By Daniel Thorpe with help from David Keith
Paleo-diet cyclists warm the planet as much as Prius drivers — but under the usual (but crazy) assumption that nothing matters beyond 100 years in the future
See our free course on edX that debates issues like this and teaches you how to do the kinds of research and calculations in this blog post.
NOTE #1: this is a back-of-the-envelope estimate of the marginal impact of biking or driving a kilometer, looking only at the fuel for each (food and gasoline). Our goal is only to stimulate quantitative thinking about what drives carbon emissions (e.g., transportation vs diet). It’s not an evaluation of whether biking or driving best overall, and it’s not peer-reviewed research. As many enthusiastic readers have pointed out, bikes provide exercise, impose less danger on others when driven, take much less energy to manufacture, etc. Please keep riding your bike, David and I do so daily :)
NOTE #2: The original version of this post on 26 May 2016 had an unreasonably high estimate of caloric expenditure of biking, 50 kcal/km, and underestimate of Paleo diet intensity of 3.8 g CO2e/kcal. After looking carefully into several references I amended the post to have 25 kcal/km for cycling and 5.4 g CO2e/kcal for a Paleo diet [i]; the impact of vegans and average-diet cyclists is now much lower, and the impact of Paleo cyclists is a bit lower too. Thanks to all the readers who brought the errors to my attention!
Is it better for the climate to bike or drive? Obviously it’s cleaner to bike. Right? Not so fast; biking can have a bigger impact than you think, depending on your diet. Long story short, if you eat enough meat the extra calories burned by biking can lead to similar emissions as driving a car with good fuel economy [ii].
First let’s start with the energy needed to travel a kilometer by bike or car. Biking takes around 25 kcal/km [iii] above basal metabolism, which is equivalent to .11 MJ/km. A typical car in the US gets 25 mpg, or 9.5L/100 km, which is equivalent to 3.3 MJ/km. The Toyota Prius takes only 5 L/100km, or 1.7 MJ/km. So a typical car takes 30x more energy per kilometer than biking, and a Prius takes 15x more. This is what we expect given how much heavier cars are than bikes.
But not all energy use has the same impact on climate. There’s a range of greenhouse gases that warm the climate at different rates and stay in the atmosphere for different lengths of time. When an activity leads to emission of several different greenhouse gases, we often combine them all into one metric, “CO2 equivalents,” by multiplying all the gases other than CO2 by their “Global Warming Potential,” which reflects how much more or less they affect the climate than CO2. This doesn’t matter a lot for estimating the impact of cars, where 90+% of the emissions are CO2, but it does matter for the agriculture powering a bike ride, where there are substantial emissions of N2O and CH4, which have GWP’s around 30 and 300, meaning we usually count 1 gram of CH4 emissions as equivalent to ~30 grams of CO2 emissions. Determining the exact value of these equivalencies a tricky exercise that involves value judgements, something that we’ll return to later.
So let’s make estimates of the climate impacts of biking and driving, in CO2 equivalents (CO2e). If we look at a typical car in the US, taking 9.5L/100km, we can use the lifecycle emissions from gasoline, ~3.2 kg CO2e/liter, to estimate 300 gCO2e per kilometer of driving. A Prius emits half as much, 150 gCO2e/km. We can do a similar analysis for biking. An “average American” eats 2600 kcal/day and their diet leads to about 2.6 gCO2e/kcal [iv]. Given that .11 MJ/km requirement for biking, this gives us an impact of 65 gCO2e/km. This is a little under half the impact of the Prius! Before writing this post, I guessed driving to have ~10x more marginal impact than riding my bike.
What about a meat-heavy diet, the Paleo diet? I looked at Paleo meal plans and academic lifecycle GHG estimates for the foods in those meal plans, and estimated the average emissions of a Paleo diet to be 5.4 gCO2e/kcal [v]. This gives us 135 gCO2e/km, very close to the Prius. What about a vegan? Vegan diets have much lower emissions, around 1.6 gCO2e/kcal [vi], for 40 gCO2e/km. This means that a biking vegan has less than a third the impact of an individual driving a Prius, and 1/7th the impact of an individual driving an average car.
