Explainer: The carbon footprint of local versus global food

Rod MacRae, a York University food expert and director of standards development at Local Food Plus, crunched the numbers to compare the environmental and economic ripple effects of buying food produced within 200 kilometres of four major Canadian cities versus food transported from Florida or California.

The science of calculating carbon footprints is imperfect, though, and some assumptions were necessary to simplify the food math. Specific numbers will vary in real-life situations depending on factors such as age and maintenance of the truck, and methods by which produce was grown. Prof. MacRae's calculations are designed to illustrate a general picture.

For his import scenarios, Prof. MacRae used a refrigerated, 18-wheel truck carrying 10 tonnes of produce (an industry norm for long-haul transport).

For the domestic scenarios, Prof. MacRae used a smaller, but still refrigerated, five-tonne truck (the industry standard for short hauls), which emits more carbon than a large truck, but makes shorter trips. For simplicity, the carbon impact of refrigeration in both scenarios was scored as a 14-per-cent addition to the emissions estimated per load.

Using the results of these calculations, Prof. MacRae was able to illustrate the environmental impact of shifting just one truck per week during feasible growing season (he allowed 30 weeks) to locally grown food. In Toronto, doing so is equivalent to taking 75 cars off the road per year. For Halifax, it means a 60-car reduction; for Vancouver, 45, and for Winnipeg, 56.

He also tried to illustrate the impact that a single family can have on the environment by shifting $10 per week (or one kilogram) of their produce purchases to local food - his calculations are based on the assumption that the average family in Canada spends about $7 a week on fresh vegetables (that buys about five kilograms).