One argument sometimes used to counteract accusations that cyclists “don’t pay road tax” is to point out that even if vehicle excise duty applied to people on bikes, they would pay nothing, just as drivers of the least polluting motor vehicles do.

The common assumption underpinning that is that someone pedalling a bike must by definition produce lower emissions than any motor vehicle.

But a climate change researcher at Harvard University’s Keith Group has challenged the idea, and says that some cyclists may actually be more harmful to for the environment than some cars.

Specifically, graduate student Daniel Thorpe singled out cyclists who follow the Paleo Diet, which have menu plans that are focused heavily on meat and animal protein, as contributing more to global warming than someone following a different diet who drives a fuel-efficient, low-emission vehicle.

His detailed findings are in published on the Keith Group’s blog on the Harvard website. He starts by noting the energy required to power a bike – 0.2 MJ/km against a typical car driven in the US, 3.3 MJ/km, and a Toyota Prius 1.7 MJ/km.  

Thorpe’s hypothesis instead uses a measure called carbon dioxide equivalents (CO2e) which enables scientists to provide a like-for-like measure of different kinds of gases based on their “Global Warming Potential” (GWP) and thereby gauge the environmental impact of complex scenarios, such as here where both mode of travel and type of diet are being compared.

As an example, 1 gram of methane, associated with livestock, is equivalent to 300 grams of carbon dioxide in terms of global warming potential, giving a reading of 300 gCO2e. Nitrous dioxide, also a factor in agriculture, has a value of 30 gCO2e. Thorpe writes:

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.

By Thorpe’s calculations, typically a car in the US will emit 300 gCO2e per kilometre driven, while a Prius emits 150 gCO2e/km. Based on average daily calorie intake of a cyclist in the US of 2,600 kcal/day he says the typical cyclist will have a reading of 130 gCO2e/km. 

Someone following the Paleo Diet, however, will emit 190 gCO2e/km, “likely higher than the Prius, though the uncertainties in these estimates are large,” admits Thorpe, who adds that a vegan’s emissions will be much lower at 80 gCO2e/km.  

The researcher said that his calculations suggested that two cyclists following the Paleo diet would actually do less damage to the environment than if they car-pooled.

He acknowledges that there are some qualifications, writing:

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.

The other qualification is that while GWP is based on a 100-year cycle, the period of radiative forcing of individual gases differs; 10 years for methane and 100 years for nitrous dioxide, but millennia for carbon dioxide.

That means that while nearly all of the impact of methane and nitrous dioxide is captured in the GWP calculation, it “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,” Thorpe argues.

He concludes that “agricultural impacts on the environment really matter,” and that “biking has a surprisingly similar impact to driving on a per kilometre basis, and depending on your diet can cause noticeably more emissions and land use.”

He adds: “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.”