Sharing rides in cars matters too. Two paleo afficianados are friendlier to the climate if they carpool together in a Prius rather than biking somewhere together. The distance-weighted average occupancy for US car travel is 1.6, so this is no minor effect (N.B. average occupancy for commuters is just 1.1, which seems awfully low; I hope someone figures out how to make carpooling more common, maybe with something like Uber Commute). If we adjust the emissions for car travel down by a factor of 1.6, the intensity of average cars is ~190 gCO2e/km, on the order of one Paleo cyclist! A Prius has an occupancy-adjusted intensity of just 100 gCO2e/km, lower than a Paleo cyclist! Check out Table 1 for a summary of these calculations.
|Mode of Transport||Energy Consumption (MJ/passenger-km)||Climate Impact (gCO2e/passenger-km)|
|Biking, vegan diet||.11||40|
|Biking, avg US diet||.11||65|
|Prius, double occupancy||.85||75|
|Biking, paleo diet||.11||135|
|Prius, single occupancy||1.7||150|
|Typical (25mpg) US car, single occupancy||3.3||300|
Table 1: Rough estimates of energy use and climate impact of different kinds of transportation.
We’ve seen that the climate impacts of a bike ride can be surprisingly similar to those of a car trip, depending on the car and your diet. But there are environmental considerations other than climate change, like land use. How much land do you think is required to fuel a car trip (in the form of oil extraction) relative to the land needed to fuel a bike ride (in the form of agriculture)? Unlike the greenhouse gas example, this doesn’t depend on your car or your diet; the bike ride almost certainly requires more land.
Estimates of land use for fossil fuel extraction vary widely, but in general they are at least 3,000 liters of oil per year for every square meter of land occupied for oil extraction, and some estimates go as high as 300,000 liters per m^2-yr for conventional oil production. These figures are equivalent to about 120 to 12,000 GJ of energy per m^2-yr, or 10 to 1000 W/m^2 [vii]. Food production per unit of land is much lower than this range. Cereal grains are at the upper end of calories per unit land out of the various types of food, but we only produce around 7500 kg of grains per hectare-year, according to the World Bank. Using the calorie density of grains (~3.6 kcal/g), that’s only 120 GJ/hectare-yr, or .4 W/m^2, at least 25 times less than the power density of fossil fuel extraction! Similar estimates for other types of food are substantially lower - fruits and vegetables are around .25 and .1 W/m^2, respectively, and chicken and beef are around .04 W/m^2 and .02 W/m^2 when accounting for the land to house the animals and grow their food [viii]. Any real diet, then, will have an average no higher than .4 W/m^2 (grain-only diet), and likely closer to .1 W/m^2, going lower with more animal product consumption.
So even though a car ride takes 15-30 times more energy, its fuel source uses at least 25-100 times less land per unit energy, giving driving a lower land footprint than biking, even when comparing a biking vegan to a standard American car.
Of course, there are differences in how fossil fuel extraction and agriculture affect the land they occupy. Images of the tar sands may seem a lot worse than what we think of when we think of farms, but the most land-efficient farms may not really be more attractive (Figure 1). Less land-efficient farms with pasture-roaming animals look gentler on the land, but by taking up more land they also have a harsher impact on large species that they displace (trees, deer, wolves, bears…). There’s no clear-cut answer to which is preferable, but it is clear that fossil fuel extraction uses little land per unit of energy extracted, and that powering our lives with alternative fuels (especially fuels derived from agriculture, like biofuels) will almost surely entail an increase in human appropriation of land.
Discussion and Conclusion
There are two important qualifications about the calculations above (besides the fact that the uncertainties are large). The first is that we found biking to have a surprisingly similar impact to driving on a per kilometer basis. But of course, cars enable you to travel much faster and much farther than bikes, so someone with a bike and no car almost surely has a much lower impact by virtue of covering a lot less distance. When I owned a car in rural Virginia I drove 20,000 km/yr, and now that I only own a bike in urban Cambridge, Massachusetts I bike about 1,500 km/yr. And there are lots of other impacts we neglect, like the energy to manufacture cars, or air pollution, or the danger car driving imposes on society.
