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I had an argument with a work colleague. He claims that me cycling to and from work is less environmentally friendly than driving because
He claims that these are more wasteful than the fuel required to drive 25km every day.
Can anyone point me to any evidence for or against this claim?
This calculation is pretty much worst-case for the bicycle. It doesn't take into account that food production has actually stored CO2 from the atmosphere which is now just released back, while fuel was originally underground and its CO2 is added to the carbon cycle. Neither does it take into account mercury/other emissions, nor the wear-and-tear in cars and bicycles, nor the enormous benefits if the bicyclist doesn't even own a car.
The question is what the bicyclist eats to get the required energy. Estimated from calculations below and this table:
Note that the fractions above are of energy, not weight.
Image by Pulpolux !!! on Flickr. CC-BY-NC 2.0
And here's the calculation. The initial assumption is that the choice between car and bicycle doesn't affect the amount of other exercise. In practice, it might be different, but this is the only way we can calculate.
CO2 emissions for the average new car in 2006 were 167.2 g/km, so let's assume this amount. For 25km the car pollutes 4.18 kg CO2.
Riding a bicycle uses up 1.62 kJ/(km∙kg). Let's assume the rider weighs 80kg, which is below or above average for a male depending on country. Allow 20kg for bike, clothes etc. and we have 100kg. Thus the 25km takes 4050kJ total -- that is 967 kcal.
Now, the food CO2 equivalent per kcal depends a lot on the type of food. For example:
So, the CO2 equivalent from the extra food needed for cycling would be:
The bicyclist also takes an extra shower. Let's use the same assumptions as a CBC campaign:
Assumes average shower time in Canada of 7.6 minutes (Source: Ipsos-Reid poll for GoBlue.org, 2008), average shower flow of 15 L per minute, and five showers per week. Also assumes 0.06 kWh electricity to heat one litre of water, and 22 g CO₂eq produced per kWh electricity used (Source: BC Hydro).
For 7.6 minutes the water usage is 114 liters, which requires 6.84kWh to heat. The CO2 equivalent from taking the shower is thus 0.15kg. This is just 3.6% of what the car is using so it's quite negligible.
For most foods other than pork, beef or lamb, the total energy consumption is below what the car uses. For example, eating only chicken, the bicyclist's total would be 1.75kg, which is significantly less than the car.
The limit where the bicycle pollutes more than the car is if the food creates more than 4.17 g/kcal CO2 equivalents. So the bicyclist can easily afford to eat some beef, if the main energy source is vegetables.
One of the questions raised by several of the commenters was that while one can estimate the calories burned by bicycling, it is hard to estimate what change in calories the cyclist requires (over what would have burned by driving to work). I designed an experiment using a Heart Rate Monitor to help me estimate just that.
Notes and assumptions
Calories were determined by a Heart Rate Monitor, which infers calories burned based on heartbeat count. Assumption: number of heartbeats are directly proportional to actual calories burned.
Calories were burned either 1) While driving to a bus stop in a car and then taking the bus to or from work or 2) while bicycling to or from work. Assumption: the cyclist will eat more on the day he cycles, corresponding to the larger quantities of calories being burned.
The car travels mostly at highway speeds to the bus park-and-ride. Distance 5.7 miles. Assumption: Nominal 30 mpg and 1 passenger were used to estimate 0.16 gallons of unleaded fuel.
Using http://www.eia.doe.gov/oiaf/1605/coefficients.html; a car fueled with unleaded gasoline produces 8.87 kg of CO2 per gallon.
The bus takes a (mostly) express route over highway. Distance 10.8 miles.
Using http://docs.wri.org/wri_co2comm_2002_commuting_protected.xls and medium haul of .20 kg CO2 per passenger mile.
CO2 is a proxy for environmental impact.
The commuter is a vegan male, weight 68 kg (150 pounds).
Design of statistical test
A difference between means (of total CO2 production) was the chosen hypothesis test.
$\mu1 represents the mean CO2 emissions required to fuel the bus, car, and the subject's body while commuting.
$\mu2 represents the mean CO2 emissions required by the foods needed to sustain the subject's body while bicycling.
Null hypothesis: $\mu1 - $\mu2 <= 0
This corresponds to the OP's friend’s assertion that bicycling produces as much or less CO2 as driving.
Alternative hypothesis: $\mu1 – $\mu2 > 0
Will choose a significance level of 0.01 Will use a one-tailed test.
Experimental data
Measurements were taken over a period of about one month. There were 18 measurements taken: 9 by bus and car, 9 by bicycle. Here is the data:
Date Mode Cal Food Bus Car Total
CO2 CO2 CO2 CO2
(kg) (kg) (kg) (kg)
24-Aug Bike 781 0.109 0.00 0.00 0.11
24-Aug Bike 830 0.116 0.00 0.00 0.12
28-Aug Bus/Car 177 0.025 2.16 1.42 3.60
30-Aug Bus/Car 326 0.046 2.16 1.42 3.62
31-Aug Bike 1148 0.161 0.00 0.00 0.16
31-Aug Bike 770 0.108 0.00 0.00 0.11
11-Sep Bus/Car 117 0.016 2.16 1.42 3.60
11-Sep Bus/Car 77 0.011 2.16 1.42 3.59
12-Sep Bike 842 0.118 0.00 0.00 0.12
12-Sep Bike 802 0.112 0.00 0.00 0.11
13-Sep Bus/Car 117 0.016 2.16 1.42 3.60
13-Sep Bus/Car 102 0.014 2.16 1.42 3.59
17-Sep Bus/Car 274 0.038 2.16 1.42 3.62
17-Sep Bus/Car 97 0.014 2.16 1.42 3.59
20-Sep Bike 850 0.119 0.00 0.00 0.12
21-Sep Bike 871 0.122 0.00 0.00 0.12
21-Sep Bike 712 0.100 0.00 0.00 0.10
24-Sep Bus/Car 381 0.053 2.16 1.42 3.63
Stats
Row Labels Average of Total CO2 (kg) StdDev of Total CO2 (kg) Count
Bike 0.118315556 0.017288929 9
Bus/Car 3.605146667 0.015794736 9
Conclusion
The data indicates that the null hypothesis is rejected at p < 0.01.
Discussion
This weakens the OP’s friend’s argument that bicycling causes as much environmental damage as commuting.
The environmental cost of heating the water for a shower was not included, as it seems to have minimal impact (see dancek's fine answer on this point).
Even with a substitution of a more typical American diet with more meats and fewer vegetables, the “bicycling is as bad as a car” hypothesis would be rejected.
This paper from the Stockholm University estimates the energy used for the production of a good hamburger is 20 MJ (the high estimate)
http://www.infra.kth.se/fms/pdf/energyuse.pdf
A liter of gasoline contains 35MJ of energy. So two cheeseburgers more per day gives 1L gasoline consumption and a change. Which is usually enough to get 25km in a light car - think Toyota aygo. Add some energy for water heating and moving the water to/from your residence
So things look pretty close. If you replace the meat - things look much better.
Your biggest contribution to saving the environment comes not from the cycling itself, but from not owning a car.
