By now, we're familiar with the usual culprits of global warming: smokestacks, cars and trucks, deforestation. But do you know the environmental impact of a bag of potato chips? British consumers do. Starting this summer, it has been listed on every bag of Walkers Crisps -- the third-most recognizable brand in Britain -- right next to the number of calories and fat content.

For each bag, that's 75 grams of carbon emissions to be exact. The number is based on a formula developed by a government-funded Carbon Trust, which takes into account everything that goes into producing and consuming the chips: from growing the potatoes to cooking them to transporting them, right up to disposing of the bag.

"We are doing it to give consumers useful information," said Neil Campbell, the CEO of Walkers Crisps. "And that's what we find. People like the intent behind it, and they like the commitment to reduce over time."

Walkers is the first company to take part in carbon labeling, but soon 20 other British companies will follow, including Tesco, Britain's largest supermarket chain, and Boots, its largest chain of pharmacies. (All those cosmetics have an environmental impact as well.)

The idea, backed by the British government, is to give consumers choices between, say, the chips and a mango-passion fruit smoothie, sold by Innocent drinks, another British company taking part in carbon labeling. You may be surprised to hear that that healthy smoothie, which has less fat and calories than the chips, contributes four times the carbon, in part because all the mangoes are shipped in from thousands of miles away.

Carbon labeling is meant to remind people that they leave behind a carbon footprint wherever they go, whatever they do, right up to virtually everything they consume inside their homes.

In Britain, food production and consumption adds a third of a ton of carbon to the atmosphere per household per year. Recreation, such as driving to a soccer game, contributes a ton. Home heating and electricity add another two tons. If that sounds like a lot, the average American household contributes twice as much carbon per year.

Surveys in Britain show that most people here like the idea of carbon labeling. That's what we found when we interviewed shoppers at a central London supermarket.

"I worry about it all the time. I'm a bit of a worrier," said Sally Bradshaw. "I think everyone has to think about it. We've got to change."

Participating companies are changing as well. To take part, they have to reduce their own carbon footprint.

"We've reduced our energy by a third over six years," said Campbell. "And we've reduced our water usage by a half."

Still, some environmental activists say labeling is simply a way of shifting responsibility from businesses to consumers.

"I think carbon labeling is useful up to a point, but it's important that we don't put too much responsibility for emissions onto consumers alone," said Kevin Smith at Carbon Trade Watch. "I think there's a danger that these sorts of voluntary initiatives are going to detract from corporations and governments taking responsibility for the large share of emissions that they're responsible for in the way goods are manufactured."

Smith also worries that carbon labeling will delay far more difficult choices ahead.

"It's not a question of consuming product A over product B." said Smith. "It's a question of consuming a whole lot less."

It's a point that becomes much clearer when you realize that one bag of chips amounts to one thousandth of 1 percent of the carbon the average person produces a year.

It's something to think about. And maybe that's the point of labeling: thinking about what we consume and the consequences.