The second qualification is something I mentioned earlier, the trickiness of equating greenhouse gases. We used the “Global Warming Potential” which adds up the radiative forcing for gases over some time horizon and compares to the sum for CO2 over that same horizon (we used the standard 100 year horizon). But this completely ignores the radiative forcing after that time horizon; this is important because CO2 stays in the atmosphere for millennia, while the main other gases we counted, N2O and CH4, have lifetimes around 100 and 10 years, respectively. So our equivalence method captured almost all of the climate impacts of N2O and CH4 but ignored hundreds of years of CO2’s influence after this century. There are reasons to think we should care more about short-term warming, since we’ll have an easier time adapting to slower changes farther in the future, but it seems odd to completely neglect everything more than 100 years away. This is a long-contested topic (e.g. see Shoemaker 2013), involving value judgements about the present and distant future, with no clear right answer; keep this in mind when you read calculations of CO2e that seem very cut-and-dry.
But these qualifications aside, we’ve seen that agricultural impacts on the environment really matter. We didn’t come to quite as strong a conclusion as Michael Pollan once did, but we came pretty close; biking has a surprisingly similar marginal impact to driving on a per kilometer basis, and depending on your diet can cause similar greenhouse gas emissions and more land use. This points to some of the important lessons from our upcoming online course, that there’s no free lunch when it comes to issues of energy and environment, and that it’s really useful to be able to make quantitative estimates of environmental impacts. Our analysis certainly doesn’t prove that you shouldn’t do more biking instead of driving, but it does help us know more clearly the environmental impacts of making the switch.
Berners-lee et al (2012) The relative greenhouse gas impacts of realistic dietary choices. Energy Policy.
Environmental Working Group (2011) Meat Eater’s Guide to Climate Change and Health http://static.ewg.org/reports/2011/meateaters/pdf/methodology_ewg_meat_e...
Fthenakis (2009) Land use and electricity generation: A life-cycle analysis. Renewable and Sustainable Energy Reviews, 13, 1465-1474
Gerbens-Leenes (2002) A method to determine land use requirements relating to food consumption patterns, Agriculture, Ecosystems, and Environment
Geus et al (2006) Determining the intensity and energy expenditure during commuter cycling. British Journal of Sports Medicine
Scarborough (2014) Dietary GHG emissions of meat eaters, fish eaters, vegetarians, vegans in UK, Climactic Change, Vol 125 Issue 2, pp 179-192
Shoemaker (2013) What Role for Short-Lived Climate Pollutants in Mitigation Policy? Science Vol 342, pg 1323-1324
Smil (2015) Power Density: A key to understanding energy sources and uses. MIT Press, Cambridge MA
Swain et al (1987) Influence of body size on oxygen consumption during bicycling. Journal of Applied Physiology
Weber (2008) Food Miles and the Relative Climate Impacts of Food Choices in the United States
Wilson (2013) The carbon foodprint of 5 diets compared, ShrinkThatFootprint.com accessed May 15 2016 http://shrinkthatfootprint.com/food-carbon-footprint-diet
Vieux et al (2012) Greenhouse gas emissions of self-selected individual diets in France. Ecological Economics
My brief notes on how I arrived at bicycling calories expenditure and carbon intensity of diets here (except for details on Paleo diet estimate, in Appendix B)
- kcal/km for biking
- My original of 50 was definitely too high
- Hard to get a definitive answer, I’m going to go with 25, see details below
- Most measures are for total calories burned while riding, need to be careful to subtract out calories for “basal” metabolism (calories burned for normal bodily functions, which person would have burned anyway even sitting still) to get additional or “net” kcal burned due to biking
- I assumed 2600 kcal/day as the basal rate, and already subtracted from numbers below
- Doing this means we don’t need to look at the calories burned by a person driving
- Popular online tools like bicycling.com, etc seem to suggest a bit over 25 kcal/km on net for 75 kg individual (US avg for adults) biking 12-15 mph
- Hard to say what’s the right speed to use, but commuters seem to be around 12-13 mph (see below) and recreational cyclists can be substantially higher
- The only academic studies I found during a short search measured oxygen consumption during a ride. One I converted to kcal burned by multiplying by 4.76 kcal/liter of oxygen, the other did the conversion themselves. This method will probably be an underestimate b/c it misses anaerobic expenditure and excess post exercise oxygen consumption
