Green math: Measuring environmental impact

Shoppers soon will be able to buy everything from meat to moccasins based on a number that purports to tell them the product's environmental impact.

Manufacturers and retailers across the globe are working to measure their products' carbon footprints for a variety of reasons, including marketing, concern for the planet, to reduce emissions and to avoid being caught flat-footed by any coming climate-change regulation. All of the efforts have one thing in common: The results have the appearance of precision.

But all the decimal points in the world can't hide the fact that measuring carbon footprints is far from an exact science. 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 drafting 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 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. And each study varies in how it counts these 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 it 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 growing 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."

A study by the U.S. dairy industry came up with a footprint that is about 15 percent lower.

What may account for some of the difference is the dizzying number of 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 -- 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 vs. the cow's carcass. If, say, a dairy farm gets 90 percent of its revenue from selling milk and 10 percent from selling the cow, then 90 percent of its emissions would be ascribed to the milk and 10 percent 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 United Kingdom. The methodology is part of a broader set of guidelines published last year by the Carbon Trust, the British government and a standard-setting organization called the British Standards Institute.

The U.S. dairy industry is updating its own study, and the newest 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.

Criticizing the U.K. approach, Thoma notes that a footprint based on milk's economic value could rise or fall just because the price of milk changes.