip commute of 10 miles, or 16 km, commuting 5 days a week, 48 weeks a year, this results in saving about 700 kg CO₂e. This is roughly equivalent to a one-way economy-class flight between San Francisco and New York.

On a larger scale, how much carbon could be saved if the number of bike commuters increased in the U.S.? According to the U.S. Census Bureau, about 805,700 Americans, ~0.5% of all commuters, biked to work in 2019. [9] With an average one-way commute time of 21.2 minutes, and assuming an average speed of 15 km h^-1, this leads to about 8.5 million km cycled cumulatively and 1.6 million kg CO₂e saved in 2019. While certainly beneficial, this amounts to only 0.1% of the total carbon emitted by commuters driving alone (calculated using the same estimation method and assuming an average speed of 50 km h^-1, or about 30 mph). Therefore, any reasonable increase in the number of commuting cyclists would make a relatively small impact on the nation's transportation-related GHG emissions.

For comparison, 13.9 million Americans, ~9% of all commuters, carpooled to work in 2019 with an average one-way commute time of 28.5 minutes. [9] Assuming a 50 km h^-1 average speed, they traveled 660 million passenger km cumulatively. Making the conservative estimate that all carpoolers traveled in pairs, this saved at least 80 million kg CO₂e in 2019, or 6% of the total GHG emissions from lone driving commuters. Thus, doubling the number of people carpooling could decrease emissions from passenger vehicle commuting by over 10%. Further, if carpoolers increased their rider density to 3 or more passengers per vehicle and/or chose to carpool with hybrid or electric vehicles, the carbon savings would be even more substantial.

Conclusion

Although it is commonly assumed that cycling has no carbon footprint, this is not the case. We find that carbon emissions from cycling are non-negligible, particularly for those with diets rich in meat. When converting a short driving commute to cycling, we find the average annual CO₂ savings are potentially significant for an individual, roughly equivalent to a one-way transcontinental flight. However, at the national level, even a substantial increase of 10 times the current number of commuter cyclists would only make a 1% level change in passenger vehicle GHG emissions. The analysis presented in this article is heavily U.S.-centric; the case for bike commuting may be stronger or weaker in countries with different transportation trends and cycling cultures. Of course, there are many other factors to consider when switching to a commute by bicycle, such as health and safety, economics, convenience, and personal enjoyment (or lack thereof). [1] This article only addresses one of these aspects, the carbon footprint, which is an important consideration for decision making at the individual and societal levels.

© Theo Schutt. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] "Fast Facts: U.S. Transportation Sector Greenhouse Gas Emissions 1990-2020," U.S. Environmental Protection Agency, EPA-420-F-22-018, May 2022.

[2] I. Ahmed, "Energy Claims of Biking to Work," Physics 240, Stanford University, Fall 2015.

[3] B. Blondel, C. Mispelon, and J. Ferguson, "Cycle More Often 2 Cool Down the Planet! Quantifying CO₂ Savings of Cycling," European Cyclists' Federation, November 2011.

[4] I. Hendriksen and R. van Gijlswijk, "Fietsen Is Groen, Gezond en Voordelig," TNO (Leiden), January 2010.

[5] B. de Geus et al., "Determining the Intensity and Energy Expenditure During Commuter Cycling," Br. J. Sports Med. 41, 8 (2007).

[6] A. Mizdrak et al., "Fuelling Walking and Cycling: Human Powered Locomotion Is Associated With Non-Negligible Greenhouse Gas Emissions," Sci. Rep. 10, 9196 (2020).

[7] M. Berners-Lee et al., "The Relative Greenhouse Gas Impacts of Realistic Dietary Choices," Energy Policy 43, 184 (2012).

[8] M. Woody et al., "The Role of Pickup Truck Electrification in the Decarbonization of Light-Duty Vehicles," Environ. Res. Lett. 17, 034031 (2022); ibid. 17, 089501 (2022).

[9] C. Burd, M. Burrows, and B. McKenzie, "Travel Time to Work in the United States: 2019," U.S. Census Bureau, ACS-47, March 2021.