Two Wheels vs. Four: How far do I have to ride my bike to pay back its carbon footprint?

I'm thinking about switching my daily commute from four wheels to two. But I'm concerned about all the energy it takes to manufacture and ship a new bicycle. How many miles would I need to substitute a bike for my car before I've gone "carbon neutral"?

It's tough to say exactly how much greenhouse gas making a bicycle requires, since none of the major manufacturers has released data on their energy consumption. Independent analysts have used a couple of different measures. Shreya Dave, a graduate student at MIT, recently estimated that manufacturing an average bicycle results in the emission of approximately 530 pounds of greenhouse gases (PDF). Umbra Fisk, a research associate at Grist, came up with a total carbon footprint of one ton of carbon dioxide-equivalents for every $1,000 of manufacturing cost. These two estimates intersect at a bike that costs $265 to build—well within the range of manufacturing costs for the wide variety of bicycles on the market.

So that's the goal: To trim about 530 pounds of CO2 emissions from your commute. There's been a lot of hemming and hawing about how biking or walking might not be so eco-friendly because your body burns more calories during those activities than while driving. But, frankly, that's bunk: As the Pacific Institute has shown, you'd have to be eating an all-beef diet to offset the environmental benefits of walking or bicycling. Given a "typical U.S. diet," you would have to ride your bike instead of driving for around 400 miles to cover the bike's initial carbon footprint.

Calculating the total environmental impact of a mode of transit, however, involves more than just the easy-to-measure metrics like mileage per gallon. To get a full sense of the comparative eco-friendliness of bicycles and automobiles, you have to consider additional factors like their toll on the roadways, useful lifetime, and maintenance costs.

Shreya Dave's research went on to measure the full carbon footprint of commuting by bike using life-cycle assessment, the analytical tool that environmental consultants employ to compare products that are often very different. She concluded that an ordinary sedan's carbon footprint is more than 10 times greater than a conventional bicycle on a mile-for-mile basis, assuming each survives 15 years and you ride the bike 2,000 miles per year (or slightly under eight miles per weekday).

A huge portion of that difference came from fuel combustion, but bicycles also require less infrastructure than cars. Even if you assume that all bicycles travel in dedicated bike lanes rather than free-ride on car-lane construction—would that we were all so lucky as to have bike lanes between our homes and work—Dave calculated building, paving, and maintaining roads for cars emits almost four times the greenhouse gases as doing the same for bike lanes.

Bikes also damage roads far less than cars do. A heavy bicycle weighs around 30 pounds, just under 1 percent of the weight of a Toyota Prius and less than 0.4 percent of the weight of a Hummer H2. Simply put, your bike isn't exactly tearing up the asphalt.

Bicycles aren't maintenance-free, but the occasional brake-pad replacement and cable adjustment are responsible for one-sixteenth as much carbon emissions as all oil changing, tire rotation, and alignment work cars require.

What about other ways to get to work? According to Dave's life-cycle analysis, the only vehicle that comes close is the peak-hour bus—and it's not really that close. A fully loaded bus is responsible for 2.6-times the carbon emissions total of a bicycle per passenger mile. But the night and weekend service ruins the bus's overall environmental credentials. Off-peak buses account for more than 20 times as many greenhouse gases as a bicycle. (Each additional passenger contributes very little to a bus's carbon footprint until all the new riders require adding another bus to the route. As a matter of convention, environmental analysts divide the overall carbon footprint by the number of passengers, rather than attributing the entire carbon output of a bus to the single passenger who forced the tipping point.) The mostly empty steel behemoths are even worse, in terms of climate change, than private sedans, SUVs, and pickup trucks.

Those of you thinking that taking the subway is just as good as riding your bike should think again. Subways and light rail systems trail behind peak-hour buses, and way behind bicycles, in life-cycle assessment. They're relatively comparable to a packed bus on fuel use, but they need their own dedicated infrastructure. Even if you prefer to ignore the energy needed to build and maintain a subway line—it is, after all, going to be there whether you take it or not—the fuel alone on Boston's Green Line accounts for almost four times the bicycle's overall carbon footprint per passenger mile.