How much fossil fuel do I use to walk a mile?
June 12, 2015 8:39 AM Subscribe
It takes 10 kilocalories of fossil-fuel to make 1kcal of food. You use 80kcal to walk a mile. There are 31520kcal per gallon of gasoline. 80kcal * 10 = 800kcal of fossil-fuel per walking-miles. 31520kcal/800kcal=39.4 walking-miles/gallon of gas? What am I missing? Can I trust the 10:1 fossil:food ratio?
Your math and inputs look fine to me. The 10:1 average ratio will be harder to source, although given how much meat we eat I'd rank it as perfectly plausible.
Another way to approach this would be to find out how much oil we use for farming in a year and then divide by (population*days/year*calories/day) but that ignores imports and exports.
posted by ftm at 9:07 AM on June 12, 2015
Another way to approach this would be to find out how much oil we use for farming in a year and then divide by (population*days/year*calories/day) but that ignores imports and exports.
posted by ftm at 9:07 AM on June 12, 2015
Check out the math in this analysis of the carbon footprint of walking versus driving. Their page with detailed calculations/assumptions is no longer online, but downloadable via Wayback Machine here.
posted by three_red_balloons at 9:16 AM on June 12, 2015 [1 favorite]
posted by three_red_balloons at 9:16 AM on June 12, 2015 [1 favorite]
gasoline does not spring from the ground ready to be burned in your car. I don't know what the ratio is, but surely it takes more than 1kcal of fossil fuels to produce 1kcal worth of gas?
Sure, but the fossil fuels that were used to produce the food were also put through some sort of extraction and refining process, right? So maybe we should cancel that out, or maybe we need to take into account the amount of energy that goes into refining industrial diesel vs. the amount that goes into making consumer-grade gasoline. Also consider that most consumer gas is cut with some percentage of ethanol, which is made from commodity grain, which took some amount of fossil fuel to grow and then more to process into ethanol.
posted by contraption at 10:01 AM on June 12, 2015
Sure, but the fossil fuels that were used to produce the food were also put through some sort of extraction and refining process, right? So maybe we should cancel that out, or maybe we need to take into account the amount of energy that goes into refining industrial diesel vs. the amount that goes into making consumer-grade gasoline. Also consider that most consumer gas is cut with some percentage of ethanol, which is made from commodity grain, which took some amount of fossil fuel to grow and then more to process into ethanol.
posted by contraption at 10:01 AM on June 12, 2015
Yeah, it's complicated, to be sure.
But not all the petroleum used to get the 10x calories for food is gas - fertilizers are made from petroleum, for example.
The analysis posted above is pretty interesting and highlights how complex it is (a 7 page PDF that is 3.5 pages of references!).
All that being said, I'm a little surprised to find that, whatever the True Numbers are, it's closer than I would have thought. A very efficient car, or scooter, might actually not be much different than walking somewhere in terms of total expenditure.
Of course, that's against sort of an average - if you personally can grow a lot of your own food, or buy food that is grown more efficiently, then I think it could - for you - swing way in advantage of walking or biking.
I ride a bike for commuting and I once did some cost estimates - totally back of the envelope - that showed that I probably wasn't saving much money on gas/wear&tear, because of the extra food and consumables I was using for the bike (although, in truth, since I was biking to work I could offload a lot of the cost onto my employer, who provides food)
posted by RustyBrooks at 10:13 AM on June 12, 2015
But not all the petroleum used to get the 10x calories for food is gas - fertilizers are made from petroleum, for example.
The analysis posted above is pretty interesting and highlights how complex it is (a 7 page PDF that is 3.5 pages of references!).
All that being said, I'm a little surprised to find that, whatever the True Numbers are, it's closer than I would have thought. A very efficient car, or scooter, might actually not be much different than walking somewhere in terms of total expenditure.
Of course, that's against sort of an average - if you personally can grow a lot of your own food, or buy food that is grown more efficiently, then I think it could - for you - swing way in advantage of walking or biking.
I ride a bike for commuting and I once did some cost estimates - totally back of the envelope - that showed that I probably wasn't saving much money on gas/wear&tear, because of the extra food and consumables I was using for the bike (although, in truth, since I was biking to work I could offload a lot of the cost onto my employer, who provides food)
posted by RustyBrooks at 10:13 AM on June 12, 2015
Can I trust the 10:1 fossil:food ratio?
