Are you ready to go on a carbon diet?

In Britain, carbon footprinting – used initially to broadly measure environmental impact across a company's entire operations – is morphing into an eco-labeling tool.

Earlier this year, the British supermarket chain Tesco began labeling some of its 70,000 products to reflect the carbon released in the their production, transport, and consumption. The 3,729 store behemoth, the world's fourth-largest retailer, now has 20 carbon-labeled items on its shelves, core items such as orange juice and laundry detergent.

The intent, said Tesco CEO Sir Terry Leahy, is to educate and empower consumers to make informed decisions about their purchases.

But some observers question whether such labels are providing consumers with information that they neither want nor understand. This past April, Forum for the Future, a green think tank in London, issued a report on carbon labeling, noting the danger in providing information without context to consumers.

"Only a handful of our focus group participants associated carbon emissions [and climate change] with what they buy in the shops," the report concluded. "The majority knew that carbon emissions are linked with cars, airplanes, and factories. They made that connection because they can 'see' the emissions, which makes them easy to interpret as being 'bad for the environment.' However, the link between products and climate change was less intuitive to them."

According to the Carbon Trust, a private company backed by the British government to accelerate development of climate-change technologies, 80 percent of a product's emissions arise further back in the supply chain – before consumer usage.

As British consumers grapple with the new labels, retailers in the United States are taking a limited approach.

Right now, "incremental change" is as far as most US companies want to go, says Joel Makower, a sustainability consultant who is also cofounder and executive editor of Greener World Media Inc., in Oakland, Calif.

Most companies don't understand their full environmental impacts, he says.

Those making efforts to examine their carbon footprints often do so without transparency – essential to generating both customer support and supply-chain innovation.

At Wal-Mart, consumer transparency is largely tied in to its corporate press releases, a growing assortment of eco-labeled products, and in-store awareness campaigns. A more robust effort is the company's "Love, Earth" jewelry, which enables customers to use the Internet to map where the jewelry's gold and silver were mined and manufactured, including information on how the mines manage cyanide and waste dumps.

Since 2005 the world's largest retailer has been attempting to embed sustainability into its corporate culture. It was the first major global retailer to announce large investments in renewable energy. Wal-Mart has given its buyers greater discretion to factor in environmental considerations while innovating with suppliers on carbon emissions. But it's only suggesting – not mandating – that suppliers measure and reduce their carbon footprint on product lines limited to DVDs, toothpaste, soap, milk, beer, vacuum cleaners, and soda.

As for carbon-labeling, Wal-Mart's senior vice president of sustainability, Matt Kistler, says that he doubted existing methodologies and the Wal-Mart customer's ability to relate carbon with consumer merchandise.

"I'm not sure the consumer will ever make a purchase based on the carbon footprint," he says, "especially the mass consumer."

At apparel companies like Gap Inc., sustainability largely focuses on packaging and store construction while the conversation with the public continues to be stymied by the fact that environmentally friendly garments cost far more than most consumers are willing to pay.

"There needs to be increased consumer education regarding clothing and the environment," says Craig Minowa, an environmental scientist at the Organic Consumers Association in Finland, Minn.

Petroleum-based pesticides and fertilizers make cotton one of the most toxic crops on the planet, says Mr. Minowa, "but to date there has been very little of this [consumer education] because many eco-friendly labeled clothes cost 50 to 100 percent more than their conventional counterparts."

In 2004, Gap Inc. launched a clean-water program to raise effluent discharge standards at its denim laundry facilities, but going back any further down the supply chain remains problematic. Gap's Better Cotton Initiative aims to provide a less costly and equally sustainable alternative to organic cotton, but so far the progress has been slow. Organic and alternative fibers remain less than 1 percent of their business, according to industry analysts.

Another option being pushed by clothing retailers centers on labels that try to convey the full impact of their manufacturing processes.

Patagonia, for example, has a new Web-based effort called the Footprint Chronicles. By showing both the "good" and "bad" on the company's interactive website, a consumer can track a Wool 2 Crew sweater as it originates in New Zealand, travels to Malaysia for combing, then to Japan, where it's spun into yarn and knitted, then to California and Nevada for sewing and production. In all, that's 16,200 miles traveled.

"This is not sustainable," the website states. The consumer also learns how the wool goes through an ozone wash, which is better for the environment than chlorine.