Doing the math: how plausible is the claim that changing what you eat makes more difference to global warming than changing what you drive?

Dave Munger pointed me to an article in the New York Times that claims "switching to a plant-based diet does more to curb global warming than switching from an S.U.V. to a Camry."

Dave is a critical consumer of information and notes that there is little given in this particular article (which appears in the "Media & Advertising" section) as far as numbers. As I'm not an agronomist, I don't have all the relevant numbers at my finger tips, but I'm happy to set up some equations into which reliable numbers can be plugged once they are located.

First, there's some unclarity in the sentence quoted above as far as what exactly is being compared when we're weighing the relative importance of choices about driving and choices about eating. Is it a comparison of greenhouse gas (GHG) emissions? Petroleum consumption? The whole bundle of factors that have an effect on global warming?

Also, as Dave pointed out to me, which SUV and which Camry are we talking about? What animal sources of protein and what plant sources of protein? There are lots of specific details that could make a difference.

Your driving decision's impact seems like it could be calculated as:

((Average miles/day)/(Average miles/gallon)) x (kg GHG emitted/gallon of gas consumed)

Of course, if you're just tracking how much gas you're burning, don't multiply by the last term.

I don't know whether there's a uniform rate of GHG emitted per gallon of gas a car consumes -- either for a single vehicle or vehicle type or across many vehicle types. In the absence of expert opinion on this, I'd probably assume it's a constant. (If you have an expert opinion on this, please chime in!)

I drive neither an SUV nor a Camry. A pretty fuel efficient SUV gets maybe 20 miles per gallon. A Camry is probably not too far from a Corolla in average fuel efficiency, so we could estimate that at around 30 miles per gallon. For someone with a daily commute about the same distance as mine (a 50 mile round-trip), this hypothetical commuter would burn 2.5 gallons of gas with the SUV and 1.7 gallons of gas with the Camry. However much crud is being emitted from the burning of the gas (assuming a constant rate of crud per gallon of gas consumed), the SUV commute puts out 47% more of it than the Camry commute.

That was the easy calculation. Now for the food.

To simplify, I'm going to deal with only one macronutrient, protein, since that's the one people seem to be most interested in getting from their meat. (The fat is like the prize at the bottom of the cereal box.) If we're sticking to GHG as the thing we're comparing, we'll be looking at:

(kg protein consumed/day) x ((GHG emissions/kg to produce the protein source) + (GHG emissions/kg to transport the protein sources))

The USDA says that my daily protein intake should be around 80 g (or 0.080 kg), but it doesn't tell me what particular source of protein to favor, or how to apportion those 80 g among different potential sources. And depending on the source, the balance sheet may come out quite differently.

For example, the GHG emissions involved in producing 1 kg of beef, pork, chicken, or (farmed) fish may all include GHG emissions related to shipping corn to feedlots. However, cows, pigs, chickens, and fish have different rates of conversion of grain input to flesh on their bones. Also, ruminant animals like cows also put out their own GHG emissions via farting; chickens don't fart as much.

Note that you can't even easily pin down GHG emissions in the production of 1 kg of a particular kind of meat. Corn fed beef will have GHG emissions from transport of the corn to the feedlot, while grass fed beef will presumably be grazing on the grass where they are, eliminating some of the GHG emissions. (Do they ship grass -- or truck cattle that are fed grass considerable distances to the grass?)

For a plant source of protein, there's less farting (by the plants, anyway), but there are still GHG emissions associated with shipping. In addition, to the extent that your plant based protein might be processed (like tofu), it's quite conceivable that some part of the processing might involve GHG emissions.

Really, without specifics about what you're eating, how it was fed or produced, and how much fuel was consumed getting it to you, it's impossible to hold up a kilogram of protein from one source and a kilogram of protein from another and say anything very sensible about which contributes more to global warming. Locally grown whole soybeans probably contribute less GHG emissions than beef from gassy cows fed corn trucked in from 1000 miles away, and then itself put on a truck for hundreds of miles to get to the store. But it's not obvious that soybeans that are shipped 1000 miles to make tofu and then shipped hundreds of miles to get to the store would lead to lower GHG emissions than an equivalent amount of beef protein from locally raised grass fed cows.

