UK Supermarket giant Tesco launched a new labeling system this week to indicate the carbon footprint of various products. With hopes for wider adoption and even a new industry code, the symbols will start out on just 20 different items. From yesterday, you'll be able to compare the store's own brand light bulbs, potatoes, orange juice and laundry powder, and choose the one with the lowest CO2 emissions. Last January the head of Tesco, Sir Terry Leahy, pledged to bring about a "revolution in green consumption", and better information for consumers is a cornerstone of his new green agenda. The company has spent the months since Sir Terry's speech working with the Carbon Trust to make a start on its 70,000 different products.

So, 15 months in and we have the footprints of 20 items. Even this has been notoriously difficult, as it's tricky to know where to start or stop. Problems included the bodily emissions of cows -- should a cow's methane-loaded farts be accounted for in a steak's carbon footprint? And what about the post-purchase shelf life of products? Researchers found that a bottle of ketchup kept in the refrigerator and used slowly over the course of a year would represent 90% more energy than one used up in a month.

Whether the system succeeds or fails now depends on consumer response. Early trials with Walker's crisps have been favorable, and other companies are now signing up to the Carbon Trust's scheme. Tesco's weight behind the labeling may now be enough to push it into the mainstream. Steve Howard of The Climate Group sums up the ambitions of the project: "Consumers have been able to count calories for a long time. This label will allow consumers to count carbon and to choose products that result in a low carbon diet. Suppliers will then start to compete for the cleanest, greenest supply chain."

It's a nice idea, but I've got mixed feelings about it so far. On the one hand you have to admire the boldness of it all, and giving consumers the information to choose for themselves has got to be a good thing. On the other hand, I'm not sure it works yet. I looked up the labeling system myself on a packet of Walkers crisps recently, and discovered that the snack-sized bag had a carbon footprint of 75 grams of CO2. That's mildly interesting, but not especially useful. 75g is more than the weight of the product itself, I notice, but without any comparison or standard, there's no way of knowing if that's good or bad. I can understand concepts like organic or fair trade because I know what they are. I can use initiatives like energy efficiency ratings, because they give me a top and a bottom. This little CO2 figure is different, it's just so abstract. Perhaps it's an early adoption problem, and when all the other products have labels too maybe I'll be able to make a genuine choice. But even then, how many people are going to choose a snack on the basis of CO2 emissions, rather than taste, or price?

Still, perhaps the market will surprise me and there'll be a race to the bottom as companies compete for the lowest possible footprint. In the meantime, I'll be continuing to apply my own guidelines for lowering CO2 emissions - buy fresh, buy local, and above all, buy less. That's a message you're unlikely to hear from a supermarket.

Incidentally, Tesco has over 1,600 stores in the UK, and hit the US recently under the name Fresh and Easy. Its corporate carbon footprint is estimated at over 4 million tonnes.