- Swain 1987 studied “experienced” cyclists with racing-style bikes on level ground, so probably a substantial underestimate for our purposes; found ~17 kcal/km on net for riders around 75 kg and 12.5 mph
- Geus 2006 used a similar method and found ~22 kcal/km on net for commuters weighing ~75 kg and going ~12.5 mph on their actual daily commutes and on their actual bikes
- I decided to go with 25 kcal/km on net since the academic studies’ methods likely underestimate a bit, but I’m not thrilled with the available data; I think ~18-30 kcal/km is the largest justifiable range, depending on speed, type of bike, terrain, and how much oxygen consumption methods underestimate
- gCO2e/kcal for diets
- I originally estimated 2.6 g CO2e/kcal for avg american and 1.6 g for vegan based on two sources (one for total emissions due to diet and one for calories); I made my own estimate for paleo diet based on paleo meal plans and LCA data on the foods therein
- I divided estimate of total emissions due to diet by total calories consumed, but the estimate of emissions included food waste whereas my estimate of calories consumed did not; thus I overestimated gCO2e/kcal using my sources, by a factor of 3700 kcal [food supply] / 2600 kcal [food consumed]
- However, after looking more carefully at more rigorous academic sources I think if anything my original estimate might have been a bit low
- Using emissions and calorie information from Scarborough 2014 we get ~ 2.8 g CO2e/kcal for average person in UK and 1.5 g CO2e/kcal for vegans
- Using Vieux 2012 (and adjusting for food waste which they ignore, with factor of 3700/2600) we get 2.7 g CO2e/kcal for average person in France
- Berners-lee 2012 gives ~2.1 g CO2e/kcal for average UK’er and 1.5 g for vegans
- Scarborough and Vieux ignore post-sale factors (transport of food to home, refrigeration, cooking…); Vieux ignores waste but I adjusted; they all ignore land use change; Berners-lee seems to ignore cooking at first glance
- Thus I feel pretty comfortable leaving my original estimates for average Americans and vegans alone; my original numbers are close to the averages from those studies above which are probably underestimates
- My original paleo estimate didn’t account for food waste, so I adjusted upwards (see Appendix B)
Daily food intake of estimated paleo diet adapted from http://paleoleap.com/paleo-meal-plan/. Meat consumption assumed to be ⅓ beef, rest from chicken, fish, and pork. Vegetables assumed to include some high-calorie vegetables like butternut squash. GHG intensities of food from Wilson (2013), Weber (2008), EWG (2011). The total diet related impact is higher than the “direct impact” calculated here due to food waste (for every kcal consumed there’s a bit of food waste); the USDA estimates average US food intake to be about 2600 kcal/day with 1100 kcal/day of additional food waste, so we estimate the total dietary impact of a paleo diet to be 3.8 gCO2e/kcal [direct impact] * 3700/2600 = 5.4 g CO2e/kcal of food consumed. This seems high relative to other diets, but it does involve dramatically more meat and egg consumption than other diets. We use .5 kg/day here which might even be low given that many Paleo meal plans call for meat or eggs at almost all meals and that the US average is already .25 kg/day; .5 kg/day is also much higher than the “high meat consumption” diet from Scarborough 2014, which included all diets over .1 kg/day and had an emissions intensity of 3.6 g CO2e/kcal.
|Servings||Weight (kg)||Caloric Intensity (kcal/kg)||Total Calories (kcal)||GHG intensity (gCO2e/kcal)||GHG Impact (kgCO2e)|
[i] It’s tricky to make a good estimate of these numbers; see Appendix A for my terse notes on how I got to my estimates. For the sake of this simple blog post I think I’m now satisfied with my estimates, but there’s room for disagreement. Let me know if you see anything egregiously wrong, or if you’ve got a substantially better, more thoroughly researched set of estimates for me to plug in.
[ii] Please note that this is only an analysis of the extra calories burned by the bike ride vs the gasoline burned by the car; it doesn’t include analysis of the energy used to make cars, or air pollution, or any of many other factors.
[iii] See Appendix A.
[iv] See Appendix A.
[v] See Appendix B.
[vi] See Appendix A.
[vii] For example, see Fthenkais (2009) and Smil (2015).
[viii] See Gerbens-Leenes (2002); for beef they estimate 21 m^2-yr/kg, or .05 kg/m^2-yr; using 2500 kcal/kg, that’s about 550 kJ/m^2-yr, or .02 Wm^2. Their estimates for vegetables and fruits (.3 m^2-yr/kg and .5 m^2-yr/kg) can similarly be converted to abouve .25 and .1 W/m^2. Their estimate for grains (1.3 m^2-yr/kg) converts to .37 W/m^2, very close to our initial estimate using World Bank data.