I got into another discussion with a journalist today about the environmental effect of eating local. I don't believe its a solution, and neither does Paul Roberts at Mother Jones, who has an interesting article on how inefficient local production is.
The most interesting line in the article is on the aggregate effect of eating local: "if we wanted to rid the world of synthetic fertilizer use—and assuming dietary habits remain constant—the extra land we'd need for cover crops or forage (to feed the animals to make the manure) would more than double, possibly triple, the current area of farmland".
Someone in the comments section of the article disagrees with this statement by discussing the output of Joel Salatin's farm Polyface in Virginia (the farm is described in detail in Omnivore's Dilemma). But this farm is the perfect example of how environmentally sustainable is not scalable. The farm does not produce at the yeild rates necessary to meet current food demands.
Which brings us to the real problem: we consume too much. As Mark Bitman discusses in his new book, cutting back on our most innefficient foods (like beef) is the only way to reach a sustainable food consumption level.
(hat tip to Matthew Yglesias)