In case you missed it, there was a bit of a kerfuffle in the blogosphere a few months back, concerning the climate impacts of walking vs. driving. Apparently, some folks—New York Times columnist and blogger John Tierney in particular—were spreading the claim that a pleasant stroll to the store might actually release more GHGs than getting behind the wheel. Other bloggers picked up the meme, including one post with the headline: “Be Green: Drive.”
The idea may sound absurd, but there’s a legitimate insight behind it. Walking burns calories, which come from food—and it takes an enormous quantity of fossil fuels to produce, process, and transport everything that we eat. Add in the other GHGs from agriculture—everything from cow manure to emissions from synthetic fertilizers—and you’ve got a potent global warming cocktail in every glass of milk.
But our doppelgangers at the Pacific Institute did their homework, compiling evidence about climate emissions from both cars and food. And they came to the conclusion that walking emits about one-quarter the GHGs of driving—earning a partial retraction from Tierney. (You go, PacInst!)
But looking at the numbers, I think that the Pacific Institute’s numbers are conservative. In fact, I think that when I take a short walk, I’m being at least 12 times as friendly to the climate as if I drove. Your mileage may vary, of course; but my shoes get about 220 miles per gallon.
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Here are a handful of reasons why I think that walking look even more climate friendly than the Pacific Institute’s estimates suggest:
What would a walker be doing otherwise?
Walking burns calories, but a person also burns calories while driving, or just loafing around. So what matters isn’t the total calories your body burns during a walk, but the marginal calories from walking vs. driving + whatever else you’d do with your time. CalorieLab gives some helpful clues: for a half-hour walking trip, they estimate that a 176 pound person (the average of the median weights for men and women in the US) burns about 106 extra food calories, compared with a 5 minute drive and 25 minutes of watching TV. This figure is slightly less than the figure the Pacific Institute used. And if you do anything more strenuous than sit on your butt for those 25 minutes, then the food-calorie “advantage” of driving narrows even further.
Score one for walking.
Where do the extra food calories come from?
If the calories to power your walk come from your waistline, then there’s no marginal food consumed—and, potentially, a long-term climate gain, since it actually takes extra food calories to maintain a heavier frame. More likely, though, you’ll find a way to eat an extra bite or two of food in the day or so after your walk; the body seems to work overtime to maintain a set-point for weight.
But if you’re like me, that food could well come from “plate waste”—food that would otherwise be thrown out. (My marginal food calories tend to come from my kids’ leftovers. Who wouldn’t want a scrap of cold, half-eaten bagel in the morning? I’m lovin’ it!) Generally speaking, there’s plenty of waste in the food system: USDA estimates that the US food system produces 3900 calories per person each day, of which roughly a third is simply thrown away or allowed to spoil. (This paper, cites a figure of 3,774 calories per day, after accounting for net exports—but who’s counting?)
Any of your calorie needs that come from your waist, or your waste, are essentially climate neutral. Score another one for walking.
What kind of food is it, anyway?
As the food chart to the right shows (see here for the source, and here for a discussion), some foods have a major climate impact, while others are comparatively benign. But my “marginal calories”—the sorts of things I eat when I load up on when I’m hungry and my willpower is low—are starchy, sweet, and/or loaded with vegetable oils. Those are the itty-bitty pink and red lines on the chart to the right, the ones with the smallest climate impacts.
So if you, like me, replenish yourself from a walk with starches or sweets, you may be doing your body no great favors—but, yet again, the climate impact of your walk falls a bit. Score yet another point for walking.
What about cars?
The Pacific Institute’s figures exclude life-cycle emissions related to manufacturing and maintaining your car. This EPA study (see table 14) suggests that vehicle manufacturing alone increases the net climate impact of driving by at least 12 percent.
The final score: Walking vs. Driving
Obviously, most of the points above are a bit mushy; but I think it’s possible to give some ballpark estimates:
- For walking energy itself, cut the Pacific Institute’s emissions estimates by about 5 percent.
- Accounting for food that would otherwise be waste, cut their estimates by another third.
- For marginal vs. average food sources, cut food emissions by another third.
- For vehicle manufacturing, increase car emissions by about 12 percent
This is all ballpark, back-of-the-envelope calculation, of course. But if it’s close to being right, walking is about 12 times better for the climate than driving. In fact, a car has to get the equivalent of about 220 miles per gallon before it matches the fuel economy of shoe leather.
So until I can buy a car that uses a quarter of the gas of a Honda Insight, I’m going to keep on walking.
Walking photo courtesy of Flickr user rogiro under a Creative Commons license.
And if you are eating calories from locally grown foods, mpg on shoes goes up even more!
Walking to your car(e) has great value in centering the driver fully into their body, and relaxing their mind. In fact, the National Hwy. Traffic Safety Council said that a rageful driver was angry before they got into their car. Therefore, when a person walks slowly to their car, and relaxs & grounds themselves into the present, the anger quotient goes down. Also, being a drummer, playing some percussion rhythms connects one to their inner rhythms and those of others, helping the drive down the road to be quite flowing and peaceful! http://www.facebook.com/truemark.
