Carbon Footprint of Walking vs. Driving

My friend Erik recently pointed out a provocative article in the New York Times, "How Virtuous is Ed Begley Jr.?" It posited that the carbon footprint of driving a car is smaller than that of walking the same distance. I thought it was surely off-base, so I did my own calculations and the results were surprising to me. Here are my calculations.

Notation: CO2e = CO2 equivalent, which includes other greenhouse gases, e.g., methane.

A. CARBON FOOTPRINT OF DRIVING A CAR FOR A MILE

Combustion of a gallon of gas in a car emits 8.5kg of CO2 [1]. But that does not include the production cost of the gasoline. When that is included, a car emits 11.6kg CO2e per gallon gas [2]. The average fuel economy of cars sold in 2004 in the US is 24.7 mpg [3]. So CO2 emissions per mile in the average 2004 car is (11.6kg/gallon) * (gallon/24.7miles) = 0.47kg/mile.

B. CARBON FOOTPRINT OF WALKING A MILE BY EATING ONLY BEEF

For a person weighing 165lbs, walking a mile at a moderate pace burns 85 calories [4].

A kilogram of ground beef contains 2590 calories [5]. So a person would need to eat 85/2590 = 3.3% of beef to get enough energy for the walk. Production of a kg of beef emits 19kg of CO2e [6], so eating only 3.3% means you're responsible for 19 * 3.3% = 0.63kg CO2e. That is more than the emissions due to driving a car for a mile.

C. CARBON FOOTPRINT OF WALKING A MILE BY EATING AN AVERAGE AMERICAN DIET

About 14% of the American diet is beef, chicken and pork [6]. Among beef, chicken and pork, beef causes the greatest greenhouse gas emissions. Let's simplify the calculation by assuming 14% of the average American diet is all red meat, and the rest is non-red-meat. This is an over-estimation of red meat (beef) consumption. Weber and Matthews (2008) identifies several non-red-meat food groups: beverages, cereal, chicken/fish/eggs, dairy, fruit/vegetable, and oils/sweets/condiments. When bundled together, the average CO2e emissions of these non-red-meat food groups is approximately 25 to 33% of red meat [7]. Let's use 33% to be conservative.

In Section B above, we assumed all 85 calories required to walk a mile came from red meat. If we assume that only 14% came from red meat and the rest came from non-red-meat, we can modify the 0.63kg of CO2e footprint from Section B above. Of the 0.63kg CO2e, we keep 14% as is, since it's still coming from red meat. The other 86% of the 0.63kg CO2e footprint can be reduced by 75%: (0.63kg * 14%) + (0.63kg * 84% * 25%) = 0.22kg CO2e. This is less than the emissions due to driving a car for a mile.

D. REFERENCES

1. US Environmental Protection Agency. Emission Facts: Average Carbon Dioxide Emissions Resulting from Gasoline and Diesel Fuel. Overview: Pollutants and Programs.
   
2. Groode, T. 2006. Review of Corn Based Ethanol Energy Use
   and Greenhouse Gas Emissions. MIT laboratory for the energy and environment report.
   
3. Carr, B. Average Gas Mileage Relatively Flat Between 1980 and 2004. Daily Fuel Economy Tip, Oct 19, 2006.
   
4. [ missing reference ]
   
5. About.com. Calories in Ground Beef.
   
6. Raloff, J. 2009. Climate-friendly dining ... meats: The carbon footprints of raising livestock for food. Science News, web edition, Feb 19, 2009.
   
7. Weber CL, Matthews HS. 2008. Food-miles and the relative climate impacts of food choices in the united states. Environmental Science & Technology 42(10):3508-3513.
   

So, the conclusion is that for the average American walking has a smaller carbon footprint than driving an average 2004 model car. In fact, for the average American, walking has a smaller carbon footprint than driving any car that gets less than 52 mpg.

But are you an average American in terms of your diet? The above conclusion is very sensitive to the proportion of red meat in one's diet. For example, if you're a full-time carnivore, your carbon footprint for walking beats only those cars getting less than 18 mpg.

Ultimately, what's striking to me is not that walking has a smaller carbon footprint than that of driving, but just how close the two numbers are. I expected the numbers to be very far apart. Moving a 165 lbs human body by itself a mile down the road should be far more efficient than dragging along a 2500 lbs chunk of steel for the same distance. The problem this highlights is that red meat production is incredibly inefficient. The message, for those who want to reduce your carbon footprint, is to eat less red meat.

Note that the above calculations do not include the carbon footprint for the manufacture and maintenance of a car and roads, not to mention the social, geopolitical and ecological impacts of driving a car. Nor do the above considerations include the benefit of walking to the human body.