It’s not so easy. WSJ’s Jeffrey Ball writes:

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.
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.

The rest here.