Let's suppose that people do decide to "buy local" with the goal of saving the world and reducing their carbon footprint. This will increase the demand for locally grown foods, but it will also have an unintended and likely deleterious consequence; it will increase the demand for farm implements and labor.

Since the decision to buy locally is essentially the decision to forsake comparative advantage, every unit of agricultural output will be more resource intensive than it would be under specialization, division of labor, and trade.

In other words, each additional unit of output will require more resources than it would under trade. To take a concrete example, this means that the cultivation of spinach in Memphis will require more fertilizer, more rakes, more tillers, and more hoes than the cultivation of spinach in California. Producing these implements will (again) require resources, which will require specialization and trade. We could push the problem back a step and say that we should only use locally produced implements, but we can only regress so far before we run into an obvious problem of definition (how "local" is "local"?), resource constraints (different regions have different natural endowments), and widespread destruction (denuded forests and gouged lands as people assemble locally produced stone tools for cultivation).

"Buy local" is, at its logical limit, a prescription for poverty and starvation