There’s no doubt that buying and eating local food is a hot trend. But is it good for the environment?

Recently, I got a press release from Wal-Mart saying that

Partnerships with local farmers have grown by 50 percent over the past two years—one example of the company’s efforts to support local economies, cut shipping costs and provide fresh food offerings.

For the 4th of July, a Wal-Mart Supercenter in DeKalb County, Ga., featured Georgia-grown Vidalia onions for burgers, Georgia cantaloupes and watermelons for fruit salad and Georgia peaches for cobbler, the company said.

Meanwhile, Chipotle Mexican Grill reports that it has stepped up its efforts to buy local produce. The fast food chain says it is the first and only national restaurant company committed to buying local on a significant scale:

Chipotle will purchase 25 percent of at least one of its produce items for each of its 730-plus restaurants from small and mid-sized local farms. The produce, which includes romaine lettuce, green bell and jalapeno peppers, and red onions, will arrive from local farms when seasonally available

What’s more, locally grown produce was voted No. 2 on a list of nearly 200 hot trends for 2008 in a survey of more than 1,200 professional chefs conducted by the Natural Restaurant Association. (Bite-sized desserts led the list.) A week or so ago, my wife and I tried a brand-new restaurant called Redwood in Bethesda, sure enough the menu is filled with beef, cheese and produce from the mid-Atlantic states.

Last week, too, The New York Times ran a story about community-supported agriculture on its front page—a reliable lagging cultural indicator, as ever.

Now, I’m a fan of local eating. Since joining a CSA last year, I’ve consumed a lifetime’s worth of Swiss chard. Buying local food supports the local economy, cuts down on shipping costs and greenhouse gases, encourages (or requires) consumers to broaden their palette of food choices (i.e., the chard) and gets fruits and vegetables to the table when they are fresher. That’s all laudable.

But before we get carried away, let’s keep a couple of things in mind. The first is that being a locavore is utterly impractical for the vast majority of people. It’s no surprise that the local-food movement is most popular in northern California, where you can get fresh produce year-round. I’m currently on vacation in Alaska where, as best as I can tell, they grow berries, catch a lot of fish and kill caribou. Not exactly a balanced diet—in fact, it makes me wonder how the Native Alaskans survived for as long as they did without imports. I enjoy a banana on my cereal or in a smoothie, and they don’t grow in the continental U.S. (Pity the farmers of Costa Rica and Ecuador if we were all to become locavores.)

The other thing to remember is that what we eat, and how it’s produced, matters a lot more to the planet that where our food is grown. As I’ve written before, the single easiest thing any of us can do to help prevent global warming is to eat less meat. This is confirmed by a life cycle assessment by Christopher Weber and Scott Matthews of Carnegie Mellon University, who found that when it comes to greenhouse gasese, food miles matter a whole lot less than agricultural production:

They found that transportation creates only 11% of the 8.1 metric tons (t) of greenhouse gases (in CO2 equivalents) that an average U.S. household generates annually as a result of food consumption. The agricultural and industrial practices that go into growing and harvesting food are responsible for most (83%) of its greenhouse gas emissions

Small changes in dietary habits can have significant environmental impacts, they report:

Replacing red meat and dairy with chicken, fish, or eggs for one day per week reduces emissions equal to 760 miles per year of driving. And switching to vegetables one day per week cuts the equivalent of driving 1160 miles per year.

That’s because everything we eat comes from plants, whether we eat the plants directly or rely on an inefficient animal intermediary to process them for us, as the excellent new website of the Peanut Butter and Jelly campaign points out:

The basic problem is that animals are inefficient at converting plants into meat, milk, and eggs. Relatively little of what they eat ends up in what you eat because animals use most of their food to keep them alive – to fuel their muscles so they can stand up and walk around, to keep their hearts beating, to keep their brains working.

That cow, pig, or chicken has to eat a lot more protein, carbohydrates, and other nutrients than it yields in meat, eggs, or milk. The result is that it takes several pounds of corn and soy to produce one pound of beef, or one pound of eggs, one pound of milk, etc. This holds true even if we’re measuring calories or protein; it takes several times the calories or protein in livestock feed to produce the calories or protein we get from the meat, eggs, or milk.

Check out the PB&J campaign. It’s the creation of a young Wesleyan graduate who we will call Bernard Brown. (He has a day job, and isn’t sure how his employer feels about his campaign on behalf of peanut butter sandwiches). “If you have a PB&J instead of a red-meat lunch like a ham sandwich or a hamburger,” he says, “you shrink your carbon footprint by almost 3.5 pounds of greenhouse gas emissions.”

Bernard’s little web startup, launched early 2007, has generated some interesting ripples, including coverage in Good Housekeeping magazine and support from, of all places, the giant food services company Sodexho which worked the PB&J campaign message into Earth Day events on several college campuses. He’s even designed a PB&J campaign T-shirt (made with organic cotton, natch) and baby bib.

No offense to the locavores, but I’ll take a PB&J sandwich over a plate of Swiss chard any day.