Of course, there are other inputs to meat and plant based proteins that are environmentally relevant -- like how much water is required to produce 1 kg of protein of each sort. But if this is included, how can we ignore the other environmentally relevant inputs (and outputs) involved in the production of the Camry and the SUV? In a fair fight between changing what you drive and changing what you eat, you'd have to take this all into account.

None of this is to say that we shouldn't bother thinking about how our choices impact our climate or environment. To me, the take-home message is really how alienated we are from where things come from, what's involved in making them and getting them to us, and what the hidden tolls might be from the choices we make on autopilot.

Milton Friedman said the only thing we need to know about a product is its price. Maybe that's true if we're just interested in buying and selling stuff. But, seeing as how we live here on the planet we're using to make and move around the products, we might be better served by the reminder (from a character in the movie Shallow Grave) that knowing how much money you paid for a product is not the same as knowing what it cost you.

Even if the complicated path our protein sources (and cars) take to come into being make straight comparisons hard, it probably wouldn't hurt us to notice some of the costs that are incurred before we make our purchase.

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I can think of one possible variant that may interfere with the constant rate of GHG emission per litre of fuel - how efficiently the vehicle is running at any given time. Would a car produce more GHG per litre when idling than when running at maximum efficiency on the open road? If so, then someone usually driving their car in the city with more stop/starting might be producing more GHG in their commute than someone in a rural area.

There is also one other obvious problem I can see in comparing a kilo of vegetable food with a kilo of animal food - how much are people likely to be eating? If everyone did change to vegetable food, would the reduction in GHG emissions per kilo be offset by the increased amount they may be eating to meet nutritional needs?

I should point out that I have no real idea about the answers to these questions, they're just ones that occurred to me while reading the post.

"Also, ruminant animals like cows also put out their own GHG emissions via farting; chickens don't fart as much."

How do you know? Have you checked?

To take an extreme, but real, example: I don't drive anything. I walk, bicycle, or (sometimes) ride the bus, train, or local tram. The bus obviously directly contributes to GHGs; both the train and tram indirectly do (via the generation of electricity which powers them (ignoring that here in France a large percentage is nuclear)); and the manufacturing, distribution, maintance, and disposal of the kit (equipment) for all the modes also contributes. Nonetheless, since the predominate modes are walking and cycling, and I buy quality stuff which lasts, I'll simplify and claim the GHG emissions are very close to nil.

In no way, shape, or form are the emissions associated with the food I eat and (this is France!) wine I drink nil, or even close to it.

If I were to start driving (anything), or (perhaps) even use the bus more, my transport-related GHG emissions would increase my orders of magnitude (I suspect). Whilst the amount might remain small, the delta is not. It'd be considerably harder to decrease the emissions (at least by my own actions) associated with food and drink. And it's bloody close to impossible to decrease any further the emissions associated with transport.

Does that make the claim absurd? Not necessarily. Arguably, for an adult in "The First World" (I hate that term!) I--and others like me--are an unusual case.

What I suspect is driving the claim here is the claim that meat (especially beef?) ranching is very "destructive", especially(?) when the cattle are not grass-fed (e.g., corn-fed). I've heard this claim for decades (I can remember hearing it back in the 1980s), and admit I sortof buy into it, but haven't really ever attempted to check/verify.

Christopher - the difference between idling and full-throttle is in the quantity of fuel used per unit time, not in the emissions per unit of fuel burned. Simple chemistry tells you that as long as you are completely burning the fuel, the amount of CO2 produced per unit of fuel must be constant. The only way you could change the emissions per unit of fuel consumed would be to screw up your engine tuning so badly that unburned fuel is coming out the tailpipe.

As for the cows farting argument, I don't buy it as a significant forcing. The important boundary to consider is the one between the active carbon cycle and the long-term (geological) carbon cycle. The carbon in the active carbon cycle is bouncing around so much that it really doesn't make much sense (to me, anyway) to consider it as it flits in and out of the atmosphere. What really matters is how much geologically-sequestered carbon is getting dumped back into the active cycle.

The big problem with considering the active carbon cycle is the question of boundaries and timescales. Depending on your choices, you can prove that growing biomass is a carbon offset, that burning biomass is carbon neutral, or that cows farting is a forcing. Of course, you can't prove all of these things using the same boundaries and timescales. Over the sort of timescales that wood-burning is carbon neutral, so are the cow farts.

The fascinating thing to observe is how unerringly people manage to select boundaries and timescales that support whatever axe they're grinding.

