Tuesday, August 07, 2007

Walking to the Store 'Damages Planet More than Going by Car'

At least according to Chris Goodall. His math?

“Driving a typical UK car for 3 miles [4.8km] adds about 0.9 kg [2lb] of CO2 to the atmosphere,” he said, a calculation based on the Government’s official fuel emission figures. “If you walked instead, it would use about 180 calories. You’d need about 100g of beef to replace those calories, resulting in 3.6kg of emissions, or four times as much as driving.
An intriguing comparison. Is it accurate?

Assuming your car gets 30 miles to the gallon and 20 lbs of CO2 per gallon, then yes, driving 3 miles emits 2 lbs of CO2. As for the beef, based on the data used comparing Hummers with Vegans (this is the source for all food based CO2 numbers below), there are 1.382 kg of CO2 emitted per 100 calories or 2.5 kg for 180 calories. This is a bit lower than his 3.6 kg estimate, possibly due to the fact that his data is based on Japanese beef raising which might be more energy intensive. But either way, eating beef as a means of calories to walk to the store does emit more CO2 than driving a car.

But, unless you are some crazy Atkins devotee, you are not likely to use beef to provide the extra calories you need in your diet for increased exercise. If we assume instead that you drink Gatorade or some other drink using corn (high fructose corn syrup) as its source of calories, you would emit approximately .02 kg of CO2 for 180 calories (based on a rate of .011 kg of CO2 per 100 calories for corn). This is much less than the .9 kg of emissions from driving.

What about drinking milk? In the article they state that 180 calories of milk emits 1.2kg of CO2. Based on the numbers I looked at previously, I put it at .5kg of CO2. Not sure what the difference is. The .9 kg of emission from driving is then either lower or higher than drinking milk depending on which set of numbers you use.

And you could always choose to bike rather than walk, as a human riding a bicycle is the most efficient means of travel of any living thing in the animal kingdom pound for pound. At 10 mph it is estimated you burn 26 calories per mile. That would be 78 calories for 3 miles, or 43% the expenditure of walking. Getting 78 calories from beef would emit 1.075 kg of CO2 (based on my numbers) which is just slightly more than the .9 kg of driving.

This comparison also assumes that you would need extra calories to walk to the store. If you use the walk as part of your daily exercise (or if you are already eating more calories than you need and should add some exercise) then no extra calories are required.

My conclusion would be that walking is still by far the preferable choice to get to the store over driving.

This also made me wonder how much additional CO2 you would emit if you were a hard core athlete like an ironman triathlete. If you ate an extra 2,000 calories a day to support your workouts, this would lead to extra annual emissions of 1.2 tonnes of CO2 if you ate the average American diet, .65 tonnes on a lacto-ovo vegetarian diet and .1 tonnes on a vegan diet. To put that in perspective, a tonne of CO2 is emitted per 113 gallons of gasoline burned. Training a year for a triathlon and getting the extra calories from an average American diet would have the same CO2 impact as driving a Hummer for 1,700 miles (which ironically is also the same distance of the actual ironman race, I believe :)).

The article also mentions other "great green myths":
Burning wood for fuel is better for the environment than recycling it, the Department for Environment, Food and Rural Affairs discovered.

Organic dairy cows are worse for the climate. They produce less milk so their methane emissions per litre are higher.

Catching a diesel train is now twice as polluting as travelling by car for an average family, the Rail Safety and Standards Board admitted recently. Diesel trains in rural Britain are more polluting than 4x4 vehicles. Douglas Alexander, when Transport Secretary, said: “If ten or fewer people travel in a Sprinter [train], it would be less environmentally damaging to give them each a Land Rover Freelander and tell them to drive.”

Trees, regarded as shields against global warming because they absorb carbon, were found by German scientists to be major producers of methane, a much more harmful greenhouse gas.
These are also thought provoking, and I would like to see how they calculated the numbers on those.

Update: Master comment critter odograph reminds me of a previous post estimating the carbon footprint of a bag of potato chips at 75g of CO2. I am not sure how many calories are in the bag of chips, but 180 sounds about right. If so, that would be a bit more than the 20g of CO2 from corn, but much less than driving, milk or beef.

via Times Online


odograph said...

Good Job. I've also been pointing people to that old calc for Walker's Crisps. That small bag of potato chips had a rigorously calculated 75g of co2 emissions from farm to store ... and I'm sure enough calories to fuel the walk.

(Though if you have data on the number of extra calories we Americans eat, or carry around our middles, that might be another interesting way to run the numbers.)

Fat Knowledge said...

Thanks odograph.

I have seen in my weblogs that you have posted that Walker's Crisp on many blogs. I kept looking at the referrals and thinking how did I get a link on that site? Then I would check it out and see that it was you. Thanks for spreading the word. :)

Man, you must read and comment on a ton of different blogs. In The Tipping Point terminology, you are one heck of a connector.

And I forgot to add the Walker's Crisps to my own post! I will go add it.

I agree with you on the extra calories angle, but unfortunately the only thing I know is that Americans consume 3,774 calories a day (which includes calories that are lost due to spoilage or other waste).

odograph said...

OK ;-)

This page claims (citing the Farmer's Almanac):

"Americans are 2,300,000,000 (over two billion) pounds overweight. We average about nine and a half pounds overweight. Overweight Americans cost about a million extra gallons of gasoline per year."

at 100 calories per mile we Americans can only walk 2,300,000,000 / 100 = 23,000,000 miles before meeting our healthy weight?

That's a good one, if true.

odograph said...

Geez I need to wake up. I didn't even do the pound to calorie conversion.

3500 calories per pound.

(2,300,000,000 * 3,500) / 100 = 80,500,000,000 miles.

That makes more sense. I'm glad I caught it before anyone else did :-/

Working from the other number ("We average about nine and a half pounds overweight."):

(9.5 * 3,500) / 100 = 332.5

We can afford to walk on average 332 miles apiece.

Who knows, maybe I made more errors there ... but maybe we're closing on the anser.

Fat Knowledge said...

I think your numbers look pretty good, except that in this example they said 3 miles = 180 calories, so that would be 60 calories per mile.

(9.5 * 3,500) / 60 = 554 miles

554 miles / 3 = 185 trips to the store

So, the average American could walk to the store 185 times before they would require any extra calories. At 1 trip per week that would take 3.5 years.

Of course this analysis relies on a simplistic look at calories, not taking into account changes in metabolism or appetite that would occur due to the extra exercise. But, interesting none the less.

odograph said...

FWIW, here's a serious table on walking and calories.

It looks like 60 is (another) corner case. Form me personally (a 150lb 3mph walker) the number is more like 90.

Fat Knowledge said...

That is a good table. Thanks for the info.

Anonymous said...

there was a paper about 10 years ago that found a person who eats beef and only beef produces GWP emission on par with flying everywhere. ...that's not even including the emissions of the medical care and drugs required to keep a complete carnivore alive!

Fat Knowledge said...


I knew that emissions from beef were high, but I didn't know they were that high.

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