While sustainability isn’t always the driving force behind everybody’s decision to cycle for transportation, it is certainly a motivating factor for a lot of people, and a feel good secondary factor for many others. Since biking burns fat and not fossil fuels, most everyday cyclists are comfortable with the notion that their commute is a daily contribution to the global fight against climate change.
But one Harvard researcher has called that notion into question. According to graduate student Daniel Thorpe, what’s important isn’t that you’re burning fat instead of fossil fuels, but what you consumed to produce that fat in the first place.
In findings published through Harvard University’s Keith Group, Thorpe singles out cyclists who consume diets heavy in animal byproducts – specifically those who follow the Paleo Diet – as being more harmful to the environment than individuals who eat plant-based diets and drive low-emission vehicles.
Basically, your bacon-fueled bike commute is killing the planet, so you best be trading in that Schwinn for a Prius and a pile of chickpeas.
Thorpe’s conclusion is drawn by using a measure called carbon dioxide equivalents (CO2e), which enables scientists to provide a unit-for-unit comparison of different kinds of gases based on their “Global Warming Potential” (GWP), and thereby analyze more complex scenarios that involve emissions of multiple different types of gases in order to more accurately gauge their environmental impact.
Thorpe began by acknowledging the different energy emissions of different modes of travel:
“Biking takes around 50 kcal/km, which is equivalent to .2 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 17x more energy per kilometer than biking, and a Prius takes 8x 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,” he wrote. “There’s a range of greenhouse gases that warm the climate at different rates and stay in the atmosphere for different lengths of time.” While this doesn’t matter much for estimating the impact of cars, where over 90% of the emissions are C02, it is very significant for estimating the environmental impact of “fat-burning” activities such as cycling which are ultimately fuelled by agriculture. The agriculture industry is responsible for substantial emissions of N20 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,” Thorpe explained.
Based on that methodology, Thorpe determined that a cyclist who consumed mostly meat could ultimately be contributing more to climate change than a vegan or vegetarian driving a low-emission vehicle, or in some cases even a typical car.
“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 2.5tCO2e/yr, or 2.6 gCO2e/kcal.[i] Given that .2 MJ/km requirement for biking, this gives us an impact of 130 gCO2e/km. This is already really close to the Prius! 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 3.8 gCO2e/kcal.[ii] This gives us 190 gCO2e/km, likely higher than the Prius, though the uncertainties in these estimates are large.”
Thorpe went on to look at the impact of the various diets and mode choices while taking car-sharing in to account, writing that two Paleo aficionados carpooling in a Prius may still be more energy-efficient than the same two individuals biking somewhere together.
Thorpe’s summary of climate impact by mode of travel.
Finally, Thorpe discussed the land use impact of the different modes, comparing the different land requirements for oil extraction vs food production. He concluded, “even though a car ride take 8-17 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.”
Thorpe did acknowledge two important qualifications to his research, besides the fact that uncertainties in the estimates are large. “The first is that we found biking to have a surprisingly similar impact to driving on a per kilometer basis,” he wrote. “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.”
The other qualification is that the Global Warming Potential is based on a 100-year cycle, when the period of radiative forcing for individual gases differs widely. While N2O and CH4 have lifetimes of around 100 and 10 years, respectively, C02 stays in the atmosphere for millennia. So their equivalence method captured only the short term impact of each gas but ignored the long-term impact C02 will continue to have on the environment for hundreds of years. “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,” Thorpe wrote.
“But these qualifications aside, we’ve seen that agricultural impacts on the environment really matter,” he wrote. “Biking has a surprisingly similar impact to driving on a per kilometer basis, and depending on your diet can cause noticeably more emissions and land use.” In the end he concluded, “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.”
So, I have a few concerns about this study. While I wouldn’t go so far as to outright question the veracity of his findings – frankly, my understanding of science and arithmetic simply isn’t sufficient to wade into that argument – it does feel like an overly-simplistic analysis of a very nuanced issue. When Thorpe writes “uncertainties in the estimates are large,” it seems like that point should be emphasized so strongly as to make the rest of the findings more or less insubstantial.
Uncertainties are very large. What if the vegan Prius drivers are Crossfitters who eat inordinately large amounts of cereals and imported tofu? Why should we assume motorists – or anyone for that matter – never consumes more than their required daily energy intake? In fact, it would probably be more accurate to assume most people eat more than their required daily energy intake, so whether they’re biking to work and burning those extra calories or not, they’re still gobbling down that afternoon snack. What if the cyclist lives in an entirely flat area and really doesn’t have to pedal too hard to get to and from work? Or what if the car-poolers in the typical vehicle are crawling along through their commute at approximately 4 mp/h, idling the entire time?
Furthermore, while there are bound to be some vegan Prius drivers and cyclists on Paleo diets, they must represent a statistically insignificant portion of the population, so the research overall seems entirely too narrow in its scope. Given what we already know about the potential environmental benefits of increased cycling and reducing our consumption of animal byproducts, the singling out of a small minority of Paleo cyclists seems like more of a diversion from progress than anything.
That said, I do agree Thorpe’s research is important if only because it drives home the point that there’s no free ride when it comes to climate change mitigation. Biking to work isn’t a green pass to then bike home, crank the AC, grill up an imported steak, and pat yourself on the back for your commitment to sustainability. Similarly, eating a vegan diet doesn’t mean you get to drive a Hummer to the grocery store to stock up on Kombucha for the good of the planet.
If there is any single takeaway I feel confident about from this study, it’s that whether you bike, walk, run, drive, fly, or pogo-stick around, you just really shouldn’t be eating a Paleo diet. It’s weird.
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