As hinted by one comment, the plant equation needs a term for additional production of edible plants, with all the related terms (irrigation, fertilizer, transportation, harvesting and their related GHG emissions). Also, I have read that cows actually produce most of their emitted methane from the other end - that is, from burps. I have also read that it is possible that different feeds can reduce the amount of methane burping. To calculate the true cost, the terms probably should also include GHG emissions related to manufacture of all the related components.

I think it is clearly much more efficient to eat plants than animals. The Owner Built Homestead by Barbara and Ken Kern on page 265 shows that the value of food from beef compared to soy is 3 to 100. Obviously some animals can be raised on poor land etc. but on average it seems like meat is a poor use of energy.

Thanks for this contribution. It seems to me your closing thoughts are especially important. Becoming more conscious of the effects of our mundane decisions seems likely to improve our quality of life in lots of ways.

Along this line, you and your readers may find value in the Ask Pablo feature at


I am particularly glad you included the contributions from flatulence in your consideration of this matter. I was surprised to learn that methane is a much more effective global-warming contributing gas than carbon dioxide.

The other problem I have with the cow farts hypothesis is that there were some really, really big herds of ruminants in the pre-industrial era. I'm not sure how the peak bison population stacks up against the number of cattle currently farmed, but I have a hard time believing it was that much lower.

I think it may be the case that the gassiness of ruminants (no matter which end is putting out the most gas) depends to some extent on their diet. So possibly a corn diet will lead to more cow methane than a grass diet. (And expert on bovine physiology would be the person to ask on this, though.)

(Do they ship grass -- or truck cattle that are fed grass considerable distances to the grass?)

As a resident of a place where grass-fed cows live: here, at least, the cows graze at higher elevations in the summer, and lower elevations in the winter. (There's maybe 4000 feet of elevation difference between the higher and lower pastures around here, within a 60-mile radius.) Here, at least, local beef (and lamb and goat) is available at the farmer's market, grown and butchered locally. Soybeans aren't grown locally, but corn and beans are, so it's also possible for a vegetarian to get protein from local sources. I'm not sure who irrigates more - cattle ranchers or corn growers. (Pinto beans are dry-land crops, though, so they just use the rain that falls, just as the cattle grazing on BLM land do.)

I imagine that eating locally and seasonally is as important a decision as meat vs vegetarian, though. A lot of vegetables are transported pretty long distances, especially in winter.


i think you are mistaken. The key thing to understand is in the choice between meat and vegetarian is that everyhting else being equal.... So for you to argue that local beef vs. imported vegetables etc. is in itself a false argument. That is not the point. I agree that local is better. I also agree that suv's use more fuel than a corolla.

I've managed to not worry about which will help the environment more by not eating meat and not driving either.

Although, I do eat dairy, and I wonder what the difference between an all veggie diet ad veggie with dairy diet is in terms of energy consumption/environmental effects.

In the 30th June 2007 issue of New Scientist, Ian Roberts (Professor of public health at the London School of Hygiene and Tropical Medicine) has an article Say no to global guzzling which is related to this posting. (My apologies for not providing any hyperlinks: I'm editing this without a connection to the Internet.) Any errors and significant omissions in the following summary are probably mine!

He postulates there is a surprising "driver of climate change: the global obesity epidemic. We tend to think of obesity as a public-health problem, but many of its causes overlap with those of global warming." He claims that, globally, food production accounts for "over 20[% of GHG] emissions--more than transport or industry.... Fats and refined sugars, which tend to dominate the diets of obese people, are particularly carbon intensive. Greater food consumption produces more organic waste, which produces methane as it decomposes."

Prof. Roberts points out an obese person is more likely to drive than walk, that that tendency increases over time (and with greater obesity), and with increasing or sustained obesity there are a number of medical and mental problems, including low self-esteem. Low self-esteem may lead to greater drinking and eating, further increasing the person's carbon footprint. (So does, he claims, the additional medical care.)

An obese person, he claims, will resort to the air conditioning sooner during warm/hot spells. Since such spells are becoming more and more frequent--this is global warming after all--the air conditioning runs for longer and longer, consuming more energy, or, more to the point, further increasing the person's carbon footprint due to the larger demand for energy to power all that air conditioning. (He does not mention food refrigeration, but a similar point would seem to apply; namely, the On, energy-consuming duty-cycle is probably increasing, as is the amount of food which must be kept chilled.)

