A lot of people have begun to lose their appetite lately at the thought that their food travels, on average, 1,500 miles from farmer to dinner plate. Buying, instead, from local farmers looks increasingly appealing: We get fresher produce (and benediction from Alice Waters), while also preserving open space and protecting local jobs.

But what’s really lifted the “buy local” movement out of the foodie realm and into general public awareness is fear of climate change: It suddenly seems dangerously profligate that we spend 36 calories of fossil fuel energy transporting one calorie of California lettuce to a consumer in New York. Likewise that apples in a New England supermarket come from New Zealand, or potatoes in Ireland from Cyprus, or flowers in the Netherlands from Kenya. Carrying carbon to Newcastle seems to be among the chief functions of modern international trade.

So my first reaction was to think that buying local makes a lot of sense. And if it’s true for food, what about the pots we cook that food in, or the furniture we sit on, or the cars we drive to the supermarket? When does “Buy American” morph from jingoism to progressivism?

And yet buying local may not be the simple answer we’re looking for. For starters, it’s more likely to hurt American farmers than help them. Agriculture is one area where the United States still enjoys a trade surplus, amounting to $5.66 billion last year. But the “buy local” movement is strongest in Europe, where it got its start, and American agricultural products feature prominently among the targets.

The “local” label also says little or nothing about a product’s actual environmental friendliness. A resident of Sacramento, for instance, can take comfort in buying “local” rice, but it’s still likely to be rice grown in a heavily irrigated desert, at huge environmental cost. In the overall carbon footprint of a product, the cost of transport often turns out to be relatively trivial. For instance, a New Zealand study recently made the case that better conditions make lamb grown there and shipped to Europe four times more energy-efficient than home-grown European lamb.

The opposite is true for products that must be air-freighted, like flowers and certain fresh seafood; good sushi probably comes with a huge carbon footprint. But sea freight can be surprisingly efficient, even for heavy manufactured goods. I asked an environmental group, the American Council for an Energy-Efficient Economy, to calculate the cost of getting an average car from Tokyo to San Diego, and we were all surprised that it came to between 1,000 and 1,800 pounds of carbon dioxide emissions. That’s close to what the same car will typically produce every month for the rest of its driving life.

But how do you factor a product’s total carbon footprint into the debate over international trade? For instance, would it make sense to impose a carbon tax at our borders, so countries that fail to control their global warming emissions, like China and India, don’t get an unfair competitive advantage over countries that take global warming seriously? Great idea. Kyoto-signatory nations in Europe are already talking about taking that kind of stand–against the United States.

Or maybe we could piggyback on cap-and-trade systems like the one already functioning in Europe. These systems impose mandatory overall limits on global warming emissions within a nation or region, but allow businesses that do better at meeting targets to sell carbon credits to businesses that do worse. To enter a market with a cap-and-trade system, an importer would have to compensate for a product with a big carbon footprint by adding the cost of carbon credits into the price. Such a system would catch countries or individual manufacturers that refuse to act on global warming (again possibly including the United States).

But either the carbon tax or the credit system is likely to lead to years of litigation through the World Trade Organization, according to Elliot Diringer of the Pew Center on Global Climate Change. That’s because the subtext in both approaches is confrontational and protectionist. Avoiding the us-and-them mindset and seeking collaborative solutions makes far more sense when scientists increasingly suggest that all of us together could soon be up to our knees in the rising consequences of global warming.

So where does all this leave the individual shopper trying to make good choices? Tesco, Britain’s largest retailer, is now working to put a “carbon label” on every product it sells. Instead of the comfortable illusion of environmentalism provided by the “buy local” idea, this label will detail the actual global warming cost of a product. And that will probably show that it makes sense to buy that compact fluorescent lightbulb, even if it was made in China. And, yes, the climate will probably be better off if you buy a Prius manufactured in Japan, not a Cadillac Escalade made in the United States.

Beneath the surface, the urge to buy local is often just a disguised version of the urge to punish someone foreign. But as a way to fix global warming, fretting about where your salad was grown is like thinking you can win a war by calling your sauerkraut “liberty cabbage.”