My understanding is that it's really not calculable. There's a reference to calculating food miles in The Omnivore's Dilemma and unless you know exactly where stuff comes from and what went into it, there's no way to figure precisely. The calculations done by the person in the book (not Michael Pollan but someone he interviews) determine that for the food he was eating, it's about even. If you were comparing local meat vs. local veg, then it would be obvious, but local meat vs. imported veg, my guess would be that they're fairly evenly matched in terms of impact.
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My understanding is that it's really not calculable. There's a reference to calculating food miles in The Omnivore's Dilemma and unless you know exactly where stuff comes from and what went into it, there's no way to figure precisely. The calculations done by the person in the book (not Michael Pollan but someone he interviews) determine that for the food he was eating, it's about even. If you were comparing local meat vs. local veg, then it would be obvious, but local meat vs. imported veg, my guess would be that they're fairly evenly matched in terms of impact.