Quite a treatise on this, referencing the sources that Michael Pollan must have used, is posted here. The research is 11 years old or older, and it cites a pretty significant shift in the patterns of energy input to food over the previous half century, so the ratio today could have changed again. But 10:1 is probably still a reasonable ballpark. It does depend on what you're eating, though. I'm guessing that pure sugar might be a lower ratio; something like lettuce or celery would probably be a higher ratio.
posted by beagle at 10:46 AM on June 12, 2015
Quite a treatise on this, referencing the sources that Michael Pollan must have used, is posted here. The research is 11 years old or older, and it cites a pretty significant shift in the patterns of energy input to food over the previous half century, so the ratio today could have changed again. But 10:1 is probably still a reasonable ballpark. It does depend on what you're eating, though. I'm guessing that pure sugar might be a lower ratio; something like lettuce or celery would probably be a higher ratio.
posted by beagle at 10:46 AM on June 12, 2015
You can also go even further down the rabbit hole if you try and then back-calculate the fossil fuel benefits of walking on health. Healthcare is incredibly fossil-fuel intensive (so. much. plastic.), and walking more is incredibly good at keeping you out of the healthcare industrial complex. If you walk to the store and other destinations less than 2-3 miles from your house on a regular basis, your risk for developing type II diabetes, heart disease, and many other chronic, expensive, fossil fuel intensive diseases is dramatically lowered.
posted by rockindata at 10:50 AM on June 12, 2015 [4 favorites]
posted by rockindata at 10:50 AM on June 12, 2015 [4 favorites]
Ah, but the longer you live, the longer you hang around consuming fossil fuels and the likelier you are to reproduce and create new consumers. If you drive, you are increasing your chances of dying a sudden violent death in a crash and not consuming any more fossil fuels ever again.
posted by contraption at 10:53 AM on June 12, 2015 [5 favorites]
posted by contraption at 10:53 AM on June 12, 2015 [5 favorites]
Also, this analysis completely ignores the embodied energy in the vehicle. Depreciate the energy used to form the steel, glass, plastic in a car over its life and add that to the number.
posted by scruss at 10:55 AM on June 12, 2015 [1 favorite]
posted by scruss at 10:55 AM on June 12, 2015 [1 favorite]
three_red_balloons' link above is where I would start. This kind of question is answered academically by what's called a "Life Cycle Analysis" (LCA). That's the search term to use.
Here's a similar analysis that compares the energy used and GHG emitted by many modes of transportation, including walking and driving (and bikes, ebikes, buses and airplanes).
Here is a blog post discussing about some recent research results as well.
The numbers for walking, in particular, are all over the map because of dietary assumptions. If you assume that you walker lives entirely on GHG-rich beef, then you can cook the numbers one way. If you assume veganism, you can cook them another. (This is all detailed in the study three_red_ballooons linked to above). However, making a middle-of the road typical USDA diet assumption, walking is still the best choice, better than any vehicle, including a bicycle.
posted by bonehead at 12:55 PM on June 12, 2015
Here's a similar analysis that compares the energy used and GHG emitted by many modes of transportation, including walking and driving (and bikes, ebikes, buses and airplanes).
Here is a blog post discussing about some recent research results as well.
The numbers for walking, in particular, are all over the map because of dietary assumptions. If you assume that you walker lives entirely on GHG-rich beef, then you can cook the numbers one way. If you assume veganism, you can cook them another. (This is all detailed in the study three_red_ballooons linked to above). However, making a middle-of the road typical USDA diet assumption, walking is still the best choice, better than any vehicle, including a bicycle.
posted by bonehead at 12:55 PM on June 12, 2015
Can I trust the 10:1 fossil:food ratio?
This will vary hugely, if you grow organic veg in your own yard without fertiliser, then its 0:1 but if you import lettuce across the Atlantic by air then thats 127:1
80kcal * 127 = 10,160 kcal of fossil-fuel per walking-miles. 31520kcal/10,160kcal = 3.1 walking-miles/gallon of gas
So somewhere between 3 miles per gallon and infinity miles/gallon of gas.
posted by Lanark at 12:56 PM on June 12, 2015 [1 favorite]
This will vary hugely, if you grow organic veg in your own yard without fertiliser, then its 0:1 but if you import lettuce across the Atlantic by air then thats 127:1
80kcal * 127 = 10,160 kcal of fossil-fuel per walking-miles. 31520kcal/10,160kcal = 3.1 walking-miles/gallon of gas
So somewhere between 3 miles per gallon and infinity miles/gallon of gas.
posted by Lanark at 12:56 PM on June 12, 2015 [1 favorite]
Yeah, I 2nd Lanark --- I think that you're actually comparing apples and oranges. Based on your first link, Pollan seems to be drawing his 10:1 ratio from
Energy used to produce food by American agriculture+food processing industries : Energy contained within the food produced
But you're asking:
Fossil fuel used to produce the food I ate : Energy expended by me derived from that food
But the amount of fossil fuel energy used to produce food can vary wildly, there's no fundamental relationship there which can be held constant. A apple picked from a tree in your backyard may have required 0 fossil fuel energy. If you're having Black Sea caviar packed in dry ice and FedExed to you overnight, there might have hundreds of thousands of kilocalories of energy expended to do that. I don't know that it makes sense as a premise to take Pollan's ratio and apply to an individual person's energy expenditure --- how much fossil fuel energy went into the food you actually ate will vary madly depending on where you source your calories, whether you buy organic produce, how processed it was, etc.
