In recent years, one part of the food business has rivaled organics as the hot growth area: “local” food (defined vaguely as coming from the same state or from less than 100 miles away, for example). It’s a market segment that has just about doubled in sales and number of outlets over the last decade. The world’s biggest food buyer, Wal-Mart, jumped on the bandwagon last fall and announced that it would double the amount of local food it sells (to 9 percent of all its food sales).

The idea of buying locally is not new, and farmers’ markets have been big for years. It’s become almost gospel that the food on our plates has traveled about 1500 miles to get to us.

So it would seem logical that the best way to shrink your food-related carbon footprint associated would be to buy from near by. But it turns out that this assumption is wrong.

Thankfully, a couple scientists took a harder look at the data and published an analysis in the Journal of Environmental Science and Technology. The abstract for this article is a prime example of clear writing and good lifecycle analysis — which don’t usually go together — so check it out. But here’s the essence:

Food is transported a long way, going about 1,000 miles in delivery and over 4,000 miles across the supply chain.

But 83% of the average U.S. household’s carbon footprint for food comes from growing and producing it. Transportation is only 11%.

Different foods have vastly different greenhouse gas (GHG) intensity, with meat requiring far more energy to produce, and red meat being particularly egregious, requiring 150% more energy than even chicken.

So the journal article adds this up to an obvious conclusion: if you want to reduce your food’s carbon footprint, eat less meat. In short, “Shifting less than one day per week’s worth of calories from red meat and dairy products to chicken, fish, eggs, or a vegetable-based diet achieves more GHG reduction than buying all locally sourced food.”

As a numbers geek, I love this kind of analysis. Now for the caveats: none of this data should dissuade anyone from eating locally also. The footprint benefits are real, even if dwarfed by food choice. And the benefits to local economies and smaller farms are very important.

But let me repeat: just moving away from meat for one day a week is more effective than buying everything you eat locally. This number will be surprising to most people, but it’s partly why the global call for “Meatless Mondays” is gaining steam, with school systems and universities adopting the approach in cities around the world, from Baltimore to Tel Aviv.

As companies keep discovering, it really helps to run the numbers. As I’ve written about before, Pepsi discovered that the largest chunk of the footprint of its Tropicana orange juice was not in production (squeezing oranges) or in distribution (shipping heavy liquids is fuel-intensive), but in growing the oranges with natural-gas-based fertilizer.

Smart, knowledgeable execs are consistently surprised when good lifecycle data trumps seemingly solid assumptions. So we shouldn’t expect consumers to figure out the right choices themselves. Buying local food seems like the obvious choice — until you run the numbers.

We have a lot of work to do, both in companies and in our homes, to tackle climate change. Good data and analysis will let us focus on the quickest paybacks and get the most out of our efforts.