Is buying local always best?

But here critics push back. Thanks to superefficient shipping systems, the amount of fuel used per unit of food is "minuscule," says Alex Avery, director of global food research at the Hudson Institute in Washington, D.C. He suggests the best way to minimize agriculture-related emissions is to buy food from the world region where it grows best.

"Efficiency is what makes the difference for the environment," because it reduces total carbon output, Mr. Avery says. "If you can leave an acre wild [by making other acreage more efficient], that's a conservation tool."

Clark takes the point one step further. He says biases in favor of local production techniques can lead not only to wasteful energy systems such as growing bananas in domestic hothouses, but also to a mistaken idea that techniques most familiar to consumers are also ecofriendly.

If local farmers "are using tractors, as they most certainly will be, then probably right from the start that means the food is less energy efficient in terms of oil use than hand-plow or ox-plow production in a developing country," Clark says. "And so it can be very deceptive to say that because it's local, it's avoiding all of these problems."

Whether buying local brings more social benefit than detriment is another point of contention. Proponents of the practice insist it is critical for maintaining strong communities, connected by neighborhood shops and sustained by their region's crops, in an age of fragmentation and alienation from one another.

A lack of connectedness "is probably why we have so much depression," says Guillermo Payet, founder and president of LocalHarvest, an Internet-based clearinghouse where small-scale farmers and consumers find one another.

To the notion that farmers overseas likewise need American dollars to keep their communities strong, Mr. Payet counters with recollections from his native Peru: "Stuff that's grown for export just goes to enrich the elites down there."

What's more, Ms. Mitchell says, to patronize local businesses is to support those companies that give most generously, per dollar in revenue, to local charities. The practice also enhances diverse thinking in a community because it supports retailers who carry books, movies, and music that aren't available in national chain stores.

Others, however, wonder about the cost – in terms of Americans' ties to foreign communities – of shunning goods made far away and, in some cases, marketed via national chains. Among those concerned is Roy Jacobowitz, senior vice president for development and communications at Acción International, a Boston-based nonprofit lender to micro-entrepreneurs in developing nations.

"The 'buy locally' argument is an isolationist argument, which I think is a dangerous one," Mr. Jacobowitz says. The danger, he says, comes in shutting the door to the reality: "Poor entrepreneurs in the emerging world need the opportunity to sell into markets that can pay fair prices for their goods." But if American consumers insist on buying local, he says, dreamers in the developing world will never reach their goals.

Voices in this debate admit few consumers stick 100 percent to any shopping policy. Avery, for instance, believes in supporting large-scale agricultural efficiencies, but he also supports one of his local cattle ranchers near Stanton, Va., by joining three neighbors and buying all the meat from one steer each year. But although he's intentionally supporting a local farm, he admits it isn't for ethical reasons.

"I don't want to see the Shenandoah Valley become another northern Virginia" in terms of converting farmland to development, he says. "It's very selfish. Am I really acting ethically if I'm acting selfishly?"

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