And then, on top of all that, groups such as the American Obesity Association are trying to remove the social stigma associated with obesity. He claims the stigma is the one of the few brakes on the trend. Removing the stigma suggests ever larger/growing carbon footprints, especially, perhaps, if "energy-guzzling escalators, moving sidewalks, [etc.]" become more common.

Finally, there's two great quotes in the article. The first is attributed to physicist Albert Bartlett: "Modern agriculture is the use of land to turn petroleum into food." The second is from Prof. Roberts himself: "We live in an environment that serves primarily the financial interests of the corporations that sell food, cars, and petroleum."

Do I accept his analysis? Well, it's an interesting point. The GHG emissions of industry and transport certainly should be reduced, and simply doing that clearly does affect modern food production. And the effects of obesity--or should that of a material wealth?--do seem to run counter to such reduction. But how much of a forcing factor is obesity? The obesity problem is, I believe, generally confined to materially wealthy countries, who are also the greatest GHG emitters. And would still be the greatest emitters, I suspect, even if only a small percentage of their populations were obese. Whilst obesity is perhaps a contributing factor, I wonder if it is a proxy rather than a cause?

A counter-example to Prof. Roberts' observations might be the mainland China, which I think now ranks as the largest emitter of GHGs, having overtaken the USA recently. However, that new status is, I presume, a case of increasing wealth, poor standards, lax enforcement (including corruption), and a much larger population; i.e., on a per-capita basis, the USA is still a vastly larger GHG emitter. So it may not actually be a counter-example, but evidence for the forcing effects of material wealth, and, perhaps, obesity?

I'm way behind in my reading, so I've no idea what, if any, responses/replies (i.e., letters or corrections) about the article New Scientist may have published in the intervening two months.

Creepy thing to read in a country blighted by an oil-man president, obese oil-man vice president, as strategized by obese Karl Rove. But didn't Henry Ford approve of a certain German dictator?

Prof. Ian Roberts: "We live in an environment that serves primarily the financial interests of the corporations that sell food, cars, and petroleum."

That triumverate reminds me of the Warren Zevon line:

"Send lawyers, guns, and money."

I went home with the waitress
The way I always do
How was I to know
She was with the Russians, too

I was gambling in Havana
I took a little risk
Send lawyers, guns and money
Dad, get me out of this, ha

I'm the innocent bystander
But somehow I got stuck
Between a rock and a hard place
And I'm down on my luck
Yes, I'm down on my luck
Well, I'm down on my luck

Now I'm hiding in Honduras
I'm a desperate man
Send lawyers, guns and money
The shit has hit the fan

Send lawyers, guns and money {4X}

It is politically incorrect to refer to obesity as a moral issue. It may be related to a viral infection. And, I ask in a politically incorrect way, how can an obese person observe his or her carbon footprint if their belly blocks a line of sight to their feet?

As MarkP alluded, fertilizer is a serious part of the equation for plants. Michael Pollen does a very good job in his book "The Omnivore's Dilemma" of showing just how much fossil fuel is used in the fertilization of plants, in his example corn. This is increased far more by the processing into corn syrup that usually follows (almost all corn is consumed as syrup).

I'm a vegetarian and think eating meat is bad (for ethical reasons), but I think sloppy science and facile comparisons in "journamalism" are bad too.

As hinted by one comment, the plant equation needs a term for additional production of edible plants, with all the related terms (irrigation, fertilizer, transportation, harvesting and their related GHG emissions). Also, I have read that cows actually produce most of their emitted methane from the other end - that is, from burps. I have also read that it is possible that different feeds can reduce the amount of methane burping. To calculate the true cost, the terms probably should also include GHG emissions related to manufacture of all the related components.

And it's also important, which plants you eat. Rice fields (O.k. that's more carbohydrates than protein) also fart, burp or whatever one would call it in this case.

I'd note that most meat consumed in the U.S. is raised on agricultural products, not free range or grass fed. This means that if plants are bad for the environment then meat is necessarily worse.


good point, I misspoke. I should have said either "most sweetcorn grown" or "most corn grown for human consumption...without passing through the chemical factory known as 'cow' or 'chicken' first" ;)

If you want a good rundown of what I'm talking about, read "The Omnivore's Dilemma", starting on page 19.