Saturday, January 20, 2007

The Carbon Footprint of a Cheeseburger

While there are carbon calculators that help you to determine how much carbon emissions are created by your consumption, they just look at all of the emissions that you use directly. They don't help you to understand how much carbon emissions are embedded in the goods and services that you purchase. If you look at electricity usage, only 1/3 is purchased directly by consumers. If you assume that ratio hold for the rest of energy use and carbon emissions in general, then 2/3 of the average American's emissions are embedded in the goods and services that they purchase. And yet there are no easily available estimates of the carbon footprints of these goods.

So, I was glad to see the analysis over at Open The Future on the Carbon Footprint of a Cheeseburger based on the Energy Use in the Food Sector report.

I like the idea of the label that he proposes and this goes along with my ongoing look into social and environmental labels. Hopefully all products will have this soon, maybe using an augmented barcode. I still think my idea of a acres and gallons label would capture most of same information as shown here, but be much simpler for the end user to understand.

Footprint of Cheeseburger and Context
The carbon footprint of a Big Mac cheeseburger is 4 kg of CO2 equivalent gases (well really 3.4 to 4.82 kg but I will call it 4, see calculations below). Of that .5 kg is from diesel emissions, .9 kg is from electricity emissions and 2.6 kg is from the cattle eructations and flatulence methane emissions.

To put this in to context, a gallon of gasoline emits 8.8 kg of CO2, so a cheeseburger has the same emissions as about 1/2 a gallon of gasoline.

On a yearly basis, the average American eats 150 burgers leading to 600 kg of emissions. This makes up a little less than 3% of the average American's 24.5 tons (or 22 metric tonnes) of emissions. If you were to eat nothing but cheeseburgers, like Don Gorske who eats 2 Big Macs a day, you would consume 730 a year. This leads to emissions of 2,920 kg or 13% of an average American's emissions.

The average American uses around 500 gallons of gasoline a year (based on this data of 140 billion gallons of gasoline and 40 billion gallons of diesel) emitting 4,400 kg of CO2. The 600 kg of burger emissions are equivalent to about a month and a half of automobile emissions.

Calories to Energy Ratio
6 times (well 3.2-8.85) as much (primarily fossil fuel) energy goes into a burger as calories in the burger. A Big Mac has 540 calories or 2.26 MJ (1 MJ = 239 kcal). Compared to the 7.3-20 MJ of energy to produce the burger that gives us a range of 3.2 to 8.85.

This is actually lower than I expected, as it has been estimated that for every calorie of food produced in the US, 7 additional calories of energy are expended and a commenter here says that Pimentel estimates it takes 28 calories of fossil fuel to grow a calorie's worth of beef.

Ways to Reduce the Footprint
The most obvious one is to eat fewer cheeseburgers. It would be useful to know the footprints of other foods, so you could understand the savings for switching. Unfortunately I don't have such data, but I will be on the lookout for it.

Since most of the emissions come from methane, the biggest gains would come from "reduced methane" beef. This could come from changing the feed to one that causes less emissions, or by capturing the methane that is emitted (I am still pushing for a tube that is inserted into one of the cow's stomachs that draws off the methane gas into a balloon that can then be captured when the cow is milked), or by genetically engineering low methane bacteria for use in the cow's stomachs.

Greater efficiency of energy would also reduce the footprint. Having a higher percentage of electricity come from non emitting sources like wind or solar energy would also help.

Potential Data Issues
These numbers are only as good as the underlying data and assumptions.

This research was done in Sweden on a Big Mac using ingredients from Europe. It is possible that the Swedish Big Mac differs from the American one. I know nothing about Swedish Big Macs other than they are the second most expensive Big Macs in the world. It is also likely that the energy intensity of farming in the US is higher than Europe which would lead to a bigger footprint.

The number of burgers the average American eats a week is debatable, as is how similar the burgers they eat are to the Big Mac used in the analysis.

Calculations of Footprint
There are .074 kg of CO2 equivalent emissions per MJ of diesel fuel energy (10.1 kg emissions per gallon / 36.4 MJ per liter * 3.785 liters per gallon ).

Each burger uses 4.7 to 10.8 MJ of diesel fuel leading to 350g to 800g of emissions. This differs from the 90 to 217 grams estimated in the Open the Future's estimate, I don't know why.

On average there are .169 kg of emissions per MJ of American electricity (1.34 lbs per kWh / 2.2 kg per lbs / 3.6 MJ per kWh).

At 2.6 to 8.4 MJ of electricity per burger this comes to 440g to 1.42kg.

In Open the Future's analysis, there are these two values for electricity emissions: Natural Gas: 37 to 119 grams and Coal: 65 to 209 grams. I chose to use the average electricity of America, which bases emissions on the percentage of energy generated from various sources (natural gas, coal, nuclear, hydro and alternative energy) at the national level. But, I was still not able to replicate his numbers. Coal emits .26kg/MJ (2.1 pounds per kWh / 2.2 kg per lbs / 3.6 MJ per kWh) and Natural Gas .16kg/MJ (1.3 pounds per kWh / 2.2 kg per lbs / 3.6 MJ per kWh). For the 2.5 to 8.4 MJ of electricity this would lead to ranges of 676 g to 2.2 kg for coal and 416 g to 1.34 kg for natural gas. Once again, I don't understand the discrepancy.

Combining we get 250g+440g= 790g to 800g+1.42kg= 2.22 kg. Adding in the 2.6 kg of methane emissions gets to 3.4 to 4.82 kg.

3 comments:

Jamais Cascio said...

I double-checked my calculations, and they looked right -- but then I noticed that you and I had wildly different values for the amount of carbon in the various fuels. Accounting for this difference pretty much accounts for the discrepancy in results.

For diese, for example, I used 2.77kg, pulled from this source at the Oak Ridge National Laboratory. You used 10.1kg, pulled from a source at the EPA.

The difference, now that I'm checking the numbers, is that my value is for carbon and yours is for carbon dioxide, which (of course) has the added weight of the two oxygen atoms.

Yours is the correct value.

Fat Knowledge said...

Thanks for double checking the numbers, Jamais.

Annelisa said...

Hello! My name is Annelisa and I am a student at Franklin High School. I am creating a documentary on the carbon footprints of a cheeseburger and I see your blog is a lot of help. If possible, please email me at amarie_023@yahoo.com because I would like to interview you via email. Thanks!

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