I'm no expert, but aren't there other reasons that local food is a good idea, at least for foods that can be grown locally (e.g. there's no such thing as a Michigan banana)? For example, there are a lot of community gardens around my neighborhood. They make the place so much more pleasant, IMO, because it turns an abandoned lot into a well manicured space with lots of different types of plants. The argument that urban farms disaggregate populations is specious, because it's typically only already unused space that is used for gardening in the first place. No one is buying high priced real estate to turn into a garden. Also, lots of perishable food can only be grown locally. Some varieties of tomatoes that taste quite a bit better do not ship well. Therefore, aggregating all agriculture would kill a lot of otherwise desirable food. Lastly, there is some (difficult to quantify) value in knowing a bit about where your food originates from.

I'm not arguing with the calculus of CO2 emissions; I've seen several reports that support the argument that local foods can be more CO2 intensive. But I think there are other factors besides CO2 that matter. I think if we quit a lot of the vast farm subsidies, then greenhouse gas emissions would drop enough (by the rising price of meat that would be inevitable) to offset those emission created by the limited number of people who drive the local food movement.