Ms. Mock discovered that how far away food was grown matters a lot less than how it was grown and whether it traveled by air. There's usually no sign in the store for these things, but she found a few tricks for picking produce in the winter.
First, just eating a more plant-based diet is a great start. Consuming less meat and dairy and reducing food waste are the two most effective ways to reduce your dietary carbon footprint, said Richard Waite, an associate in the food program at the World Resources Institute.
When buying off-season produce, be skeptical of anything that's perishable but super fresh, Mr. Waite said. While transportation accounts for only about 6 percent of food-related greenhouse gas emissions globally, not all modes of travel are created equal. Anything that travels by air, like fresh blackberries or asparagus from South America, is going to burn a lot of carbon.
Also avoid fruits and vegetables that were likely to have been grown in a heated greenhouse, said Martin Heller, who studies sustainable food systems at the University of Michigan. For example, if you live in New York and are choosing between a fresh tomato grown in wintry Canada versus one trucked in from Mexico or California, go with the one from southern climes. Even having been shipped farther, those southern tomatoes probably have a smaller footprint than the hothouse variety because it takes a lot of energy to keep a greenhouse warm during the cold months.
Root vegetables, winter squash and hearty greens are all good winter bets, Dr. Heller said. Carrots, potatoes and squash have long shelf lives, so they can sprout up in summer and still hit stores in the winter. And many greens can thrive under covered, unheated greenhouse conditions, growing fresh well into the colder months without the need for carbon-intensive heating.
"Food choice is one of the easiest things for people to change," said Chris Weber, a scientist at the World Wildlife Fund who has studied food shipping miles and climate impacts. "Because when you compare it to something like your transportation choices or home energy use, food is something you choose every day."