Carbon Dioxide and the Farmer’s Market

It says here on Al Gore’s website that:

Number of gallons of gasoline is multiplied by the emissions factor of 19.6 to convert to pounds CO2, or 22.4 for diesel vehicles. These emissions factors come from the Energy Information Administration, Fuel and Energy Source Codes and Emissions Coefficients.

According to Terrapass, the fuel efficiency of my 2000 Jetta with a manual transmission and standard gasoline is 24 miles per gallon in the city (which is where I usually drive it).

This means that for every one mile I drive I use 0.0416 gallons of gasoline and emit 0.81536 pounds of C02. Assuming my car is out of tune, and has aged somewhat after 40,000 km of driving, and so has poorer gas mileage than that, a figure of 1 pound per mile driven is probably a reasonable ballpark.

The round-trip from our house to the Charlottetown Farmer’s Market is 3.2 miles, so if we drive we emit roughly 3.2 pounds of C02. Over the course of the year, assuming we drive every week, that adds up to 166 pounds of C02 (or 0.08 tons).

The carbon calculator tells me that my personal carbon emissions are 8.1 tons per year (the big contributors to that are flights to Europe and New England and oil heat in our house).

While we’re heavily abstracted from real numbers by this point, it seems like that driving to the Farmer’s Market every week would contribute about 1% of my annual C02 emissions.

It’s interesting to note that when we moved from rural Kingston to urban Charlottetown seven years ago, I cut out a 6 day a week commute of 20 miles return, resulting in a reduction of roughly 6,240 pounds (or 3 tons) of C02 emissions per year.

Please point out the errors in my math and logic if you find any.


Kevin's picture
Kevin on April 2, 2007 - 17:22 Permalink

Something about “tons of carbon” has always bothered me:

From my high school chemistry: Carbon weighs about 12 grams per mole (6.022*10^23 atoms). Average gasoline has 8 carbons per molecule and a mole of molecules per litre. Therefore each litre of gasoline has eight moles of carbon or 96 grams of carbon. Since people like to speak in tons (I’ll lave out the math steps, but) you must burn just under 9500 litres of gas to produce a ton of carbo
Now, unless my math/science is WAY off it would take me nearly eight years of daily driving to produce a ton of carbon (because I burn a little less than 100 litres of fuel per month). Conventional wisdom would indicate I am grossly wrong in my calculation.

Another way to look at this is: **Your carbon cannot outweigh your fuel.** Carbon, in fact, must be less than the weight of the fuel since none is added from the atmosphere. How much does a tank of gas weigh? A ton of gasoline is 231 gallons. How far can you drive on that? At 30 mpg you’ll get over six thousand miles. With gasoline I’d produce a ton of carbon (max) per year, and with diesel it’s less.

Anyway, again this disagrees with conventional wisdom.

Another way of looking at this is to look at the ratio of carbon to hydrogen (there’s only those two things in gas), multiply each by their atomic weight, assign that much of the weight of gas to each element and work it out. Again, it will not agree with conventional wisdom.

I just don’t know how they come up with our tonnage numbers.

Kevin's picture
Kevin on April 2, 2007 - 17:29 Permalink

It turns out there’s about 320 gallons per ton of gasoline — that means I wouldn’t produce a ton of carbon per year under normal daily driving. Anyway, either conventional wisdom is wrong or I am, but how can the carbon weigh more than the fuel? Impossible!

oliver's picture
oliver on April 2, 2007 - 17:49 Permalink

You didn’t reap savings right away in moving, because you spent carbon to move, plus it might do well to compare not just your driving but your utility usage. I’m not sure what else is liable to differ. If you lived off locally produced, processed and packaged food in one place almost exclusively and not at all in the other, or did a lot of canning and root cellar storage in one place but only bought and refrigerated in the other, perhaps those would be other big ticket carbon items? Nationally, those lifestyle choices certainly matter, but for an individual, I don’t know how high they stack up versus the CO2 you put out with your car or by air conditioning your poorly insulated ranch-style home.

Peter Rukavina's picture
Peter Rukavina on April 2, 2007 - 18:40 Permalink

Kevin — 100 litres of fuel per month is 26.4 gallons (US) of fuel per month, is 517.7 pounds of C02 emitted per month (26.4 x 19.6 = 517.7).

517.7 pounds of C02 is 0.25 tons (US).

Which means your driving, in theory, is responsible for 3 tons per year of C02.

Derek Martin's picture
Derek Martin on April 2, 2007 - 19:26 Permalink

Peter, do those figures include CO2 from the production and shipping of the gas too?

Peter Rukavina's picture
Peter Rukavina on April 2, 2007 - 20:17 Permalink

I can only assume the figures include only the actual CO2 from the burning of the fuel itself, not the CO2 burned in its production, as this would vary widely given the source of the original oil, the place it was refined, how it was shipped, etc.

Peter Rukavina's picture
Peter Rukavina on April 2, 2007 - 20:21 Permalink

Commute Solutions, a California group, has an excellent brochure on the “true cost” of driving. On the issue of “External Resource Consumption Costs” they say:

Automobile use is one of the largest consumers of natural resources
Ann's picture
Ann on April 2, 2007 - 21:52 Permalink

Commute solutions makes a very good point. When we figure the “cost* of driving, I wonder if we should figure the cost of the war in Iraq.

Kevin's picture
Kevin on April 2, 2007 - 23:32 Permalink

Pete: “100 litres of fuel per month is 26.4 gallons (US) of fuel per month, is 517.7 pounds of C02 emitted per month (26.4 x 19.6 = 517.7).”

How? How can the carbon content of a gallon of gas actually weigh more than the gas? I mean really; if I’m wrong about this I would really like to know why because carbon emission figures have been irrational (to me) for years now — they just don’t make sense because:

If gas is 6.2** pounds / gallon (which is the best figure I can find), then the carbon in 26.4 gallons of gas cannot exceed 164 pounds; it’s total weight (at STP). **(avg, gasoline varies *a lot* with temperature)

I think I know the reference for the conversion number you’re using but I just don’t believe it, I think when experts issue a number like this they are obliged to explain it *in detail* and I haven’t seen that as yet.

Peter Rukavina's picture
Peter Rukavina on April 3, 2007 - 01:13 Permalink

Kevin, you will find the answer here. To quote a bit:

It seems impossible that a gallon of gasoline, which weighs about 6.3 pounds, could produce 20 pounds of carbon dioxide (CO2) when burned. However, most of the weight of the CO2 doesn’t come from the gasoline itself, but the oxygen in the air.
Andrew MacPherson's picture
Andrew MacPherson on April 3, 2007 - 14:46 Permalink

A factor on the plus side is that at the Farmer’s Market you have purchased locally produced food which would cost less carbon than food shipped in to Sobeys or Superstore from California, Mexico, New Zealand, etc.

Kevin's picture
Kevin on April 3, 2007 - 19:12 Permalink

Thanks Pete. There’s the answer -=- and I was looking for it.

Anyway, I guess I should learn to not take anyone literally — not even scientists. When I hear a scientist use a phrase like, “carbon emission”, or “tons of carbon” I assume they mean C, not CO2. Anyway, I guess it’s legit to measure the carbon dioxide emissions AS LONG AS carbon monoxide does not combine with anything other than oxygen to produce another greenhouse gas. If that were the case then it would be more appropriate to 1) count the carbon only and 2) drastically reduce the units that are considered safe for the environment, or to say that another way: to set appropriate benchmarks based on elemental carbon.

oliver's picture
oliver on April 3, 2007 - 22:13 Permalink

Methane has carbon and it’s a potent greenhouse gas. But “complete” combustion (as well as complete or metabolism) gives you C02, because thermodynamically it’s the most stable compound in which to combine earthly atoms in earthly abundances at earthly temperatures, pressures, etc. So arguably it’s a natural currency, although I wouldn’t be surprised if scientists talk about the mass of carbon alone too, because some atoms will go into methane and other organic molecules important to the atmosphere or the carbon cycle.

Kevin's picture
Kevin on April 5, 2007 - 15:10 Permalink

Experts (and journalists) should choose accuracy over cadence; but alas ‘tis often not the case… Anyway, C or CO2, it matters not as long as it is explicit which term is being used. It’s like hearing a politician or journalist talking about an expenditure in dollars but they leave out the “per”; as in “per year”, “per month”, “in total over the life of the project”, etc… this simple omission — which is quite frequent — makes an entire point meaningless to anyone who really cares about it. Confusion ensues when experts, rather than being accurate, use the term “carbon emissions” (a nod to Barbie no doubt…”carbon dioxide sounds too, like, well, y’know, scientific, right?”).