WalkingIn 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?

    food CO2 intensityAs 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.