I stopped by my favorite boutique grocery store to pick up a red onion today. The young clerk running the cash register wore a t-shirt with the slogan "Eat Local." Oddly, the shop's shelves and coolers were stuffed with cheeses, sausages, olives, jams, cookies, and crackers from California, France, Italy, Spain, Belgium, Germany, and many other exotic locales. As I walked home, I mused over the fact that I needed the onion to go with the organic Irish salmon and the Spanish capers my wife and I were having for dinner. The salmon was a gift from a visiting friend from Dublin. Now, I enjoy seeking out and eating locally produced foods. My wife and I make it a habit to shop at our town's weekly farmers market for fresh fruits and vegetables.

But for some activists, eating local foods is no longer just a pleasure—it is a moral obligation. Why? Because locally produced foods are supposed to be better for the planet than foods shipped thousands of miles across oceans and continents. According to these activists, shipping foods over long distances results in the unnecessary emission of the greenhouse gases that are warming the planet. This concern has given rise to the concept of "food miles," that is, the distance food travels from farm to plate. Activists particularly dislike air freighting foods because it uses relatively more energy than other forms of transportation. Food miles are supposed to be a simple way to gauge food's impact on climate change.

In their recent policy primer for the Mercatus Center at George University, however, economic geographer Pierre Desrochers and economic consultant Hiroko Shimizu challenge the notion that food miles are a good sustainability indicator. As Desrochers and Shimizu point out, the food trade has been historically driven by urbanization. As agriculture became more efficient, people were liberated from farms and able to develop other skills that helped raise general living standards. People freed from having to scrabble for food, for instance, could work in factories, write software, or become physicians. Modernization is a process in which people get further and further away from the farm.

Modern technologies like canning and refrigeration made it possible to extend the food trade from staple grains and spices to fruits, vegetables, and meats. As a result, world trade in fruits and vegetables—fresh and processed—doubled in the 1980s and increased by 30 percent between 1990 and 2001. Fruits and vegetables accounted for 22 percent of the exports of developing economies in 2001. If farmers, processors, shippers, and retailers did not profit from providing distant consumers with these foods, the foods wouldn't be on store shelves. And consumers, of course, benefit from being able to buy fresh foods year around.

So just how much carbon dioxide is emitted by transporting food from farm to fork? Desrochers and Shimizu cite a comprehensive study done by the United Kingdom's Department of Environment, Food and Rural Affairs (DEFRA) which reported that 82 percent of food miles were generated within the U.K. Consumer shopping trips accounted for 48 percent and trucking for 31 percent of British food miles. Air freight amounted to less than 1 percent of food miles. In total, food transportation accounted for only 1.8 percent of Britain's carbon dioxide emissions.

In the United States, a 2007 analysis found that transporting food from producers to retailers accounted for only 4 percent of greenhouse emissions related to food. According to a 2000 study, agriculture was responsible for 7.7 percent of total U.S. greenhouse gas emissions. In that study, food transport accounted for 14 percent of the greenhouse gas emissions associated with agriculture, which means that food transport is responsible for about 1 percent of total U.S. greenhouse gas emissions.

Food miles advocates fail to grasp the simple idea that food should be grown where it is most economically advantageous to do so. Relevant advantages consist of various combinations of soil, climate, labor, capital, and other factors. It is possible to grow bananas in Iceland, but Costa Rica really has the better climate for that activity. Transporting food is just one relatively small cost of providing modern consumers with their daily bread, meat, cheese, and veggies. Desrochers and Shimizu argue that concentrating agricultural production in the most favorable regions is the best way to minimize human impacts on the environment.

Local food production does not always produce fewer greenhouse gas emissions. For example, the 2005 DEFRA study found that British tomato growers emit 2.4 metric tons of carbon dioxide for each ton of tomatoes grown compared to 0.6 tons of carbon dioxide for each ton of Spanish tomatoes. The difference is British tomatoes are produced in heated greenhouses. Another study found that cold storage of British apples produced more carbon dioxide than shipping New Zealand apples by sea to London. In addition, U.K. dairy farmers use twice as much energy to produce a metric ton of milk solids than do New Zealand farmers. Other researchers have determined that Kenyan cut rose growers emit 6 metric tons of carbon dioxide per 12,000 roses compared to the 35 tons of carbon dioxide emitted by their Dutch competitors. Kenyan roses grow in sunny fields whereas Dutch roses grow in heated greenhouses.

Nevertheless, organic food activists in Britain's Soil Association argued for lifting the organic certification from Kenyan food exports because they are brought into Britain on airplanes. Some high-end British retailers have begun slapping a label featuring an airplane on various food products to indicate that they have been air freighted. Kenyan growers cannily responded by launching their own "Grown Under the Sun" label, pointing out that their agricultural production methods emit far less greenhouse gases than many crops grown in Britain do.

A die-hard response to the above studies would be: Don't eat either British or Spanish tomatoes out of season; don't cold store apples, dry them in the sun instead; don't ever eat dairy products; and give your true love a bouquet of in-season root vegetables for Valentine's Day. In order to reduce your food miles, the National Sustainable Agriculture Information Service makes these recommendations: Eat foods that are in season; eat minimally processed, packaged, and marketed food; use public transportation when grocery shopping; can and dry fruits and vegetables yourself; and plant a garden and grow as much of your own food as possible. In other words, spend more time and effort finding, growing, and preparing food at the expense of other productive or leisure activities.

Desrochers and Shimizu demonstrate that the debate over food miles is a distraction from the real issues that confront global food production. For instance, rich country subsidies amounting to more than $300 billion per year are severely distorting global agricultural production and trade. If the subsidies were removed, far more agricultural goods would be produced in and imported from developing countries, helping lift millions of people out of poverty. They warn that the food miles campaign is "providing a new set of rhetorical tools to bolster protectionist interests that are fundamentally detrimental to most of humankind." Ultimately, Desrochers and Shimizu's analysis shows that "the concept of food miles is...a profoundly flawed sustainability indicator."

Ronald Bailey is reason's science correspondent. His book Liberation Biology: The Scientific and Moral Case for the Biotech Revolution is now available from Prometheus Books.