To choose a locally grown apple over an apple trucked in from across the country might seem easy. But this decision ignores economies of scale. To take an extreme example, a shipper sending a truck with 2,000 apples over 2,000 miles would consume the same amount of fuel per apple as a local farmer who takes a pickup 50 miles to sell 50 apples at his stall at the green market. The critical measure here is not food miles but apples per gallon.At our house, we're seasonal locavores - especially during the late summer when the farmers' market is full of sweet corn and veggies. We do it not to save the planet, but to enjoy the unparalleled taste of sweet corn picked the same day as it is eaten (when it sits on a shelf in the store, the sugar turns to starch).
The Forbes article linked above addresses some of the complexity involved in deciding when and whether local food is more carbon-friendly than imported food. I would take issue with their apple-shipping example above, because the two trucks might not get the same mpg, but their point is that if all carbon balance is considered, there are situations when export/import is better for the planet than local production. I've seen the same argument apparently validated in terms of flowers - that's it's better for the planet to grow them in tropical areas and ship rather than in greenhouses in northern climes. Food quality and local economics are factors that aren't directly addressed in the Forbes article.