Thank you for staying on this. Like you say, analysis can get mushy, depending on who is worried about maintaining their lifestyle. Is it worth extending the analysis of the GHG cascade associated with getting around to the producers of the particular food, fuel or transportation mode? Shouldn’t the energy producer’s own energy usage be added to our GHG footprint? Without our demand they would need to find a different niche in the economy. And maybe, just maybe, recognize that it might be prudent to develop a mindset that reduces consumption and the number of children they bring into the world and raise to feel entitled to consume at such a high level.
A (definitely not fact-checked) joke email contends that “A recent study conducted by Harvard University found that the average American walks about 900 miles a year. Another study by the American Medical Association found that Americans drink, on average, 22 gallons of alcohol a year. So Americans average about 41 miles per gallon.”More seriously, Hal’s comment is right on the money. If we are to consider not just the caloric content of food but the energy embodied in growing, transporting, and processing food, then we must also consider not just the energy contained in gasoline but also the energy used to discover, prove, recover, refine, and transport the oil that went into the gasoline. As we go to ever more remote regions and drill the deep sea and retort bituminous sand for oil, the energy “embodied” in gasoline grows rapidly.
This is a really fantastic discussion. Even the original (erroneous) post from John Tierney reminds us that GHG are hidden in everything we do, not just in the obvious places. This is the reason that the ONLY effective solution to GHG is a carbon tax or universal cap-and-trade system. By putting the tax on the back end, at the point of production, this cost would get passed down the chain to every activity which contributes to climate change. We don’t have to worry about calculating the full life cycle energy hidden in various activities. It will all be presented to us right there in the final price.Obviously, the cost of gas would rise, but so would the cost of red meat, electricity, and energy intensive manufactured goods. The price of food shipped from far away would rise relative to the price of food grown locally. Then all people have to do is do what they are already very good at, which is to pick the cheapest alternative, because we will have aligned the price signal with the damage it causes.
Hal & Denis -Just FYI, the Pacific Institute folks do include estimates for “upstream” emissions from fossil fuels—everything from extraction to shipping to refining to distribution. They’re derived from the GREET model maintained by Argonne National Labs. Link is here: http://www.transportation.anl.gov/pdfs/TA/339.pdfI don’t know of any better estimate than the GREET model. But if you guys do, by all means, let me know.Tony – I agree, a cap or tax are, in my view, the only ways to make sure that we don’t promote policies that inadvertently make GHG emissions worse.
All of which seems to indicate that population density is a factor and that if we lived a more rural lifestyle with greater self reliance we may have to consider reducing population growth.
What would a walker be doing otherwise?The fact that drivers’ bodies are burning calories when they are driving is a point I’ve always thought is important in this debate. Walking at a slow stroll burns the same number of calories as driving a car ((2 kcal/kg/hr). The driver of a more demanding vehicle like a truck, RV or bus burns even more calories than the slow stroller. But your analysis treats walking and driving as if they are comparable travel modes. They aren’t—driving is used for longer trips, on average, than walking. So the duration of walking trips is a bit less than driving trips. At the NHTS website you can create custom tables. Make a table of “Average Person Trip Duration” by mode and the result is 18.6 minutes for cars and 16.4 minutes for walking. So even before you consider vehicle fuel use, it looks like walking is little more efficient than driving on a per trip basis.What about the energy costs of motor vehicle infrastructure (roads, highways, parking, etc.) and operations (policing, emergency response, maintenance, lighting, etc.)? Those are higher for motor vehicles on a per traveler basis.On the other side of the ledger, I don’t understand how “plate waste” can be reclassified as food that’s eaten. If it’s plate waste, by definition it is not eaten. My two cents, anyway.
Laurence -Concerning “comparable travel modes”—I’m not sure I get you. The question I’m trying to look at, and that Tierney & the Pacific Institute were addressing: if you’re interested in taking a 1-to-2 mile trip, and you’re looking to minimize the climate impact of that trip, should you walk or drive? Tierney initially suggested that you should drive. The Pacific Institute debunked that. I think that they were, if anything, too gentle on Tierney. The question of whether one should take a 15 minute walk or a 15 minute drive, or an average-length walk vs. an average-length drive, are separate questions. It might be interesting; but given the mile-to-mile comparison, it’s not likely to turn up anything that we don’t know.And on plate waste—I’m assuming that food that would have been plate waste, but instead gets eaten to power a walk, doesn’t affect one’s carbon footprint. In my case, table scraps that I’d throw away, but instead decide to eat, shouldn’t be counted as contributing to upstream energy or related GHGs. And in my household, there happen to be plenty of scraps that don’t get eaten—the result of some finicky kids with unpredictable appetites.