On the surface, it appears that riding a bike produces no emissions, because it’s human-powered.

A bicycle weighs about a ton (or three) less than a car, and at speeds of 15-25 mph uses a tiny fraction of the energy per mile consumed by a car.

However, humans use fuel too, and when you start looking at the way gas is used in farm equipment for human food production, bicycling might have a significant environmental impact, too.

Carbon Footprint: Bike vs Car

Some people have even made some claims that bicycling produces more greenhouse gases and consumes more oil than driving, because of the inefficiencies of our food production systems.

Here is one calculation that shows otherwise, this is probably the most balanced calculation I’ve found on the web:

http://bicycleuniverse.info/transpo/energy.html

This calculation shows that a rider like me (around 20mph, 195 lbs including the bike and whatnot) gets the equivalent of around 35 mpg while biking and eating a “standard American diet”. (Which isn’t very impressive mileage in my book; I can do slightly better than that in my ancient Corolla, and significantly better in my not-so-ancient Civic.)

However I can think of several reasons that even this calculation is pessimistic:

  • I need the exercise anyway. What if I didn’t bike anywhere and stood on the treadmill for an hour and got 0 “miles per gallon” instead?
  • Even if my diet is largerly “standard,” and I increase my diet to compensate for bicycling calories, what is the distribution of foods in my increase? Do I actually eat more meat or do I eat more grains, which have a comparatively low carbon footprint? This is quite important.
  • There is a certain amount of exercise that you can do without increasing your diet at all. This is because your body uses Calories more efficiently as a consequence of exercise. This means that some of the miles I ride are completely free in terms of carbon output.
  • There is quite a bit of exercise that I can do without increasing my diet, since I have a few pounds of reserves to use up.
  • There is also a bit of exercise that I can do without increasing my diet, since holidays invariably involve consuming more Calories than I need, which puts an anual deposit into my reserves.
  • I followed Bluejay’s source to nutristrategy.com and I think the number of Calories given applies to heavy mountain bikes, but road bikes are more efficient. (If I compute my power using this: http://www.noping.net/english/, assume my muscles are 22% efficient, and convert power and speed to Cal/mi, a road bike involves 36-45 Cal/mi instead of 58 Cal/mi derived from nutristrategy.com.) This makes Bluejay’s calculator somewhere between 28% and 61% pessimistic for me.
  • Another important point I want to make is that although our existing food production system is carbon intensive, there is no inextricable link between them. My tiny back yard is big enough to grow hundreds of thousands of Calories per year with practically no carbon footprint at all… which also means that gardening probably has as great or greater greenhouse benefits than bicycling (well, hang on, where do you get fertilizers and water? Gardening carbon output depends on methodology).

Next to gardening, potatoes have a pretty low carbon footprint per Calorie.

One thing that is pretty clear in that article: different foods have different carbon footprints. Beef in particular is extremely high because cattle are grain-fed, and the meat production is very innefficient in terms of the feed. (How many calories did the cow eat in order to produce one calorie of roast? About 14.) Consciously choosing food types can be as environmentally effective as hypermiling.

All things considered, I would frame the problem like this: bicycling solves the environment problem for commuting, what remains are the environmental problems with food production.

Smog Emission

Human power emits close to zero smog emissions at the “point of use”. In other words, a cyclist’s breath only has carbon dioxide and water in it, without any NOx, CO, or hydrocarbon emission, and so it doesn’t contribute to bad air quality. Except that bicycles do require point-of-use lubrication, which does emit some hydrocarbons. I’m guessing my cycling smog emission in one month from my chain lube is less than the gasoline that escapes into the air when I fillup my car. In other words, pretty close to zero.

However, there are still some indirect smog-type emissions produced by cycling, in food production and (maybe) in sewage treatment.

Nevertheless, smog is not like carbon, and point-of-use is the only smog emission that matters.

Manufacturing

A bike has 15-25 lbs of material on it instead of 1-3 tons of material in a car. It’s that simple.

Well, okay, maybe with a helmet, bike clothes, bike racks, trailers, lights, etc etc, the cyclist’s material total might be up to 100 lbs.

And it is true that different materials have different environmental impacts. I.e. an aluminum bike probably has a greater footprint per pound than a steel car. But still, we’re talking about only 1-5% of the weight of a car.