Hate Calculus? Try Counting Cow Carbon

Companies Are Measuring the Environmental Impact of Their Products, but the Math is Fraught With Complexity and Imprecision
By JEFFREY BALL
Shoppers soon will be able to buy everything from meat to moccasins based on a number that purports to tell them the products' environmental impact.
Manufacturers and retailers across the globe are working to measure their products' carbon footprints for a variety of reasons, and all of the efforts have one thing in common: The results have the appearance of precision.
See a breakdown of the carbon footprint for a gallon of U.S. milk
But all the decimal points in the world can't hide the fact that measuring carbon footprints is inexact. It is clouded by varying methodologies and definitions -- not to mention guesses.
"There are no clear rules for the time being," says Klaus Radunsky, who co-chairs a group within the Geneva-based International Organization for Standardization that is producing a guideline for measuring products' environmental impacts. "It depends very much on how you do the calculations."
Few products demonstrate the messiness of this effort more than a simple carton of milk. Several studies in various countries have already sought to tally the impact of milk from its production on a farm to the disposal of its carton. In between, the studies try to measure such intricacies as the energy used to make the fertilizer to grow feed for the cows, to fuel trucks delivering the milk, and to power refrigerators cooling it in kitchens.
It isn't surprising that each of these studies sizes milk's footprint differently, in large part because each varies in the way it counts one or more of those factors.
Milk is among the first products that Wal-Mart Stores Inc. is trying to measure as part of a broad effort by the retailer to assess the environmental impact of the products its sells. It intends to begin labeling certain products with a "sustainability" score -- a single number that would take into account not only carbon emissions, but also water use and waste production. That is doubly complicated because it involves weighing the relative importance of different kinds of environmental impact. Which is worse: that a tomato uses lots of water or lots of pesticide?
Wal-Mart is working with academics and environmentalists to decide both how to tally that score and how to display it. It might be a number from 1 to 10, and it might be a color in a range of hues, says Matt Kistler, the retailer's senior vice president of sustainability. The challenge is to come up with something that is understandable and accurate. "Can we get there overnight? No, because a lot of the information doesn't exist yet," he says. "But I think we can get there."
See how the New York City Department of Education is making simple changes to make big cuts in its energy use and carbon footprint.
Among the reasons driving the measurement efforts by manufacturers and retailers are concern for the planet, marketing, to reduce emissions and, in some cases, to avoid being caught flat-footed by any coming climate-change regulation.
Tesco PLC, the big U.K. retailer, began last month labeling milk sold under its store brand. Its studies concluded that a pint of whole milk generates an amount of greenhouse gas equivalent to about two pounds of carbon dioxide. Tesco prints the metric equivalent of that number, 900 grams, on its whole-milk labels.
Another study by the U.S. dairy industry came up with a preliminary footprint that is about 15% lower, when expressed in terms of a comparably sized container of milk.
What may account for some of the difference is another set of dizzying variables in the carbon calculation. Some farms have more energy-efficient machinery. Some cows eat less corn, which typically is grown with petroleum-based fertilizers. And some kinds of feed cause cows to burp more methane, a potent source of carbon. That bovine belching is widely agreed to be the biggest source of carbon emissions in milk production.
But some parts of the equation are subjective. Cows produce multiple sellable goods: milk while they are alive, and, once they are slaughtered, products including beef, leather and bones. So how much of the emissions from the dairy farm should be blamed on the milk, and how much on the making of the steak and shoes?
Tesco attempts to resolve that question by splitting the emissions according to the relative economic value of the milk versus the cow's carcass. If, say, a dairy farm got 90% of its revenue from selling milk and 10% from selling the cow, then 90% of its emissions would be ascribed to the milk and 10% to the other products.
That is the route recommended as most practical by the Carbon Trust, a London-based company established by the British government to help curb carbon emissions in the U.K. The methodology is part of a broader set of carbon-measuring guidelines published last year by the Carbon Trust, the U.K. government and a standard-setting organization called the British Standards Institute.
Euan Murray, who oversees carbon-footprint studies at the Carbon Trust, which Tesco hired to conduct its milk study, says allocating emissions based on economic value makes intuitive sense to most people. But, he adds, "there's no absolutely right way of doing it."
The U.S. dairy industry is updating its own study, and the new version uses a more-complicated calculation preferred by the International Organization for Standardization. It seeks essentially to look inside the cow, separating the portion of the animal's biological functions that go to producing milk from the portion that go to producing the cow itself. Those functions include the cow's eating, burping, flatulence and waste.
"It becomes extremely difficult to do," says Greg Thoma, a chemical-engineering professor at the University of Arkansas. He is one of a team of professors at the university contracted by the Innovation Center for U.S. Dairy, a dairy-industry group, to produce the carbon-footprint study.
The Arkansas researchers see their method as more accurate than the approach used in the U.K. A footprint based on milk's economic value, Mr. Thoma notes, could rise or fall just because the market price of milk changes.
The Carbon Trust recommends updating footprint studies if conditions change significantly. The Trust's Mr. Murray says it makes sense for products that contribute more to a farm's economic activity to be tagged with more of the carbon responsibility.
The U.S. method is open to plenty of uncertainties, too. It requires knowing, for instance, what mix of feed a cow ate on the dairy farm, because each kind of feed -- corn, say, or almond hulls -- brings with it a different carbon footprint. It also requires knowing the weight of each cow when it left the farm to be slaughtered. In reality, researchers don't know those figures for every cow. "So I have to make a guess," Mr. Thoma says. "It's not going to be exact."

Write to Jeffrey Ball at jeffrey.ball@wsj.com