Also, when Pollan talks of "fossil fuel energy used to produce food" --- that seems to me a sneaky little phrase. Because nitrogen-based fertilizers are ultimately derived from fossil fuels. These fertilizers are a big part of what allows the Earth to feed 7 billion people and counting without the same frequent mass famines that occurred periodically through most of human history. How many of Pollan's 10 kilocalories go into the fertlizer, and how many to the transport?
posted by maggiepolitt at 2:33 PM on June 12, 2015
Energy used to produce food by American agriculture+food processing industries : Energy contained within the food produced
But you're asking:
Fossil fuel used to produce the food I ate : Energy expended by me derived from that food
But the amount of fossil fuel energy used to produce food can vary wildly, there's no fundamental relationship there which can be held constant. A apple picked from a tree in your backyard may have required 0 fossil fuel energy. If you're having Black Sea caviar packed in dry ice and FedExed to you overnight, there might have hundreds of thousands of kilocalories of energy expended to do that. I don't know that it makes sense as a premise to take Pollan's ratio and apply to an individual person's energy expenditure --- how much fossil fuel energy went into the food you actually ate will vary madly depending on where you source your calories, whether you buy organic produce, how processed it was, etc.
Also, when Pollan talks of "fossil fuel energy used to produce food" --- that seems to me a sneaky little phrase. Because nitrogen-based fertilizers are ultimately derived from fossil fuels. These fertilizers are a big part of what allows the Earth to feed 7 billion people and counting without the same frequent mass famines that occurred periodically through most of human history. How many of Pollan's 10 kilocalories go into the fertlizer, and how many to the transport?
posted by maggiepolitt at 2:33 PM on June 12, 2015
So, I’m assuming (and it appears others above are assuming) that the next step in this analysis is “if my car gets 40miles to the gallon that’s more efficient than walking.” As stated above, that’s comparing the energy it took to produce one energy source (your food) to the energy contained in another (that gallon of gasoline). So there has to be an adjustment based on the energy it takes to drill, refine, and ship the gas, and perhaps even some consideration of the energy embodied in the car itself.
Another point that gets elided in this type of analysis is that the physical structure of the built environment is shaped by (among other things) the dominant mode of transportation. That is, sure, if everyone drives to work and there are direct roads from anywhere to anywhere, a twenty-mile commute via auto may turn out to cause fewer emissions than walking the same distance. But over the long haul, a society that primarily walks or bikes to work will become much more compact. Commutes of more than ten miles would become effectively impossible (I’m pretending mass transit doesn’t exist for purposes of simplicity). Such a society will live at densities orders of magnitude higher than US suburbia, with a resulting decrease in food energy needed to power the average individual’s commute.
So the most relevant question isn’t so much “could we save energy if everyone ditched their cars tomorrow and walked everywhere”, because many of our current cities would literally die out if people had to walk 50+ miles a day to and from work. The relevant question is “would a city based around walking as the dominant mode of transportation use less energy than the auto-dependent cities we have today?” That, in turn, depends on further impacts of density such as home energy use.
posted by five toed sloth at 4:14 PM on June 12, 2015 [1 favorite]
Another point that gets elided in this type of analysis is that the physical structure of the built environment is shaped by (among other things) the dominant mode of transportation. That is, sure, if everyone drives to work and there are direct roads from anywhere to anywhere, a twenty-mile commute via auto may turn out to cause fewer emissions than walking the same distance. But over the long haul, a society that primarily walks or bikes to work will become much more compact. Commutes of more than ten miles would become effectively impossible (I’m pretending mass transit doesn’t exist for purposes of simplicity). Such a society will live at densities orders of magnitude higher than US suburbia, with a resulting decrease in food energy needed to power the average individual’s commute.
So the most relevant question isn’t so much “could we save energy if everyone ditched their cars tomorrow and walked everywhere”, because many of our current cities would literally die out if people had to walk 50+ miles a day to and from work. The relevant question is “would a city based around walking as the dominant mode of transportation use less energy than the auto-dependent cities we have today?” That, in turn, depends on further impacts of density such as home energy use.
posted by five toed sloth at 4:14 PM on June 12, 2015 [1 favorite]
I was kind of musing over this the other day while I was riding my bike, and I thought, I wonder if we could convince people to do some kind of gardening or food production as a form of exercise - you'd probably have to try to make it good enough exercise to sub in for running or cycling. Competitive apple picking or something.
posted by RustyBrooks at 1:24 PM on June 15, 2015
posted by RustyBrooks at 1:24 PM on June 15, 2015
This thread is closed to new comments.
I think my BMR is around 1650, so 69 kcal/hour or therabouts. If I walk 3 miles per hour then my BMR "per mile" would be 23 kcal, making my extra expenditure per mile 57/mile. By that calculation it's 570kcal of fossil fuel per walking mile, or 54.8 "mpg"
Also, consider that gasoline does not spring from the ground ready to be burned in your car. I don't know what the ratio is, but surely it takes more than 1kcal of fossil fuels to produce 1kcal worth of gas?
posted by RustyBrooks at 9:05 AM on June 12, 2015 [3 favorites]