We've talked about this before but I found a link to this really interesting article on Andrew Sullivan's blog.
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.
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."