Sheesh. Wouldn’t you know it, the "walking is bad for the planet" meme has reared its head yet again, this time in a British newspaper:

Food production is now so energy-intensive that more carbon is emitted providing a person with enough calories to walk … than a car would emit over the same distance. The climate could benefit if people avoided exercise, ate less and became couch potatoes.

This made its way to the top of Digg over the weekend, and it’s little wonder. It’s got all the characteristics of a “sticky idea“: it’s simple, it’s memorable, it seems credible, and most of all, it’s unexpected — which makes it perfect for passing around at the water cooler.

Yet it’s actually nothing new. Versions of this idea have been circulating since at least the 1980s. I blogged about a similar claim a year ago. Moreover, as I found out when I ran the numbers, there’s a good reason this claim is so counterintuitive: it’s false!

I won’t rehash my year-old calculations here. (Lucky you.) But unless I’m crazy or just badly mistaken, the propagators of the walking-is-worse-than-driving meme are probably skipping a few steps in their math.

First, driving emits more global warming pollution than most people allow for. Burning a gallon of gasoline — the only thing that some people count when considering the climate impacts of cars — releases a little under 20 pounds of CO2 per gallon. But refining and transporting petroleum releases an additional 4 to 5 pounds per gallon. Plus, CO2 is emitted in car manufacturing and repair; in building and maintaining roads and parking spaces; and even running car insurance offices. You also have to add in climate-warming NOx emissions from vehicle exhaust. All of those extras add up. (To be fair, the same is true for agriculture. There’s a lot of methane and NOx released from cows and rice fields and fertilizers, which might not get captured in an energy analysis.)

Second, walking consumes fewer calories than most exercise hounds think. The right measure isn’t how many calories you burn by walking; it’s how many you burn walking, minus how many you burn just loafing around. Without that correction, you’re likely to overestimate the calorie demands of hoofing it — and thus underestimate the "miles per gallon" you get from shoe leather.

And third, adding a short walk to your daily routine doesn’t necessarily increase your consumption of food, or of food-system energy. Sadly, most of us already eat more than we need, so upping our exercise may just keep us a bit slimmer, rather than encouraging us to eat more. Even if we do eat a bit extra, some of it will likely represent a reduction in food waste (Americans waste something like 1,200 food calories per day!). Plus, things like refrigerators and dishwashers don’t consume any more energy if we eat a bite or two of extra food each day. All in all, powering a short walk may not raise food system energy all that much, above and beyond what we use in a typical day.

When I ran the numbers a year ago, I only looked at the first two points above … and I still found that walking gets more miles per gallon than any car on the market. (The third point is just, er, gravy.) Plus, obviously enough, your personal mileage will vary, depending on what you choose to eat. A vegan locavore pedestrian might be even more fuel efficient than an intercity bus. (There’s a sentence you won’t read every day.)

Still, there’s an important truth at the heart of the walking-is-bad meme: it really does take a lot of energy to put food on the table. But it’s not just agriculture itself. According to a University of Michigan study, only about a fifth of total food energy — or about 2 percent of total energy consumption in the U.S. — is used for fertilizer, tractor fuel, and other farm inputs. Even "food miles" aren’t such a huge deal, at least on average. Perhaps a tenth of food energy — one percent of the U.S. total for all sectors — is for long-distance food transport. Most of the rest of the energy in the food system is used for home refrigeration and cooking, food-industry processing and packaging, running restaurants and supermarkets, and driving to the store.

Oh right, driving to the store. That raises a completely separate question. If everyone drives everywhere — if, "for the good of the planet," every trip requires a car — then stores have to be surrounded by parking; local commercial districts give way to distant malls; downtowns cede space to suburbs. In that kind of world, where the car is the organizing principle of our lives, we all have to travel much farther to go about our daily business. And that’s where the real — and downright dangerous — energy inefficiency comes in.

So if you think that walking a mile a day is bad for the planet, think hard about the alternative. It’s probably worse.