In a working paper entitled “The Environmental Paradox of Bicycling“, Karl Ulrich at the University of Pennslyvania reports that shifting people from their cars to bicycles offers almost no benefit to the environment.
We’ll dig into this paradox in just a second, but first a little background. Ulrich is the man behind TerraPass, the Wharton professor who challenged his students to bulid a viable business around consumer carbon offsets. Ulrich is also an avid cyclist himself and creator of a number of human-powered personal transportation products, including the Xootr Scooter and Xootr Swift folding bicycle.
Bicycles do have large first-order environmental benefits over cars as a means of transportation. Ulrich’s analysis considers the case in which a formerly sedentary person begins bicycling 10 km per day, 5 days per week. In this scenario, about one ton of CO2 is spared every year in the form of reduced fuel consumption.
This reduction in fuel use is partially offset by the increased food consumption of a cyclist. Although typically we think of food as carbon neutral — because the plants at the bottom of our food chain regrow after we harvest them — this view overlooks the fact that most of us don’t feed ourselves by hunting and gathering. The energy required to grow, harvest, process, package, and transport food to your nearest Whole Foods significantly outweighs the actual caloric content of your meal, by a factor of almost six. In other words, only about 15% of the energy we consume when we eat is actually in our food. The rest is contained in the fossil fuels used to bring our food to us.
But increased food consumption is a relatively minor effect when compared to the overall gas savings of cycling over driving. The real culprit in Ulrich’s analysis is the increased lifespan of people who ride bikes. Regular exercise helps you live longer, which points to an unsettling fact. One of the single best things you can do for the planet is to limit your time here.
Population is one of the primary drivers of energy consumption. And there are only two ways to increase the rate of global population growth: bump up the birth rate, or bump down the death rate. In effect, cycling does the latter. (If we look at the population of individual countries rather than the entire planet, immigration is a third way to affect population growth. We’ve previously discussed some of the energy implications of immigration.)
Ulrich estimates that every year of sustained bicycle use adds about 10.6 days to the average person’s lifespan, even accounting for the increased accident risk that cyclists face.
The result, in Ulrich’s analysis, is basically a wash. Each of us, simply by participating in the economy, uses a significant amount of energy. Bicycling rather than driving causes a large first-order decrease in the amount of energy a person uses, but the increased longevity of that person almost entirely negates the savings.
Interesting. But how well does theory map to reality? Personally, I have strong doubts about the practical implications of this analysis. The first issue is that most people who opt to cycle rather than drive cars are likely to be fairly fit already. These cyclists will see less health benefits on the margin than the hypothetical sedentary person, and therefore the first order CO2 reductions will dominate.
The second issue is subtler but possibly far more important. In Ulrich’s analysis, the population effects of cycling occur immediately (which is mathematically accurate in his hypothetical example). But I strongly suspect that the actual demographics of bicycle usage mean that the population bump from improved fitness won’t be seen for a number of years. In effect, riding a bicycle shifts energy consumption from today to an unspecified point in the future.
In private communication, Ulrich ballparks the delay in the population bump as maybe ten years, which he points out is an insignificant amount of time when compared to the climactic changes that are already underway. True enough, but ten years could be long enough for highly significant changes to occur in the energy intensity of our lifestyles. If, for example, in ten years all of our electricity is produced by wind (yea!) or coal (boo!), shifting our consumption into the future will have real consequences for our rate of carbon emissions.
My advice: keep cycling, certainly for your health, and for the environment too.
Update: Ulrich’s paper got picked up in Andrew Leonard’s How the World Works column in Salon. Fun read, although Leonard’s main complaint seems to be that he doesn’t really like the conclusion of the paper.