Please read the updated and complete version of the cheeseburger footprint story, found here.

This is, I hope, the last post on cheeseburgers for a long while.

I need to address a couple of issues here about the numbers used in the Cheeseburger Footprint articles. The first will clarify the one moderately controversial assumption. The second is to acknowledge and rectify a math mistake.

But before I begin, I want to say this: the Cheeseburger Footprint is about much more than raw numbers. It's about how we live our lives, and the recognition that every action we take, even the most prosaic, can have unexpectedly profound consequences. The article was meant to poke us in our collective ribs, waking us up to the effects of our choices.

It just happens to turn out that the effects are even more dramatic than I first thought.

Three per Week?!?
Of all of the numbers tossed around in the Cheeseburger Footprint posts and audio bits, the one that draws the most attention is the assertion that the average American eats three burgers every week. That's an extraordinary number, if you think about it, and since it's an average, for every one of us that rarely or never eats a burger, someone must be gobbling them down like candy. Where does that number come from, and (perhaps more importantly) is it accurate?

My reference for that figure was an article in the Economist, but other sources include Google Answers (ascribing it to a Dr. Kathleen Aicardi) and The Encyclopedia of American Food and Drink (as cited here). But you'll notice in all of these, the 3/week number is offered as an assertion; I couldn't dig up a direct research or study reference for the number.

It turns out that the claims for burger consumption in the US are all over the map, and 3/week -- or about 40-45 billion burgers -- seems to be at the high end. Eric Schlosser, in Fast Food Nation, is at the other end of the spectrum, claiming 13 billion in the US annually. It's not hard to find groups asserting 15 billion, 25 billion, 30 billion, etc. -- there really doesn't seem to be a hard number for this.

100,000 SUVs?
Now I have to admit a math error. A reporter asked about the 100,000 SUVs number, whether I mean the annual output or something different, and in going back to double-check my notes I found something that made my heart sink. In my calculations, I left out a step, and the 100,000 SUVs parallel is wrong by a large margin.

The real number -- if we stick with the 3/week assumption -- is closer to 13 million SUVs. Even if we cut that by two-thirds to go with the Fast Food Nation number, the American cheeseburger habit still puts out every year the equivalent to the annual greenhouse gases of over 4 million SUVs

Here's the math, so that my calculations are clear. The kg/burger value is from the original Cheeseburger Footprint post, and is the low end of the range. A Hummer H3 SUV emits 11.1 tons (imp.) of CO2 over a year; this converts to about 10.1 tonnes, so we'll call it 10 to make the math easy:

300,000,000	citizens
* 150 		burgers/year (~Economist, Dr. Alcardi)
* 2.85 		kilograms of CO2-equivalent per burger
/ 1000		kilograms per tonne (metric ton)
= 128,250,000	annual tonnes of CO2-equivalent for all US burgers
/10		tonnes of CO2-equivalent per SUV
=12.8 million SUVs 
 ----or----
300,000,000	citizens
* 50		burgers/year (~Fast Food Nation)
* 2.85		kilograms of CO2-equivalent per burger
/ 1000		kilograms per tonne
= 42,750,000	annual tonnes of CO2-equivalent for all US burgers
/10		tonnes of CO2-equivalent per SUV
=4.3 million SUVs

In my original post, the calculations I used missed the step of multiplying by the number of burgers per year -- the final results shown at the time actually amounted to the output if everyone only ate one cheeseburger a year. So I was off by anywhere from a factor of 40 to a factor of over 100. Whoops -- my apologies. Things are much worse than we thought.

But does that really matter? These are all pretty fuzzy numbers to begin with, and even these revised values are themselves likely to be off by a significant margin. And how many people would be thinking, "4 million? That's different, then. A hundred thousand SUVs, that's nothing." These are big numbers either way, and both 100K and 12M SUVs offer stark comparisons for our cheeseburger footprint.

Ultimately, this is about awareness, not math. Too few of us recognize the climate and environmental impact of our day-to-day activities, and if the cheeseburger post woke some people up -- as it seems to have done -- then it's accomplished its task, even if the real numbers are far worse than initially thought.

But a mistake is a mistake, and I apologize for mine.