It's something of a green goal: Reduce your carbon footprint. And if you can do so as you trek along the grocery aisles, all the better on this 42nd Earth Day.
That was the thinking behind splashy announcements that started coming out a couple of years ago from a handful of corporations such as PepsiCo and smaller firms saying they had carbon-footprinted cartons of orange juice, six-packs of beer and other goods.
Tesco, the British supermarket giant, even began slapping items with big black footprints stating the number of grams of carbon dioxide supposedly emitted while churning out that swig of juice or mouthful of chips.
A low rating is intended to indicate a product you can eat without guilt, because its production generated fewer of the greenhouse gases that are responsible for climate change.
Two years on, handicapped by uncertainties about how to calculate those ratings - or whether it's even possible - carbon-footprinting schemes struggle to be recognized as the standard stamps of eco-consciousness that the FairTrade, Energy Star and LEED systems have become.
Nonetheless, several countries have labeling schemes in the works, and U.S. proponents of labeling argue that even flawed systems are better than none. More and more U.S. consumers crave such information, proponents say, and in the absence of government action to reduce carbon emissions, pushing consumers toward lower-carbon choices might be the best climate change-fighting tool available.
"We only need fairly small changes in consumer behavior to make a dent in the growth curve of carbon emissions," said Michael Vandenbergh of Vanderbilt University, who advocates carbon labeling of products.
So far, though, few people agree on how to measure the carbon footprint of, say, a kilogram (2.2 pounds) of beef. "All those companies making announcements in 2009 bit off more than they could chew," said Rita Skank, who directs a rapidly growing professional organization of number-crunchers who calculate ecological impacts of goods.
Green-leaning consumers - hoping to estimate the carbon impacts of eating more vegetables and less meat, as well as activities such as choosing to ride a bicycle instead of driving - are left to supplement what little information U.S. companies offer with online carbon-footprint calculators.
A recent study of carbon emissions generated by Brazilian beef cattle illustrates the difficulties in calculating carbon footprints. Estimated emissions vary greatly depending on how many environmental ripples are included in the equations.
And deciding that is a matter of choice, not math.
Consider: Brazilian beef raised on long-established pastures generates about 28 kilograms of carbon dioxide (or its equivalent in other greenhouse gases, such as belched methane) for each kilogram of beef under wrap at the grocery store.
But the study's international team of authors say that this figure fails to account for a dramatic indirect cost of Brazilian beef: the deforestation of the lower Amazon region to create grazing land for cattle.
Growing demand for beef both within Brazil and for export has skyrocketed, driving large-scale deforestation. Adding the carbon dumped into the atmosphere by burning or otherwise dispatching those trees pushes the beef figure dramatically skyward, to 726 kilograms of carbon dioxide for each kilogram of edible meat raised on newly deforested land.
When buying beef, though, there's no easy way to tell whether the animal lived on old pasture land or new. So the study authors calculated an average for all Brazilian beef after accounting for deforestation. That figure is 44 kilograms of carbon dioxide per kilogram of beef.
Three different calculations, three different answers, all valid.
"You can calculate correctly and arrive at different figures," said study author Sverker Molander, a professor of environmental studies at Chalmers University of Technology in Sweden.
Lack of agreement about what to include in those calculations, said Skank and other experts, is slowing the development of international standards.
The uncertainties inherent in calculating carbon emissions means consumers will "never be able to tell the difference between Coke and Pepsi," said Christopher Jones of the University of California, Berkeley.
Despite these complexities, a few companies have worked through the calculations, turning up surprises about where in the life cycle of a product most of the carbon comes from.
Take New Belgium Brewing Co. of Fort Collins, Colo., which carbon-footprinted its Fat Tire Amber Ale.
First, the company figured the amount of carbon dioxide emitted to make the electricity consumed by its brewery. It added on employee commuting and jet travel. Widening the circle, it included impacts from water pumping, wheat and hops growing, glass manufacturing and paperboard making. Emissions of trucks hauling those six-packs were factored in, as was the electricity used by retail refrigerators keeping them cold for a week. Finally, the company summed the energy costs of recycling the bottles and trashing the packaging.
It arrived at a figure of 3.2 kilograms of carbon dioxide per six-pack. The surprise: Just 4 percent of that came from the company's own operations. The rest was squirreled away in the chains of supply and distribution.
The company then had another decision: what to do with the data. Stamping sixers with "Only 3.2 kilograms of carbon dioxide!" would "not really educate the consumer," said Katie Wallace, a sustainability specialist at New Belgium. It's a meaningless number, floating above the heads of beer drinkers who have nothing to anchor it on or compare it with. So the company chose not to label its packages.
Still, the exercise offered insights into where the company could save energy and pressure suppliers to do the same. It also provided an "indirect" marketing boost, Wallace said, by positioning the company as transparent and forward-thinking.