Food grown far away has to ride to you on pollution-belching trucks or ships or coal-fired bulldozers, all of which result in carbon dioxide entering the atmosphere. Consequently, the theory goes, we can dramatically reduce the environmental impact of what we eat by buying food grown locally. That's sort of true, although don't look surprised when I tell you there's a "but."
The "but" is that it takes a lot more carbon to simply grow the food, with production accounting for something like 83 percent of the total carbon footprint. This means that a farm that's even slightly more efficient at growing might be the better choice, even if it's farther away. For example, if you're buying lamb in the U.K., one study has calculated that it's four times more carbon intensive to buy locally grown lamb than lamb shipped all the way from New Zealand. That study was admittedly performed by a New Zealand university, which might not be entirely unbiased on the subject of lamb exports, but the same thing has been found in a lot of other cases as well.
Pictured: A New Zealand university.
Also, these "food mile" calculations elevate carbon dioxide above all other pollutants and environmental impacts. A full life-cycle consideration of the environmental impacts of food might reveal a dramatically different conclusion from "local food is better." But seeing as that kind of spreadsheet heavy-nerd-lifting is the kind of thing people don't want to do in a grocery store, here's a super simple shorthand trick you can use to reduce the environmental impact of your food: