Environmental "food-prints" are determined not just by how far food travels, but also by what we eat and how it was produced, explains DeWeerdt.
"In 1993, a Swedish researcher calculated that the ingredients of a typical Swedish breakfast - apple, bread, butter, cheese, coffee, cream, orange juice and sugar - travelled a distance equal to the circumference of the Earth before reaching the Scandinavian table," she writes.
"In light of such contrasts, the admonition to 'eat local' just seems like common sense," she states. But "what exactly is 'local food' in the first place? How local is local?," she asks.
"Sometimes local means food grown within a county, within a state or province, or even, in the case of some small European nations, within a country," DeWeerdt explains.
"All of those are perfectly valid ways of thinking […] local. But they don't have all that much to do with environmental costs and benefits," she adds.
The environmental impact of food "depends on how the food was transported, not just how far," underlines DeWeerdt.
"For example, trains are 10 times more efficient at moving freight, ton for ton, than trucks are. So you could eat potatoes trucked in from 100 miles away, or potatoes shipped by rail from 1,000 miles away, and the greenhouse gas emissions associated with their transport from farm to table would be roughly the same," she explains.
"The environmental impact of food also depends on how it is grown," the author adds, citing the fact that from a greenhouse-gas perspective, it is better for Swedes to buy Spanish tomatoes than Swedish ones. This is because Spanish tomatoes are grown in open fields, while local ones are grown in fossil-fuel-heated greenhouses.
"Life-cycle analysis (LCA) [the phases of food production, transport and consumption] reveals that food miles represent a relatively small slice of the greenhouse-gas pie," DeWeerdt explains.
Moreover, "what you eat matters at least as much as how far it travels," she adds. "A large portion of emissions associated with meat and dairy production take the form of methane and nitrous oxide, greenhouse gases that are respectively 23 and 296 times as potent as carbon dioxide," she explains.
Thus, local food is not necessarily greener, but "if you're a consumer interested in greener food, the local food economy is currently a good place to find it," De Weerdt concludes.
She also calls for greater investment in rail infrastructure, better fuel economy standards and a "carbon-pricing system [which incorporates] some of the environmental costs of agriculture that are currently externalised" to reduce environmental food-prints.