In this issue of Fun With Statistics we’ll tackle that new consumer term du jour, the "carbon footprint." Most of us know by now that the carbon footprint is the amount of carbon dioxide and other greenhouse gases that are created when a product is created, shipped, stored, and eventually used. But is it a reliable gauge? Yes and no.
There are three primary issues, the first being how the number is calculated. Depending on who is doing the calculation and their perspective on the product, the number may not include the entire supply chain.
Several studies of milk’s carbon footprint are under way in the U.S. Each has come up with a different number, largely because each is counting things differently.
A recent study by National Dairy Holdings, a Dallas-based dairy, found that the carbon footprint of a gallon of its milk in a plastic jug is either 6.19 pounds or 7.59 pounds. The difference rests in what kind of cases the jugs are placed in during transport from the milk-processing plant to the distribution center. Plastic cases, because they take more energy to produce, yield more carbon-dioxide emissions than do cardboard ones.
But National Dairy Holdings’ study doesn’t count all the emissions created by a gallon of milk. It includes those from the cows themselves (more than half of the total), from the processing of the milk and from the transport of the milk to a distribution center. It doesn’t count the emissions earlier in the process: growing the cows’ feed. Nor does it count the emissions later in the process: transporting the milk from the distribution center to the store and refrigerating it there.
But the second, and perhaps more annoying problem, is what to do after the analysis is complete and a number is agreed upon.
When New Belgium Brewing Co. set out last year to compute the carbon footprint of a six-pack of its Fat Tire Amber Ale, it figured it would find transportation was the biggest problem. That’s the emission source New Belgium thinks about most often. The microbrewer, based in Fort Collins, Colo., has been expanding into more states, necessitating more trucking of its beer.
When the numbers came in this summer, they showed that a six-pack’s carbon footprint was about seven pounds. The real surprise was where the bulk of that number came from: the refrigeration of the beer at stores. Transportation came in fourth, behind manufacturing the glass bottles and producing the barley and malt. "It seems that in every [carbon-footprint study] I’ve come across, people are surprised," says Jennifer Orgolini, New Belgium’s sustainability director.
A third significant issue is the difference between like products, or how the term "product" is used. If the "product" is "car" then there’s a problem similar to what Toyota discovered.
Sometimes, the differences between models can be substantial. For one overview of how cars stack up, consider a new computer model paid for by Toyota Motor Corp. that computes the lifetime carbon footprints of about 400 auto models from multiple manufacturers.
To narrow things down, consider a handful of Toyota’s own models. The Prius, a hybrid gasoline-and-electric car that averages 42 miles per gallon, has a lifetime carbon footprint of 44 metric tons, according to the updated computer model done for Toyota by Kreider & Associates, a consultant based in Boulder, Colo. The Corolla, a small sedan with a conventional gasoline engine rated at 29 miles per gallon, has a footprint of 64 tons. The Camry, a larger car rated at 23 miles per gallon, has a footprint of 95 tons. And the 4Runner, an SUV rated at 16 miles per gallon, has a footprint of 118 tons.
That’s probably to be expected, and definitely something to be aware of. The carbon footprint is emerging as an important metric, but it is critical that we be aware of some of the nuances of the calculation.
And that’s today’s Fun With Statistics.